NEOShield-2
Science and Technology for Near-Earth Object Impact Prevention
Grant agreement no:
640351
Project Start:
1 March 2015
Project Coordinator
Airbus Defence and Space DE
Project Duration:
31 Months
WP 10
Deliverable D10.2
Intermediate observations and analysis progress report
WP Leader
OBSPM
Due date
M13, 31 Mar 2016
Delivery date
30.03.2016
Issue
1.0
Editor (authors)
D. Perna, M.A. Barucci, S. Eggl, M. Birlan, E. Dotto, S. Ieva, M. Delbo, V. Ali-Lagoa
Contributors
A. Di Paola, R. Speziali, E. Mazzotta Epifani, M. Lazzarin, S. Magrin, I. Bertini J.
Hanus, M. Popescu, E. Perozzi
Task Leader
OBSPM
Verified by
Document Type
R
Dissemination Level
PU
The NEOShield-2 Consortium consists of:
Airbus DS GmbH (Project Coordinator)
Deutsches Zentrum für Luft- und Raumfahrt e.V.
Airbus Defence and Space SAS
Airbus Defence and Space Ltd
Centre National de la Recherche Scientifique
DEIMOS Space Sociedad Limitada Unipersonal
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
GMV Aerospace and Defence SA Unipersonal
Istituto Nazionale di Astrofisica
Observatoire de Paris
The Queen’s University of Belfast
This project has received funding from the European
Union’s Horizon 2020 research and innovation
programme under grant agreement No 640351.
ADS-DE
DLR
ADS-FR
ADS-UK
CNRS
DMS
EMI
GMV
INAF
OBSPM
QUB
Germany
Germany
France
United Kingdom
France
Spain
Germany
Spain
Italy
France
United Kingdom
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Change Record
Issue
Date
Section, Page
Description of Change
0.1
15/3/2016
First Draft ; Distributed to Consortium
1.0
30/3/2016
Incorporating comments from Consortium
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Table of Contents
1
2
Introduction ............................................................................................................................................. 4
1.1
Scope and objective ...................................................................................................................... 4
1.2
List of Abbreviations.................................................................................................................... 4
1.3
Applicable Documents ................................................................................................................ 5
1.4
Reference Documents ................................................................................................................. 5
Photometric observations................................................................................................................... 8
2.1
Colours and phase functions (INAF) ...................................................................................... 8
2.2
Lightcurves and rotational properties (OBSPM-IMCCE).............................................. 11
2.2.1
French facilities ..................................................................................................................................... 11
2.2.2
International Facilities, the characterization of 2004 BL86 ............................................... 12
2.2.3
The YELP campaign ............................................................................................................................. 13
2.2.4
Space based photometry using NASA's Kepler spacecraft .................................................. 14
2.3
3
4
Spectroscopic observations (OBSPM-LESIA) ............................................................................. 18
3.1
A literature study of the Potentially Hazardous Asteroid (PHA) population....... 18
3.2
Guaranteed Time Observations at ESO-NTT .................................................................... 20
3.3
Further ESO observations ....................................................................................................... 23
Thermal IR observations (CNRS) ................................................................................................... 24
4.1
Modeling techniques................................................................................................................. 24
4.1.1
Determination of sizes and albedos of NEAs from simple thermal models ................. 24
4.1.2
Thermal inertia ...................................................................................................................................... 25
4.1.3
Thermophysical modelling of near-Earth Asteroids.............................................................. 25
4.1.4
Asteroid 3D shapes as input for thermophysical models .................................................... 26
4.1.5
Hybrid thermal model ........................................................................................................................ 27
4.2
Results............................................................................................................................................ 27
4.2.1
TPM of (3200) Phaethon ................................................................................................................... 27
4.2.2
TPM of (1685) Toro ............................................................................................................................. 28
4.2.3
Hybrid thermal model of (1685) Toro ......................................................................................... 29
4.2.4
Comparison with other thermal inertias found in the literature...................................... 29
4.3
5
Precovery of NEOs and discovery apparition photometry (QUB) ............................ 17
Conclusions and Perspectives ............................................................................................... 30
Summary................................................................................................................................................. 31
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1
Introduction
1.1 Scope and objective
At the moment of this writing the number of known NEOs exceeds 14000 and new objects are
currently being discovered at the rate of 3 per day. However less than 10% of NEOs have been
investigated to retrieve their physical properties. The lack of data is particularly evident for the
smaller sizes, those of interest for NEOShield-2. For example, less than 100 NEOs in the size
range 50-300 m had been taxonomically characterized prior of our observations.
The present/near future NEO discovery rate is mostly due to the detection of small objects
approaching the Earth. They represent a significant source of potential targets for physical
characterization satisfying the PROTEC-2 requirements on size and accessibility, thus
complementing the observation of already known objects. As discussed in Section 3.1 of the
NEOShield-2 Deliverable 11.1 “Report on a Future NEO physical properties database” [AD3],
more than 2/3 of NEOs which had a close approach with the Earth in 2012, hence an
observational opportunity, were discovered within the same year, and among them almost the
totality are small (< 300 m). Hence the possibility of a quick (~weeks) physical follow-up of
newly discovered objects could provide a significant contribution to the characterization of the
NEO population, especially in the size range of our interest. As the discoveries peak sharply
around V=20, in order to exploit this opportunity at best, large telescopes (4-m class) available
on short notice are needed. That’s why, as reported in [AD2], an agreement with the European
Southern Observatory (ESO) was signed on 1/3/2015 to obtain Guaranteed Time Observations
(GTO) at the 3.6-meter New Technology Telescope (NTT). Further observing time has been
obtained via the standard biannual proposals at large telescopes, as well as with our small
guaranteed-access telescopes.
In this document, we report about all observations and data analysis carried out so far (M13) by
all NEOShield-2 participating observers.
Section 2 deals with photometric observations and data analysis (INAF, OBSPM-IMCCE, QUB).
Section 3 deals with spectroscopic observations and data analysis (OBSPM-LESIA).
Section 4 deals with thermal infrared observations and data analysis (CNRS).
1.2 List of Abbreviations
AD
Applicable Document
AU
Astronomical Unit
EARN
European Asteroid Research Node
EPIC
Ecliptic Plane Input Catalog
ESO
European Southern Observatory
GA
Grant Agreement
GTO
Guaranteed Time Observations
IRTF
InfraRed Telescope Facility
LBT
Large Binocular Telescope
MOID
Minimum Orbit Intersection Distance
NEA
Near Earth Asteroid
NEATM
Near-Earth Asteroid Thermal Model
NEO
Near Earth Object
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NTT
New Technology Telescope
OHP
Observatoire Haute Provence
PDM
Pic du Midi
PHA
Potentially Hazardous Asteroid
PHO
PHA
RD
Reference Document
SSO
Solar System Object
TIR
Thermal InfraRed
TNG
Telescopio Nazionale Galileo
TPM
Thermo-Physical Model
WISE
Wide-field Infrared Survey Explorer
WP
Work Package
Potentially Hazardous Object
1.3 Applicable Documents
[AD1]
NEOShield-2: “Science and Technology for Near-Earth Object Impact Prevention”, Grant
Agreement no. 640351, 28.10.2014.
[AD2]
NEOShield-2 Deliverable 10.1 “Report on observation procedures and tools”, v1.0,
7.9.2015.
[AD3]
NEOShield-2 Deliverable 11.1 “Report on a Future NEO physical properties database”,
v0.1, 14.3.2016.
1.4 Reference Documents
[RD1]
Alí-Lagoa V., et al. (2014) Astron. Astrophys., 561, A45.
[RD2]
Altmann, M., A. H. Andrei, U. Bastian, Sebastien Bouquillon, F. Mignard, R. Smart, I.
Steele, Paolo Tanga, and Francois Taris (2010). "Ground Based Optical Tracking of
Gaia." In Workshop Gaia Fun-SSO: follow-up network for the Solar System Objects, vol.
1, p. 149.
[RD3]
Berthier, J., Carry, B., Vachier, F., Eggl, S., Santerne, A. (2016) ,Prediction of transits of
solar system objects in Kepler/K2 images: An extension of the Virtual Observatory
service SkyBoT, MNRAS (accepted), ArXiv e-prints, arXiv:1602.07153.
[RD4]
Birlan, M., Barucci, M. A., Vernazza, P., Fulchignoni, M., Binzel, R. P., Bus, S. J., Belskaya, I.,
& Fornasier, S. (2004), New Astronomy, 9, 343.
[RD5]
Birlan M., Popescu M., Nedelcu D. A., Turcu V., Pop A., Dumitru B., Stevance F., Vaduvescu
O., Moldovan D., Rocher, P., Sonka A., Mircea, L. (2015), Characterization of (357439)
2004 BL86 on its close approach to Earth in 2015, Astronomy & Astrophysics vol 581,
id.A3, 7pp.
[RD6]
Birlan M., Nedelcu A., Sonka A., Popescu M., Dumitru B. (2016) Observations for secure
and recovery Near-Earth Asteroids, Rom Astron. J., vol 26, n 1 (accepted).
[RD7]
Bottke W. F. J., Vokrouhlický D., Rubincam D. P., et al. (2006) Annu. Rev. Earth Planet.
Sci., 34, 157–191.
[RD8]
Carry, B. (2012,) Planetary and Space Science, 73, 98.
[RD9]
Carry, B., Solano, E., Eggl, S., & DeMeo, F. E. (2016). Spectral properties of near-Earth and
Mars-crossing asteroids using Sloan photometry. Icarus.
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[RD10] Delbo’ et al. (2015) In Asteroids IV (P. Michel. et al., eds.). Univ. of Arizona, Tucson.
[RD11] DeMeo, F. E., Binzel, R. P., Slivan, S. M., & Bus, S. J. (2009), Icarus, 202, 160.
[RD12] Drube L., Harris A. W., Hoerth, T., Michel, P., Perna, D., Schäfer, F. (2015). In “Handbook
of Cosmic Hazards and Planetary Defense”, ed. Joe Pelton & Firooz Allahdadi, Springer.
[RD13] Durech et al. (2005) Earth Moon Planets, 97, 179–187.
[RD14] Durech et al. (2007) In Near Earth Objects, Our Celestial Neighbors: Opportunity and
Risk (A. Milani et al., eds.), p. 191. Cambridge Univ., Cambridge.
[RD15] Durech et al. (2009) Astron. Astrophys., 493, 291–297.
[RD16] Eggl, S., Hestroffer, D., Cano, J. L., Avila, J. M., Drube, L., Harris, A. W., ... & Michel, P.
(2016). Dealing with Uncertainties in Asteroid Deflection Demonstration Missions:
NEOTwIST. IAU Symposium, 318, 231-238.
[RD17] Fulvio, D., Perna, D., Ieva, S., et al. (2016), MNRAS, 455, 584.
[RD18] Hanuš J., et al. (2011) Astron. Astrophys., 530, A134.
[RD19] Hanuš J., et al. (2013a). Icarus, 226, 1045–1057.
[RD20] Hanuš J., et al. (2013b) Astron. Astrophys., 551, A67.
[RD21] Hanuš J., et al. (2015) Icarus, 256, p. 101-116.
[RD22] Harris A. W. (1998), Icarus, 131, 291–301.
[RD23] Harris et al. (2013), Acta Aeronautica 90, 1, p. 80-84.
[RD24] Harris A. W. and Lagerros J. S. V. (2002) In Asteroids III (W. F. Bottke Jr. et al., eds.).
Univ. of Arizona, Tucson.
[RD25] Ieva, S., Dotto, E., Lazzaro, D., et al. (2016), MNRAS, 455, 2871.
[RD26] Kaasalainen (2001) Astron. Astrophys., 376, 302–309.
[RD27] Kaasalainen & Torpa (2001) Icarus, 153, 24–36.
[RD28] Kaasalainen (2004) Astron. Astrophys., 422, L39–L42.
[RD29] Licandro et al. (2016), Astron. Astroph. 585, A10, 4pp.
[RD30] Mainzer et al. (2011) Astrophys. J., 743, 156.
[RD31] Masiero et al. (2011) Astrophys. J., 741, 68.
[RD32] Micheli, M., Tholen, D. J., Jenniskens, P. (2016). Icarus, 267, pp. 64-67.
[RD33] Moskovitz, N. et al. (2014), in AAS/Division for Planetary Sciences Meeting Abstracts,
Vol. 46.
[RD34] Pal, A., Szabo, R., Szabo, G. M., Kiss, L. L., Molnar, L., Sarneczky, K., & Kiss, C. (2015), The
Astrophysical Journal Letters, 804, L45.
[RD35] Perna, D., Dotto, E., Ieva, S., et al. (2016). Grasping the Nature of Potentially Hazardous
Asteroids. The Astronomical Journal, 151, 11, 14 pp.
[RD36] Pravec, P. & Harris, A. W. (2000). Icarus, 148, pp. 12-20.
[RD37] Rozitis et al. (2014) Nature, 512, 174–176.
[RD38] Szabo, R. et al. (2015), The Astronomical Journal, 149, 112.
[RD39] Thuillot, W., Carry, B., Berthier, J., David, P., Hestroffer, D., & Rocher, P. (2014). GaiaFUN-SSO: a network for ground-based follow-up observations of Solar System Objects.
In SF2A-2014: Proceedings of the Annual meeting of the French Society of Astronomy
and Astrophysics (Vol. 1, pp. 445-448).
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[RD40] Thuillot, W., Bancelin, D., Ivantsov, A., Desmars, J., Assafin, M., Eggl, S., ... & Abe, L. (2015).
The astrometric Gaia-FUN-SSO observation campaign of 99942 Apophis. Astronomy &
Astrophysics, 583, A59.
[RD41] Wright et al. (2010) Astron. J., 140, 1868–1881.
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2
Photometric observations
2.1 Colours and phase functions (INAF)
During this first year of activity, we have analyzed and interpreted photometric data acquired on
July 2014 at Telescopio Nazionale Galileo (TNG) and we have prepared and submitted several
proposals to TNG and the Large Binocular Telescope (LBT).
At TNG we have obtained a long-term program devoted to the characterization of surface color
indexes (B-V-R-I) for a large number of NEAs. The program started in September 2015 and
spans on 4 semesters (up to August 2017). For each semester, we have been awarded of 6 runs
of 4 hours each one. So far we have acquired data on 58 targets: for 30 of them data have been
already reduced and analysed and a preliminary taxonomy has been obtained. For further 28
targets, data are presently under reduction.
At LBT, we have obtained telescope time to observe NEAs in the B-V-R-I-g-r-z filters. 6 targets
have been so far observed and data are presently under reduction.
In the framework of a collaboration with the Observatorio Nacional (Rio de Janeiro - Brasil, D.
Lazzaro) for the use of the telescope OASI – Itacuruba, we have performed observations of NEAs
at different phase angles, for phase curves characterization. Five targets have been so far
observed: for 1 of them data are presently under reduction, for 4 of them phase curves have
been obtained.
Table 2-1: runs, allocated time and comments.
Telescope
TNG
TNG
run
July 2014
October 2015
Allocated time
4h
4h
TNG
November 2015
4h
TNG
December 2015
8h
TNG
January 2016
4h
TNG
OASI – Itacuruba
February 2016
September 2015
4h
8n
OASI – Itacuruba
October 2015
---
OASI – Itacuruba
November 2015
---
OASI – Itacuruba
December 2015
1n
OASI – Itacuruba
January 2016
---
OASI - Itacuruba
February 2016
---
LBT
January 2016
2h
LBT
February 2016
2h
Comments
9 targets – data reduced
6 targets – data reduced
No observations, bad
weather
15 targets – data reduced
13 targets – data under
reduction
No observations, bad
weather
15 targets – data under
reduction
4 targets – data reduced
No observations, bad
weather
No observations, bad
weather
1 target – data under
reduction
No observations, bad
weather
No observations, bad
weather
2 targets – data under
reduction
4 targets – data under
reduction
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For 4 objects observed at OASI – Itacuruba it was possible to compute the absolute magnitude H
from the phase curve characterization.
Table 2-2: dates of observation and computed absolute magnitude (and albedo value
from literature) of the 4 targets observed at OASI-Itacuruba.
Date
Object
H
ρv
9-18/9/2015
14.43 ± 0.24
4055 Magellan
0.31(1)
9-18/9/2015
17.62 ± 0.38
333889 1998SV4
0.19 (2)
9-18/9/2015
17.77 ± 0.47
337118 1999TX2
0.11(3)
9-18/9/2015
20.40 ± 0.35
446833 2001RB12
--References: (1) Thomas et al. 2014; (2) Harris et al. 2011; (3) Trilling et al. 2010.
Table 2-3: date of observation, color indexes and the obtained taxonomy of the 30 objects
observed at TNG.
Object
2014ER49
2005UK1
2010NY65
2004LJ1
2008LV16
1994CJ1
2002SR41
2010LE15
1995SA
65690
423709
2006BE55
2011AK5
445974
2013QJ10
2013UG5
2015RT83
155110
138852
442243
2012XA133
142563
162273
2008VQ4
443880
2011YH6
174806
date
V
1/07/2014
1/07/2014
1/07/2014
1/07/2014
1/07/2014
1/07/2014
1/07/2014
1/07/2014
1/07/2014
18.61 ± 0.03
18.94 ± 0.03
18.63 ± 0.02
18.53 ± 0.02
18.54 ± 0.01
18.54 ± 0.01
18.77 ± 0.02
19.23 ± 0.03
0.88
0.51
0.77
0.6
0.8
0.94
0.76
0.86
0.52
0.58
0.56
0.49
0.46
0.61
0.44
0.67
1.04
0.82
1.11
0.9
0.83
1.01
0.94
0.88
18.24 ± 0.02
20.33 ± 0.06
21.74 ± 0.15
21.17 ± 0.05
19.57 ± 0.05
21.78 ±0.14
20.25 ± 0.04
20.17 ± 0.04
19.51 ± 0.03
19.13 ± 0.03
19.77 ± 0.05
19.95 ± 0.04
19.97 ± 0.04
19.92 ± 0.04
19.52 ± 0.04
19.78 ± 0.04
20.05 ± 0.07
0.74
0.68
0.61
0.5
0.81
0.62
0.89
0.43
0.66
0.74
0.7
0.8
0.56
0.78
1.02
0.64
0.61
0.33
0.46
0.59
0.53
0.65
0.58
0.3
0.33
0.45
0.34
0.4
0.38
0.49
0.42
0.58
0.37
0.28
0.81
0.43
1.25
1.4
0.88
0.85
0.79
0.73
0.86
0.92
0.85
0.88
0.64
0.57
0.77
0.51
0.46
19.96 ± 0.06
19.94 ± 0.03
0.7
0.72
0.25
0.43
0.84
0.87
13/10/2015
13/10/2015
13/10/2015
13/10/2015
13/10/2015
13/10/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
10/12/2015
B-V
V-R
V-I
Preliminary
Taxonomy
S-complex
S-complex
S-complex
Sv
S-complex
A
D, S-complex
S-complex, A
C-complex, Xcomplex
Xc
A/V
S-complex
S-complex
S-complex
S-complex
C-complex
D/X
X-complex
X-complex
S-complex
Q
O,B
S-complex
C-complex
B, C-complex
X-complex, Scomplex
T, S-complex
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2000YK4
194126
2012XD112
10/12/2015
10/12/2015
10/12/2015
20.33 ± 0.05
19.87 ± 0.04
19.67 ± 0.03
0.69
0.79
0.79
0.29
0.39
0.28
C-complex
X-complex
C-complex
0.77
0.78
0.55
The whole sample of objects observed at TNG were divided in four classes according to their
composition: carbonaceous (C), siliceous (S), basaltic (V) and miscellaneous (X).
The distribution of different taxa was then analyzed according to NEAs orbital parameters.
In Fig. 2-1 we show, as an example, the Minimum Orbital Intersection Distance (MOID) with our
planet as a function of the orbital inclination of the observed NEAs. In our sample, siliceous
objects seem to have the lowest MOID, while C and X-complex objects seem to have higher
inclination. Two carbonaceous objects show low MOID and very high inclination, representing
the most dangerous objects in the present set of bodies.
0.35
V
0.3
C
MOID (au)
0.25
X
S
0.2
0.15
0.1
0.05
0
0
5
10
15
20
25
30
35
i (deg)
Fig. 2-1 Minimum Orbital Intersection Distance (MOID) of compositional groups
(Carbonaceous, Silicaceous, Basaltic and Miscellaneous) of our sample of NEAs,
plotted vs their orbital inclination.
As above mentioned, data acquired in the temporal range December 2015 - February 2016 are
presently under reduction; new observations will be soon available. Thus will allow us to
implement the number of objects analyzed and upgrade our statistical work.
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2.2 Lightcurves and rotational properties (OBSPM-IMCCE)
2.2.1
French facilities
The IMCCE partakes in the data collection and analysis endeavor making use of international as
well as French facilities like the Observatoire Haute Provence (OHP) and the Pic du Midi (PDM)
observatories. The telescopes at those sites available for asteroid research are in the one-meter
class supporting a relatively large fieldview (FOV) which makes the excellent tools for and
astrometric follow up of newly discovered asteroids. However, only a few relatively bright
objects can be studied spectroscopically and photometrically at reasonable signal-to-noise
ratios. The observatories that are available to the IMCCE for astrometric and photometric
observations of NEOs are the Observatoire Haute Provence (OHP, MPC Code 511) and Pic du
Midi (PDM, MPC Code 586). The accessible telescopes at both sites have apertures of 120cm and
106cm, respectively. While both telescopes are used to generate robust ground based
astrometry for fainter sources on a regular basis, only relatively bright NEOs (around apparent
magnitude 17-18) can be targeted with respect to photometry, if a signal-to-noise ratio around
50 is aimed for. Pic du Midi is accessible throughout the year. However, the observation program
has to be funded through the NEOShield-2 travel and/or IMCCE team budget. A shared campaign
was organized at PDM from September, 4th-9th, 2015. Unfortunately, the weather and shared
time was allowing only for one target to be observed, namely 2001 RB12. In contrast, time at the
OHP site was more plentiful as it could be shared with Gaia-FUN-SSO (Thuillot et al. 2014, 2015)
and GBOT (Altmann et al. 2010) programs. In total we were able to conduct four campaigns at
the OHP in 2015 yielding 13 objects where light curves could be measured fully or at least in
part. Table 2-4 summarizes the preselected NEOShield-2 targets compatible with the brightness
limits of OHP and PDM which were observed so far. The data and calibration frames are
currently saved at IMCCE servers but will be made available for NEOShield-2 consortium
members as soon as the corresponding facilities are online. Since the GBOT astrometric pipeline
has become available astrometric data reduction is currently tested on a semiautomated basis.
The current GBOT pipeline does extract photometric data. However, it has not been designed to
provide high precision photometric results. While some work has been done in this respect, the
GBOT photometric pipeline sill needs to be worked on in this respect to allow for a proper rereduction of the acquired data. We foresee a re-reduction of all data by mid 2016. The Gaia-FUNSSO campaign continues through 2016. Shared NEOShield-2 observation time at the OHP has
already been allocated for April 4th-8th 2016. One observing run (PI M. Birlan) is also scheduled
for April 2016 at Pic du Midi for observing colors of NEAs.
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Table 2-4. Targets observed at French observatories so far. Astrometric (AM) and
photometric light curve (LC) observations are shown. The corresponding program name
is also given. One can see that time sharing with Gaia FUN-SSO (GFSSO) has resulted in a
substantial increase in observed targets. YELP is a program speficially dedicated to detect
changes in the rotation of NEOs with a measured astrometric orbit drift.
TARGET
2000 LF6
2010 EV45
1998 AX4
2003 NZ6
2015 JH2
2015 KJ7J
2015 KL122
2006 WP127
2015 NZ13
2010 PR66
2012 NP
2001 RB12
2015 SA17
2015 TE
Geographos
1998VW36
2000 NL10
2000 SU318
2003 XO15
1991 CS
2016 CB138
2016 DV1
A100hUP
2016 DN2
2016 ED
2.2.2
PROGRAM
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
NEOShield 2
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
GFSSO
YELP
YELP
YELP
YELP
YELP
YELP
TYPE
LC
LC
LC
AM
AM
AM
AM
LC
AM
LC
LC
LC
AM
LC
LC
AM
LC
AM
LC
LC
AM
LC
AM
AM
AM
DATE
11-15.06.2015
11-15.06.2015
15.06.2015
14.06.2015
15.06.2015
15.06.2015
15.06.2015
22-23.07.2015
20.07.2015
10.-13.08.2015
10.-13.08.2015
9.-14.09.2015
10.10.2015
10.-15.10.2015
10.-15.10.2015
10.-15.10.2015
10.-15.10.2015
10.-15.10.2015
10.-15.10.2015
2.-5.03.2016
2.-5.03.2016
02.03.2016
02.03.2016
03.03.2016
03.03.2016
PERIOD (prel.) [h]
14.9
3.5
2.9
tbd
tbd
19.6
5.3
tbd
5.2
6.9
tbd
2.4
tbd
International Facilities, the characterization of 2004 BL86
The potentially hazardous asteroid (PHA) (357439) 2004 BL86 grazed Earth on January 26,
2015 at a distance of about 1.2 million km. The favorable geometry during its closest approach
to Earth in January-February 2015 allowed to derive its physical and dynamical parameters.
(357439) 2004 BL86 was previously estimated to be a 500 m body. Spectral VNIR and
photometry of asteroid binary asteroid (357439) 2004 BL86 was obtained. (357439) 2004 BL86
was classified as V-type asteroid, which are particularly rare among binary PHAs, see Figure 2-2.
Near-infrared (NIR) spectral observations (0.8-2.5μm) were carried out using the upgraded
SpeX instrument mounted on the InfraRed Telescope Facility (IRTF), located on Mauna Kea,
Hawaii. The remote observing technique was used from CODAM-Paris Observatory (Birlan et al.
2004). The upgraded SpeX (uSpeX) instrument was used in low-resolution prism mode, with a
0.8×15” slit oriented north-south. Spectra of the asteroid and solar analogs were obtained
alternatively in two distinct locations on the slit (A and B); this is referred to as the nodding
procedure. The visible (V) spectrum (0.4-0.9 μm) was obtained using the IDS instrument
mounted on Isaac Newton Telescope (INT), located at El Roque de Los Muchachos Observatory,
Canary Islands. These observations were obtained remotely in a first remote observing run
between th e ROC-Astronomical Institute, Romanian Academy and INT. The IDS instrument was
used in low-resolution mode (R150 grating) with a slit width of 1.5” and the RED +2 CCD
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detector. Based on an average value of the thermal albedo for a V-type object, its diameter was
estimated to be 290 ± 30 m. The mineralogical analysis revealed similarities to HED meteorites.
The band analysis revealed that the object is more similar to an eucritic and howarditic
composition and that it originated from the crust of a large parent body. The analysis tends to a
mineralogical solution with an errorbar of 4%. A dynamical analysis showed a chaotic behavior
of (357439) 2004 BL86. The result of integrating backward in time for 500 000 yr showed that
this object was part of the NEA population. However, even if its MOID is 0.007 au, no direct
correlation with HED meteorite falls was found. The rotational period of the asteroid was
estimated to be 2.637±0.024 h and 2.616±0.061 h, respectively. These observations were crucial
since the next favorable geometry for ground-based observations of (357439) 2004 BL86 will
not occur before January-February 2050. More details can be found in Birlan et al. (2015).
Figure 2-2: (Top) Composite VNIR spectrum of (357439) 2004 BL86 normalized to unity at 0.55
μm. (Bottom) Mineralogical parameters in a howardite–eucrite–diogenite (HED) diagram. The
composition is more like eucritic and howarditic mineralogy.
2.2.3
The YELP campaign
Proposed to the OHP, YELP (Yarkovsky effect Estimation via Light-curve derived Physical
modeling) is an observation campaign, initiated by a consortium of astronomers at the IMCCE,
LESIA Observatoire de Paris and the OCA in Nice with the aim of improving the correspondence
between model predictions and observations in order to allow for a more reliable long term
impact monitoring of potentially hazardous objects. A brief summary of the project is given
below. A proper physical characterization is important for an accurate prediction of the
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influence of non-gravitational accelerations on asteroid orbits. Fortunately, nongravitational
effects such as the Yarkovsky-effect – an acceleration due to emission of thermal radiation tend
to cause only relatively small alterations in an asteroid’s orbit. However, in combination with
close encounters with e.g. terrestrial planets, even small drift effects can lead to significant
changes in predicted future positions. For near-Earth asteroids (NEAs) and especially for
potentially hazardous objects (PHO), an accurate understanding of the nongravitational forces
is, thus, vital in order to assess a potential impact threat. In fact, one of the main limitations of
long term orbit prediction and impact monitoring for asteroids is our lack of knowledge of the
physical and spin properties of asteroids. This becomes clear when looking at the drift rate in
the asteroid’s semimajor axis caused by the Yarkovsky effect
where S, c, n, r, A, γ, P , Theta, T , Γ, σ are the solar flux at 1 au, the speed of light, the mean orbital
motion, the heliocentric distance in au, the bolometric Bond albedo, and the spin axis obliquity,
the rotational period, the emissivity, the subsolar temperature, the thermal inertia (see Section
4), and the Stefan-Boltzmann constant, respectively. From the above equation it becomes clear
that the thermal properties and the spin state directly influence the non-gravitational orbit drift,
even in the most simple models. It has been difficult so far to find a reliable link between first
principle predictions and actually measured values. In fact, basically all detections of the
Yarkovsky effect have been achieved via fitting a constant transversal acceleration to
astrometric and radar data. It remains, however, unclear whether this transversal acceleration is
indeed a manifestation of the Yarkovsky effect. This is partly due to the fact that the physical
parameters for asteroids where the Yarkovsky effect was determined via astrometry alone
remain largely unknown. Thus, no reasonable comparison between model predictions and
observed drift rates can be performed. The observation program aims to collect vital physical
data on a sample of asteroids with already detected Yarkovsky drift rates. The goal is to
accumulate enough photometric and thermal data to compare realistic shape and thermal model
predictions with the actually measured drifts. Observing time at the 120cm telescope at the OHP
gives access to vital photometry and astrometry measurements that will be used to determine
and improve current period as well as drift rate estimates. Additional thermal infrared
observations would have been carried out at ESO. However, this proposal was unfortunately not
selected. Nevertheless, the program should enhance our understanding of how the Yarkovsky
effect works in detail and, thus, contribute to an improvement in the global asteroid impact risk
assessment process. Two OHP observation runs in 2016 were attributed to the YELP campaign,
one from March 1st-5th 2016 and the other one from March 31st to April 4th 2016. The first
observation run featured the target 1991 CS, a kilometer sized asteroid with a period of roughly
2.4h with a detectable astrometric drift rate.
2.2.4
Space based photometry using NASA's Kepler spacecraft
Given the limited number of objects accessible via OHP and PDM, other options for asteroid
photometry have been considered, in order to catch up with US based programs such as MANOS
(Moskovitz et al. 2014). The basic idea was to use photometric data of asteroids acquired by
NASA’s Kepler spacecraft during the K-2 mission. Kepler is known to produce high accuracy
photometry for stars. The possibility of extracting photometry of Solar System Objects (SSOs)
passing the FOV of Kepler has been discussed e.g. by Pal et al. (2015), Szabo et al. (2015). During
the K-2 mission, the Kepler spacecraft enacts step and stare phases along the ecliptic, see Figure
2-3. However, not all of the SSOs in K-2’s FOV have actually been observed due to telemetry
constraints. Only small areas around the stars, so called ”boxes” or ”imagettes” around the
ecliptic plane input catalog (EPIC) target objects, are scanned on a regular basis. In contrast to
the previous works, we are using Virtual Observatory tools such as SKYBOT and MIRIADE
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developed at the IMCCE to scan K-2 FOVs for passing asteroids. As soon as FOV crossing
asteroids have been identified, they are checked in more detail with regard to whether they have
actually been recorded in the imagettes of EPIC targets. As examples for such events, we present
Figures 2-4 and 2-5. Those show the crossing of EPIC image boxes by the asteroid 484
Pittsburghia. The results of this query are directly fed into an image reduction pipeline
providing photometric data on those frames which contain the crossing asteroid. From those
frames light-curve data can be extracted, such as presented in Figure 2-5. The SKYBOT query
that identifies potential targets in Kepler’s FOV was previously only valid for Earth-based
observations. As the parallax between the Earth and the Kepler S/C become non-negligible over
the mission’s lifetime, however, FOV crossing predictions for NEOs was to be rather inaccurate.
This may lead to a loss of possible targets. An update of the SKYBOT service was necessary in
order to be able to predict all FOV crossings of NEOs from the viewpoint of the Kepler spacecraft.
This update of the SKYBOT service has been performed. The results are published in Berthier et
al. (2016). The quality of the extracted asteroid light-curves for the long cadence data (exposure
time of 30mins) were unfortunately too sparse to produce reliable period estimates themselves.
In fact from 10 bright objects with known periods only 3 could be recovered ab initio, i.e. using
Kepler data alone. Linking single measurements to already existing light curves is also difficult,
since the absolute photometric calibration for Kepler SSOs is not an easy task. Hence, we shall
explore algorithms that may allow for a better prediction of rotation periods from sparse data
on the one hand, and we shall look into short cadence data (exposure time in the order of one
minute). While there are fewer and only relatively bright targets in short cadence data, the
quantity of photometric measurements should allow for a better period determination.
Figure 2-3: The FOVs of the K-2 mission campaigns C0-C3 are already public. SSOs that cross the
FOV can happen to be recorded. Image credits: NASA.
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Figure 2-4: Time series of K-2 Campaign 0 images of the target EPICID 202137697. The asteroid
484 Pittsburghia enters the field at the lower left corner and leaves it at the upper right corner. The
cadence is 30 minutes. Photometry can be extracted when the asteroid is totally in the field of view.
Figure 2-5: Light-curve data extracted form images such as presented in Figure 3.4 for 484
Pittsburghia. The blue curve symbolizes the light-curve prediction based on the current shape
model. The Grey dots represent the data extracted from K-2 image crossings (Berthier et al. 2016).
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2.3 Precovery of NEOs and discovery apparition photometry (QUB)
Re-analysis of NEOs observed by Pan-STARRS identified from Physical Properties Priority Lists
has been done.
Time for additional photometry with 2-m Liverpool Telescope has been allocated.
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3
Spectroscopic observations (OBSPM-LESIA)
3.1 A literature study of the Potentially Hazardous Asteroid (PHA) population
On April 2015, we retrieved the European Asteroid Research Node (EARN)1 database of NEO
physical properties, selecting those 255 PHAs with published taxonomic classifications.
Combining the literature with our unpublished spectroscopic observations for further 7 PHAs, a
total sample of 262 PHAs has been considered in our analysis. The full results have been
published in Perna et al. (2016). Hereafter we summarize the main results.
We found that the taxonomic distribution of PHAs is similar to that of NEOs in general (i.e.,
dominated by the S/Q complex, though observational biases surely affect such distribution; Fig.
3-1). Given a number of uncertainties about their taxonomy and composition, we defined four
“groupings” of objects: the “silicaceous” (types S, Q, A, and O – 184 objects in total), the “basaltic”
(V-types – 12 objects), the “carbonaceous” (types B, C, D, P, T, and Xc – 40 objects in total), and
the “miscellaneous” (types X, Xe, Xk, K, and L – 25 objects in total) PHAs. Then we analyzed the
distribution of such groupings in terms of dynamical and physical properties (Tab. 3-1). The
primitive, carbonaceous asteroids seem to pose a special danger to our planet: not only are the
most mature techniques for deviating an asteroid from a hazardous orbit less efficient for such
objects (e.g., Drube et al. 2015), but their low MOID and inclination values indicate that these
PHAs will have close approaches with the Earth more frequently than those belonging to the
other groupings. Based on their low albedo and Tisserand parameter with respect to Jupiter (we
remind that Tj roughly distinguishes asteroids with typical Tj>3 from Jupiter-family comets,
with typically 2<Tj<3), we also identified two candidate extinct cometary nuclei within the
carbonaceous PHAs, which could present extremely low porosities: 2001 XP1 and 2002 BM26.
The possible cometary origin of 2001 ME1 and (4015) Wilson–Harrington was already pointed
out in previous works. The basaltic PHAs also deserve special attention, as the dynamical routes
from Vesta to the near-Earth region seem to put them on orbits characterized by low MOID
values and frequent close approaches with our planet, as also suggested by the latest findings
about the lack of space weathering on the surfaces of V-type NEOs (Fulvio et al. 2016; Ieva et al.
2016). Because of their rapid rotations and elongated shapes, suggesting an important internal
strength, additional objects that we identified as particularly hazardous are the silicaceous 2011
XA3, 2011 BT15, 1998 WB2, and 2002 NV16. The X-types (29075) 1950 DA and (367248) 2007
MK13 also deserve attention because of their possible metallic nature and extreme rotational
properties (as well as the Xe-type (144898) 2004 VD17). Conversely, no fast rotators are found
within the carbonaceous PHAs, suggesting low cohesions (Fig. 3-2).
Table 3-1: Orbital and physical parameters median values (Median Absolute Deviation in
parentheses) for the four PHA compositional groupings defined in the text. From Perna et
al. (2016).
1
http://earn.dlr.de
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Figure 3-1: Distribution of PHAs within the different taxonomic classes. The color coding is relative
to the grouping scheme introduced in the text (red for the “silicaceous” PHAs, magenta for the
“basaltic” PHAs, black for the “carbonaceous” PHAs, green for the “miscellaneous” PHAs).
Figure 3-2: Distribution of PHA groupings in rotational period and light-curve amplitude. The
rotational break-up limits for cohesionless bodies (e.g. Pravec & Harris 2000) are also reported for
different densities.
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3.2 Guaranteed Time Observations at ESO-NTT
As reported in [AD2], Guaranteed Time Observations (GTO) at the 3.6-meter ESO-NTT telescope
are currently conducted by OBSPM-LESIA (programme “Characterizing the small near-Earth
asteroid population in the framework of the NEOShield-2 EC project”, PI: D. Perna).
This GTO programme is mainly devoted to visible spectroscopic observations to derive the
taxonomic type of the targets, hence some information on their surface composition. Our
observing strategy is to prepare the target list for each run a few days before of the
observations, in order to consider all of the small NEOs that are discovered near their close
approaches with the Earth. We usually limit the observations to objects with absolute magnitude
equal or fainter than H=20, which corresponds to a maximum diameter of 300 m assuming a
value of 0.20 for the albedo. An exception has been the km-sized 2009 WN25, observed because
of its low Tisserand parameter suggesting a cometary nature. Indeed, 2009 WN25 has been
identified (Micheli et al. 2016) as the likely progenitor of the November i-Draconids, a recently
detected weak annual meteoroid stream. The primitive nature of this body is confirmed by our
spectroscopic observations (taxonomy: X/Xc/T type).
Up to present, 12 out of 30 nights have been carried out. The data acquired in April, June, July
and November 2015 have been reduced and analysed (10 nights in total, though the
observations in April 2015 have been strongly concerned by poor weather conditions, resulting
in a very limited number of objects for which we could acquire useful data). The data acquired
on 13-14 December 2015 for eleven additional small NEOs are currently under reduction. The
next four runs of the GTO (3 nights for each run) are already scheduled for 29-31 March, 9-11
May, 28-30 June, 28-30 August 2016. The final 6 nights will be scheduled within the next months
for execution in 2016-2017.
Figure 3-3 reports an example spectrum, obtained for 2009 FD. The taxonomic classification has
been derived for 69 objects, which already almost double the available literature prior of
NEOShield-2. Table 3-2 reports, for each object:







the type of the orbit (Aten/Apollo/Amor)
if the object is a Potentially Hazardous Asteroid
the necessary “change in velocity” Δv for a spacecraft rendezvous2
the Tisserand’s parameter with respect to Jupiter Tj3
the epoch of the observations
the absolute magnitude H
the derived taxonomic class
Figure 3-3: NTT spectrum of 2009 FD, one of the most hazardous NEOs currently known (0.29%
impact probability in 2185-2196). Derived taxonomy: Xc-type.
2
3
http://echo.jpl.nasa.gov/~lance/delta_v/delta_v.rendezvous.html
http://echo.jpl.nasa.gov/~lance/tisserand/tisserand.html
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Table 3-2: Preliminary results from the GTO programme at ESO-NTT telescope.
NEO
2001 HY7
2014 TF17
2014 WP365
2015 KU121
2015 JY1
2013 BO73
2011 AM24
2015 HP43
2015 HB117
Asclepius
2011 OL5
2007 RQ17
2015 GF
2010 LN14
2015 BY310
2001 XP88
2015 HA1
2015 FD134
2011 KD11
2003 KZ18
2015 JJ2
2014 YS34
2009 XO
2015 LH
2004 EW
2015 KS121
2000 YJ11
2001 XP88
2002 RB
2007 WU3
2008 JV19
2010 NY65
2012 NP
2012 PG6
2012 RS16
2014 OE338
2014 QK362
2015 AY245
2015 HM10
2015 JJ2
2015 LH14
2015 LN21
2015 LU24
2015 MN44
1993 HA
2000 WN10
2001 RV17
2002 VV17
2005 UO5
2005 XT77
2007 WQ3
Orbit PHA Delta-V
AT
y
8.961
AP
n
13.547
AM
n
9.327
AP
n
9.08
AM
y
6.3
AP
n
6.18
AM
y
5.02
AM
n
7.77
AM
n
5.24
AP
y
7.03
AM
n
6.7
AP
n
4.89
AM
n
7.35
AP
n
7.45
AP
y
5.32
AM
n
5.15
AT
n
8.91
AM
n
6.77
AP
n
7
AT
n
10.83
AM
n
6.42
AP
y
5.37
AP
n
6.27
AP
n
5.61
AT
n
6.73
AM
n
8.86
AM
y
4.767
AM
n
5.148
AM
n
6.979
AP
n
5.45
AT
y
6.477
AT
y
9.242
AM
n
6.393
AT
n
13.029
AM
n
6.539
AP
n
11.329
AP
n
8.788
AM
y
6.361
AM
n
6.364
AM
n
6.419
AM
n
6.863
AM
n
6.362
AM
n
8.399
AM
n
5.974
AM
n
5.302
AP
n
9.742
AT
n
7.913
AT
n
9.744
AP
n
7.921
AT
y
9.607
AM
n
7.379
Tj
6.453
3.747
4.656
5.6
3.55
4.82
5.34
3.41
4.68
5.91
5.18
4.32
4.76
5.33
4.29
4.85
6.19
3.33
4.24
6.22
3.51
4.35
3.81
3.79
6.1
4.06
4.932
4.854
3.53
6.008
6.113
6.01
3.6
6.844
3.617
5.79
5.687
5.516
3.174
3.514
3.43
3.581
3.94
3.993
5.043
5.976
6.467
6.927
5.361
6.927
4.76
Run
April 2015
April 2015
April 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
June 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
July 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
H
20.5
20.7
20.3
22.9
20.8
20.1
20.5
21.1
23.6
20.7
20.2
22.6
20.7
21.1
21.7
20.7
21.2
20.4
20.1
21.2
21.9
20.8
20.5
27.2
20.8
22.8
20.8
20.7
20.8
23.8
20.8
21.5
21.3
20.3
21.2
21
21.6
21.2
23.6
21.9
20.1
23
20.4
22.4
20.0
20.2
20.5
20.2
20.7
20.9
21.1
Taxonomy
Q,R (noisy)
Q,Sq,Sr
Sq, Sr
Q, Sq
R
L, C
L?
Q
R, Sa
Cg
C
A
Q
Q
Q
Xk, Q
C, D, L?
V
R
Xc, C
Xc
A, Sv
X, Xc
A
X, Xe, Xc
C
S,Sv
Cb, Cgh
Cb, C, Cgh
Sq, Q
Ch (noisy)
Sv, S
A,Sa
X, Xc
Q,V (noisy)
Cb (noisy)
C-complex (noisy)
C-complex (noisy)
Xk,Xc,X
D,T;
Xe
Sv, S
V,R,Sa
O,Q (noisy)
T, D;
Sv, S;
S, Sv;
Q
Q
Sq, K
Sq, Sr
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2009 FD
2009 WN25
2012 TS
2015 JD1
2015 QQ3
2015 RD37
2015 RG36
2015 TA
2015 TA25
2015 TB179
2015 TG238
2015 TK238
2015 TL143
2015 TM143
2015 TW144
2015 TY144
2015 TZ237
2015 UC
2015 UJ51
AP
AM
AT
AP
AM
AM
AM
AM
AP
AM
AM
AP
AM
AP
AM
AM
AM
AM
AP
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
7.661
21.018
7.744
7.497
6.942
6.709
5.597
8.404
11.727
7.342
8.369
6.497
6.544
5.675
6.189
6.712
5.647
6.181
8.012
5.291
1.968
6.081
5.166
3.342
3.892
4.54
4.757
5.092
3.192
3.588
3.237
3.135
5.471
3.951
3.531
3.96
3.43
3.149
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
November 2015
22.10
18.4
20.8
20.6
21.3
20
20.3
21.6
20.0
20.3
22.7
21.9
23
23.6
21
21.2
24
24.5
21.2
X, C, Xc, Cb
X, Xc, T
T,D
-- (Too noisy)
Q
V
Sr, Sq, R
Q,V (noisy)
Sv;S
A,L
Q (noisy)
Q,Sq
Sv, S;
Cb
A,Sv
Sr,R,Sq
Xk,Xc,X
Sr,Sq
O,Q
As for the literature data, rather than analyzing each taxon separately, we define four major
groupings to increase the significance of our analysis (also in terms of impact risk mitigation
purposes): the “silicaceous” asteroids, including the whole S-complex together with objects
classified as Q-, A-, or O-type; the “basaltic” V-type asteroids; the “carbonaceous” asteroids,
consisting of NEOs belonging to the B, C, D, P, T, and Xc classes; the remaining “miscellaneous”
asteroids, i.e. those classified in the X, Xe, Xk, K, and L taxa (such a grouping will therefore
include objects of either silicaceous, carbonaceous, enstatitic, or metallic nature).
While it is still too early to draw firm conclusions from our observations and data analysis, we
can note that, in comparison with the available literature for NEOs of all sizes (from EARN), our
sample of “small” NEOs seem to include a higher fraction of carbonaceous objects, possibly
because of their greater fragility (Fig. 3-4).
Figure 3-4: Distribution of NEOs based on their compositions, for small (10-300 m, left) and all sizes
(right). The carbonaceous asteroids seem more abundant at small sizes.
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3.3 Further ESO observations
Besides the preparation and execution of GTO runs, and the reduction and analysis of the
obtained data, we have also submitted in October 2015 a proposal to ESO (PI: D. Perna) to use
the (UV-to-NIR) X-Shooter and (visible) FORS2 spectrographs for studying asteroid Ryugu. This
potentially hazardous asteroid is the target of the sample return mission Hayabusa 2 by JAXA,
which will reach Ryugu in July 2018, and will return samples of its surface back to Earth in
December 2020. The Hayabusa 2 project will represent a breakthrough in our understanding of
the nature of primitive asteroid material, with obvious consequences for the mitigation of the
impact risk from this kind of objects. However, the physical properties of Ryugu are still
somewhat puzzling, and the July 2016 observing opportunity is the only left before mission
arrival. Our proposal has been accepted, and we have been assigned 5 hours of X-Shooter
observing time and 1 night of FORS2 observing time (both runs will be carried out in July 2016)
for solving the current uncertainties about the surface composition and possible heterogeneity
of Ryugu, as well as to secure the determination of its rotational period and to help assessing the
orientation of its rotation axis. Such detailed characterization will be fundamental to optimize
the Hayabusa 2 mission operations and scientific return.
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4
Thermal IR observations (CNRS)
Sizes and albedos are within the most fundamental physical properties of asteroids. Although
some NEAs have been observed by radar, the majority of NEA sizes and corresponding albedos
have been derived from the analysis of thermal infrared (TIR) fluxes (Delbo’ et al. 2015, and
references therein). Thermal inertia is another extremely important parameter, not only
because it gives us information about the physical nature of the surface material, but because it
also modulates the Yarkovsky effect, a non-gravitational force susceptible of affecting the orbital
evolution of asteroids < 40 km (for a review, see Bottke et al. 2006).
Obtaining TIR data is especially complicated, since we require the largest ground-based
telescopes on Earth, but after the decommissioning of CanariCam at the GranTeCan (La Palma,
Spain), VISIR at the VLT stands as the only thermal infrared instrument currently available to
Europeans. Although we have successfully gained access to using these facilities until recently
(Licandro et al. 2016), our two last ESO proposals, submitted in collaboration with other
members of WP10 in April 2015 and in October 2015, have not been successful. Thus, we have
had to resort to the literature, namely to the WISE catalog, in search for TIR data from which we
can expand our knowledge of the properties of some NEAs. In particular, we have used TPM to
constrain the thermal inertia of two bodies, and we illustrate an approach that can potentially be
used to constrain the thermal inertia of objects that do not have determined shape.
In the next section, we lay out some concepts relevant to our modeling techniques, in Section 4.2
we present our results and a brief discussion, and in Section 4.3 we summarize our conclusions.
4.1 Modeling techniques
4.1.1
Determination of sizes and albedos of NEAs from simple thermal models
To better explain the thermophysical model, it is convenient to introduce a less sophisticated
thermal model first. The near-Earth Asteroid Thermal Model (NEATM, Harris 1998) has been
widely applied to infer asteroid sizes from TIR data in cases in which information about the
shape and rotational state of the asteroids are not known, i.e., the vast majority of them. The
model assumes that the asteroid has a spherical shape and does not rotate. In this sense, the
computed sizes are the diameters of the spheres that produce the best-fitting values of TIR
fluxes. The other fundamental assumption of NEATM is that the surface is always in
instantaneous equilibrium with the fraction of the incident solar radiation that it absorbs, which
allows one to calculate the temperature of each illuminated surface element of the surface. The
amount of energy absorbed by each surface element of the sphere depends on the object’s
heliocentric distance and Bond albedo (the ratio of total absorbed to incident energy), and on
each element’s inclination with respect to the sunward direction.
Typically, the NEATM allows a robust estimation of asteroid diameter, but does not provide any
direct information about other physical properties of the material (see Harris & Lagerros 2002
for a review). Nonetheless, knowledge of the object’s H-value allows one to estimate its visible
geometric albedo (pV) given its diameter. This provides a very coarse idea of composition, since
low-pV objects are usually associated with spectrally classified C- and/or X-complex asteroids,
thought to be primitive bodies, whereas higher-pV ones usually fall in the S-complex asteroids,
whose spectra indicate they have undergone igneous processing.
A large fraction of the currently known sizes and albedos of NEAs have been estimated from the
use of NEATM to model WISE/NEOWISE data (Mainzer et al. 2011). WISE stands for Wide-field
Infrared Survey Explorer, a survey carried out in 2010 that provided photometric observations
of more than 150,000 asteroids (Wright et al. 2010; Masiero et al. 2011), i.e., two orders of
magnitude more than its predecessor, the Infrared Astronomical Satellite (IRAS), the major
source of asteroid diameter and albedos for over two decades.
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4.1.2
Thermal inertia
A very important parameter that is largely unknown for near asteroids but highly relevant for
the NEOShield-2 objectives is thermal inertia. It is related to the nature of the material
constituting the regolith and governs the surface temperature over a rotational period.
Therefore, it controls non-gravitational Yarkovsky effect, the modeling of which is necessary to
accurately determine NEA orbits. In turn, accurate orbits are essential to adequately assess
impact risks, which one of the major goals of NEOShield-2. Furthermore, the nature of regolith is
essential parameter if one wants to estimate the restitution coefficient after a kinetic impact.
To give an intuitive notion of thermal inertia, consider that the temperature of any material
capable of efficiently conducting incident solar energy towards its interior will not respond
quickly to changes in illumination. Unlike the instantaneous equilibrium case, the surface will
remain colder for a longer time in the morning, and warmer in the night. Thermal inertia
increases with the conductivity of the material, its density, and its specific heat capacity, and
thus is related to the material’s composition as well as physical structure, for example, its
porosity. In Figure 4-1 we show two paradigmatic examples of terrains with different thermal
inertias. For a recent review, see Delbo et al. (2015).
This parameter has been traditionally estimated for asteroids by means of thermophysical
models (TPMs), which we briefly introduce in the following section.
Figure 4-1. (A) Close-up image of (433) Eros from
the NEAR Shoemaker mission reveals coarse
regolith with grain size in the mm-range Γ~150 J m-2
s-0.5 K-1 for Eros. (B) Image from the Hayabusa
mission of the surface of (25143) Itokawa displaying
gravel-like regolith and a correspondingly higher
thermal inertia of Γ~700 J m-2 s-0.5 K-1.
4.1.3
Thermophysical modelling of near-Earth Asteroids
Asteroid thermophysical models (TPMs) are computer numerical codes that allow one to
calculate the temperature of asteroids’ surface and immediate sub-surface. These temperatures
depend on absorption of sunlight, multiple scattering of reflected and thermally emitted
photons, and heat conduction. Physical parameters such as albedo (or reflectivity), thermal
conductivity, heat capacity, emissivity, density and roughness, along with the shape (e.g.,
elevation model) of the body, its orientation in space, and its previous thermal history are taken
into account. From the synthetic surface temperatures, thermally emitted fluxes (typically in the
medium-infrared) can be calculated. Physical properties are constrained by fitting model fluxes
to observational data. Typically TPMs produce the value of the thermal inertia averaged over the
whole surface of the body.
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Figure 4-2 (a) example of a triangulated 3D shape model as
typically used in TPMs taken from Delbo’ et al. (2015).
Temperatures are colour coded: white corresponds to the
maximum and dark-grey corresponds to minimum
temperature. Three different roughness models are sketched
in the bottom of the figure: (b) hemispherical section
craters; (c) Gaussian surface; (d) fractal surface. Adapted
from Delbo et al. (2015).
4.1.4
Asteroid 3D shapes as input for thermophysical models
The lightcurve inversion method developed by Kaasalainen et al. (2001) and Kaasalainen and
Torppa (2001) is a powerful tool that allows us to derive basic physical properties of asteroids
(the sidereal rotation period, spin vector orientation and its shape) from their disk-integrated
photometry. This photometry can be dense-in-time, sparse-in-time or combination of both.
These shape models are used as inputs for the TPM.
To obtain a unique spin and shape solution, one needs a set of at least a few tens of dense
lightcurves observed during three or more apparitions for an asteroid: this is the first/classical
approach. Kaasalainen (2004) showed that one can also use only sparse data for the inversion
technique. In such case, a unique model can be derived from more than about one hundred
calibrated measurements observed during 3–5 years if the photometric accuracy is better than
5% (Durech et al. 2005, 2007). Sparse data available so far are not that accurate. Nevertheless,
for many asteroids with high lightcurve amplitudes, it is possible to derive their shape models
from contemporary sparse data (covering usually time of ~15 years). First results coming from
this approach were shown by Durech et al. (2009), where sparse data from the US Naval
Observatory in Flagstaff (USNO-Flagstaff station) were used. If one combines sparse and dense
data together, the shape model can be already derived from few dense lightcurves and about
100 sparse data points. This approach led to a significant increase of derived shape models from
~100 to ~400 (Hanus et al. 2011, 2013a, 2013b).
Kaasalainen et al. (2001) validated the lightcurve inversion method on asteroids (243) Ida,
(433) Eros, and (951) Gaspra and demonstrated that convex shape models well represents the
convex hulls of the real shapes. Experience shows that shape models derived from only sparse
data are much coarser than those based on dense data, and should be refined by additional
dense lightcurves prior applying them for thermophysical modeling, for example.
Most of the asteroid models are publicly available in the Database of Asteroid Models from
Inversion Techniques (DAMIT, http://astro.troja.mff.cuni.cz/projects/asteroids3D, Durech et al.
2010). In March 2015, models of almost 400 asteroids were included in DAMIT, about a hundred
based only on dense data.
More recently, Hanus et al. (2015) have examined for the first time how uncertainties associated
with the pole and shape determination affect the best-fitting values of thermal inertia. Their
approach is based on bootstrapping the visible data from which the shapes are obtained, i.e.,
they randomly remove a portion of the data used in each case and recomputed a new shape. In
turn, the new shape is used in the TPM to recalculate new best-fitting value of thermal inertia.
Doing this procedure several times, one can estimate an average value and a more realistic
interval of uncertainty than in previous works.
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4.1.5
Hybrid thermal model
The scarcity of both requisites for TPM, namely high-quality IR data and asteroid shapes and
rotation states, is critically limiting our possibility to exploit WISE data with TPMs, as evidenced
by the fact that only two works have been published so far (Alí-Lagoa et al. 2014, Rozitis et al.
2014).
Here, we also propose to combine the use of TPM with spherical shapes plus partial but
potentially critical information about the rotational states of some objects. The idea is based on
the notion that NEAs coming from the 6 secular resonance must be retrograde (Bottke et al.
2006). This makes a retrograde pole orientation for a sphere (i.e., any axis with a negative
ecliptic latitude) a reasonable assumption. While this does not purely constitute a TPM, it is still
an improvement over the near-Earth asteroid thermal model NEATM that may help constrain
the thermal inertia and improve the size estimate. In addition, it has also been shown by Hanus
et al. (2015) that, in some cases, changing the 3-D shapes within the uncertainties of the pole
and shape determination increases the 2 of the best-fitting thermal inertia but does not change
the best-fitting thermal inertia value itself. In these circumstances, ignorance of the shape may
not be a limitation, but the feasibility of this approach still needs to be evaluated, which is our
purpose here.
4.2 Results
4.2.1
TPM of (3200) Phaethon
We used combined newly acquired visible data to produce a 3D model of this object (Figure 43), with which we constrained its thermal inertia. The TIR data were collected from different
works in the literature. In Figure 4-4 we show the reduced 2 of the fit versus different values of
thermal inertia of the model (Hanus et al., in preparation). The large circle shows the minimum
corresponding to the original shape, and the smaller circles those of the varied shapes, which
inform about the uncertainty in the thermal inertia determination associated to the
uncertainties in the 3D model.
Figure 4-3: shape model of (3200)
Phaethon (Hanus et al., in preparation).
Shape models that correspond to the
first (top) and second (bottom) pole
solutions derived from dense data only.
Each panel shows the shape model at
three different viewing geometries: the
first two are equator-on views rotated by
90◦, the third one is a pole-on view.
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Figure 4-4: reduced 2 versus thermal
inertia for (3200) Phaethon (Hanus et al.,
in preparation). The circles indicate the
minima of the different models. The
varied-shape models help quantify the
uncertainty in the thermal inertia
associated to the uncertainty in the shape
and pole orientation.
4.2.2
TPM of (1685) Toro
We used a still unpublished shape model for this asteroid (Figure 4-5) and fitted its available
WISE data. Preliminary analysis with TPM shows a minimum reduced 2 at a thermal inertia of
100+40
−30 SI units at the 1-sigma level (based on standard statistical analysis, Figure 4-6).
Figure 4-5: shape model of (1685) Toro (tri_model_02_1).
Figure 4-6: Reduced 2 versus thermal inertia for NEA (1685) Toro
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4.2.3
Hybrid thermal model of (1685) Toro
Dynamical models suggest Toro has >70% probability of having been delivered to NEA space
through the 6 secular resonance. If this is the case, it should have a retrograde spin. To test this
prediction, we applied TPM with a sphere to several prograde and retrograde spin orientations,
changing both ecliptic latitude and longitude. TPMs have frequently helped constrain multiple
solutions obtained from lightcurve inversion, and we find that this is the case for Toro and our
hybrid thermal model. In Figure 4-7 we show the reduced 2 versus thermal inertia for three
retrograde spheres, one with spin pole perpendicular to the ecliptic, one with the spin pole
obtained from lightcurve inversion, one with the same ecliptic latitude but zero eclipctic
longitude, and finally a prograde sphere rotating perpendicularly to the ecliptic. The fact that we
obtain a maximum in 2 for a thermal inertia close to the TPM solution (see above) provides a
strong basis to reject the prograde solution, as well as a good corroboration of the dynamical
model’s prediction about the object’s spin axis orientation. The three other models give similar
best-fitting values of thermal inertia at ~150 SI units, which are also consistent within the
uncertainties with the value obtained from TPM.
Toro + retrograde sphere
90
lambda=0,beta=-90
lambda=0,beta=-61
Pole (lambda=75.6, beta=-61)
prograde (lambda=0, beta=90)
85
80
Reduced chi2
75
70
65
60
55
50
45
40
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Thermal inertia (SIu)
Figure 4-7: Reduced 2 versus thermal inertia for NEA (1685) Toro using a
sphere instead of a 3D shape. Three different retrograde spheres
4.2.4
Comparison with other thermal inertias found in the literature
Figure 4-8, adapted from Delbo’ et al. (2015), shows a plot of thermal inertia versus diameter for
all objects with known thermal inertias. Different spectral types are indicated with different
symbols. We have updated the figure to include our results. While the thermal inertia of (1685)
Toro lies within the values spanned by other similarly-sized asteroids, (3200) Phaethon
presents a higher thermal inertia. This may be an interesting clue about the different nature of
the surface material on Phaethon, possibly containing very coarse-grained particles.
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Figure 4-8: Thermal inertia versus diameter
of all asteroids with estimated values. It is an
update of Delbo’ et al. (2015) that includes
our results for Phaethon (Hanus et al. in
preparation) and Toro (this report).
4.3 Conclusions and Perspectives
We constrained the thermal inertia of NEAs (3200) Phaethon and (1685) Toro based on newly
derived shapes obtained from light curve inversion and thermophysical modelling of WISE
thermal infrared data. Our results for Toro are consistent with other asteroids with similar sizes,
but the thermal inertia of Phaethon is high for its size, which may hint that the surface material
on this object is.
Motivated by the scarcity of available NEA shapes, we also proposed a strategy to constrain the
thermal inertia of asteroids with known rotational periods for which no 3-D models are
available by using a sphere. Our approach requires partial but potentially crucial knowledge of
the object’s pole orientation, which is based on the fact that NEAs coming from the 6 secular
resonance must be retrograde (Bottke et al. 2006). We explored this idea with (1685) Toro as a
test case, which shows that prograde spheres cannot fit the thermal inertia (Figure 4-7) but
retrograde spheres may succeed, albeit with some inaccuracy.
In future work, we will survey the literature to find all NEAs for which this idea may be applied
and discuss its feasibility further.
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Summary
While we are not yet at half of the NEOShield-2 project duration, within the WP10 activity we
already acquired/analysed a good clump of data to characterize the small NEA population:




Eight observing runs have been carried out at TNG (6 runs) and LBT (2 runs) telescopes
to acquire the photometric colors of a total of 64 targets. Data have been analysed for
30 of them, and the corresponding taxonomic type derived. A collaboration has been
established with the Observatorio Nacional (Brazil) to make use of the OASI telescope, to
acquire phase curves of NEAs. Five objects have been observed so far, with the data
analysed and the corresponding absolute magnitude H derived for four of them.
Light-curves of 13 NEAs have been taken at OHP and PDM telescopes, where additional
astrometric data have been also acquired for 12 asteroids (these data could help to
enhance our understanding of the Yarkovsky effect). The light-curve and spectrum of
2004 BL86 have been also obtained at the IRTF telescope. Efforts are in progress to
make use of the photometric data acquired by NASA’s Kepler space telescope to obtain
further light-curves of asteroids and NEAs in particular.
Twelve out of the 30 observing nights of our GTO programme at the ESO-NTT telescope
have been carried out: reflectance spectra of 80 small NEAs have been acquired. Data
have been analysed for 69 of them, and the corresponding taxonomic type derived
(suggesting that the primitive, carbonaceous objects are more common within the
smaller NEA population). An analysis of the available literature of spectroscopic data of
the PHA population has also been carried out, to identify those particularly hazardous
objects requiring a special attention in the near future.
The thermal inertia of NEAs (3200) Phaethon and (1685) Toro has been constrained
based on newly derived shape models and thermophysical modelling of WISE data.
The results obtained for Phaeton suggest a very coarse-grained surface. A novel strategy
is also proposed (and successfully tested on Toro) to constrain the thermal inertia of
asteroids with retrograde rotations and no 3-D models, assuming a spherical shape (e.g.,
for objects coming from the 6 secular resonance).
In summary, NEOShield-2 new observations already more than doubled the available data for
what concerns the surface composition (taxonomy) of small NEAs, and several objects of
particular interest have been identified and/or studied. A number of observing runs are already
foreseen for the next 1.5 years at several worldwide telescopes. Novel strategies are being
developed by WP10 partners for the rotational and thermophysical modelling of extended
samples of NEAs.