The Solar System at ~10 mas
perspectives for a Fresnel imager
Paolo Tanga
Marco Delbò
Laboratoire Cassiopée, OCA
Observational challenges
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Hi-res atmospheric activity & aurorae (UV) on Jupiter, Saturn
Atmospheric activity on the remote planets
Evolution of fine structures in Saturn’s rings
…
Trans-Neptunian Object population:
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Asteroids:
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Inventory
Size (albedo)
Anomalous orbits
Inventory at small sizes
Internal structure (mass, size, shape)
Cometary activity
Comets
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Evolution of chemicals after sublimation
Giant planets
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UV features:
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HST, H2 emission (160 nm)
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Aurorae
Upper atmospheric
features (40-100 mb)
Interest
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Connection to
magnetosphere
Coupling to other
wavelengths
Our nearby prototype of
extrasolar planets!
The importance of asteroids…
The great issues:
 Origin: collisional life, related physics
 Dynamical processes: transport, mixing in
the primitive nebula, origin of meteorites
 Impact risks and mitigation strategy
The problem…
Very limited knowledge of basic properties:
density, porosity…
 Spectral types and connection to composition
 Shapes, satellites
 Size distribution
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Itokawa as seen by Hayabusa
Is this a cohesionless « gravitational aggregate »?
540 m
How much is/will be known
Property
today
after Gaia
shapes, poles
100
~100,000
rotation periods
1000
10,000
satellites
~ 20 (MBA)
? New constraints!
surface properties
~ 1000
~200,000
astrometry
~ 0"5
0"005
masses, s < 50%
~ 50
150
~2000
3000
size / albedos
Ceres
~200 asteroids
1 x 106 (?)
Neptune
Jupiter
Mars
1 x 106 (?)
Angular size as a function of distance for objects of 10 and 100
km. The two « p » curves represent the limit size at V=20 (at
opposition) for two extreme albedo values.
Areas open to a Fresnel imager
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Size inventory/improvement 10-100 km
Macroscopic shapes 10-100 km
Cometary activity (OH at 308 nm)
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Binary asteroids
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Discovery
 Orbits
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Binary asteroids
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Importance:
Linked to the collisional physics  past history of
the belt
 Period + separation  mass
 If size is known  density (internal properties)
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Properties:
Wide range of separations
and size ratios!
 16% of objects at D<30 km?
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Binary asteroids – the Fresnel imager domain
radar/
lightcurves
Imaging (AO)
How many asteroids at V=20 ?
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Evolution of the number of entries H < Hlim
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old files retrieved by D. Hestroffer in the IMCCE archives
Geometric observability of orbits
Sun
45°
L2
Velocity distribution
simulation
5
4
on 5,000
objects
3
%
main-belt,
2
NEOs
1
0
-40
-30
-20
-10
0
mas/s
10
s ~ 12 mas/s
20
30
40
 Possible problems
related to motion
Possible strategies:
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Priority given to Solar System objects
Exceptional events (comets, storms on the main
planets…)
 Specific long-term monitoring programs
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« Opportunity » targets
~50 asteroids V<20 in 1 sq. degree
 Few requirements on pointing
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Conclusions
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Asteroids offer a wide variety of targets
Binary objects
 Cometary activity
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Giant planets
Interesting features at all wavelengths
 Can help in modeling extrasolar planet
observations
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Configuration space period-diameter
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no very fast rotator due to centrifugal force
lack of global cohesion
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Today
The occultation revival
poor predictability for objects <50 km
 bright Hipparcos/Tycho stars favoured
 ~0.1 events/objects/year
 Current practical limit: 100 km at 10%
accuracy
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After Gaia (100 X orbit
improvement):
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Uncertainty smaller than the asteroid at
>20 km
Light curves
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Asteroid’s magnitude function of:
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shape, rotation period, direction of spin axis
Direct problem:
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model of light curves for different shapes and
rotation
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Inverse problem:
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find the rotation parameters from photometric
data
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strongly non linear
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not well conditioned if period unknown
Animation: M. Delbo