In comments about my previous bounty on the Space SE question Which deep-space spacecraft flew closest by Earth during a gravitational assist?, I started to look at the Galileo mission and ran across the following factoid. From here for example:

On August 28, 1993 Galileo came within 2,400 kilometers of Ida, the second asteroid ever encountered by a spacecraft. They passed each other at a relative velocity of 12.4 km/sec (28,000 mph). At the time of the encounter, Ida and Galileo were 441 million kilometers from the Sun.

The greatest discovery from the Galileo fly-by was that Ida has a natural satellite. The moon has been named Dactyl by the International Astronomical Union. Dactyl is the first natural satellite of an asteroid ever discovered and photographed. The tiny moon is about 1.2 by 1.4 by 1.6 km across.

From Wikipedia's 243_Ida; Discoveries:

The discovery of Ida's moon Dactyl, the first confirmed satellite of an asteroid, provided additional insights into Ida's composition.36

36Chapman, Clark R. (October 1996). "S-Type Asteroids, Ordinary Chondrites, and Space Weathering: The Evidence from Galileo's Fly-bys of Gaspra and Ida". Meteoritics. 31 (6): 699–725. https://ui.adsabs.harvard.edu/abs/1996M%26PS...31..699C/abstract and https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.1996.tb02107.x

Delay-Doppler radar can also generate images of (rotating) asteroids and detect and image any satellites they may have. So I'd like to ask:

Question: What was the first satellite of an asteroid (or double asteroid) imaged by delay-Doppler radar?

From this answer to What causes "North-South ambiguity" when doppler radar imaging a planet surface equator? in Astronomy SE (see also What is the physical geometry of this apparent "eclipse" of a tiny moon of Asteroid Florence?)

Left: A very slow GIF from Emily Lakdawalla's How radio telescopes get "images" of asteroids

Right: A radar image shows asteroid 3122 Florence and tiny echoes from its two moons. Here is an animation that shows them more clearly. The direction of the radar illumination (and thus the direction toward Earth) is at the top." From here. NASA / Jet Propulsion Laboratory. This is a small subset of the frames contained in the original 36 MB GIF, and the size has been decreased by a factor of 2 in order to fit in SE's 2 MB limit. From the question What is the physical geometry of this apparent "eclipse" of a tiny moon of Asteroid Florence?

Note: These two images do not necessarily have the same orientation.

Delay-Doppler Imaging Emily Lakdawalla enter image description here

  • 2
    $\begingroup$ +1 Tough question! I tried a quick 5min search for digging up, found nearly nothing! I'll keep trying tho :3 $\endgroup$
    – DialFrost
    Commented Oct 15, 2022 at 8:49
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    $\begingroup$ There are also other ways to find asteroid satellites. On Mar 5 1977, Paul Maley detected a satellite of 6 Hebe during an asteroid occultation. He unofficially named it Jebe, as in heebie-jeebie. It was the first modern day suggestion that asteroids could have satellites. $\endgroup$
    – John
    Commented Oct 15, 2022 at 19:03
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    $\begingroup$ @John Indeed! Measuring stellar occultations by asteroids, especially with a closely spaced array of portable telescopes is now a very important technique (cf. Winking Stars Help Map Asteroids — And Discover A New Moonlet), but I don't know anything about it's early history and firsts. Maybe that topic can benefit from a new question? $\endgroup$
    – uhoh
    Commented Oct 15, 2022 at 21:44

1 Answer 1


The second table here provides the radar detections RADAR DETECTIONS IN CHRONOLOGICAL ORDER

The first radar binary asteroid detection was in 1985 by continuous wave radar but not reported until 2007. Here are some cut and pastes from Wikipedia's 1866 Sisyphus:

1866 Sisyphus /ˈsɪsɪfəs/ is a binary stony asteroid, near-Earth object and the largest member of the Apollo group, approximately 7 kilometers in diameter. It was discovered on 5 December 1972, by Swiss astronomer Paul Wild at Zimmerwald Observatory near Bern, Switzerland. It was named after Sisyphus from Greek mythology. In 1985, this object was detected with radar from the Arecibo Observatory at a distance of 0.25 AU. The measured radar cross-section was 8 square kilometers. During the radar observations, a small minor-planet moon was detected around Sisyphus, although its existence was not reported until December 2007.

From the JPL website above:

1866 Sisyphus is now listed as a binary based on CW data obtained at Arecibo in December, 1985. The CW signature matches those subsequently seen many times with other binary NEAs. Attempts to detect the secondary with ranging observations in 1985 were unsuccessful.

The first reported range-Doppler radar binary asteroid was 2000 DP107 ([archived]. Since the question asks specifically which was the first Doppler radar detection of a binary asteroid, DP107 is the answer.


Fig. 2. Mosaic of eight range-Doppler images of binary asteroid 2000 DP107 obtained at Arecibo Observatory from 30 September to 7 October 2000.

Fig. 2. Mosaic of eight range-Doppler images of binary asteroid 2000 DP107 obtained at Arecibo Observatory from 30 September to 7 October 2000. Each frame is 5.8 km vertically by 12.2 Hz horizontally, with range, or distance from the observer, increasing downward and Doppler, or line-of-sight velocity with respect to the observer, increasing to the right. The resolutions are 75 m in range and 0.24 Hz (15 mm s-1) in Doppler. Rotation and revolution are counterclockwise. A roughly spherical primary is visible, as well as a secondary at different phases of the orbital cycle. In range-Doppler space the secondary appears much smaller than the primary because its spin rate is lower. The actual size ratio is ~8:3. The rotational and orbital motions of the objects during the 1-hour period over which data were accumulated each day lead to perceptible smearing in range-Doppler space.


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