An optimal search strategy for Trojan
asteroids and science follow-up of GAIA alerts
with the Zadko Telescope, Western Australia
Michael Todd
May 4, 2011
M. Todd1, D. Coward2 and M.G. Zadnik1
Email: [email protected]
1
2
Curtin University, Western Australia
The University of Western Australia
Part 1
The Zadko Telescope
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Zadko Telescope - Introduction
Rapid response optical telescope
Fully robotic
Unique location
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Zadko Telescope – Specifications
Telescope:
Primary mirror aperture
Focal length
Focal ratio
Camera:
Model
CCD array
Pixel size
Operating temperature
Field of view
Limiting magnitude
Location:
Longitude
Latitude
Altitude
1.0 m
4.0 m
f/4.0
Andor iKon DW436BV
2048 x 2048 pixels
13.5 x 13.5 µm
-50°C
23.5 x 23.5 arc-minutes
R≈21 (180 s exposure)
115°42’47.2” E
31°21’21.5” S
50 m ASL
(Coward et al. 2010)
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Zadko Telescope - Location
About 70 km north from Perth
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Zadko Telescope - Location
Co-located with Australian LIGO, the Gravity Discovery Centre (a science education
outreach facility) and the Leaning Tower of Gingin (Torre pendente di Gingin)
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GREAT-SSO, Pisa, 4 May 2011
TAROT
TAROT (Télescopes à Action Rapide pour les Objets Transitoires)
 a network of fully robotic rapid response telescopes
(Klotz et al. 2008)
Zadko Telescope + TAROT
 a global fast response robotic telescope network for the
study of multispectra transients and potentially dangerous
Earth-orbiting space debris
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TAROT
TAROT Calern: first light 1998. 15 GRBs observed since 2004.
TAROT La Silla: first light 2006. 6 GRBs observed since 2006.
(http://tarot.obs-hp.fr)
Zadko Telescope: first light 2009. 7 GRBs observed since 2009.
robotised and networked with TAROT in 2010
1998
2006
2010
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Automatic vs Robotic
Automatic telescope
•
•
•
•
•
Surveys
Scheduling done before night
Routine Supernova search, variable stars
1+ operators
e.g. OGLE, EROS, LSST
Robotic telescope
•
•
•
•
•
Targets of Opportunity
Rescheduling during the night
GRB (early detections), confirmations
no operator required
e.g. ROTSE, TAROT, ASAS
}
Can interrupt
schedule from
external triggers
-GRB
-Gravity Wave
-Neutrino...
(Klotz 2008)
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Robotic Software Structure
Not telescope dependent!
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Current Projects
Research projects
Spectrum
Partners
Status
Gamma ray bursts
GRB optical
follow-up
TAROT (France), UWA,
Curtin
Current
Gamma ray bursts
GRB astrophysics
TAROT/NASA
Current
Gravitational waves
searches
GW triggers
LIGO/VIRGO
Current
MOU in place
Extra-Galactic Neutrino
searches
Neutrino triggers
ANTARES, TAROT
Pilot program 2011
Binary asteroid studies
Optical
UWA, OCA, Curtin
Current
Education outreach
Optical
UWA, Curtin,
Polly Farmer Foundation
Current
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Future Projects
Research projects
Spectrum
Partners
Status
Optical follow-up of
radio transients
Radio triggers
ICRAR/ASKAP
Proposed 2012
GAIA Satellite follow-up
Optical
ESA, OCA, Obs. Paris
Proposed 2012
GBOT (GAIA)
Optical
ESA, OCA, Obs. Paris
Proposed 2012
Space-debris tracking
Optical
TAROT, ICRAR, CNRES,
ESA
Pilot program 2010
Proposed 2012
(Australian SKA Pathfinder)
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Part 2
Trojan asteroids in the
inner Solar System
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Trojans - Introduction
There are about 570,958 known1 asteroids in the Solar System
Of these, there are:

Jupiter Trojans: 4832

Mars Trojans: 4 (predicted ~50)

Earth Trojans: 0 (predicted ~17)
1
as of April 18, 2011 (www.minorplanetcenter.org)
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What is a Trojan?
Trojans are those asteroids which:
• share an orbit with a planet, and
• are located in regions around L4 and L5 Lagrangian points
These have 1:1 mean motion resonance (coorbital), which only
occurs if the semi-major axis is similar to the planet
and
the eccentricity must be close to e = 0 for them to remain in the
Lagrangian region during their orbits and so be considered to be
Trojans.
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Earth Trojans
Earth Trojans (may) exist near the L4 and L5 Lagrangian points of
Earth’s orbit.
Known: 0
Predicted: 0.65 ± 0.12 (diam. > 1 km)
16.3 ± 3.0 (diam. > 100 m)
Known asteroids having a≈1 AU (grey) compared to
stable inclinations for Earth Trojans (red),
from Morais & Morbidelli (2002)
(Morais & Morbidelli 2002)
Regions in which a body may exist
in co-orbital motion with a planet
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Earth Trojans
Synthesis of orbit inclination model (Morais & Morbidelli 2002) and
heliocentric longitude model (Tabachnik & Evans 2000) to identify
probability regions
Normalised probability contour for Earth Trojan
bodies by Inclination and Heliocentric Longitude.
Earth Trojan (L4) target field.
>63% probability that Trojan will occupy this
region.
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Earth Trojans
Earth Trojans – Observing Constraints
• Need to observe at elongations close to the Sun
• Small observing window after sunset and before sunrise
(Image: NASA)
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Earth Trojans
Earth Trojans – Field survey options
Option 1: Survey entire field
Solid angle of field is 3490 deg2.
Telescope
Limiting
mag.
Exp.
FOV
FOVs Time
Zadko
R ~ 21
180s
0.15 deg2
23267
1160h
TAROT
R ~ 18
60s
3.5 deg2
998
16.6h
SkyMapper
g ~ 21.9
110s
5.7 deg2
613
18.7h
Catalina
V ~ 20
30s
8.0 deg2
437
3.6h
PTF 1.2m
R ~ 20.6
60s
8.1 deg2
431
7.2h
Pan-STARRS
R ~ 24
30s
7.0 deg2
499
4.2h
LSST
r ~ 24.7
30s
9.6 deg2
364
3.0h
GAIA
V ~ 20
0.45 deg2
7756
Note 1
Note 1: GAIA to operate in continuous scanning mode
Only possible to observe entire field with large survey telescope!
Will take several days.
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Earth Trojans
Earth Trojans – Field survey options
Option 2: Survey field within inclination limits
Solid angle of field is 1300 deg2.
Telescope
Limiting
mag.
Exp.
FOV
FOVs
Time
Whole
field
Zadko
R ~ 21
180s
0.15 deg2
8667
433h
1160h
TAROT
R ~ 18
60s
3.5 deg2
372
6.2h
16.6h
SkyMapper
g ~ 21.9
110s
5.7 deg2
228
7.0h
18.7h
Catalina
V ~ 20
30s
8.0 deg2
163
1.4h
3.6h
PTF 1.2m
R ~ 20.6
60s
8.1 deg2
161
2.7h
7.2h
Pan-STARRS
R ~ 24
30s
7.0 deg2
186
1.6h
4.2h
LSST
r ~ 24.7
30s
9.6 deg2
136
1.2h
3.0h
GAIA
V ~ 20
0.45 deg2
2889
Note 1
Note 1: GAIA to operate in continuous scanning mode
Can be done in 1 day with large survey telescope.
Requires pairs of observations, repeated at 3-month intervals..
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Earth Trojans
Earth Trojans – Field survey options
Option 3: Survey in ecliptic plane ±10°
Solid angle of field is ~900 deg2
Telescope
Limiting
mag.
Exp.
FOV
FOVs
Time
Whole
field
Zadko
R ~ 21
180s
0.15 deg2
5840
292h
1160h
TAROT
R ~ 18
60s
3.5 deg2
257
4.3h
16.6h
SkyMapper
g ~ 21.9
110s
5.7 deg2
157
4.8h
18.7h
Catalina
V ~ 20
30s
8.0 deg2
112
56m
3.6h
PTF 1.2m
R ~ 20.6
60s
8.1 deg2
111
111m
7.2h
Pan-STARRS
R ~ 24
30s
7.0 deg2
128
64m
4.2h
LSST
r ~ 24.7
30s
9.6 deg2
94
47m
3.0h
GAIA
V ~ 20
0.45 deg2
400
•
•
•
•
Look for Trojans crossing ecliptic plane
Requires 2 observing sessions per 2-3 weeks for half a year
Less time per session compared to whole field survey
Still requires large FOV telescope
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Earth Trojans
Earth Trojans – Field survey options
Option 4: Survey a swath of the field
For a 10˚ swath, area ~90 - 140 deg2
Telescope
Limiting
mag.
Exp.
FOV
FOVs
Time
Zadko
R ~ 21
180s
0.15 deg2
590 – 930
29.5 – 46.5h
TAROT
R ~ 18
60s
3.5 deg2
26 – 40
26 – 40m
SkyMapper
g ~ 21.9
110s
5.7 deg2
16 – 25
30 – 46m
Catalina
V ~ 20
30s
8.0 deg2
12 – 18
6 – 9m
PTF 1.2m
R ~ 20.6
60s
8.1 deg2
12 – 18
12 – 18m
Pan-STARRS
R ~ 24
30s
7.0 deg2
13 – 20
7 – 10m
LSST
r ~ 24.7
30s
9.6 deg2
10 – 15
5 – 8m
GAIA
V ~ 20
0.45 deg2
200 - 300
•
•
•
•
Use Earth’s revolution about Sun to sweep out field
Requires 2 observing sessions per week for up to a year
Minimal time per session compared to whole field survey
Observations made at end of twilight before/after primary science
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Earth Trojans – Variation in magnitude
• Apparent magnitude for 1 km object ranges from 17.9 to 19.5
• Assumed albedo 0.20
• No atmospheric extinction
Variation in apparent magnitude across field.
Inverse square law dominant over phase angle.
Earth Trojan (L4) target field.
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Mars Trojans
Mars Trojans exist near the L4 and L5 Lagrangian points
of Mars’ orbit.
Known: 4
Predicted: ~50 (diam. > 1 km)
(Tabachnik & Evans 1999)
Inclinations of 72 known asteroids (grey) with 𝑎≈1.52 AU (similar to Mars) compared to prediction
from Trojan model (red [L4] / blue [L5] lines), from Tabachnik and Evans (1999)
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Mars Trojans
Synthesis of orbit inclination model (Scholl, Marzari & Tricarico 2005)
and heliocentric longitude model (Tabachnik & Evans 2000) to
identify probability regions.
Normalised probability contour for Mars Trojan
bodies by Inclination and Heliocentric Longitude.
Mars Trojan target field at opposition.
>48% probability that Trojan will occupy this
region.
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Earth Trojans
Mars Trojans – Field survey options
Field at opposition subtends 9450°
- nearly 3x larger than Earth Trojan field!
Best approach:
• survey a swath of the field (L4 / L5)
• use Earth’s and Mars’ revolutions about the Sun to sweep
out the field during the ~4 months the fields are visible.
Apparent magnitude for 1km
object ranges from 16.9 to 19.3
across field
• Assumed albedo 0.20
• No atmospheric extinction
Mars Trojan target field at opposition.
Indicated angles of longitude and latitude are
heliocentric angles.
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Earth Trojans
Conclusions
Zadko Telescope
• Unique location in the Southern Hemisphere
• Most suited for optical follow-up tasks
• With TAROT forms a global network of robotic telescopes
• Can respond to external triggers – automatic scheduling
Trojan asteroid search
• Trojan fields occupy significant sky area
• Most efficient use of telescope time:
• divide search field into strips
• use Earth’s revolution about Sun to sweep out area
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Thanks for your attention
Zadko Telescope
http://www.zt.science.uwa.edu.au/
Key contacts:
David Coward (Director) UWA
Email: [email protected]