(66391) 1999 kw4

(66391) 1999 KW₄ (also written 1999 KW4) is an Aten and Mercury-crosser asteroid discovered by LINEAR in 1999. It is interesting for two reasons. First, 1999 KW₄ is one of the few asteroids whose orbit crosses that of Mercury, the innermost planet (another is 1566 Icarus). Its unusual, eccentric orbit suggests that the asteroid may in fact be a comet that has lost its surface ice and can no longer produce a tail. Second, despite its small size (about 1.2 km in diameter), 1999 KW₄ has a small moon orbiting it. The moon, designated S/2001 (66391) 1 is only 360 m in diameter, and orbits 1999 KW₄ in 0.758 d (16 hours) at a distance of 2.6 km and a leisurely speed of 0.25 m/s (0.9 km/h). The companion was first suggested by an apparent eclipsing binary signal in photometric observations made June 19-27, 2000 by Petr Pravec and Lenka arounov at Ondrejov Observatory. It was confirmed by radar observations from Arecibo Observatory from May 21-23, 2001 by Lance A. M. Benner, Steven J. Ostro, Jon D. Giorgini, Raymond F. Jurgens, Jean-Luc Margot and Michael C. Nolan, announced on May 23, 2001.

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1999 KW4 1999 KW4

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