YALE News Release
CONTACT: Jacqueline Weaver 203-432-8555 #151

New Haven, Conn. — The number of “near-Earth asteroids” that have
a chance, though miniscule, of colliding with Earth this century is half
what was originally estimated, a Yale researcher says.

“They are the kind of asteroids you hear about in movies that are about
two-thirds of a mile in diameter and could easily obliterate a city
because they are coming in at cosmic velocities,” said Research Associate
David Rabinowitz. “The main thing we want to stress is that none of
the known asteroids are in imminent danger of falling to earth and no
impacts are predicted in the near future.”

It was previously estimated that there are 1,000 to 2,000 such asteroids
in chaotic orbits. Rabinowitz and his fellow researchers estimate that
these asteroids actually total half that, or 500 to 1,000. Each has a 0.5
percent chance of colliding with Earth in the next million years, he said
in the article published this week in the journal “Nature.”

“The reduced number doesn’t make us feel that much safer, but it does
allow us to plan more accurately,” Rabinowitz said. “The goal is to find
the asteroids hundreds to thousands of years before they even come close.”

The other researchers were Eleanor Helin, Kenneth Lawrence, and Steven
Pravdo, all of the Jet Propulsion Laboratory at the California
Institute of Technology. They made their observations through a U.S. Air
Force telescope in Hawaii that was designed to look for artificial

Rabinowitz said smaller asteroids the size of a large house or hotel
generally burn up or blow up before they hit the ground.

“But an asteroid the size of a city block is more dangerous because it
could punch through the atmosphere and raise a lot of dust, which would
change the climate of the Earth,” he said. “It would be like a nuclear
bomb coming in.”

Efforts are underway around the world to survey this asteroid population
because most of the threatening bodies remain undiscovered and their
number is uncertain, the researchers said.

The observations were made using a large-format, charge-coupled device and
a one meter aperture telescope based on the summit of Haleakala Crater on
the island of Maui. Earlier photographic methods required a trained
observer to identify asteroids by visual inspection and the accuracy
varied according to the skill of the observer. The automated method is
more consistent and provides a record of every detectable asteroid.

The researchers said that, at the current rate of discovery, about 90
percent of the asteroids probably will be identified in the next 20 years.
The goal, however, is to double the current worldwide detection rate to
complete the program in 10 years.

“If we can find the asteroids in 10 years, that’s plenty of time,”
Rabinowitz said. “If you wait 100 or 1,000 years, that’s too much time.”

Rabinowitz is currently working with Professor Charles Baltay,
chairman of the Yale Physics Department,
to develop one of the world’s largest electronic cameras. It will be used
by Yale astronomers and physicists to study the properties of distant
galaxies and supernova, and to study the expansion of the universe. The
new camera also will be used in cooperation with astronomers at the Jet
Propulsion Lab to continue the survey for near-Earth asteroids.

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