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

7494

.811

4500

000

m

10.31765 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)

7494

.811

45 m

10.31765 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.

7494

.811

45 m

10.31765 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 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.

2001 SN263• We chose near-Earth asteroid 2001 SN263 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 SN263

12 13 14 18 21 23 24 26

Date in February 2008

2001 SN263

12 13 14 18 21 23 24 26

2001 SN263

2001 SN263Primary Outer Inner

Diameter 2.8 km 1.2 km .5 km

Orbit a ~17 km / sin i ~4 km / sin i

12.5 Rp 2.8 Rp

Orbit Period ~147 h ~17 h

Rotation 3.434 h ~ 15 h * sin i ~15 h * sin i

Synchronous No Probably

Sin i Close to 1 Close 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)