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Huge Asteroid Due for a Close Call in 2028
By Kathy Sawyer, Washington Post Staff Writer
Thursday, March 12, 1998; page A2

A mile-wide asteroid first detected in December is virtually certain to sweep past Earth within the distance of the moon 30 years from now, and there's a small chance it could strike Earth directly with potentially disastrous effects, astronomers reported yesterday. "The chance of an actual collision is small, but one is not entirely out of the question," said Brian G. Marsden, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

By far the higher probability appears to be that, on Thursday, Oct. 26, 2028, about 1:30 p.m. Eastern time, the object will hurtle harmlessly past at a distance of about 30,000 miles from Earth's center (or 26,000 miles from its surface), he said, based on the most recent observations and computations of the "miss distance." This would be the closest approach by a cosmic object this large in modern history. "That evening, the object should be visible with the naked eye," Marsden wrote in a statement issued through the International Astronomical Union's Central Bureau for Astronomical Telegrams, which he operates. "In Europe, where it would be dark by that time, the object should be a splendid sight as it moves from northwest to southeast across the sky over a couple of hours."

Based on its projected track and its large size, the object has been added to the list of "potentially hazardous objects" that are monitored to determine whether they are destined to come dangerously close to Earth over the next several centuries. There are 108 such objects on the list, but astronomers say they have detected only about a tenth of the estimated number of asteroids and comets larger than a kilometer in diameter (0.6 mile) whose orbits could intersect with Earth. The newly detected intruder, known as 1997 XF11, was discovered on Dec. 6 by Jim Scotti, of the Spacewatch program at the University of Arizona, which uses a 77-year-old 36-inch telescope at Kitt Peak.

Follow-up observations by two Japanese amateur astronomers during the next two weeks first alerted scientists to the unusually small minimum distance between the path of 1997 XF11 and Earth. Peter Shelus, using a 30-inch telescope at the McDonald Observatory in Texas, made observations March 3-4 that further refined the object's threatening track. Experts say that an impacting object about a mile in diameter is right on the murky dividing line, in terms of size, between the threat of merely local devastation and the potential for truly global ecological damage, with widespread disruption of civilization. "This is a pretty close approach. This is quite spectacular. That's a dangerous diameter," said Kevin Zahnle of NASA's Ames Research Center in California. "It was sure to happen sooner or later." He noted that the comet that struck Jupiter in 1994, known as Shoemaker-Levy 9, was about a mile in diameter before it split into a train of smaller pieces.

Zahnle calculated the odds that this asteroid would collide with Earth at 1 in 1,000. An object this size would release the energy equivalent of 1 million megatons of TNT at impact, he said. If it landed in an ocean, it could kick up tidal waves that would race across the water and, as they climbed ashore, build as high as 100 feet. If it struck land, it could raise a global cloud of stratospheric dust that would dim the sun over one or both hemispheres for months, producing a global cooling effect. Locally, he added, in addition to a destructive shock wave, the thermal radiation could light fires over an area of 6,000 square miles around the impact site. "This is unprecedented, if true," said Steven Pravdo of the Jet Propulsion Laboratory in Pasadena, Calif., project manager and researcher for NASA's NEAT (Near-Earth Asteroid Tracking) system.

Scientists were careful to note the uncertainties in the calculations of the asteroid's arc through space, which will be refined over coming years with new observations and also by checking old photographs to see if the object appears in any of them. They may also refine their estimates of the object's size up or down, as they learn more about its composition. Coal black asteroids reflect much less sunlight for their size than stony ones. As it orbits the sun, the object will move out of view to all but the largest telescopes over the next few months but will become more visible again in 2000. And on Halloween 2002, it is expected to sweep within 6 million miles of Earth. Then it will swing far out into space, returning to Earth for a rendezvous of an undetermined nature.

Copyright 1998 The Washington Post Company