Planetary Radar Imaging of Binary Asteroids
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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.
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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.
![Page 5: Planetary Radar Imaging of Binary Asteroids](https://reader036.fdocuments.us/reader036/viewer/2022062422/5681389f550346895da05a0c/html5/thumbnails/5.jpg)
Absolute size and velocity
• No scale uncertainty
• sin i (velocity)
• SNR matters
7494
.811
4500
000
m
10.31765 m/s
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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.
![Page 7: Planetary Radar Imaging of Binary Asteroids](https://reader036.fdocuments.us/reader036/viewer/2022062422/5681389f550346895da05a0c/html5/thumbnails/7.jpg)
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
![Page 8: Planetary Radar Imaging of Binary Asteroids](https://reader036.fdocuments.us/reader036/viewer/2022062422/5681389f550346895da05a0c/html5/thumbnails/8.jpg)
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.
![Page 9: Planetary Radar Imaging of Binary Asteroids](https://reader036.fdocuments.us/reader036/viewer/2022062422/5681389f550346895da05a0c/html5/thumbnails/9.jpg)
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
![Page 10: Planetary Radar Imaging of Binary Asteroids](https://reader036.fdocuments.us/reader036/viewer/2022062422/5681389f550346895da05a0c/html5/thumbnails/10.jpg)
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.
![Page 13: Planetary Radar Imaging of Binary Asteroids](https://reader036.fdocuments.us/reader036/viewer/2022062422/5681389f550346895da05a0c/html5/thumbnails/13.jpg)
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 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).
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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 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?
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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
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2001 SN263
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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
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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)