A Close Visitor Tumbles By

The little asteroid 1998 WT24, about 1 kilometer wide, was radar-imaged by NASA's Goldstone antenna last week when it flew within five Moon-distances of Earth. Click on image for animated movie of the asteroid's rotation.
Courtesy Steve Ostro, Lance Benner, Jon Giorgini (JPL), Jean-Luc Margot (Caltech), and Mike Nolan (Arecibo Observatory).
Amateur astronomers got an unusual treat several days
ago when an Earth-crossing asteroid, 1998 WT24, passed just
1.9 million kilometers from Earth and brightened to magnitude 9.5. Subscribers
to Sky & Telescope's AstroAlert service for minor planets were notified that the object would be crossing
Gemini, Auriga, and Perseus at a speed of up to 1 degree per hour.

That was fast enough for the asteroid to
show visible motion in real time in amateur telescopes. Sure enough, in a
12.5-inch reflector at 60 power, 1998 WT24 shone bright
yellow-white at its predicted location on December 15th and actually
appeared to creep along as I watched, especially when it passed close to a
star. It looked like a super-slow satellite — and it was a little
chilling to realize that this tiny object would cause a
civilization-destroying impact if it ever struck Earth. (It will continue
to miss Earth for the foreseeable future.)

1998 WT24 is the brightest 'star' in this frame from a movie made by amateur astronomer Nick James in England on December 17th. He took 350 ten-second CCD exposures while his 12-inch Newtonian reflector tracked the asteroid. Click on image for animation. James's longer movies are also available. S&T's Russell Sipe also has an animation. Here are other movies of the flyby.

The last time a kilometer-sized object came so close to
Earth was on August 27, 1969, when 1999 RD32 passed within
3.7 Moon distances of our planet, according to Donald Yeomans (Jet Propulsion
Laboratory). And no one noticed it because it hadn't been discovered
yet. Only one other known near-Earth asteroid, 4179 Toutatis, will become
brighter than 10th magnitude before 2027 (during a flyby in 2004).
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