Icarus 179 (2005) 174–194
www.elsevier.com/locate/icarus
Physical characteristics of Comet Nucleus C/2001 OG108 (LONEOS)
Paul A. Abell a,∗,1,2 , Yanga R. Fernández b,3 , Petr Pravec c , Linda M. French d,4 ,
Tony L. Farnham e , Michael J. Gaffey f,1 , Paul S. Hardersen f,1 , Peter Kušnirák c ,
Lenka Šarounová c , Scott S. Sheppard g , Gautham Narayan d
a Planetary Astronomy Group, Astromaterials Research and Exploration Science, NASA Johnson Space Center, Mail Code KR,
Houston, TX 77058-3696, USA
b Institute for Astronomy, University of Hawai’i, Honolulu, HI 96822, USA
c Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-25165, Ondřejov, Czech Republic
d Department of Physics, Illinois Wesleyan University, Bloomington, IL 61702, USA
e Department of Astronomy, University of Maryland, College Park, MD 20742, USA
f Department of Space Studies, University of North Dakota, Grand Forks, ND 58202, USA
g Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
Received 26 September 2004; revised 28 May 2005
Available online 24 August 2005
Abstract
A detailed description of the Halley-type Comet C/2001 OG108 (LONEOS) has been derived from visible, near-infrared, and mid-infrared
observations obtained in October and November 2001. These data represent the first high-quality ground-based observations of a bare
Halley-type comet nucleus and provide the best characterization of a Halley-type comet other than 1P/Halley itself. Analysis of time series
photometry suggests that the nucleus has a rotation period of 57.2 ± 0.5 h with a minimum nuclear axial ratio of 1.3, a phase-darkening slope
parameter G of −0.01 ± 0.10, and an estimated H = 13.05 ± 0.10. The rotation period of C/2001 OG108 is one of the longest observed
among comet nuclei. The V -R color index for this object is measured to be 0.46 ± 0.02, which is virtually identical to that of other cometary
nuclei and other possible extinct comet candidates. Measurements of the comet’s thermal emission constrain the projected elliptical nuclear
radii to be 9.6 ± 1.0 km and 7.4 ± 1.0 km, which makes C/2001 OG108 one of the larger cometary nuclei known. The derived geometric
albedo in V -band of 0.040 ± 0.010 is typical for comet nuclei. Visible-wavelength spectrophotometry and near-infrared spectroscopy were
combined to derive the nucleus’s reflectance spectrum over a 0.4 to 2.5 µm wavelength range. These measurements represent one of the
few nuclear spectra ever observed and the only known spectrum of a Halley-type comet. The spectrum of this comet nucleus is very nearly
linear and shows no discernable absorption features at a 5% detection limit. The lack of any features, especially in the 0.8 to 1.0 µm range
such as are seen in the spectra of carbonaceous chondrite meteorites and many low-albedo asteroids, is consistent with the presence of
anhydrous rather than hydrous silicates on the surface of this comet. None of the currently recognized meteorites in the terrestrial collections
have reflectance spectra that match C/2001 OG108 . The near-infrared spectrum, the geometric albedo, and the visible spectrophotometry
all indicate that C/2001 OG108 has spectral properties analogous to the D-type, and possibly P-type asteroids. Comparison of the measured
albedo and diameter of C/2001 OG108 with those of Damocloid asteroids reveals similarities between these asteroids and this comet nucleus,
a finding which supports previous dynamical arguments that Damocloid asteroids could be composed of cometary-like materials. These
observations are also consistent with findings that two Jupiter-family comets may have spectral signatures indicative of D-type asteroids.
C/2001 OG108 probably represents the transition from a typical active comet to an extinct cometary nucleus, and, as a Halley-type comet,
* Corresponding author. Fax: +1 281 483 5276.
E-mail address: [email protected] (P.A. Abell).
1 Visiting astronomer at the Infrared Telescope Facility, which is operated by the University of Hawai’i under Cooperative Agreement no. NCC 5-538 with
the National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy Program.
2 National Research Council Associate.
3 Visiting astronomer at the W.M. Keck Observatory, which is jointly operated by the California Institute of Technology and the University of California.
4 Visiting astronomer at Lowell Observatory.
0019-1035/$ – see front matter  2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2005.06.012
Characteristics of C/2001 OG108 (LONEOS)
175
suggests that some comets originating in the Oort cloud can become extinct without disintegrating. As a near-Earth object, C/2001 OG108
supports the suggestion that some fraction of the near-Earth asteroid population consists of extinct cometary nuclei.
 2005 Elsevier Inc. All rights reserved.
Keywords: Comets, composition; Infrared observations; Visible observations; Comets, origin; Near-Earth asteroids, origin
1. Introduction
Cometary nuclei represent objects which have remained
relatively pristine since the early formation of the Solar System. Due to their large heliocentric formation distances (>4 AU), they probably have not been as thermally
processed as those objects residing in the inner asteroid
belt, and are more likely to contain a significant amount of
volatiles (Wyckoff, 1982). In addition, they may have sampled entirely different reservoirs of material that condensed
from the late solar nebula than their asteroid counterparts,
which formed in the innermost regions of the Solar System. Therefore, observations of cometary nuclei could give
some insights into the probable material species that existed
in these outer regions of the early Solar System, giving investigators a better understanding of the possible physical
properties of these objects, and a more complete inventory
of the distribution of materials within the early Solar System
(Kidger, 2002). Such observations should not only provide
clues about the possible compositional characteristics of
comets, but also provide information on their evolutionary
history, and potential relationship to other bodies, such as
interplanetary dust particles, meteorites, and asteroids.
For the past few decades, scientists have been exploring
possible connections between specific populations of Solar System bodies and cometary nuclei. One such group of
objects that has been the focus of this kind of investigation is the near-Earth asteroid population. These asteroids
have been recognized as some of the more scientifically interesting objects in the Solar System because, unlike the
orbits of asteroids in the mainbelt population, near-Earth
asteroid orbits are not stable over the 4.6 billion-year age
of the Solar System (Öpik, 1951). Their dynamical lifetimes are only on the order of ∼106 to 108 years due to
interactions with other objects in the inner Solar System
that cause them to either impact one of the inner planets or the Sun, or be ejected from the Solar System altogether (Morbidelli and Gladman, 1998). Hence the presence of these bodies requires a mechanism(s) and source
region(s) to replenish and maintain the near-Earth asteroid population over time. The mainbelt asteroids have been
recognized as one of the primary sources of material for
the near-Earth asteroid population (McFadden et al., 1985;
Morbidelli et al., 2002), but several investigators have suggested that a nonnegligible portion of the near-Earth asteroid
population could also be replenished by cometary nuclei that
have evolved dynamically into the inner Solar System from
such reservoirs as the Edgeworth–Kuiper belt and the Oort
cloud (Öpik, 1961, 1963; Wetherill, 1971; Kresák, 1979;
Shoemaker et al., 1979; Degewij and Tedesco, 1982; Weissman et al. 1989, 2002; Harris and Bailey, 1998).
Evidence used to support the hypothesis of a cometary
component to the near-Earth asteroid population was based
on: observations of asteroid orbits and associated meteor
showers (e.g., 3200 Phaethon and the Geminid meteor
shower) (Whipple, 1983; Fox et al., 1984; Olsson-Steel,
1988; Williams and Wu, 1993); low activity of shortperiod comet nuclei, which implied nonvolatile surface
crusts (e.g., 28P/Neujmin 1, 49P/Arend-Rigaux) (A’Hearn,
1988); and detection of possible transient cometary activity in a near-Earth asteroid (e.g., 4015 Wilson–Harrington)
(Cunningham, 1950; Bowell et al., 1992; McFadden, 1993;
Fernández et al., 1997). Previous dynamical studies have
concluded that as much as 40–50% of the near-Earth asteroid population could be due to extinct comets (Wetherill,
1988, 1991; Binzel et al., 1992), but more recent investigations based on physical and dynamical evidence have
suggested that approximately 5–10% of the near-Earth asteroid population may be extinct comets with the remaining
fraction made up of fragments from parent bodies within
the mainbelt asteroid population (Fernández et al., 2001;
Bottke et al., 2002).
The uncertainty of the cometary contribution to the nearEarth asteroid population is partly due to the lack of an
observational discriminator that will distinguish between an
extinct comet and a “true” asteroid. In addition, the observational techniques successfully used to study asteroid surfaces
are usually not applicable to comets because their comae
dominate any signal from the nucleus during their perihelion
passages (Weissman et al., 2002). In order to examine known
comet surfaces directly, it is either necessary to study them
during their quiescent phase, when they are at large heliocentric distances and thus extremely faint (i.e., low signal-tonoise), or to develop spacecraft missions to rendezvous with
them. Both of these observational strategies have produced
results, but only a few cometary nuclei have been adequately
studied in enough detail to constrain their physical characteristics and compositions through near-infrared spectroscopy
(Soderblom et al., 2002; Licandro et al., 2002, 2003).
However, if a significant fraction (∼5–10%) of the nearEarth asteroid population is actually composed of extinct
cometary nuclei, there should be some objects within this
subset of the population that demonstrate low-levels of
coma. These low-activity comets may represent objects
undergoing the transition from active comets to extinct
cometary nuclei. Such objects may have been nearly de-
176
P.A. Abell et al. / Icarus 179 (2005) 174–194
pleted of their entire volatile content, or at least their nearsurface volatiles, leaving behind a nonvolatile and poorly
conducting surface crust that could prevent solar insolation from sublimating subsurface ice (Whipple, 1950;
Weissman, 1980; A’Hearn, 1988; Levison and Duncan,
1994, 1997; Weissman and Levison, 1997; Weissman et al.,
2002). Coma from these objects would be frequently undetectable or nonexistent, and these comet nuclei would
resemble asteroids. Therefore, low-activity comets may help
constrain the estimated contribution of cometary objects to
the near-Earth asteroid population and lead to a better understanding of the mechanisms related to the last stages of
cometary evolution. The detections of coma from near-Earth
object 2001 OG108 , and its subsequent re-classification from
an asteroid to a comet, have therefore renewed interest in
determining the physical properties of comet–asteroid transition objects.
Near-Earth object 2001 OG108 was discovered by the
Lowell Observatory Near Earth Asteroid Search (LONEOS)
program (described by Stokes et al., 2002), on July 28, 2001.
Marsden (2001) soon after published an orbit for the object
with a 50-year period and an inclination almost perpendicular to the ecliptic. It was quickly apparent that this object
had an orbit similar to the Halley-type comets and thus
was a member of the so-called “Damocloids,” i.e., asteroids
like (5335) Damocles on high-inclination and large semimajor axis orbits (Asher et al., 1994; Bailey and Emel’yanenko, 1996; Bowell, 2001). Almost 20 Damocloids are now
known. Subsequent astrometry of 2001 OG108 over the next
few months refined the orbit (Marsden, 2002) to a period
of 48.5 years, semi-major axis of 13.31 AU, eccentricity of
0.925, and orbital inclination of 80.2 degrees.
Due to its orbital similarity to the Damocloid asteroids and Halley-type comets, it was thought that 2001
OG108 might possibly be an inactive or dormant comet
nucleus. Therefore, several groups began monitoring the object as it proceeded toward perihelion in the hopes that it
would show signs of cometary activity. However, examination of the initial discovery observations and those obtained several months later showed no evidence of coma
(Marsden, 2001; French, 2002). Hence the asteroid classification of 2001 OG108 remained. It was not until observations were obtained during January and February
2002, which showed that the object had developed a slight
amount of coma as it approached perihelion (Nakamura
et al., 2002), that 2001 OG108 was re-classified and designated as comet C/2001 OG108 (LONEOS) (Marsden,
2002).
All the data presented in this paper are based on multiple studies that took place during October and November
2001, while the object was still relatively bright, but before
any coma was detected (French, 2002). C/2001 OG108 ’s first
report of activity was roughly at a heliocentric distance of
1.4 AU in January 2002. This activity continued through
May 2002, and stopped when the comet was at heliocentric distances of approximately 1.4 to 1.5 AU (Filonenko and
(a)
(b)
Fig. 1. (a) A 120-second R band image of C/2001 OG108 (LONEOS) taken
with the Lowell Observatory 1.1-m Hall telescope on October 23, 2001 at
03:30 UT. The comet is located next to the white arrow in this image. North
is up and east is to the left. Linear width of the image at the position of
the comet is approximately 292,000 km (277.2 arcsec). Note that no coma
is evident from the object in this image. (b) A 1200-second R band image
of C/2001 OG108 (LONEOS) taken with the University of Hawai’i 88-inch
telescope on June 1, 2002 at 06:45 UT. North is up and east is to the left.
Linear width of the image at the position of the comet is approximately
75,000 km (76.1 arcsec).
Churyumov, 2003). The appearance of the comet before and
after activity is shown in Fig. 1a and 1b. The core of the
comet is point-like in each case, indicating the absence of a
“canonical” cometary coma. There may be some lingering
cometary tail in the 2002 image (Fig. 1b), but the comet’s
photocenter mimics the point-spread function. In any case,
a study of the 2002 behavior of this comet will be presented in a future paper. In the present work, the evidence
indicates that the 2001 observations are of a bare cometary
nucleus.
Characteristics of C/2001 OG108 (LONEOS)
177
Table 1
Observing parameters of C/2001 OG108
Observing
date (2001)
Start
time (UT)
End
time (UT)
RA
(h m)
Dec (◦ ’)
Solar
distance
Earth
distance
# of
obs.
Visible observations (Ondřejov Obs. 0.65-m, Lowell Obs. 1.1-m, McDonald Obs. 2.7-m, Univ. of Hawai’i 88-inch)
Oct. 4
11:06
12:04
00 19
+22 00
17.00
1.050–1.175
7.17
Oct. 10
18:02
0:36
23 59
+22 24
16.94
1.124–1.574
8.84
Oct. 12
1:19
1:30
23 56
+22 29
16.94
1.596–1.660
9.38
Oct. 12
17:49
2:19
23 54
+22 34
16.93
1.122–2.124
9.68
Oct. 13
17:38
1:14
23 50
+22 38
16.93
1.121–1.644
10.14
Oct. 14
17:50
2:07
23 47
+22 42
16.93
1.121–2.141
10.63
Oct. 15
17:25
0:13
23 44
+22 46
16.94
1.120–1.574
11.13
Oct. 19
17:43
0:46
23 31
+22 57
16.95
1.118–1.728
13.28
Oct. 23
3:17
6:36
23 21
+23 02
16.97
1.021–1.131
15.21
Oct. 24
4:06
6:36
23 18
+23 03
16.98
1.022–1.146
15.80
Nov. 9
18:28
18:28
22 32
+22 46
17.11
1.122–1.222
24.61
Nov. 10
16:50
22:16
22 29
+22 43
17.12
1.121–1.725
25.03
Nov. 12
4:12
4:45
22 25
+22 37
17.13
1.168–1.273
25.68
2.527
2.459
2.446
2.436
2.425
2.414
2.403
2.358
2.321
2.309
2.120
2.109
2.090
1.558
1.505
1.497
1.491
1.485
1.480
1.475
1.459
1.451
1.450
1.476
1.480
1.485
12
14
2
11
7
8
7
5
5
4
1
5
21
Near-infrared observations (NASA Infrared Telescope Facility)
Oct. 9
10:01
10:25
00 04
+22 16
Oct. 10
9:21
9:45
00 01
+22 22
9:45
10:01
00 01
+22 22
16.94
16.94
16.94
1.020–1.032
1.003–1.013
1.013–1.016
8.22
8.58
8.58
2.475
2.465
2.464
1.516
1.509
1.508
10
10
6
Mid-infrared observations (Keck I)
Oct. 4
11:16
11:48
17.00
1.068–1.132
7.17
2.527
1.558
4
00 19
+22 00
Visual
mag.
Airmass
range
Phase
angle
Note. Solar distance and Earth distance are measured in astronomical units (AU). Phase angle is measured in degrees. RA and Dec are in J2000 coordinates.
Fig. 2. A lightcurve plot for C/2001 OG108 showing its rotation period of 57.2 ± 0.5 h. The up-arrows indicate the times of the NASA IRTF SpeX observations.
2. Observations and reduction
2.1. Visual spectrophotometry
Observations of C/2001 OG108 span three wavelength
regimes: visible, near-infrared, and mid-infrared. The details of the observations are listed in Table 1, along with
the object’s geometry at the time of the specific observations.
Visual photometric observations were acquired of C/2001
OG108 during October and November 2001 in B, V , R,
and I bandpasses (Bessell, 1990). All measurements were
calibrated and transformed to the absolute Johnson–Kron–
Cousins system using standards from Landolt (1992). Several different telescopes were used to collect these measure-
178
P.A. Abell et al. / Icarus 179 (2005) 174–194
ments, which were subsequently used to generate a complete
lightcurve of the comet nucleus (Fig. 2).
2.1.1. Lowell Observatory
Observations obtained on October 23 and 24, 2001 Universal Time (UT) were carried out with the Lowell Observatory 1.1-m Hall telescope on Anderson Mesa near Flagstaff,
Arizona. CCD photometry was carried out using a SITe
back-illuminated 2K × 2K CCD with 24 µm pixels. The
frames were binned 2 × 2, resulting in an image scale of
1.1 arcsec per pixel. The usable field of view was 19 arcmin
on a side. Whenever a suitable guide star could be found,
the telescope was tracked at the object’s nonsidereal rate of
motion.
The weather was photometric on both nights with seeing estimated to be about 2.7 arcsec. Observations of standard stars were obtained to derive extinction, zero-point, and
color transformations, which were in good agreement with
other recent observations at the Hall telescope (P. Massey,
personal communication). The images were bias-corrected
and flat-fielded using twilight flats. Aperture photometry
was done with the PHOT task in the IRAF DIGIPHOT package. For all objects, the radius was taken to be 5 pixels
and a 5.5 arcsec radius aperture was used. The sky value
was found using an annulus from 15–25 pixels radius. Faint
background objects were removed by finding the mode of
the sky measurements and removing excess values. In all
the images C/2001 OG108 appeared to be a point source and
showed no evidence of coma. More detailed image analyses were performed to investigate whether a small degree of
coma could have been present at the time the observations
were obtained. Line profiles of the comet nucleus and comparison stars generated from the data demonstrate similar
point-spread functions, indicating that no source extension is
observed in the line profile of C/2001 OG108 (Fig. 3). Hence
no coma was present during the observations of this comet
nucleus.
2.1.2. McDonald Observatory
C/2001 OG108 was observed on November 12, 2001 UT
from the 2.7-m Harlan J. Smith telescope at the McDonald
Observatory located atop Mt. Locke near Fort Davis, Texas.
The viewing conditions were good with seeing estimated to
be 1.3 arcsec and the skies clear. Measurements of the object
were obtained by the Imaging Grism Instrument (IGI) with
a 5:1 focal reducer, a Mould R filter, and a TeK 1024 × 1024
CCD. In this configuration the pixel scale is 0.57 arcsec with
a usable field of view of approximately 7 arcmin.
IRAF photometry routines were used with a 3 arcsec radius aperture to measure the brightness of the object and 10
to 15 reference stars in each image. Relative photometry was
used to correct for variations in the extinction of the reference stars from one image to the next. Standard stars were
observed over several airmasses, and used to calculate the
extinction coefficient and zero point offset in the instrument
magnitudes. This information provided a means for calibrat-
Fig. 3. Line profile data of C/2001 OG108 and field stars from images acquired from the Lowell Observatory 1.1-m Hall telescope on October 23
and 24, 2001 UT. The point-spread function (PSF) curve is made by taking a line profile cut through a stack of 8 stars that have been registered
and added together. The line profile is taken perpendicular to the direction
of motion to minimize the effects from tracking the comet at nonsidereal
rates. The solid horizontal line marks the zero point and the dashed lines
mark the +1 and −1 sigma levels of the noise in the sky counts associated
with the comet image. Note that the pixel values of the comet follow the
PSF curve and do not show any evidence of an extended source (i.e., coma)
at the time the observations were obtained. There is some deviation from
the PSF curve, at distances greater than 4 to 5 arcsec, but this is due to noise
in the comet signal which dominates at low count values.
ing the reference stars in each image to an absolute system,
which in turn could be used to calibrate the object.
To specifically see if C/2001 OG108 exhibited any coma,
the radial profile of the stars and the object were compared
to each within the same image. In order for this type of comparison to be valid, the images of the stars and the object
should not show any signs of trailing. Hence the exposure
times were kept short given that the comet nucleus was moving at a significant non-siderial rate. In all the images, neither
the object nor the reference stars were trailed by more than
half a pixel in each image. No coma of C/2001 OG108 was
detected in any of the images during this time.
2.1.3. Ondřejov Observatory
Observations of C/2001 OG108 were obtained over seven
nights from October 10 to 19, 2001 UT and two nights from
November 9 to 10, 2001 UT with the Ondřejov Observatory
0.65-m telescope. CCD photometry was carried out using a
Characteristics of C/2001 OG108 (LONEOS)
SITe back-illuminated 512 × 512 CCD with 24 µm pixels
and a resulting image scale of 2.2 arcsec per pixel. The observational and reduction techniques were essentially identical to those used for earlier near-Earth asteroid photometric
work described in Pravec et al. (1996). Color transformations used the measured color indices determined by the
participating observatories described in this section and Section 2.2 below.
2.2. Simultaneous visible and mid-infrared observations
The October 4, 2001 UT visible dataset was taken with
a Tektronix CCD at the University of Hawaii 88-inch telescope on Mauna Kea, Hawai’i. The detector has a pixel scale
of 0.22 arcseconds and a field of view of over 7 arcmin.
A Johnson V -band and a Cousins R-band filter were used
to observe the object, which appeared as a point source in
all of the images. Atmospheric conditions were good with
the seeing measured to be approximately 0.8 arcsec. The
CCD bias level was determined from the median of several zero-exposure images and was subsequently subtracted
from each image. All the images of the object also had a
flat-field, obtained from the median of several dithered exposures of the blank twilight sky, divided into them. Several
calibration stars from Landolt (1992) were observed at various airmasses during the night with stars in the PG+2231,
PG+2213, and PG+0918 fields, as well as stars in the vicinity of SA 113-265, SA 92-252, and SA 98-966 being used.
The absolute flux calibration, airmass correction, and color
correction from the photometry of these stars were simultaneously solved. A total of five exposures in R-band and two
in V -band of C/2001 OG108 were obtained; there was negligible change in the photometry from the nucleus’s rotation
over the course of the observations. A V -band magnitude of
16.756 ± 0.014 and a V -R color of 0.456 ± 0.016 were derived.
The mid-infrared data were taken with the LWS instrument on Keck-I atop Mauna Kea, Hawai’i on October 4,
2001 UT. The detector has a pixel scale of 0.080 arcseconds
and a field of view of only 10.2-by-10.2 arcsec. 10%-wide
filters centered at 10.7 and 17.9 µm were used to observe the
object, and it appeared as a point source in all the images.
Seeing was about 0.5 arcsec at both wavelengths. Field flattening was unnecessary since the standard star and the object
were observed at the same location on the detector. For absolute flux calibration, the standard star Beta Andromeda
was used, which was less than 20◦ from our target and very
close in airmass. The star was assumed to have a flux density
of 85.9 Jy at 17.9 µm and 235.7 Jy at 10.7 µm; these numbers
are derived from the results of Tokunaga (1984). The target
was measured in each wavelength twice and from these data,
flux densities of 421 ± 20 mJy at 10.7 µm and 742 ± 34 mJy
at 17.9 µm were derived.
179
2.3. Near-infrared observations
Near-infrared observations of C/2001 OG108 were obtained using the SpeX instrument (Rayner et al., 2003),
a medium resolution near-infrared spectrograph, developed
by the Institute for Astronomy for the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawai’i. Operating
SpeX in the low-resolution (or asteroid) mode and using a
0.8 arcsecond wide slit provides a spectral resolution of ∼93
over the entire ∼0.7 to 2.5 µm wavelength range. Signal-tonoise values of the data are contingent on the brightness of
each object observed, the total integration time, and the atmospheric conditions at the summit during observations, but
are attainable in excess of 80 to 100 for most objects given
good observing conditions.
C/2001 OG108 was observed for two nights in October
2001 at an estimated V -Mag = 17 and phase angle of ∼8◦ .
Twenty-six 120 second near-infrared spectra were taken of
this object between 1.00 and 1.02 airmass on October 9
and 10 UT (Table 1). Summit weather for both nights provided excellent opportunities for observing: the skies were
clear with seeing at 0.8 arcseconds or less, the winds calm,
and relative humidity levels at values less than 10%. Guide
camera images from SpeX showed that these near-infrared
spectra were obtained when no coma was detected. Other
efforts to study this object two weeks later during its perihelion approach also showed no detection of coma (French,
2002) (Fig. 1). Therefore, the observations of C/2001 OG108
obtained in October were of the surface of the nucleus rather
than the dust and gas coma that is typically seen in most
comet observations.
Observations of a local standard star, SAO 91809, were
obtained over a similar airmass range in order to model the
atmosphere at Mauna Kea during the observing run. This
allows for a more accurate determination of extinction coefficients over the entire spectral interval obtained by the SpeX
instrument. These coefficients are contained in starpacks
(see Gaffey et al., 2002, for a detailed explanation), which
are used to remove the spectral absorption effects of the atmosphere and reduce the noise in the final spectra of the
object. Without this type of correction, the spectra will have
spurious artifacts due to strong telluric water vapor features,
especially at ∼1.4 and ∼1.9 µm, in regions where most of
the common materials found in extraterrestrial objects have
near-infrared absorptions. Observations were also obtained
of a solar analogue star, SAO 93936 (Hyades 64), to correct for any slight differences in the local standard star’s
spectrum relative to that of the Sun (Hardorp, 1978). Furthermore, each of the object and standard star spectra were
channel-shifted to a registered reference spectrum in order to
account for any slight instrument flexure due to the changing orientation of the telescope as it tracked the object/star
over the course of the night.
All of the C/2001 OG108 , SAO 91809, and SAO 93936
spectra were extracted using the Image Reduction and
Analysis Facility (IRAF) program written and distributed
180
P.A. Abell et al. / Icarus 179 (2005) 174–194
Fig. 4. A spectral reflectance plot of C/2001 OG108 with combined visible photometry and near-infrared spectral measurements. The plot is scaled to the
determined albedo value of 0.040 at 0.55 µm. The near-infrared data represent a two-night average spectrum of C/2001 OG108 which was taken at the NASA
IRTF on October 9 and 10, 2001 UT. The errors are commensurate with the point-to-point scatter in the data.
by the National Optical Astronomy Observatories (NOAO).
The raw spectra files were imported into the Spectral
Processing Routine (SpecPR) program for processing and
analysis where the starpack and channel-shifting corrections are performed (Clark, 1980; Gaffey et al., 2002;
Gaffey, 2003). The two-night average near-infrared spectrum of C/2001 OG108 is shown in Fig. 4. A brief examination of the spectrum of C/2001 OG108 demonstrates that this
comet nucleus has a nearly linear spectrum with no discrete
absorption features, and is generally red in terms of its reflectance at longer wavelengths. Although the overall slope
of the spectrum is red, the slope tends to gradually decrease
with increasing wavelength.
The relative point-to-point scatter of the data shown in
Fig. 4 provides a good estimate of the uncertainty associated with these measurements. However, more accurate error
analyses using the standard error of the mean indicate that
the average uncertainty in the data is roughly 3–5% at wavelengths shorter than 1.8 µm and approximately 8–10% for
those points at wavelengths up to 2.3 µm. At wavelengths
greater than 2.3 µm, the noise increases to values above
15–20% due to the decreased response of the SpeX detector and the relatively faint (V -Mag = 17, see Table 1) signal
from the comet nucleus at the time of the observations.
3. Discussion and analysis
Combining the data sets from the three separate wavelength regimes described above gives the opportunity for a
more complete study of the comet nucleus. The discussion
and analysis that follows provides a detailed characterization
of C/2001 OG108 , and is to date the most comprehensive
study of a Halley-type comet nucleus attained via groundbased sensors.
3.1. Lightcurve and pole position
All the R-band measurements obtained at the four observatories have been compiled and analyzed for periodicity using the Fourier series fitting procedure developed by
Harris et al. (1989) and implemented by Pravec et al. (1996).
A unique, single-period solution is obtained of 57.2 ± 0.5 h,
with a phase-darkening slope parameter G of −0.01 ± 0.10,
and an estimated absolute V magnitude H = 13.05 ± 0.10.
The light-travel time corrected lightcurve data (folded with
the best fit period, reduced to unit geocentric and heliocentric distances, at a phase angle of 10 degrees, and with the
best fit G parameter) is shown in Fig. 2. The rotation phase
of 0.0 in Fig. 2 corresponds to JD 2452195.5 = 2001 Oct.
13.0 UT. The data fit well with a single-period Fourier series of the third order and their residuals are consistent with
estimated calibration errors. The observed lightcurve is consistent with a principal-axis rotation of the comet nucleus.
If there had been any contribution from a nonprincipal rotation (precession) component to C/2001 OG108 ’s observed
lightcurve, its amplitude was not greater than a few 0.01
magnitudes.
These visible observations indicate that C/2001 OG108 is
an elongated object on the basis of its lightcurve with a minimum axial ratio of approximately 1.3 (Fig. 2). The simple
sinusoidal and periodic nature of the derived lightcurve suggests that this object shows no evidence of precession, so
if any nonprincipal rotation component was present at the
time of observation, its effect on the overall amplitude of
the lightcurve was negligible (Pravec et al., 2005). A de-
Characteristics of C/2001 OG108 (LONEOS)
rived rotation period of 57.2 ± 0.5 h is somewhat atypical
for similarly-sized asteroids (Pravec and Harris, 2000), but
is consistent with the measured rotation periods of other
Halley-type comets (Samarasinha et al., 2004). Based on the
values found in Samarasinha et al. (2004), the rotation period of C/2001 OG108 is the third longest among all of these
comets.
In addition to deriving the rotation period, it may be possible to constrain the spin axis direction using the fact that
the comet was active for only a few months near perihelion.
If one assumes that the activity of the comet is driven by seasonal changes on the surface (e.g., coma exists only when an
active region on the nucleus’s surface is in sunlight), then
the pole’s orientation can be estimated. With the first report of activity coming in early January (Nakamura et al.,
2002) and knowing that the activity had ceased by the end
of May (Fig. 1), these dates can be adopted to represent the
time when the source is in sunlight. Interestingly, this range
of dates is centered at approximately the time of perihelion
(mid-March) and spans about 75 days before and after.
If a pole orientation is to be considered viable, then it
must be one in which the source becomes illuminated by
January 1 and falls into shadow after late May. Furthermore, the comet’s subsolar latitude should be about the same
around January 1 and June 1. Given this configuration, the
subsolar latitude must reach its highest value about the time
of perihelion, so to first order, one can assume that the pole
is pointing toward the Sun at perihelion and the active region is at a high latitude in the illuminated hemisphere. (If
the source were near the equator, it would be more likely
to produce coma for a longer range of times than was observed.) For this orientation, which corresponds to a possible
end state for spin axis migration (Samarasinha and Belton,
1995), the pole is pointed to an ecliptic latitude of −62 degrees and a longitude of 352 degrees (or in equatorial coordinates, RA = 32 degrees, Dec = −57 degrees). Note that
this result is only approximate and tests indicate that deviations of up to 40 degrees in some directions can still produce
viable configurations for reproducing the observed coma.
Furthermore the short-lived coma could simply be the result
of an outburst event, which would make our fundamental assumptions invalid.
If the above solution is indeed the true pole position,
then the sub-Earth latitude during the time the lightcurve
measurements were taken (about 150 days prior to perihelion) was located within a few degrees of the equator.
Hence, the observed lightcurve amplitude is the maximum
possible and the axial ratio of C/2001 OG108 is close to
1.3 (Fig. 2). However, the uncertainties in the pole position
means that the observed amplitude could represent the rotation of a body that is somewhat foreshortened. Previous
observations of lightcurves have been used to constrain the
axial ratios of comets, even when the pole position is not
precisely known, as demonstrated by the following equation
taken from (Fernández et al., 2000)
(a/c)2 + tan2 l
,
δm = 1.25 log
(b/c)2 + tan2 l
181
(1)
where a/c and b/c are the axial ratios of the nucleus and l
is the sub-Earth latitude on the comet’s surface. Therefore
a difference of 40 degrees in sub-Earth latitude would give
an upper limit to the axial ratio of approximately 1.5 for
C/2001 OG108 . Either axial ratio value discussed above is
consistent with observations obtained from other Oort cloud
comets (e.g., 1P/Halley).
3.2. Color indices and phase slope parameter
Visual photometric observations of C/2001 OG108 in
R-filter combined with several additional points obtained in
B, V , and I -filters (Johnson–Cousins system) give color indices of: B-V = 0.76 ± 0.03; V -R = 0.46 ± 0.02; V -I =
0.90 ± 0.02 and a phase slope parameter G of −0.01 ± 0.1.
The values of V -R collected in this study are almost identical to those determined for extinct comets (0.44 ± 0.02)
and comet nuclei (0.45 ± 0.02) (Jewitt, 2002). In addition, the color indices of C/2001 OG108 are similar to
those determined for Trojan D-type asteroids and Damocloids (Table 2), and are not unlike the average values determined for Damocloids by Jewitt (2005). These results
suggest that there may be some similarities in the surface materials of all of these objects as has been noted
by many others (Hartmann et al., 1987; A’Hearn, 1988;
Fitzsimmons et al., 1994; Di Martino et al., 1998; Davies
et al., 1998, 2001; De Sanctis et al., 2000; Hicks et al., 2000;
Harris et al., 2001; Licandro et al., 2002, 2003).
Some centaurs show a certain degree of affinity in terms
of their color indices to C/2001 OG108 , whereas others, such
as (5145) Pholus, clearly are not similar in terms of composition to this comet based on this criterion (Bauer et al.,
2003). Kuiper belt objects generally do not exhibit similar
color indices to this object and are not suitable analogues for
the composition of this Halley-type comet (Table 2) (Jewitt
and Luu, 2001).
The determined value of the phase slope parameter G of
−0.01 ± 0.1 was calculated by fitting the visual photometric
data to an H –G phase relationship. A phase slope parameter G was fitted simultaneously with the measured period
of the comet nucleus and a range of values from −0.2 to
0.5 was grid-searched in increments of 0.01 while the period was solved for in each increment. The best fit solution
was found to be G = −0.01 and a formal estimate of the error in G based on a chi2 statistic of the fit gave a 1 sigma
uncertainty of 0.02. A more realistic uncertainty of G, taking into account the possible systematic errors due to absolute calibration errors in the datasets taken at the different
observatories, as well as possible minor aspect changes during the observational interval, is likely a few times greater.
Therefore, 0.1 is a more realistic uncertainty of the derived
182
P.A. Abell et al. / Icarus 179 (2005) 174–194
Table 2
Color indices and slope parameters of outer Solar System objects
Object name
B-V
V -R
V -I
G
Notes
Reference
C/2001 OG108
0.76 ± 0.03
0.46 ± 0.02
0.90 ± 0.02
−0.01 ± 0.1
Comet
This work
(624) Hektor
0.77
0.44
0.950
–
Trojan (D Type)
a,b
1996 PW
1998WU24
–
0.78 ± 0.034
0.56 ± 0.04
0.53 ± 0.037
1.03 ± 0.06
0.99 ± 0.035
Low (0.05)
–
Damocloid
Damocloid
c
1P/Halley
2P/Encke
28P/Neujmin 1
49P/Arend-Rigaux 1
0.73 ± 0.03
–
0.86 ± 0.04
–
0.44 ± 0.03
0.46 ± 0.02
0.41 ± 0.03
0.47 ± 0.01
0.92 ± 0.07
–
0.99 ± 0.04
0.98 ± 0.02
–
−0.252
+0.41 ± 0.08
–
Comet
Comet
Comet
Comet
e
(5145) Pholus 1992 AD
(10199) Chariklo 1997 CU26
(10370) Hylonome 1995 DW2
(49036) Pelion 1998 QM107
(52872) Okyrhoe 1998 SG35
–
–
–
–
–
0.78 ± 0.05
0.49 ± 0.02
0.50 ± 0.07
0.63 ± 0.12
0.42 ± 0.08
1.59 ± 0.05
1.00 ± 0.02
1.02 ± 0.07
1.27 ± 0.11
0.88 ± 0.06
+0.16 ± 0.02
+0.08 ± 0.05
+0.18 ± 0.10
−0.11 ± 0.08
+0.12 ± 0.07
Centaur
Centaur
Centaur
Centaur
Centaur
k
(26375) 1999 DE9
(38628) Huya 2000 EB173
(79360) 1997 CS29
0.94 ± 0.03
0.93 ± 0.04
1.16 ± 0.06
0.57 ± 0.03
0.65 ± 0.03
0.61 ± 0.05
1.13 ± 0.03
1.24 ± 0.03
1.27 ± 0.05
−0.34 ± 0.05
+0.10 ± 0.03
−1.42 ± 0.30
Kuiper belt object
Kuiper belt object
Kuiper belt object
l,m
a
b
c
d
e
f
g
h
i
j
k
l
m
d
f,g
h,i
j
k
k
k
k
l,m
l,m
Dunlap and Gehrels (1969).
Degewij and Van Houten (1979).
Davies et al. (1998).
Davies et al. (2001).
Thomas and Keller (1989) deduced by Davies et al. (1998) from reflectivity gradients.
Fernández et al. (2000).
Jewitt (2002).
Campins et al. (1987) deduced by Davies et al. (1998) from reflectivity gradients.
Delahodde et al. (2001).
Luu (1993) deduced by Davies et al. (1998) from reflectivity gradients.
Bauer et al. (2003).
Jewitt and Luu (2001).
Sheppard and Jewitt (2003).
G parameter for C/2001 OG108 . Such a value of G is indicative of an object with a low albedo, and is consistent with
the albedo value derived from the simultaneous visible and
mid-infrared data (see Section 3.3). This value is also consistent with those derived from other primitive, low albedo
objects such as comets, centaurs, and Kuiper belt objects
(Table 2) (Fernández et al., 2000; Sheppard and Jewitt, 2003;
Buratti et al., 2004).
3.3. Albedo and size
The simultaneous visible and mid-infrared observations
allow for a relatively robust determination of the object’s radius and albedo. This technique has been used for over 30
years and basically involves solving two equations, one for
the visible flux and one for the mid-infrared flux, with two
unknowns (Allen, 1970, 1971). The method is described by
Lebofsky and Spencer (1989) in an asteroid context and by
Lamy et al. (2004) in a comet context, and it basically rests
on knowing the object’s surface temperature map at the time
of observation. This map is derived from a thermal model,
and in the vast majority of cases the slow-rotator thermal
model of Lebofsky et al. (1986), with an update by Harris
(1998), gives an adequate description of the temperature [see
the review by Harris and Lagerros (2002) for details]. The
slow-rotator model name is somewhat misleading since for
most objects the thermal inertia is more important than the
actual rotation rate. In any case, the largest uncertainty in
the model comes from the so-called beaming parameter (η),
which accounts for the fact that the surface of a real object
has topography and will not radiate the same way a smooth
sphere or ellipsoid will. The model has a few other parameters, all of which are either better known or less influential on
the results; a detailed discussion of them is given by Lamy
et al. (2004).
As shown by Harris et al. (1998) and Delbo et al. (2003),
the actual value of η can vary from object to object, and may
also be somewhat dependent on the phase angle. Thus it is
difficult to confidently assume a single value for η without
any other prior knowledge of the object in question; instead
a more conservative approach would be to use a range of
values for η. For example, if η = 1.0 is applied to the observations of C/2001 OG108 , a value that seems appropriate for
Comet 2P/Encke (Fernández et al., 2004), then the comet has
a V -band geometric albedo pV = 0.030 ± 0.005, an R-band
geometric albedo pR = 0.032 ± 0.005, and an effective radius RN = 8.8 ± 0.2 km. However, Fernández et al. (2005)
suggest that a very low value of η of approximately 0.52
Characteristics of C/2001 OG108 (LONEOS)
to 0.75 may be warranted for C/2001 OG108 instead, which
would produce a pV = 0.050 ± 0.008, a pR = 0.054 ± 0.008,
and an RN = 6.8 ± 0.5 km. Such low values for η are somewhat unusual, and given the difficulties of observing in the
Q-band, a slight error in the mid-IR measurements could
affect the assumed range of η used in the thermophysical
modeling process, and thus produce a larger albedo value
for the comet nucleus.
Because of the uncertainty associated in determining the
correct value for the beaming parameter of C/2001 OG108 ,
the following mid-range values with appropriate errors will
be used to characterize this object: pV = 0.040 ± 0.010,
pR = 0.043 ± 0.010, and RN = 7.6 ± 1.0 km. This value of
the radius applies to the actual time of observations, which
were done at a rotation phase of 0.43 (Fig. 2). Given the
photometric range of the light curve, this effective radius
can be converted to the effective radius at the maximum
of the light curve. For a prolate ellipsoid with semi-major
axes a > b = c, the cross section at the light curve maximum should be proportional to a ∗ b. Thus from the mid-IR
data, the individual semi-major axes can be calculated as follows: the light curve maxima occur at phases 0.23 and 0.73,
and the object is about 0.22 mag brighter than at a phase of
0.43 (Fig. 2). Since the mid-IR flux is proportional to the
square of the radius, if C/2001 OG108 was observed in the
mid-IR at the maximum of the light curve, a cross section
of a ∗ b = (7.6 ± 1.0 km)2 ∗ 10(0.4∗0.22) = (8.4 ± 1.0 km)2
would have been obtained. For axial ratio a/b = 1.3, this
leads to a = 9.6 ± 1.0 km and b = 7.4 ± 1.0 km. Note that an
important uncertainty in this radius estimate, aside from the
beaming parameter discussed above, is the viewing geometry of the nucleus at the time the observations were obtained.
If the comet nucleus was not observed at an equatorial viewing geometry, then the nucleus’s true axial ratio could be
larger than 1.3 (see Section 3.1).
The albedo measurement of C/2001 OG108 agrees well
with previous values obtained from asteroids that have been
identified as good candidates for possible extinct comet nuclei (Harris et al., 2001; Fernández et al., 2005). Six of
these objects are also Damocloids and their radii and albedos
are shown for comparison in Table 3. Note that the albedo
of C/2001 OG108 falls within the range of the error bars
183
for all but one of the objects listed, and is quite similar
in terms of size to Damocloids (65407) 2002 RP120 , 1999
LE31 , and 2000 DG8 . As discussed above, these types of asteroids may be possibly related to the Halley-type comets
on the basis of their orbital parameters (see Section 1).
The data from Table 3 suggests that there may not only be
a similarity in orbital parameters with these types of objects, but that some Damocloid asteroids and Halley-type
comets may also have similar physical properties in terms
of size and albedo. In addition, the albedo value determined
for C/2001 OG108 is also consistent with those derived for
the nuclei of active comets (Campins and Fernández, 2002;
Lamy et al., 2004). There seems to be as yet no obvious
difference in the albedo distribution of the nuclei of active
comets and that of Damocloids/extinct comet candidates observed to date.
3.4. Possible meteorite affinities
The determined albedo value of 0.040 ±0.010 and visible measurements obtained are applied to the best average
near-infrared spectrum of C/2001 OG108 (Fig. 4) so that
a comparison of its spectral reflectance to those of other
extraterrestrial materials with similar albedo values can be
performed. Before the albedo adjustment was made, all the
visible data were scaled to the near-infrared data at approximately 0.79 microns, which is the center wavelength for the
Cousins I-filter. This was the only wavelength overlap between the two spectral data sets obtained for C/2001 OG108 .
Extraterrestrial objects that have similar albedos to this
comet, and that have reliable visible and near-infrared spectral measurements, are the carbonaceous chondrite meteorites (Gaffey, 1976). Apart from a few exceptions, these
meteorites are easily distinguished from most other chondritic meteorite groups due to their dark visual appearance
in hand sample. A more detailed examination of these meteorites reveals the presence of a fine-grained (5 µm),
opaque matrix, which may contain 1 to 5 weight percent
carbon (Brearley and Jones, 1998) and nickel-iron sulfides
(M. Zolensky, personal communication). The combination
of the fine-grained matrix, carbon, and/or nickel-iron sulfides produces the low albedos observed in these meteorites
Table 3
Radii and albedos of Damocloids and C/2001 OG108
Object
C/2001 OG108
(20461) Dioretsa 1999 LD31
(65407) 2002 RP120
1999 LE31
2000 DG8
2000 HE46
2003 WN188
Effective radius (km)
Geometric albedo
Reference
7.6 ± 1.0
0.040 ± 0.010
This work
14.0 ± 3.0
7.3 ± 1.4
8.4 ± 2.1
7.8 ± 1.3
3.2 ± 0.6
5.0 ± 1.1
0.030 ± 0.010
0.090 ± 0.036
0.052 ± 0.026
0.049 ± 0.017
0.041 ± 0.016
0.046 ± 0.021
a
Note. Errors listed are the maximum range of values that yield acceptable fits to a standard thermophysical model.
a Harris et al. (2001).
b Fernández et al. (2005).
b
b
b
b
b
184
P.A. Abell et al. / Icarus 179 (2005) 174–194
(a)
(b)
Fig. 5. (a) A comparison of C/2001 OG108 ’s spectral reflectance with carbonaceous meteorites. All meteorite data, except those of Tagish Lake, are from
Gaffey (1976). Tagish Lake data are from Hiroi et al. (2001). (b) The same comparison of C/2001 OG108 ’s spectral reflectance with carbonaceous meteorites
as shown in (a), but with the vertical scale expanded for clarity.
(Gaffey, 1976; Clark and Lucey, 1984; Buseck and Hua,
1993; M. Zolensky, personal communication).
A plot of the spectral reflectance of C/2001 OG108 versus several carbonaceous chondrite meteorites is shown in
Fig. 5a. It should be noted that the data obtained of C/2001
OG108 is of a whole disk integrated average, which could
mask any albedo and/or spectral variations due to differences
in particle size, mineralogy, or abundance, whereas the meteorite spectra are of well-characterized powders measured
under known laboratory conditions.
Upon comparing the spectra of select meteorites from
this group to the data of C/2001 OG108 , it is immediately
clear that no meteorite sample currently in the terrestrial collections matches the spectrum of this comet nucleus. All
of the characterized meteorite types listed in Fig. 5a, with
the exception of Tagish Lake, have albedos that fall above
the value of 0.040 ± 0.010 that has been determined for
C/2001 OG108 . In addition, all of the carbonaceous meteorites show evidence of absorption features in their spectra,
whereas the comet nucleus does not appear to have any such
features at the detection limit of these data (Fig. 5b). Those
meteorites, belonging to the CV3 class (e.g., Allende and
Mokoia), the CM2 class (e.g., Murchison and Murray), and
the CI1 class (e.g., Orgueil), have evident features due to
Characteristics of C/2001 OG108 (LONEOS)
the presence of olivine, pyroxene, or phyllosilicate minerals, which produce features in the 1 and 2 µm regions of
their near-infrared spectra and strong absorption edges in
their UV-visible spectra (Gaffey, 1976; Gaffey et al., 1993;
Calvin and King, 1997). Tagish Lake, on the other hand,
does not appear to have features as obvious as those seen in
the other meteorites shown in Fig. 5b, but has a broad, weak
feature located near 1 µm seen in the spectrum measured by
Hiroi et al. (2001).
Although none of the meteorites shown in Fig. 5a match
the spectral response of C/2001 OG108 , Tagish Lake seems
to have a spectrum that is more similar to the comet nucleus
than any of the other samples. It has the same slight increase
in slope in the visible portion of the spectrum as the comet
nucleus, which is unlike the steeply rising spectra seen in the
other carbonaceous meteorites over this same wavelength region, and has a similar response in the near-infrared to the
spectrum of C/2001 OG108 . However, the presence of the
broad, weak feature located near 1 µm seen in the Tagish
Lake spectra (Fig. 5b), and the observed hydrated phases of
this meteorite (Brown et al., 2000), may preclude any mineralogical association with this comet nucleus.
It is important to note that C/2001 OG108 is not a plausible parent body for the Tagish Lake meteorite, even though
some similarities exist between their spectra, due to the significant differences in their orbital parameters. Brown et al.
(2000) report that the derived orbital elements place Tagish Lake and its parent body in an orbit similar to that of
an Apollo asteroid, with a semi-major axis well inside the
main asteroid belt. C/2001 OG108 has a much larger semimajor axis and its orbital characteristics are similar to the
Halley-type comets (Asher et al., 1994; Bailey and Emel’yanenko, 1996). In addition, the initial entry velocity of Tagish
Lake is estimated to be 15.8 ± 0.6 km/s (Brown et al., 2000),
185
whereas any entry velocity of a similar sized fragment of
C/2001 OG108 would be approximately 48 km/s and would
almost certainly cause the fragment to vaporize in the upper
atmosphere (Flynn, 1989).
3.5. Possible asteroid affinities
Other extraterrestrial objects that have similar low albedo
values with relatively featureless spectra are asteroids of
the F, C, P, T, and D taxonomic classes (Tholen, 1984;
Tholen and Barucci, 1989). These asteroids have albedos
typically ranging from 3 to 8% (Tedesco et al., 1992, 2002)
and may be good analogues for a comparison to the spectral reflectance of C/2001 OG108 . F-type asteroids generally have flat, reddish spectra at wavelengths longwards of
0.4 µm, but differ from the C-type asteroids in the visible
portion of their spectra even though the C-types have a similar flat to slightly red reflectance at longer wavelengths.
The other three remaining groups have a much more reddish spectral response in the near-infrared regions of their
spectra than either the F- or C-types (Gaffey et al., 1993).
P- and T-type asteroids both have significant red slopes to
their spectra, but the T-types tend to flatten out with increasing wavelength (Tholen and Barucci, 1989; Britt et al., 1992;
Gaffey et al., 1993). Of the five listed classes, the reddest
spectrum belongs to asteroids in the D taxonomic class.
These asteroids have spectra that are slightly red in the visible wavelengths up to 0.55 µm, but extremely red in visible
and near-infrared wavelengths longer than 0.55 µm (Bell
et al., 1988; Tholen and Barucci, 1989; Vilas and Gaffey,
1989; Gaffey et al., 1993).
Several asteroid spectra belonging to the F, C, P, T,
and D taxonomic classes have been plotted in comparison to the spectrum of this comet nucleus and are shown
Fig. 6. Representative asteroid spectra for each of the low albedo taxonomic classes is plotted here and compared with the spectrum of C/2001 OG108 . All
spectra are normalized to unity at 0.55 µm and then offset for clarity. Asteroid spectra are taken from Zellner et al. (1985) and Bell et al. (1988).
186
P.A. Abell et al. / Icarus 179 (2005) 174–194
in Fig. 6. The spectra of F- and C-type asteroids and this
comet nucleus are clearly quite dissimilar as the slopes
of these asteroid spectra are generally much flatter than
that of C/2001 OG108 . Spectra belonging to members of
the T-type asteroids match the steep visible response of
C/2001 OG108 , but tend to flatten out their spectral slopes
with increasing wavelength. P-type asteroids have similar slopes in the near-infrared portions of their spectra to
the comet nucleus, but do not match the visible portion
of its spectrum, which has a much steeper spectral response.
Of all the asteroid taxonomic types, the reflectance spectrum of C/2001 OG108 appears to most closely resemble
that of a D-type asteroid (Fig. 6). The D-type spectra presented here match both the visible and near-infrared portions
of the comet’s nucleus, unlike the T- and P-type spectra,
which only match either the visible or near-infrared portions of the spectrum, respectively. The observation that
this comet nucleus has a reflectance spectrum similar to
those of D-type asteroids is consistent with previous observations of other cometary nuclei and extinct comet candidates. Several investigators have suggested similarities between the characteristics of D-type asteroids and comet nuclei in terms of their spectral measurements and albedos
(e.g., Hartmann et al., 1987; A’Hearn, 1988; Fitzsimmons et
al., 1994; Di Martino et al., 1998; Davies et al., 1998, 2001;
De Sanctis et al., 2000; Hicks et al., 2000; Harris et al.,
2001; Licandro et al., 2002, 2003). D-type asteroids typically have low albedos (0.05) and are similar to values
seen for most comet nuclei (0.03) (A’Hearn, 1988). Although some comets have been reported to have slightly
higher albedos (0.04 to 0.06), they are still within range
of the albedos seen for D-types given the reported uncertainty of 0.01 to 0.03 in the data for those observations
(Campins and Fernández, 2002). Asteroids of the T taxonomic class have a higher range of albedos (0.10), whereas
the P-types have a similar albedo range (0.04) to those
seen for the D-types and cometary nuclei (Gaffey et al.,
1993).
In addition to the spectral and albedo similarities between C/2001 OG108 and these asteroids, it also seems that
D-type asteroids have a plausible range of compositions
that is suited to the current understanding of what comet
nuclei may be composed of in terms of organic content,
anhydrous silicates, and water ice (A’Hearn et al., 1995;
Joswiak et al., 2001; Soderblom et al., 2002). However, Ptype asteroids may also fit these compositional criteria and
should not be excluded as potential comet analogues on this
basis alone as future observations of comets may show some
similarities to these objects. It is important to note that not
all spectra of comet nuclei may resemble those of D-type asteroids, given the limitations of the taxonomic classification
schemes used, but the similarity of the spectra shown here
does suggest the possibility that C/2001 OG108 , a Halleytype comet, has similar characteristics to this taxonomic
class of asteroids.
3.6. Spectral comparison with interplanetary dust particles
Detailed examination of the average spectrum of C/2001
OG108 (Fig. 4) reveals that there is no indication of the presence of weak 0.8 to 1.0 µm features, at a 5% detection limit,
such as are seen in the spectra of carbonaceous chondrites
and many low-albedo asteroids (Vilas and Gaffey, 1989;
Vilas et al., 1994; Calvin and King, 1997). This would be
consistent with the presence of anhydrous rather than hydrous silicates, similar to the results of measurements of dust
from Comet 1P/Halley made by the Giotto spacecraft and
results from observations made by the Deep Space 1 probe
of Comet 19P/Borrelly (Brownlee, 1988; Soderblom et al.,
2002). It is important to note that although the possibility exists for C/2001 OG108 to have near-infrared spectral features
below the detection limit for this data set, any features are
likely to be due to anhydrous silicates, and may not be readily detected via ground-based observations due to the low
albedo of this comet nucleus.
No known recognized macroscopic sample of a comet nucleus exists in the terrestrial collections. However, small 5 to
15 µm particles, referred to as interplanetary dust particles
(IDPs), have been identified as possibly having cometary
origins on the basis of their heating profiles from high
velocity (V > 18 km/s) atmospheric entry (Flynn, 1989;
Jackson and Zook, 1992; Nier and Schlutter, 1993; Brownlee et al., 1994, 1995; Joswiak et al., 2001). To date, of the
13 IDPs that have been identified as having an entry velocity
consistent with a cometary origin, 12 have anhydrous silicate
mineralogies with 11 members of this group having significant amounts of amorphous carbon (Joswiak et al., 2001).
The sole remaining high velocity IDP (V = 20.4 km/s) is
reported to be phyllosilicate-rich, and investigators Joswiak
et al. (2001) suggest that it may either be a unique type of
cometary IDP or an asteroidal IDP whose parent body was
perturbed into a high velocity orbit relative to Earth prior to
the particle’s atmospheric entry.
Most anhydrous IDPs that have been measured through
reflectance spectroscopy appear to have low albedos (<10%)
in the visible wavelength range and tend to have red slopes
longwards from 0.5 µm (Bradley et al., 1996). Bradley et al.
(1996) also state that the spectral response of these dust
particles is very similar to what is observed for the P- and
D-type asteroids. Some of these IDPs have been observed
to have high concentrations of carbon, with one particular IDP containing greater than 90% by volume (Bradley
et al., 1996). The proposed compositions of comet nuclei
and these asteroid taxonomic classes also fit the description
that some of these particles have a composition of anhydrous
silicates with high concentrations of carbon. These properties are consistent with the observations obtained of the
albedo and spectral reflectance of C/2001 OG108 . To further
illustrate this similarity, the spectral reflectance of a typical anhydrous IDP measured from Bradley et al. (1996) is
shown for comparison to that of C/2001 OG108 (Fig. 7). Although the spectral coverage of the IDP and C/2001 OG108
Characteristics of C/2001 OG108 (LONEOS)
187
Fig. 7. A spectrum of an anhydrous IDP plotted with that of C/2001 OG108 . Both spectra are normalized to unity at 0.55 µm. The IDP spectrum is taken from
Bradley et al. (1996) and is described as pristine chondritic porous IDP W7030A5.
is not identical, the two spectral signatures agree quite
well where the spectral coverage overlaps in the visible region.
3.7. Dynamical connection with D-type asteroids
Like the rest of the Halley population, C/2001 OG108
presumably spent most of its lifetime in the Oort cloud.
However, these objects probably did not form in situ within
the Oort cloud given the inefficiency of accretion rates at
such large heliocentric distances; rather, they must have originated in the planetary region of the Solar System (Oort,
1950). It was originally thought that the source material for
the Oort cloud predominately arose from the perturbation
of material from the Uranus–Neptune region because gravitational interactions with Jupiter and Saturn would have
ejected most planetesimals into hyperbolic orbits and thus
out into interstellar space (Shoemaker et al., 1989). More
recent dynamical studies have shown, however, that the entire giant planet-forming region could have been responsible
for populating the Oort cloud, and at least a few percent of
this population could consist of asteroidal objects formed
within the orbit of Jupiter (Weissman and Levison, 1997;
Weissman, 1999).
Therefore, if a significant fraction of the planetesimals
in or near the orbit of Jupiter were dispersed to the Oort
cloud, the Trojan asteroids at the Lagrange L4 and L5
points of Jupiter could represent some remnant population
of these original planetesimals. Compositions represented
among these objects could be similar to those found within
the Oort cloud population. Previous taxonomic assessments
of the Trojan population have discovered that they are dominated by the D-type (60%) and P-type (37%) asteroid classes
(French et al., 1989). Hence, it is plausible that a Halley-type
comet such as C/2001 OG108 , an object from the Oort cloud,
would have spectral properties and compositions similar to
D- or P-type asteroids.
However, it should be noted that the Jupiter-forming region likely provided only some of these objects to the Oort
cloud. A majority of these planetesimals probably formed
in the Saturn–Uranus and Uranus–Neptune regions before
they were gravitationally perturbed and ejected to the Oort
cloud (Weissman, 1999). If this hypothesis is correct, material originating from 5 to 35 AU was ejected from the Solar
System. This material could represent a variety of compositions, but based on the physical properties of the surfaces
of Damocloids and Halley-type comets (e.g., albedo, V -R
index, and spectral reflectance), it appears that these Oort
cloud members have similar compositions. There are several
possible explanations for this:
(1) the objects in question formed from the same reservoir
of material in the solar nebula;
(2) current observational data may be biased and represent
only a small subset of the total population;
(3) beyond heliocentric distances greater than 4.0 AU, planetesimal formation processes are dominated by ice, anhydrous silicates, and organics, and therefore produce
objects that are virtually identical (Shoemaker et al.,
1989);
(4) the dynamical processes that occurred in the giant planets’ region during the latter stages of the primordial solar
nebula resulted in high degrees of collisional processing,
which homogenized the compositions of planetesimals
(Weissman, 1999); and
(5) the current taxonomic classification schemes are too
broad to distinguish compositional differences between
objects with such low albedos and featureless spectra
(Gaffey et al., 1993).
188
P.A. Abell et al. / Icarus 179 (2005) 174–194
It is unlikely that the first possible explanation is valid
given that the zone of material that populated the Oort cloud
spanned ∼5 to 35 AU. Observations of the inner Solar System show a wide range of oxygen fractionation trends over a
much shorter range of heliocentric distance, and it is reasonable to assume that this was also the case in the outer Solar
System. Therefore, all the objects observed would have to
have originated from a specific region, and dynamic modeling suggests this was not the case (Weissman, 1999). In
addition, data from Mumma et al. (2001) demonstrate that
not all Oort cloud comets have similar chemical signatures
in their comae, which is indicative that these objects formed
in separate reservoirs during the formation of the early Solar
System.
The second explanation concerning observational bias
is more difficult to dismiss given the small sample of the
comet nuclei and Damocloids studied. Hence, additional
high signal-to-noise spectral observations of Halley-type and
Long Period comet nuclei are needed to support or disprove
this possibility. The third option is improbable given the
spectral analysis of centaur object (5145) Pholus. This object formed beyond 30 AU and demonstrates a steeply sloped
spectrum with strong absorption bands (Cruikshank et al.,
1998). In comparison, C/2001 OG108 has no absorption
bands in its near-infrared spectrum at a 5% detection limit;
therefore, the surface compositions of these two objects are
much different even though they presumably formed within
the same 5 to 35 AU heliocentric range. The fourth explanation, which refers to the homogenization of the planetesimals, is unlikely based on new dynamic models that suggest
such collisional processing would result in high erosive mass
loss rather than significant accretion of material (Stern and
Weissman, 2001). Accretion of the fragments during collisions would be necessary to fully homogenize the Oort cloud
progenitor population before they were ejected from the giant planet region.
The fifth option listed above, barring an observational
bias, is the most plausible given the previously recognized
failure of taxonomic classifications to distinguish between
the varying compositions of asteroids belonging to the S, T,
and M taxonomies (Gaffey et al., 1993; Hardersen, 2003). It
also appears that studies of low albedo C-type asteroids are
showing distinct compositional differences when examined
via high quality visible spectra (Vilas et al., 1993, 1994). In
addition, near-infrared spectral studies of Trojan asteroids,
predominantly P- and D-types, also show some degree of
compositional variability (Emery and Brown, 2003, 2004).
Thus these observations suggest that future studies of lowalbedo objects, comprising the asteroids of the inner Solar System, Damocloids, and comets, should rely on high
signal-to-noise data involving visible and near-infrared spectroscopy, as well as lightcurve and thermal measurements to
fully characterize their physical properties. One possible target for future detailed investigations for extinct comet candidates would be the near-Earth asteroid (3552) Don Quixote,
which has been identified as a D-type object with a low
albedo, and resides in a cometary-like orbit (Tholen, 1984;
Hahn and Rickman, 1985; Hartmann et al., 1987; Veeder
et al., 1989).
3.8. Spectral comparison with Damocloids and comets
As stated above, there is considerable interest in the
possible relationship between Damocloid asteroids and the
Halley-type comets due to the dynamical similarities of
their orbits (Asher et al., 1994; Bailey and Emel’yanenko,
1996). It also appears that there are some similarities between these objects in terms of size and albedo (Table 3).
In addition, based on observations of other Damocloid asteroids, it seems that there may also be a connection to their
surface compositions in terms of spectral reflectance. Three
Damocloids, 1996 PW, 1998 WU24 , and (20461) Dioretsa
1999 LD31 , were observed by several different observers at
various telescopes and found to be spectrally alike. Optical
photometry, infrared photometry, and infrared spectroscopy
measurements were obtained of 1996 PW and combined to
produce a reflectance spectrum that is typical for D-type
asteroids (Davies et al., 1998; Hicks et al., 2000). Similar
optical and infrared measurements were obtained of asteroid
1998 WU24 , which also appears to have surface materials
similar to those of D-type asteroids (Davies et al., 2001).
(20461) Dioretsa 1999 LD31 was observed using the eightcolor asteroid filter (ECAS) set (Tedesco et al., 1982), and is
reported to have characteristics that are most consistent with
asteroids of the D taxonomic class (Harris et al., 2001). Separate observations of this object were also made via 0.5 to
1.0 µm reflectance spectroscopy, which revealed no absorption features and a spectral slope similar to those seen for
D-type asteroids (Binzel, 2000).
The spectral signatures of these three Damocloids are
plotted and compared to that of C/2001 OG108 (Fig. 8).
Note that although these Damocloids have redder slopes
over near-infrared wavelengths, they all fall within the range
of values of the taxonomic D-type asteroids. These results
suggest that all the Damocloid asteroids observed to date
have surface properties that have similar spectral responses
to those of D-type asteroids. However, given the parameters
used to define the different taxonomies (Tholen, 1984) and
the relatively low spectral resolution of these measurements
as described above, some materials or combinations of materials may seem spectrally similar enough that the criteria
used for taxonomic classification cannot readily distinguish
the differences between their compositions (Gaffey et al.,
1993, 2002). Therefore, these objects may not necessarily
have identical compositions. On the other hand, this does
not preclude the possibility that these objects may also be
quite alike in terms of composition.
It should be noted that in addition to making observations
of Damocloids, several investigators have recently reported
ground-based near-infrared spectra from the nuclei of Jupiter
family Comets 28P/Neujmin 1 and 124P/Mrkos (Licandro
et al., 2002, 2003). These objects have also been suggested
Characteristics of C/2001 OG108 (LONEOS)
189
Fig. 8. Spectra of three Damocloids, 1996 PW, 1998 WU24 , and (20461) Dioretsa 1999 LD31 , compared to the spectral signature of C/2001 OG108 . All
the Damocloid spectra have been previously identified as being consistent with those of D-type asteroids. Spectral data for these Damocloids are taken from
Davies et al. (1998, 2001) and Harris et al. (2001).
Fig. 9. A comparison of the spectrum of comet 124P/Mrkos with the spectrum of C/2001 OG108 . Both spectra are normalized to unity at 1.6 µm. Comet
124P/Mrkos spectrum is taken from Licandro et al. (2003).
to have similarities to the D-type asteroids on the basis of
their steeply sloped and featureless near-infrared spectra.
The relationship of these comets to D-type asteroids is highlighted in a plot comparing the spectra of 124P/Mrkos and
C/2001 OG108 and is by no means proven on the basis of
these data (Fig. 9). Because the spectrum of 124P/Mrkos
was normalized to 1.6 µm, C/2001 OG108 is also normalized to the same wavelength in order to enhance the comparison. Given the range of the 124P/Mrkos data, there is
a reasonable possibility that this object could be similar
to the T-, P-, or D-type asteroids. The spectral reflectance
curves of these three taxonomies could easily fall within
the scatter of these measurements. However, even though
these data have lower resolution, limited wavelength coverage, and lower signal-to-noise as compared to the data
obtained of C/2001 OG108 in this study, they still provide
a constraint on the possible compositional nature of these
objects.
4. Conclusions
The data presented in this study have characterized
C/2001 OG108 to such an extent that it is the best understood
190
P.A. Abell et al. / Icarus 179 (2005) 174–194
nucleus among the Halley-type comets, except for 1P/Halley
itself. Furthermore, there are very few comets (among all the
dynamical groups) that are as well studied as C/2001 OG108 .
The first high quality ground-based visible, near-infrared,
and mid-infrared measurements of a bare Halley-type comet
nucleus indicate that C/2001 OG108 (LONEOS) has the following physical characteristics:
(1) Color indices of B-V = 0.76 ± 0.03, V -R = 0.46 ±
0.02, and V -I = 0.90 ± 0.02, which are nearly identical
to those observed for extinct comet candidates and other
comet nuclei. These values suggest that these types of
objects appear to be very similar in terms of their surface properties.
(2) A phase-darkening slope parameter of −0.01 ± 0.10,
which is consistent with the low albedo determined for
this object and those slope parameters previously derived for other comets and Kuiper belt objects.
(3) A rotation period of 57.2 ± 0.5 h that shows no evidence
of complex rotation. This would suggest that at the time
of observation, the comet nucleus rotated around its
principal axis (Pravec et al., 2005). In addition, the rotation period is one of the longest observed among all
comet nuclei, but not atypical for Halley-type comets.
(4) A nuclear axial ratio estimated to be a minimum of 1.3
based on the lightcurve amplitude and the orientation of
the comet’s tentative pole position to be approximately
−62 degrees latitude and 352 degrees longitude. However, the axial ratio for C/2001 OG108 could be as much
as 1.5 given the uncertainties of the exact pole position. Either set of axial ratios makes this comet nucleus
somewhat elongated, but well within the distribution for
previously observed cometary nuclei.
(5) An albedo of 0.040 ± 0.010, which is typical for
Damocloids, low-Tisserand asteroids, and other active
cometary nuclei. This value constrains the radius of
C/2001 OG108 to 7.4 ± 1.0 km by 9.6 ± 1.0 km for an
axial ratio of 1.3. Hence this comet nucleus is one of the
largest known to date.
(6) A red near-infrared spectrum which demonstrates no
discernable absorption features (at a 5% detection limit)
across the entire wavelength interval from ∼0.7 to
2.5 µm. The lack of any discernable features in the
0.8 to 1.0 µm region such as are seen in the spectra
of carbonaceous meteorites and many low-albedo asteroids suggests that the comet’s surface is consistent with
the presence of anhydrous rather than hydrous silicates
on the surface of this comet. This finding is similar to
the results obtained by the Giotto spacecraft of Comet
1P/Halley (Brownlee, 1988), data gathered by the Deep
Space 1 probe of Comet 19P/Borrelly (the only other nucleus with a high signal-to-noise reflectance spectrum)
(Soderblom et al., 2002), and analysis of cometary dust
particles collected from the stratosphere (Bradley et al.,
1996).
The average near-infrared spectrum produced from two
nights of data taken in October 2001, combined with a measured albedo of 0.040 ± 0.010 and visible spectrophotometry, indicate that C/2001 OG108 has spectral properties that
are analogous to the taxonomic D-type and possibly P-type
asteroids found within the mainbelt and Trojan asteroid populations. No similarities are seen with respect to the C-, F-,
or T-type asteroids. Comparison of measured albedos and diameters of Damocloid asteroids reveal similarities between
these types of asteroids and this comet nucleus, which supports previous dynamical arguments that these Oort cloud
objects could be composed of similar materials. These observations are also consistent with findings that two Jupiter
family comets may have spectral signatures indicative of
D-type asteroids (Licandro et al., 2002, 2003).
None of the currently recognized meteorites in the terrestrial collections have reflectance spectra that match C/2001
OG108 , but the closest resemblance seems to be the spectrum of the Tagish Lake meteorite. However, Tagish Lake
is not related to C/2001 OG108 on the basis of its composition. This meteorite is composed of phyllosilicates, which
suggest that its parent body experienced significant aqueous
processing, whereas multiple lines of evidence suggest that
comets are composed of anhydrous silicate materials. While
the spectral signature of C/2001 OG108 shows no evidence
for any hydrated species on its surface at the detection level
of the data, higher signal-to-noise spectra would be required
to completely rule out the possibility of such species existing
on the comet. More recent work suggests that Tagish Lake
may be more similar to the hydrated T-type asteroid (308)
Polyxo (Hiroi et al., 2003).
Given that C/2001 OG108 is a Halley-type comet, this object probably represents the transition from a typical highly
active short-period comet to an extinct comet nucleus. The
existence of such objects suggests that some comets can
survive many passages into the inner Solar System without
splitting or disintegrating. Therefore, a fraction of the nearEarth asteroid population may actually be extinct cometary
nuclei and thus may exhibit spectral properties similar to
those observed for C/2001 OG108 . A detailed survey combining visible and near-infrared spectroscopy coupled with
lightcurve and thermal measurements of low-albedo objects
among the near-Earth population would be useful to identify extinct comet candidates and possible future spacecraft
targets (e.g., (3552) Don Quixote). Similar projects should
be undertaken for low-albedo asteroids in the inner Solar
System, Damocloids, and comet nuclei. Results from such
investigations could be expected to further insights into the
possible composition(s) of cometary materials, identify possible source relationships between the giant planets’ region
and the Oort cloud, and help constrain the relative proportion
of extinct cometary nuclei that are thought to exist within the
near-Earth object population.
Characteristics of C/2001 OG108 (LONEOS)
Acknowledgments
The authors are grateful for the constructive reviews of
Lucy McFadden and an anonymous reviewer, which greatly
helped to improve this paper. P.A.A. also expresses many
thanks to Alan Tokunaga, John Rayner, and Bobby Bus
for several useful discussions concerning the SpeX instrument, and to Bill Golisch for his help during the collection of the near-infrared spectra at the NASA IRTF.
Both the NASA Planetary Geology and Geophysics Program Grant NAG5-10345 [M.J.G.] and NASA Exobiology
Program Grant NAG5-7598 [M.J.G.] supported aspects of
P.A.A.’s research. Y.R.F. would like to thank John Dvorak and Cynthia Wilburn for their help obtaining observations at the University of Hawai’i 88-inch and Keck telescopes, respectively. NASA and NSF grants which provided
funds for Y.R.F.’s observations are appreciated. P.P.’s work
at the Ondřejov Observatory has been supported by the
Grant Agency of the Czech Academy of Sciences, Grant
A3003204, and by the Grant Agency of the Czech Republic, Grant 205-99-0255. L.M.F. thanks Larry Wasserman and
Phil Massey for help in obtaining observations at Lowell Observatory, and John Dickel for assistance in copying backup
tapes. L.M.F. is also grateful for support from the Provost
of Illinois Wesleyan University. T.L.F.’s research has been
supported by NASA Grant NAG5-4384. The bulk of this research was performed while P.A.A. held a National Research
Council Research Associateship Award at the NASA Johnson Space Center.
References
A’Hearn, M.F., 1988. Observations of cometary nuclei. Annu. Rev. Earth
Planet. Sci. 16, 273–293.
A’Hearn, M.F., Millis, R.L., Schleicher, D.G., Osip, D.J., Birch, P.V., 1995.
The ensemble properties of comets: Results from narrowband photometry of 85 comets, 1976–1992. Icarus 118, 223–270.
Allen, D.A., 1970. Infrared diameter of Vesta. Nature 227, 158–159.
Allen, D.A., 1971. The method of determining infrared diameters. In:
Gehrels, T. (Ed.), Physical Studies of Minor Planets, National Aeronautics and Space Administration Report SP-267. National Aeronautics
and Space Administration, Washington, DC, pp. 41–44.
Asher, D.J., Bailey, M.E., Hahn, G., Steel, D.I., 1994. Asteroid 5335 Damocles and its implications for cometary dynamics. Mon. Not. R. Astron.
Soc. 267, 26–42.
Bailey, M.E., Emel’yanenko, V.V., 1996. Dynamical evolution of Halleytype comets. Mon. Not. R. Astron. Soc. 278, 1087–1110.
Bauer, J.M., Meech, K.J., Fernández, Y.R., Pittichova, J., Hainaut, O.R.,
Boehnhardt, H., Delsanti, A.C., 2003. Physical survey of 24 centaurs
with visible photometry. Icarus 166, 195–211.
Bell, J.F., Davis, D.R., Hartmann, W.K., Gaffey, M.J., 1988. The 52-color
asteroid survey: Final results and interpretations. Lunar Planet. Sci. 19,
57–58 (abstract).
Bessell, M.S., 1990. UBVRI passbands. Publ. Astron. Soc. Pacific 102,
1181–1199.
Binzel, R.P., 2000. 1999 LD31 . In: Green, D.W.E. (Ed.), IAUC #7376.
Central Bureau for Astronomical Telegrams, Smithsonian Astrophysical Observatory, Cambridge, MA.
Binzel, R.P., Xu, S., Bus, S.J., Bowell, E., 1992. Origins for the near-Earth
asteroids. Science 257, 779–782.
191
Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J., Levison, H.F., Michel, P.,
Metcalfe, T.S., 2002. Debiased orbital and absolute magnitude distribution of the near-Earth objects. Icarus 156, 399–433.
Bowell, E., 2001. Largest Earth-approaching asteroid discovered? Lowell
Observatory News Release, August 14, 2001.
Bowell, E., Skiff, B.A., West, R.M., Heyer, H.H., Quebatte, J., 1992. (4015)
1979 VA = Comet Wilson–Harrington (1949 III). In: Marsden, B.G.
(Ed.), IAUC #5585. Central Bureau for Astronomical Telegrams, Smithsonian Astrophysical Observatory, Cambridge, MA.
Bradley, J.P., Keller, L.P., Brownlee, D.E., Thomas, K.L., 1996. Reflectance
spectroscopy of interplanetary dust particles. Meteorit. Planet. Sci. 31,
394–402.
Brearley, A.J., Jones, R.H., 1998. Chondritic meteorites. In: Papike, J.J.
(Ed.), Reviews in Mineralogy. In: Planetary Materials, vol. 36. Mineralogical Society of America, Washington, DC, pp. 1–398.
Britt, D.T., Tholen, D.J., Bell, J.F., Pieters, C.M., 1992. Comparison of
asteroid and meteorite spectra: Classification by principal component
analysis. Icarus 99, 153–166.
Brown, P.G., Hildebrand, A.R., Zolensky, M.E., Grady, M., Clayton, R.N.,
Mayeda, T.K., Tagliaferri, E., Spalding, R., MacRae, N.D., Hoffman,
E.L., Mittlefehldt, D.W., Wacker, J.F., Bird, J.A., Campbell, M.D., Carpenter, R., Gingerich, H., Glatiotis, M., Greiner, E., Mazur, M.J., McCausland, P.J., Plotkin, H., Mazur, T.R., 2000. The fall, recovery, orbit,
and composition of the Tagish Lake meteorite: A new type of carbonaceous chondrite. Science 290, 320–325.
Brownlee, D.E., 1988. A comparison of Halley dust with meteorites, interplanetary dust, and interstellar grains. In: Hanner, M.S. (Ed.), Infrared
Observations of Comets Halley and Wilson and Properties of the Grains.
Report 66B. National Aeronautics and Space Administration, Washington, DC, pp. 66–67.
Brownlee, D.E., Joswiak, D.J., Love, S.G., Bradley, J.P., Nier, A.O., Schlutter, D.J., 1994. Identification and analysis of cometary IDPs. Lunar
Planet. Sci. 25, 185–186 (abstract).
Brownlee, D.E., Joswiak, D.J., Schlutter, D.J., Pepin, R.O., Bradley, J.P.,
Love, S.G., 1995. Identification of individual cometary IDPs by thermally stepped He release. Lunar Planet. Sci. 26, 183–184 (abstract).
Buratti, B.J., Hicks, M.D., Soderblom, L.A., Britt, D., Oberst, J., Hillier,
J.K., 2004. Deep Space 1 photometry of the nucleus of Comet
19P/Borrelly. Icarus 167, 16–29.
Buseck, P.R., Hua, X., 1993. Matrices of carbonaceous chondrite meteorites. Annu. Rev. Earth Planet. Sci. 21, 255–305.
Calvin, W.M., King, T.V.V., 1997. Spectral characteristics of iron-bearing
phyllosillicates: Comparison to Orgueil (CI1), Murchison and Murray
(CM2). Meteorit. Planet. Sci. 32, 693–701.
Campins, H., Fernández, Y.R., 2002. Observational constraints on surface
characteristics of comet nuclei. Earth Moon Planets 89, 117–134.
Campins, H., A’Hearn, M.F., McFadden, L.A., 1987. The bare nucleus of
Comet Neujmin 1. Astrophys. J. 316, 847–857.
Clark, R.N., 1980. A large-scale interactive one-dimensional array processing system. Publ. Astron. Soc. Pacific 92, 221–224.
Clark, R.N., Lucey, P.G., 1984. Spectral properties of ice-particulate mixtures and implications for remote sensing. 1. Intimate mixtures. J. Geophys. Res. 89, 6341–6348.
Cruikshank, D.P., Roush, T.L., Bartholomew, M.J., Geballe, T.R., Pendleton, Y.J., White, S.M., Bell III, J.F., Davies, J.K., Owen, T.C., de Bergh,
C., Tholen, D.J., Bernstein, M.P., Brown, R.H., Tryka, K.A., Dalle Ore,
C.M., 1998. The composition of Centaur 5145 Pholus. Icarus 135, 389–
407.
Cunningham, L.E., 1950. Periodic Comet Wilson–Harrington. In: Hansen,
J.M.V. (Ed.), IAUC #1250. Central Bureau for Astronomical Telegrams,
Smithsonian Astrophysical Observatory, Cambridge, MA.
Davies, J.K., McBride, N., Green, S.F., Mottola, S., Carsenty, U., Basran,
D., Hudson, K.A., Foster, M.J., 1998. The lightcurve and colors of unusual minor planet 1996 PW. Icarus 132, 418–430.
Davies, J.K., Tholen, D.J., Whiteley, R.J., Green, S.F., Hillier, J.K., Foster,
M.J., McBride, N., Kerr, T.H., Muzzerall, E., 2001. The lightcurve and
colors of unusual minor planet 1998 WU24 . Icarus 150, 69–77.
192
P.A. Abell et al. / Icarus 179 (2005) 174–194
Degewij, J., Tedesco, E.F., 1982. Do comets evolve into asteroids? Evidence
from physical studies. In: Wilkening, L.L. (Ed.), Comets. Univ. Arizona
Press, Tucson, pp. 665–695.
Degewij, J., Van Houten, C.J., 1979. Distant asteroids and outer jovian
satellites. In: Gehrels, T. (Ed.), Asteroids. Univ. Arizona Press, Tucson,
pp. 417–435.
Delahodde, C.E., Meech, K.J., Hainaut, O.R., Dotto, E., 2001. Detailed
phase function of Comet 28P/Neujmin 1. Astron. Astrophys. 376, 672–
685.
Delbo, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K., 2003. Keck
observations of near-Earth asteroids in the thermal infrared. Icarus 166,
116–130.
De Sanctis, M.C., Lazzarin, M., Barucci, M.A., Capria, M.T., Coradini, A.,
2000. Comet P/Gehrels 3: Spectroscopic observations and nucleus models. Astron. Astrophys. 354, 1086–1090.
Di Martino, M., Carusi, A., Dotto, E., Lazzarin, M., Marzari, F., Migliorini,
F., 1998. Dynamical and physical properties of comet–asteroid transition objects. Astron. Astrophys. 329, 1145–1151.
Dunlap, J.L., Gehrels, T., 1969. Minor Planets. III. Lightcurves of a trojan
asteroid. Astron. J. 74, 796–803.
Emery, J.P., Brown, R.H., 2003. Constraints on the surface composition of trojan asteroids from near-infrared (0.8–4.0 µm) spectroscopy.
Icarus 164, 104–121.
Emery, J.P., Brown, R.H., 2004. The surface composition of trojan asteroids: Constraints set by scattering theory. Icarus 170, 131–152.
Fernández, Y.R., McFadden, L.A., Lisse, C.M., Helin, E.F., Chamberlin,
A.B., 1997. Analysis of POSS images of comet–asteroid transition object 107P/1949 W1 (Wilson–Harrington). Icarus 128, 114–126.
Fernández, Y.R., Lisse, C.M., Käufl, H.U., Peschke, S.B., Weaver, H.A.,
A’Hearn, M.F., Lamy, P.P., Livengood, T.A., Kostiuk, T., 2000. Physical
properties of the nucleus of Comet 2P/Enke. Icarus 147, 145–160.
Fernández, Y.R., Jewitt, D.C., Sheppard, S.S., 2001. Low albedos among
extinct comet candidates. Astrophys. J. 553, L197–L200.
Fernández, Y.R., Lisse, C.M., Schleicher, D.G., Bus, S.J., Kassis, M., Hora,
J.L., Deutsch, L.K., 2004. Bull. Am. Astron. Soc. 36. Abstract 21.04.
Fernández, Y.R., Jewitt, J.C., Sheppard, S.S., 2005. Albedos of asteroids in
comet-like orbits. Aston. J. 130, 308–318.
Filonenko, V.S., Churyumov, K.I., 2003. Peculiarities of light curves of
some new bright comets. In: Physical Properties and Morphology of
Small Solar System Bodies, 25th Meeting of the IAU, Joint Discussion
19, Sydney, Australia (abstract).
Fitzsimmons, A., Dahlgren, M., Lagerkvist, C.I., Magnusson, P., Williams,
I.P., 1994. A spectroscopic survey of D-type asteroids. Astron. Astrophys. 282, 634–642.
Flynn, G.J., 1989. Atmospheric entry heating: A criterion to distinguish between asteroidal and cometary sources of interplanetary dust. Icarus 77,
287–310.
Fox, K., Williams, I.P., Hughes, D.W., 1984. The “Geminid” asteroid (1983
TB) and its orbital evolution. Mon. Not. R. Astron. Soc. 208, 11P–15P.
French, L.M., 2002. Pre-activity BVRI colors of high-inclination Comet
C/2001 OG108 (LONEOS). Bull. Am. Astron. Soc. 34. Abstract 16.01.
French, L.M., Vilas, F., Hartmann, W.K., Tholen, D.J., 1989. Distant asteroids and Chiron. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.),
Asteroids II. Univ. of Arizona Press, Tucson, pp. 468–486.
Gaffey, M.J., 1976. Spectral reflectance characteristics of the meteorite
classes. J. Geophys. Res. 81, 905–920.
Gaffey, M.J., 2003. Observational and data reduction techniques to optimize mineralogical characterizations of asteroid surface materials. Lunar Planet. Sci. 34. Abstract #1602 [CD-ROM].
Gaffey, M.J., Burbine, T.H., Binzel, R.P., 1993. Asteroid spectroscopy:
Progress and perspectives. Meteoritics 28, 161–187.
Gaffey, M.J., Cloutis, E.A., Kelley, M.S., Reed, K.L., 2002. Mineralogy of
asteroids. In: Bottke, W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.),
Asteroids III. Univ. of Arizona Press, Tucson, pp. 183–204.
Hahn, G., Rickman, H., 1985. Asteroids in cometary orbits. Icarus 61, 417–
442.
Hardersen, P.S., 2003. Near-IR reflectance spectroscopy of asteroids and
study of the thermal history of the main asteroid belt. Ph.D. thesis. Rensselaer Polytechnic Institute, Troy, 387 pp.
Hardorp, J., 1978. The Sun among the stars. I. A search for solar spectral
analogs. Astron. Astrophys. 63, 383–390.
Harris, A.W., 1998. A thermal model for near-Earth asteroids. Icarus 131,
291–301.
Harris, A.W., Lagerros, J.S.V., 2002. Asteroids in the thermal infrared. In:
Bottke, W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.), Asteroids
III. Univ. of Arizona Press, Tucson, pp. 205–218.
Harris, A.W., Young, J.W., Bowell, E., Martin, L.J., Millis, R.L., Poutanen,
M., Scaltriti, F., Zappalà, V., Schober, H.J., Debehogne, H., Zeigler,
K.W., 1989. Photoelectric observations of Asteroids 3, 24, 60, 261, and
863. Icarus 77, 171–186.
Harris, A.W., Davies, J.K., Green, S.F., 1998. Thermal infrared spectrophotometry of the near-Earth Asteroids 2100 Ra-Shalom and 1991 EE.
Icarus 135, 441–450.
Harris, A.W., Delbo, M., Binzel, R.P., Davies, J.K., Roberts, J., Tholen,
D.J., Whiteley, R.J., 2001. Visible to thermal–infrared spectrophotometry of a possible inactive cometary nucleus. Icarus 153, 332–337.
Harris, N.W., Bailey, M.E., 1998. Dynamical evolution of cometary asteroids. Mon. Not. R. Astron. Soc. 297, 1227–1236.
Hartmann, W.K., Tholen, D.J., Cruikshank, D.P., 1987. The relationship of
active comets, “extinct” comets, and dark asteroids. Icarus 69, 33–50.
Hicks, M.D., Buratti, B.J., Newburn Jr., R.L., Rabinowitz, D.L., 2000. Physical observations of 1996 PW and 1997 SE5 : Extinct comets or D-type
asteroids? Icarus 143, 354–359.
Hiroi, T., Zolensky, M.E., Pieters, C.M., 2001. The Tagish Lake meteorite:
A possible sample from a D-type asteroid. Science 293, 2234–2236.
Hiroi, T., Kanno, A., Nakamura, R., Abe, M., Ishiguro, M., Hasegawa, S.,
Miyasaka, S., Sekiguchi, T., Terada, H., Igarashi, G., 2003. The Tagish
Lake meteorite as a possible sample from a T or D type asteroid. Lunar
Planet. Sci. 34. Abstract #1425 [CD-ROM].
Jackson, A.A., Zook, H.A., 1992. Orbital evolution of dust particles from
comets and asteroids. Icarus 97, 70–84.
Jewitt, D.C., 2002. From Kuiper belt object to cometary nucleus: The missing ultrared matter. Astron. J. 123, 1039–1049.
Jewitt, D.C., 2005. A first look at the Damocloids. Astron. J. 129, 530–538.
Jewitt, D.C., Luu, L.X., 2001. Colors and spectra of Kuiper belt objects.
Astron. J. 122, 2099–2114.
Joswiak, D.J., Brownlee, D.E., Pepin, R.O., Schlutter, D.J., 2001. Characteristics of asteroidal and cometary IDPs obtained from stratospheric
collectors: Summary of measured He release temperatures, velocities,
and descriptive mineralogy. Lunar Planet. Sci. 31. Abstract #1500 [CDROM].
Kidger, M.R., 2002. The round table discussion: How should we take best
advantage of the new generation of large telescopes for cometary research? Earth Moon Planets 90, 515–522.
Kresák, L., 1979. Dynamical interrelations among comets and asteroids. In:
Gehrels, T. (Ed.), Asteroids. Univ. of Arizona Press, Tucson, pp. 289–
309.
Lamy, P.L., Toth, I., Fernández, Y.R., Weaver, H.A., 2004. The sizes,
shapes, albedos, and colors of cometary nuclei. In: Festou, M., Keller,
H.U., Weaver, H.A. (Eds.), Comets II. Univ. of Arizona Press, Tucson,
pp. 223–264.
Landolt, A.U., 1992. UBVRI photometric standard stars in the magnitude
range 11.5–16.0 around the celestial equator. Astron. J. 104, 340–371.
Lebofsky, L.A., Sykes, M.V., Tedesco, E.F., Veeder, G.J., Matson, D.L.,
Brown, R.H., Gradie, J.C., Feierberg, M.A., Rudy, R.J., 1986. A refined
“standard” thermal model for asteroids based on observations of 1 Ceres
and 2 Pallas. Icarus 68, 239–251.
Lebofsky, L.A., Spencer, J.R., 1989. Radiometry and thermal modeling of
asteroids. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids
II. Univ. of Arizona Press, Tucson, pp. 128–147.
Levison, H.F., Duncan, M.J., 1994. The long-term dynamical behavior of
short-period comets. Icarus 108, 18–36.
Levison, H.F., Duncan, M.J., 1997. From the Kuiper belt to Jupiter-family
comets: The spatial distribution of ecliptic comets. Icarus 127, 13–32.
Characteristics of C/2001 OG108 (LONEOS)
Licandro, J., Guerra, J.C., Campins, H., Di Martino, M., Lara, L.M., GilHutton, R., Tozzi, G.P., 2002. The surface of cometary nuclei related
minor icy bodies. Earth Moon Planets 90, 495–496.
Licandro, J., Campins, H., Hergenrother, C., Lara, L.M., 2003. Nearinfrared spectroscopy of the nucleus of Comet 124P/Mrkos. Astron.
Astrophys. 398, L45–L48.
Luu, J.X., 1993. Spectral diversity among the nuclei of comets. Icarus 104,
138–148.
Marsden, B.G., 2001. MPEC 2001-P40, Minor Planet Center, Smithsonian
Astrophysical Observatory.
Marsden, B.G., 2002. MPEC 2002-C04, Minor Planet Center, Smithsonian
Astrophysical Observatory.
McFadden, L.A., 1993. The comet–asteroid transition: Recent telescopic
observations. In: Milani, A., Di Martino, M. (Eds.), Asteroids, Comets,
Meteors. Proceedings of the 160th Symposium of the International Astronomical Union. Kluwer Academic, Dordrecht, pp. 95–110.
McFadden, L.A., Gaffey, M.J., McCord, T.B., 1985. Near-Earth asteroids:
Possible sources from reflectance spectroscopy. Science 229, 160–163.
Morbidelli, A., Gladman, B., 1998. Orbital and temporal distributions of
meteorites originating in the asteroid belt. Meteorit. Planet. Sci. 33,
999–1016.
Morbidelli, A., Bottke, W.F., Froeschlé, Ch., Michel, P., 2002. Origin and
evolution of near-Earth objects. In: Bottke, W.F., Cellino, A., Paolicchi,
P., Binzel, R.P. (Eds.), Asteroids III. Univ. of Arizona Press, Tucson,
pp. 409–422.
Mumma, M.J., Dello Russo, N., DiSanti, M.A., Magee-Sauer, K., Novak,
R.E., Brittain, S., Rettig, T., McLean, I.S., Reuter, D.C., Xu, L.-H.,
2001. Organic composition of C/1999 S4 (LINEAR): A comet formed
near Jupiter? Science 292, 1334–1339.
Nakamura, A., Wakuda, S., Oribe, T., 2002. Comet C/2001 OG108 (LONEOS). In: Green, D.W.E. (Ed.), IAUC #7814. Central Bureau for Astronomical Telegrams, Smithsonian Astrophysical Observatory.
Nier, A.O., Schlutter, D.J., 1993. The thermal history of interplanetary dust
particles collected in the Earth’s stratosphere. Meteoritics 28, 675–681.
Olsson-Steel, D., 1988. Identification of meteoroid streams from Apollo
asteroids in the Adelaide radar orbit surveys. Icarus 75, 64–96.
Oort, J.H., 1950. The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin. Bull. Astron. Inst.
Netherlands 11, 91–110.
Öpik, E.J., 1951. Collision probability with the planets and the distribution
of planetary matter. Proc. R. Irish Acad. 54A, 165–199.
Öpik, E.J., 1961. The survival of comets and cometary material. Astron.
J. 66, 381–382.
Öpik, E.J., 1963. The stray bodies in the Solar System. Part I. Survival of
cometary nuclei and the asteroids. Adv. Astron. Astrophys. 2, 219–262.
Pravec, P., Harris, A.W., 2000. Fast and slow rotation of asteroids.
Icarus 148, 12–20.
Pravec, P., Šarounová, L., Wolf, M., 1996. Lightcurves of 7 near-Earth asteroids. Icarus 124, 471–482.
Pravec, P., Harris, A.W., Scheirich, P., Kušnirák, P., Šarounová, L., Hergenrother, C.W., Mottola, S., Hicks, M.D., Masi, G., Krugly, Yu.N.,
Shevchenko, V.N., Nolan, M.C., Howell, E.S., Kaasalainen, M., Galád,
A., Brown, P., DeGraff, D.R., Lambert, J.V., Cooney Jr., W.R., Foglia,
S., 2005. Tumbling asteroids. Icarus 173, 108–131.
Rayner, J.T., Toomey, D.W., Onaka, P.M., Denault, A.J., Stahlberger,
W.E., Vaca, W.D., Cushing, M.C., Wang, S., 2003. SpeX: A mediumresolution 0.8–5.5 micron spectrograph and imager for the NASA Infrared Telescope Facility. Publ. Astron. Soc. Pacific 115, 362–382.
Samarasinha, N.H., Belton, M.J., 1995. Long-term evolution of rotational
states and nongravitational effects for Halley-like cometary nuclei.
Icarus 116, 340–358.
Samarasinha, N.H., Mueller, B.E.A., Belton, M.J., Jorda, L., 2004. Rotation
of cometary nuclei. In: Festou, M., Keller, H.U., Weaver, H.A. (Eds.),
Comets II. Univ. of Arizona Press, Tucson, pp. 281–299.
Sheppard, S.S., Jewitt, D.C., 2003. Hawaii Kuiper belt variability project:
An update. Earth Moon Planets 92, 207–219.
193
Shoemaker, E.M., Williams, J.G., Helin, E.F., Wolfe, R.F., 1979. Earthcrossing asteroids: Orbital classes, collision rates with Earth, and origin. In: Gehrels, T. (Ed.), Asteroids. Univ. of Arizona Press, Tucson,
pp. 253–282.
Shoemaker, E.M., Shoemaker, C.S., Wolfe, R.F., 1989. Trojan asteroids:
Populations, dynamical structure, and origin of the L4 and L5 swarms.
In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ.
of Arizona Press, Tucson, pp. 487–523.
Soderblom, L.A., Becker, T.L., Bennett, G., Boice, D.C., Britt, D.T.,
Brown, R.H., Buratti, B.J., Isbell, C., Giese, B., Hare, T., Hicks, M.D.,
Howington-Kraus, E., Kirk, R.L., Lee, M., Nelson, R.M., Oberst, J.,
Owen, T.C., Rayman, M.D., Sandel, B.R., Stern, S.A., Thomas, N.,
Yelle, R.V., 2002. Observations of Comet 19/P Borrelly by the Miniature Integrated Camera and Spectrometer aboard Deep Space 1. Science 296, 1087–1091.
Stern, S.A., Weissman, P.R., 2001. Rapid collisional evolution of comets
during the formation of the Oort cloud. Nature 409, 589–591.
Stokes, G.H., Evans, J.B., Larson, S.M., 2002. Near-Earth asteroid search
programs. In: Bottke, W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.),
Asteroids III. Univ. of Arizona Press, Tucson, pp. 45–54.
Tedesco, E.F., Tholen, D.J., Zellner, B., 1982. The eight-color asteroid survey: Standard stars. Astron. J. 87, 1585–1592.
Tedesco, E.F., Veeder, G.J., Fowler, J.W., Chillemi, J.R., 1992. IRAS Minor Planet Survey. Phillips Laboratory Report PL-TR-2049. Phillips
Laboratory, Directorate of Geophysics, Air Force Materiel Command,
Hanscom Air Force Base, Massachusetts.
Tedesco, E.F., Noah, P.V., Noah, M., Price, S.D., 2002. The supplemental
IRAS minor planet survey. Astron. J. 123, 1056–1085.
Tholen, D.J., 1984. Asteroid taxonomy from cluster analysis of photometry.
Ph.D. thesis. University of Arizona, Tucson, 150 pp.
Tholen, D.J., Barucci, M.A., 1989. Asteroid taxonomy. In: Binzel, R.P.,
Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press,
Tucson, pp. 298–335.
Thomas, N., Keller, H.U., 1989. The colour of Comet P/Halley’s nucleus
and dust. Astron. Astrophys. 213, 487–494.
Tokunaga, A.T., 1984. A reevaluation of the 20-µm magnitude system. Astron. J. 89, 172–175.
Veeder, G.J., Hanner, M.S., Matson, D.L., Tedesco, E.F., Lebofsky, L.A.,
Tokunaga, A.T., 1989. Radiometry of near-Earth asteroids. Astron.
J. 97, 1211–1219.
Vilas, F., Gaffey, M.J., 1989. Phyllosilicate absorption features in main-belt
and outer-belt asteroid reflectance spectra. Science 246, 790–792.
Vilas, F., Hatch, E.C., Larson, S.M., Sawyer, S.R., Gaffey, M.J., 1993. Ferric iron in primitive asteroids: A 0.43-µm absorption feature. Icarus 102,
225–231.
Vilas, F., Jarvis, K.S., Gaffey, M.J., 1994. Iron alteration minerals in the
visible and near-infrared spectra of low-albedo asteroids. Icarus 109,
274–283.
Weissman, P.R., 1980. Physical loss of long-period comets. Astron. Astrophys. 85, 191–196.
Weissman, P.R., 1999. Diversity of comets: Formation zones and dynamical
paths. Space Sci. Rev. 90, 301–311.
Weissman, P.R., Levison, H.F., 1997. Origin and evolution of the unusual
object 1996 PW: Asteroids from the Oort cloud? Astrophys. J. 488,
L133–L136.
Weissman, P.R., A’Hearn, M.F., McFadden, L.A., Rickman, H., 1989. Evolution of comets into asteroids. In: Binzel, R.P., Gehrels, T., Matthews,
M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 880–
920.
Weissman, P.R., Bottke, W.F., Levison, H.F., 2002. Evolution of comets into
asteroids. In: Bottke, W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.),
Asteroids III. Univ. of Arizona Press, Tucson, pp. 669–686.
Wetherill, G.W., 1971. Cometary versus asteroidal origin of chondritic meteorites. In: Gehrels, T. (Ed.), Physical Studies of Minor Planets. SP267. National Aeronautics and Space Administration, Washington, DC,
pp. 447–460.
194
P.A. Abell et al. / Icarus 179 (2005) 174–194
Wetherill, G.W., 1988. Where do the Apollo objects come from? Icarus 76,
1–18.
Wetherill, G.W., 1991. End products of cometary evolution: Cometary origin of Earth-crossing bodies of asteroidal appearance. In: Newburn,
R.L., Neugebauer, M., Rahe, J.H. (Eds.), Comets in the Post-Halley Era.
Kluwer Academic, Dordrecht, pp. 537–556.
Whipple, F.L., 1950. A comet model. I. The acceleration of Comet Encke.
Astrophys. J. 111, 375–394.
Whipple, F.L., 1983. 1983 TB and the Geminid meteors. In: Marsden, B.G.
(Ed.), IAUC #3881. Central Bureau for Astronomical Telegrams, Smithsonian Astrophysical Observatory, Cambridge, MA.
Williams, I.P., Wu, Z., 1993. The Geminid meteor stream and Asteroid 3200
Phaethon. Mon. Not. R. Astron. Soc. 262, 231–248.
Wyckoff, S., 1982. Overview of comet observations. In: Wilkening, L.L.
(Ed.), Comets. Univ. of Arizona Press, Tucson, pp. 3–55.
Zellner, B., Tholen, D.J., Tedesco, E.F., 1985. The eight-color asteroid survey: Results for 589 minor planets. Icarus 61, 355–416.