Unexpected asteroid: a close call from space.

Unexpected asteroid: A close call from space

It passed about twice the distance to Earth's moon, but an asteroid that missed the planet by some 500,000 miles on March 23 was only the second such object ever cataloged that came so close. The first, called Hermes, was discovered in 1937 but never located again. Others are known only because they left craters where they struck Earth's surface.

Henry E. Holt (a retired geologist with the U.S. Geological Survey in Flagstaff, Ariz.) and Norman G. Thomas (a retired astronomer from Lowell Observatory in Flagstaff) discovered the newcomer on April 6 in photos taken earlier with the 18-inch Schmidt telescope on Palomar Mountain in California. No astronomers saw the asteroid as it approached, because it came in through the daytime sky. Holt estimates the big boulder probably measured several hundred yards across, assuming that its albedo or reflectivity represents a surface like that of Earth's moon and some meteorites.

The object's orbit tilts about 5[deg.] from those of most planets in the solar system. So, although it is not in the orbital planes of Venus and Mars when it is as those planets' distances from the sun, the asteroid is approximately in plane when it crosses Earth's orbit, Holt says. Unless the asteroid's present orbit is perturbed by Earth or another object, its closest approaches to Earth should occur about every 25 to 30 years, since its year is only 14 to 15 days longer than Earth's. Next April, says Holt, it will probably pass "several million miles" away. More precise estimates of when and at what distance it will pass again will require extended observations to establish a more accurate orbit, since the object was seen only from March 31 to April 9.