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1999 JM8

Sky & Telescope, December 1999, News Notes, p. 26:

Another odd rotator is far different in character and can be best summed
up as "big, battered, and slow." On May 13th the Lincoln Near-Earth
Asteroid Research (LINEAR) team discovered 1999 JM8, whose 3.5-km
diameter makes it one of the largest near-Earth objects known. In late
July the newfound object passed our planet at relatively close range,
8,500,000 km away. This provided enough lead time to ready the radar
systems at both Goldstone and Arecibo, and both facilities obtained
high-quality views of 1999 JM8 during its brief visit. "The discovery of
this object weeks before its closest approach was a stroke of luck,"
notes team leader Lance A.M. Benner (Jet Propulsion Laboratory). "The
asteroid won't come this close again for more than a thousand years."
Fortunately, an actual collision with Earth is not possible, at least in
the foreseeable future.
The Goldstone and Arecibo radar maps show that 1999 JM8 bears many
craters, some as small as 100 meters across. This implies that the
surface is quite old, so this object is not just a recent chip off a
larger body. Moreover, 1999 JM8 spins very slowly, about once per week.
Slow rotation is also a hallmark of 4179 Toutatis, which is similar in
size. However, it is not yet clear how such exaggerated rotation periods
come about.

Astronomy Now, October 1999, News Update, p. 8: Radar images near-Earth
asteroid

Astronomers have used the world's two most powerful radar telescopes to
produce the most detailed images of an Earth-grazing asteroid. The new
images of asteroid 1999 JM8 were obtained in late July and early August
from observations with NASA's Goldstone Solar System Radar in California
and the Arecibo Observatory in Puerto Rico.
Although the near-Earth object was originally designated 1990 HD l after
its discovery in 1990 by JPL astronomer Eleanor Helin, it was
subsequently 'lost' until the rediscovery on 13 May 1999 by the US Air
Force LINEAR telescope. Astronomers seized the opportunity to study this
relatively large body as it approached to a distance of 8.5 million km
(5.3 million miles) - its closest Earth flyby for the foreseeable
future. After many days of computer processing the data, the results
show that 1999 JM8 bears a striking resemblance to Toutatis, another
asteroid previously studied by groundbased radar.
Further analysis is required to determine the object's exact shape and
rotation state, but the images clearly show a peculiarly shaped object
with an average diameter of about 3.5 km (2 miles) and an unusually
slow, possibly complex, rotation. The high resolution radar imaging
technique was able to show features as small as 15 metres (49 ft)
across, better than any previous spacecraft or ground-based asteroid
observations. Impact craters with diameters down to 100 metres (330 ft)
can be seen, as well as a few in the 1 km (0.6 mile) size range.
Although 1999 JM8 passes quite close to Earth in celestial terms, it is
unlikely to prove a direct threat to the planet for at least the next
few centuries (by Peter Bond).


Best wishes,

Bernd

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