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.)

1999 KW4 viewed in orbit plane

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.

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.

Absolute size and velocity No scale uncertainty sin i (velocity) SNR matters m m/s

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.

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) m m/s

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.

Unambiguous Detection Don’t need to wait for mutual event. SNR Shape Uncertain if at same range, but that’s when it’s moving fast m m/s

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

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

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.

Slowly rotating secondaries Secondaries typically rotate slowly, giving narrow Doppler width and high brightness.

Very Fast Rotator

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

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 that’s unusual. Only Arecibo and Goldstone, difficult to get long windows on short notice. –Goldstone’s primary missions is spacecraft communications. –Arecibo heavily oversubscribed. I can occasionally say “We need this one” (2000 DP107 and 2001 SN263).

Detectability Radar “Matched” SNR  D 3/2 P 1/2 R -4 SNR reduced (linearly) if object is resolved in range. SNR reduced (sqrt) if object is over- or under-resolved in Doppler.

2001 SN 263 We chose near-Earth asteroid 2001 SN 263 for an extensive campaign because of it’s 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?

2001 SN Date in February 2008

2001 SN

2001 SN 263

PrimaryOuterInner Diameter2.8 km1.2 km.5 km Orbit a~17 km / sin i ~4 km / sin i 12.5 R p 2.8 R p Orbit Period~147 h~17 h Rotation3.434 h~ 15 h * sin i SynchronousNoProbably Sin iClose to 1

2001 SN263 These values give density 0.7 to 1.0 for a sphere –Fairly uncertain –KW4-like shape volume < sphere Size consistent with albedo of 0.04 from thermal model (E. Howell)

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

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 don’t know whether this was an “exhaustive” search for funding, but I am exhausted.