Asteroid Buzzes Earth
Highlighting Cosmic Blind Spot

By Robert Roy Britt
Senior Science Writer

"An asteroid large enough to have flattened a city buzzed Earth earlier this month and was not seen until after if flew harmlessly by.

The space rock approached Earth in the glare of the Sun, a blind spot that made it impossible to see during the day or night from any terrestrial vantage point. The event illustrates the potential of a surprise hit by an asteroid, astronomers said.

The object, now named 2002 EM7, was probably between 40 and 80 meters (130-260 feet) in diameter, said Gareth Williams, associate director of the International Astronomical Union's Minor Planet Center.

On March 8, the asteroid passed within 298,400 miles (480,200 kilometers) of our planet, or about 1.2 times as far away as the Moon -- considered a relatively close shave by cosmic yardsticks. It was not discovered until March 12, however. After the rock was detected, scientists calculated its orbit and determined the path it had taken.

No Way to See It

In a telephone interview, Williams explained there was no way to see the asteroid until it moved out of the Sun's glare and to the opposite side of Earth in relation to the Sun -- Earth's night side.

To spot such an object earlier would require a telescope elsewhere in space, he said. Ideas have been floated to put an observatory in orbit around Mercury, where it could observe the portion of sky that is not visible to terrestrial telescopes or even to Earth-orbiting observatories like the Hubble Space Telescope.

But a telescope at Mercury, given the likely limitations to its budget and size, would not be able to see asteroids as small as 2002 EM7. It could, however, spot large asteroids that might cause global destruction.

No firm plans exist for a Mercury-orbiting telescope.

Meanwhile, few asteroids this large have ever been known to pass so close to Earth. Asteroid 2002 EM7 is the ninth closest brush known, said Williams, who helps with the Minor Planet Center's task of cataloguing all data on asteroids.

"Of the objects that have come closer, only one is bigger," he said.

Months or years of warning have sometimes preceded close passes in the past. Other times, rocks have been found just days before they zoomed past.

Williams adds that there have no doubt been many, many other close shaves by small asteroids that went entirely unnoticed because the objects zipped back out into the solar system without ever being detected.

Telescopes devoted to asteroid tracking scan just portions of the sky on any given night.

Asteroid 2002 EM7 carves an elliptical path around the Sun. It has a remote chance of hitting Earth on a future pass, odds that will likely be reduced even further as researchers continue to track the object and refine their orbital calculations.

Another Blind Spot

Researchers have used similar close brushes in the past as opportunities to remind politicians that many potentially threatening asteroids remain undiscovered and more money is needed to find them. About 1,000 asteroids larger than 1 kilometer (0.6 miles) are thought to lurk in orbits that might one day threaten Earth with planet-wide chaos. About 500 of them have been found.

The bulk of search efforts are conducted in the United States, much of it financed by NASA in a Congressionally mandated program. Somewhat like the blind spot created by the Sun, skies below the equator are poorly surveyed, though in that case it is due to the fact that no telescopes are devoted to the task.

A recent plea by scientists to the Australian government to fund a search of the southern skies fell on unsympathetic ears, however. Australian science minister Peter McGauran said he was not convinced the threat of impact was real enough to warrant spending government money.

Williams, of the Minor Planet Center, stressed that no amount of searching, north or south, would have spotted 2002 EM7.

Copyright © 2002