Discovered by LINEAR Alternative names none Aphelion 1.0845 AU (162.24 Gm) Discovered 20 May 1999 Orbits Sun | Discovery date 20 May 1999 Observation arc 6238 days (17.08 yr) Orbital period 188 days Inclination 38.884° Asteroid group Aten asteroid | |
 | ||
Minor planet category Aten asteroid,
Mercury-crosser asteroid,
Venus-crosser asteroid Discoverer Lincoln Near-Earth Asteroid Research Similar Solar System, Sun, (33342) 1998 WT24, (136617) 1994 CC, 69230 Hermes | ||
(66391) 1999 KW4 (also written (66391) 1999 KW4) is an Aten and Mercury-crossing binary asteroid discovered by LINEAR in 1999.
1999 KW4 has a moon orbiting it. The moon, designated S/2001 (66391) 1 or "1999 KW4 Beta", is ~360 m in diameter, and orbits 1999 KW4 'Alpha' in 0.758 d (16 hours) at a distance of 2.6 km. The presence of a companion was suggested by photometric observations made June 19–27, 2000 by Petr Pravec and Lenka Šarounová at Observatoř Ondřejov (Ondřejov Observatory) and 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.
The shapes of the two bodies and their dynamics are complex. Among other bizarre properties, the equatorial regions of Alpha are very close to breakup: raising a particle a meter above the surface would put it into orbit. As seen in the image at above right, the gravitational effects between the moon and the asteroid create a gigantic mountain extending in the equatorial plane around the entire asteroid.
On May 25, 2036, it will pass 0.0155 AU (2,320,000 km; 1,440,000 mi) from Earth.