The asteroid Didymos is only 780m in "diameter" with a moon, Dimorphos only 170m across. Is Didymos the smallest object known to have a natural satellite? Also, contact binaries where the asteroid and moon touch each-other should be excluded as the moon is not in a free orbit.


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This may not be complete or up to date, but sorting the List of minor planets with moons for smallest primary shows 2015 TD144 as having a 0.1 km diameter primary and 0.1 km diameter secondary. That Wikipedia article links to Asteroids with Satellites by Wm. Robert Johnston last updated 7 November 2021 which lists 2015 TD144 having an effective diameter of 0.09 km and secondary unknown.

The source for that data is W19e: Warner, B. D., and R. D. Stephens, 2019, Near-Earth asteroid lightcurve analysis at the Center for Solar System Studies, Minor Planet Bulletin, 46(4):423-438 which says at the top of page 435:

As of 2019 July, the binary with the smallest primary is 2015 TD144 (D = 0.09 km; Radar Team, 2015).


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