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Pasquale Tricarico
Planetary Science Institute
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Pasquale Tricarico, Ph.D.

Senior Scientist
Planetary Science Institute

The Peculiar Flyby of Asteroid 2012 KT42 -- May 29, 2012

The tiny asteroid 2012 KT42, approximately 5-10 meters in diameter, was discovered just about one day ago by observer A.R. Gibbs of the Catalina Sky Survey, using a 1.5 meter diameter telescope. This asteroid is experiencing a very close flyby with Earth literally as I write this post, minimum altitude of about 14,500 km over Earth's surface on May 29, 2012 at 7:10 UTC, much closer than i.e. the artificial satellites in the GPS constellation. 2012 KT42 is approaching the Earth from the night side, which is one of the conditions for discovery, and as a consequence will cross to the day side during this flyby and become unobservable for a long time.
So far nothing new. It has become almost routine to have these smaller asteroids experience a flyby immediately before or after discovery, approximately once a month, merit of the active near-Earth objects surveys that have been constantly improving their techniques to pick smaller and smaller objects. As a matter of fact, when the asteroid is so small, the flyby is a requirement for discovery, because otherwise the asteroid is too far from Earth and then too faint to be detected.
But one aspect is unusual about 2012 KT42: it experiences not only a flyby, but also an eclipse and a transit during the same flyby. The transit has been already reported elsewhere, and Aldo Vitagliano from Italy has produced a map of the transit, even if the objects is far too small to produce a visible spot on the solar disk. This transit is due to the very small inclination the orbit of the asteroid, only 0.35 degrees. So if you do the math, you can see that the gravity of the Earth is strong enough, and the flyby low enough, that the asteroid crosses the Sun-Earth line not only once, but twice. So I went and checked using my asteroid dynamics simulator, and sure enough I found that the asteroid 2012 KT42 is partially eclipsed by the Earth. This happened on May 28, 2012 between 14:05 UTC and 21:45 UTC, before the flyby. Now, I have looked a bit into eclipses experienced by asteroids while approaching the Earth-Moon system, and I don't recall this ever happening before, so it's a first. So far no observations have been reported during the partial eclipse period, but it will be interesting to see how the brightness changes.

This first animation shows the Sun and the Earth as observed from the asteroid 2012 KT42 as it is approaching. From the point of view of the asteroid, the Sun is partially obstructed by the Earth, causing a partial eclipse of the asteroid. Also notice how the apparent size of the Earth is increasing, sign that the asteroid is approaching the Earth.

In this second animation, the observer is at the Sun position, looking towards the Earth. The asteroid 2012 KT42 first passes behind the Earth, causing the partial eclipse, and then accelerates under the influence of Earth's gravity, inverts its apparent motion (relative to Earth), and passes between the Earth and the Sun, causing the solar transit.