Planetary Radar Imaging of Binary Asteroids

1
Planetary Radar Imaging of Binary Asteroids

• Michael C. Nolan, Ellen S. Howell, (Arecibo
  Observatory), Lance A. M. Benner, Steven J.
  Ostro, Jon D. Giorgini (JPL/Caltech),
• Chris Magri (U. Maine, Farmington), Jean-Luc
  Margot (Cornell), Michael Shepard (Bloomsburg U.)

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1999 KW4 viewed in orbit plane
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Radar Imaging of Binaries

• Absolute ranges and radial velocities
• Scaled by sin i, but no reflectivity assumptions
  for scales or sizes.
• Geometry not very important for detection.
• Pathological cases exist, but mutual events are
  not required.
• Unambiguous detection in a single night.
• Common trend for slowly-rotating secondaries
  makes detection likely.
• Rapidly rotating secondaries would be harder to
  detect.
• Fairly easy to quantify detection limits.

4
Radar Imaging of Binaries

• Absolute ranges and radial velocities
• Scaled by sin i, but no reflectivity assumptions
  for scales or sizes.
• Geometry not very important for detection.
• Pathological cases exist, but mutual events are
  not required.
• Unambiguous detection in a single night.
• Common trend for slowly-rotating secondaries
  makes detection likely.
• Rapidly rotating secondaries would be harder to
  detect.
• Fairly easy to quantify detection limits.

5
Absolute size and velocity

• No scale uncertainty
• sin i (velocity)
• SNR matters

6
Radar Imaging of Binaries

• Absolute ranges and radial velocities
• Scaled by sin i, but no reflectivity assumptions
  for scales or sizes.
• Geometry not very important for detection.
• Pathological cases exist, but mutual events are
  not required.
• Unambiguous detection in a single night.
• Common trend for slowly-rotating secondaries
  makes detection likely.
• Rapidly rotating secondaries would be harder to
  detect.
• Fairly easy to quantify detection limits.

7
Geometry not very Important

• Radar beam is 4000 km across at 0.1 AU.
• A satellite in the plane of sky would be
  invisible.
• Mutual event could hide satellite (low measure)

8
Radar Imaging of Binaries

• Absolute ranges and radial velocities
• Scaled by sin i, but no reflectivity assumptions
  for scales or sizes.
• Geometry not very important for detection.
• Pathological cases exist, but mutual events are
  not required.
• Unambiguous detection in a single night.
• Common trend for slowly-rotating secondaries
  makes detection likely.
• Rapidly rotating secondaries would be harder to
  detect.
• Fairly easy to quantify detection limits.

9
Unambiguous Detection

• Dont need to wait for mutual event.
• SNR
• Shape
• Uncertain if at same range, but thats when its
  moving fast.

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Unambiguous Detection?

• Is this an object with a satellite, or a
  weird-shaped object?

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Unambiguous Detection?

• Is this an object with secondaries, or a
  weird-shaped object?
• 73P/Schwachmann-Wachmann 3 (B)

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Radar Imaging of Binaries

• Absolute ranges and radial velocities
• Scaled by sin i, but no reflectivity assumptions
  for scales or sizes.
• Geometry not very important for detection.
• Pathological cases exist, but mutual events are
  not required.
• Unambiguous detection in a single night.
• Common trend for slowly-rotating secondaries
  makes detection likely.
• Rapidly rotating secondaries would be harder to
  detect.
• Fairly easy to quantify detection limits.

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Slowly rotating secondaries

• Secondaries typically rotate slowly, giving
  narrow Doppler width and high brightness.

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Very Fast Rotator
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Changing Frequency Resolution

• Can rescale frequency to increase SNR of fast
  rotator.
• Eye is pretty good at picking out linear
  structure anyway.

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Radar Imaging of Binaries

• Must come near the Earth (0.1 AU) for sufficient
  SNR.
• Relatively short observing windows
• 2001 SN263 had 14 days, but thats unusual.
• Only Arecibo and Goldstone, difficult to get long
  windows on short notice.
• Goldstones primary missions is spacecraft
  communications.
• Arecibo heavily oversubscribed. I can
  occasionally say We need this one (2000 DP107
  and 2001 SN263).

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Detectability

• Radar Matched SNR ? D3/2P1/2R-4
• SNR reduced (linearly) if object is resolved in
  range.
• SNR reduced (sqrt) if object is over- or
  under-resolved in Doppler.

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2001 SN263

• We chose near-Earth asteroid 2001 SN263 for an
  extensive campaign because of its large size
  (2km) and long Arecibo view window.
• Got lucky with schedule only conflict was very
  flexible.
• Discovery of first near-Earth triple asteroid
  system, the only one where we have images of the
  components
• Orbits will reveal density of primitive material
  (near-IR spectrum suggests carbonaceous
  chondrite-like)
• Is this a stable system, or is it young and
  evolving? How common are multiple systems?

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2001 SN263
12 13 14 18
21 23 24 26
Date in February 2008
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2001 SN263
12 13 14
18 21 23
24 26
21
2001 SN263
22
2001 SN263
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2001 SN263

• These values give density 0.7 to 1.0 for a sphere
• Fairly uncertain
• KW4-like shape volume lt sphere
• Size consistent with albedo of 0.04 from thermal
  model (E. Howell)

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Funding

• The planetary community has made their support of
  the program known to the agencies and to
  Congress.
• Division of Planetary Science of the AAS
  statements support the radar program.
• AAS/DPS policy supports the SR.
• Congressional hearing Nov. 8. Members were very
  supportive.
• HR 3737 and SR ???? submitted (no action to
  date).
• The 2008 omnibus appropriations bill directs NASA
  and NSF to fund the radar program and the
  observatory, and to have NRC review the program.
• NASA planning for review

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Funding

• Submitted 2009 NASA budget contains no new
  funding with the NEO observations program
  receiving an increase near inflation (300k).
• Submitted 2009 NSF AST budget contains an 8 cut
  for the observatory.
• Congressmen Fortuño (PR) and Rohrbacher (CA) are
  working on changing the 2009 appropriations bill
  to support the radar as a program within NASA.
• Relevant officials at NASA and NSF maintain their
  stance each agency believes that the program is
  valuable and should be supported by the other
  one.
• I dont know whether this was an exhaustive
  search for funding, but I am exhausted.