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Pasquale Tricarico
Planetary Science Institute
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tel: +1 520 547-3954
email: tricaric@psi.edu

Pasquale Tricarico, Ph.D.

Senior Scientist
Planetary Science Institute

Asteroid 2012 XE54 to Cross Earth's Shadow -- December 10, 2012

The asteroid 2012 XE54 was announced in MPEC 2012-X37. It has an estimated diameter between 15 and 50 meters, and is quickly approaching the Earth-Moon system, reaching on December 11, 2012 around 10:10 UTC a minimum distance of approximately 0.23 million kilometers, little over half-way the average distance between the Earth and the Moon.

A close analysis of the trajectory of this asteroid reveals that it will likely cross the Earth's shadow, causing a partial eclipse of the asteroid a few hours before reaching its minimum distance with the Earth. Asteroids eclipsing during an Earth flyby are relatively rare, with the first known case of asteroid 2008 TC3 which was totally eclipsed just one hour before entering Earth's atmosphere over Sudan in 2008, and asteroid 2012 KT42 experiencing both an eclipse and a transit during the same Earth flyby in 2012.

Ingress into the Earth's penumbra cone of asteroid 2012 XE54 could start as early as 01:22 UTC on December 11, 2012, with egress at approximately 02:00 UTC. At the time of writing, the largest uncertainties are in the orbit of the asteroid, and the nominal orbit solution comes close to totality without reaching it. The maximum of the eclipse should be at about 01:41 UTC, when the asteroid is at about 0.46 million kilometers from the Earth, just beyond the average Moon distance.

This animation shows the Sun and the Earth as observed from the asteroid 2012 XE54, using the nominal orbit solution. If we improve the brightness and contrast of the movie at the central time of the eclipse, we see that it should be easy to follow for observers in Europe and most of North and South America and Africa.

The asteroid should be getting close to the 13th apparent magnitude before and after the eclipse period, and of course much dimmer during the eclipse. The plot below shows the expected average apparent magnitude of asteroid 2012 XE54 (line), the filter-corrected magnitudes reported (blue boxes) and the dip due to the eclipse (red curve). Note how, in absence of an eclipse, the asteroid would show a small peak in magnitude, due to the opposition effect (zero phase angle). Also, the brightness keeps increasing after the expected eclipse because the asteroid is still getting closer to the Earth for a few hours, and maximum brightness is reached before reaching minimum distance. As a side note, the Moon is close to new, so a possible contribution to brightening the asteroid is negligible.

Updates are being posted as more data become available.

Update: Eclipse of Asteroid 2012 XE54 Confirmed -- December 11, 2012

At this time, two observers have reported the observation of the partial eclipse of the near-Earth asteroid 2012 XE54.

Elia Cozzi of the New Millennium Observatory in Italy reported at 1:45 UTC:

In two images taken at 01:30:16 and 01:31:18, 60sec exposure, 2012 XE54 appeared as a very faint and long track, then... nothing. In the following images there is no visible track. Wonderful!
and then at 1:53 UTC:
at 01:43:43 a very faint track is visible, at 01:48:43 a very bright track is visible.

Peter Birtwhistle of the Great Shefford Observatory in United Kingdom reported at 3:47 UTC:

Great prediction! I've put up a very preliminary lightcurve covering the eclipse earlier today, showing minimum centred around approx. 01:37:00 UT, probably around mag ~ +17.5 (but still to analyse all the images from the deepest part of the eclipse properly)
Peter has posted a very preliminary lightcurve of the asteroid during the eclipse.

The Minor Planet Center has issued an orbit update, but most of the observations made during the eclipse will likely need a few more days to get processed and submitted for publication. This is necessary also because the asteroid was a very fast mover and appears as a long trail in most images acquired during the Earth flyby. We will update again in the next days after most of the observations have been published.