2010 WC9, unofficially designated ZJ99C60, is a sub-kilometer near-Earth asteroid of the Apollo group, approximately 100 meters (330 feet) in diameter. First observed for eleven days by the Catalina Sky Survey in 2010, the asteroid was recovered in May 2018 during its sub-lunar close encounter with Earth.[4]
First observation and recovery
2010 WC9 was first observed by astronomers with the Catalina Sky Survey on 30 November 2010 with a 1-day observation arc[6] and was observed through 10 December 2010.[7] By 10 December 2010, the asteroid was more than 24 million kilometers from Earth[6] at apparent magnitude 21.8[1] and was becoming too faint to be practical to track.
The preliminary 10-day observation arc generated a line of variation roughly 15 million km long for May 2018 that did not intersect Earth's orbit and thus was not a 2018 impact threat.[8] The 10-day observation arc showed the asteroid would pass about 0.026 AU (3,900,000 km; 2,400,000 mi) from Earth around late 14 May 2018. The asteroid was recovered on 8 May 2018 when it was 8 million kilometers from Earth and given the temporary NEOCP designation ZJ99C60.[2] It was removed from the Sentry Risk Table on 10 May 2018[9] and is not an impact threat for the next 100 years or more. The asteroid now has a secure 7-year observation arc.[4]
On 15 May 2018, 22:05 UT, the asteroid approached Earth at just over 0.5 LD, the closest approach of this asteroid in nearly 300 years.[3] It was expected to reach apparent magnitude +11 at closest approach,[10] bright enough to be seen in a small telescope if you have a custom ephemeris for your location. At closest approach, it was best seen from the Southern hemisphere such as South Africa and southern South America. The asteroid passed Earth going 12.81 km/s (28,700 mph).[4]
This was the third closest approach ever observed by an asteroid with absolute magnitude (H) brighter than 24.[11]
Flyby in 2018: geocentric trajectory in the sky (top) and flyby from north to south, shown with hourly motion (bottom)
Animation of 2010 WC9's orbit
Around the Sun
Around the Earth
2010 WC9· Sun· Earth· Moon
History of closest approaches of large near-Earth objects since 1914 (less than H 24 and 1 LD)(A)
(A) This list includes near-Earth approaches of less than 1 lunar distances (LD) of objects with H brighter than 24. (B)Nominal geocentric distance from the center of Earth to the center of the object (Earth has a radius of approximately 6,400 km). (C) Diameter: estimated, theoretical mean-diameter based on H and albedo range between 0.05 and 0.25. (D) Reference: data source from the JPL SBDB, with AU converted into LD (1 AU≈390 LD) (E) Color codes: unobserved at close approach observed during close approach upcoming approaches Note: All close approaches between 1900 and 2200 are listed (with H<24 at less than 1 LD). Objects not observed during the approach, and simply estimated to have approached on this date, are colored grey. Generically estimated asteroid diameters are given in italics.
Physical characteristics
Diameter
As the asteroid has not been directly resolved by telescope, its diameter can only be estimated based on the distance and brightness. Based on a generic magnitude-to-diameter conversion, it is estimated to measure between 60 and 130 meters in diameter, for an absolute magnitude of 23.5,[4] and an assumed albedo of 0.04–0.20.[3][5]
^Earth Impact Risk Summary: 2010 WC9 (10 day arc; computed on Sep 20, 2015) "2010 WC9 Impact Risk". Archived from the original on 15 December 2016. Retrieved 13 May 2018.
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