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2011 XC2
(also written 2011 XC2) is a near-Earth asteroid roughly in diameter that passed less than 1 lunar distance from Earth on 3 December 2011.〔
From mid October 2011 until 3 December 2011 15:00 UT the small dim asteroid had an elongation less than 60 degrees from the Sun.〔 (While less than 18 degrees from the Sun any dim asteroid can be lost in astronomical twilight, and many observatories can not see below ~40 degrees from the horizon.) On 3 December 2011 at 15:20 UT the asteroid passed from Earth and at 16:20 UT passed from the Moon.〔 The asteroid was then discovered on 8 December 2011 by Lincoln Near-Earth Asteroid Research (LINEAR) at an apparent magnitude of 19 using a reflecting telescope.〔 At the time of discovery the asteroid was near opposition to the Sun.〔
It has an observation arc of 22 days with an uncertainty parameter of 7.〔 Virtual clones of the asteroid that fit the uncertainty region in the known trajectory show a 1 in 455,000 chance that the asteroid will impact Earth on 2 December 2056.〔 With a 2056 Palermo Technical Scale of −4.35,〔 the odds of impact by in 2056 are about 22387 times less〔 than the background hazard level of Earth impacts which is defined as the average risk posed by objects of the same size or larger over the years until the date of the potential impact.〔 Using the nominal orbit, JPL Horizons shows that the asteroid will be from Earth on 2 December 2056.〔
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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