(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.

Radar images of asteroid 1999 KW₄ and its moon. The 'streaks' on the image are the moon's trail as it moved while the images were created.

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 Šarounová 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.

References

• http://www.johnstonsarchive.net/astro/astmoons/am-66391.html