MONTEREY, Calif.-Of the approximately 1,000 near-Earth asteroids larger than 1
kilometer in diameter, one strikes the Earth on average once every 600,000
years, MIT
Lincoln Laboratory researcher J. Scott Stuart reported today (Sept. 5) at the
35th annual
meeting of the American Astronomical Society Division for Planetary Sciences.

By combining discovery statistics with new observations of the surfaces of
near-Earth
asteroids (NEAs), Stuart also determined that impacts similar to the Tunguska
event-in which an asteroid destroyed more than 2,000 square kilometers of
forest in a
remote part of Siberia in 1908-are much less frequent than previously thought.
Events
of this magnitude occur, on average, once every 2,000 to 3,000 years.

Near-Earth asteroids are small solar system objects whose orbits around the sun
may
bring them close to Earth. While all currently known NEAs are in safe orbits
that will
not cause them to collide with the Earth in the foreseeable future, many NEAs
remain
undetected and could be on a collision course with Earth, Stuart said.

The Lincoln Near-Earth Asteroid Research (LINEAR) Project has been scanning the
skies to discover and catalog NEAs and to provide advance warning of any that
are
bound for Earth. Since March 1998, LINEAR, whose principal investigator is
Lincoln
Lab associate division head Grant H. Stokes, has found 70 percent of all
near-Earth
asteroids discovered worldwide and is a major contributor to the NASA goal of
cataloguing 90 percent of NEAs larger than 1 kilometer (0.6 miles) in diameter
2008.

The amount of damage that would be caused by an asteroid depends on its size.
Impacts
of asteroids bigger than 1 kilometer are thought to be capable of causing
long-term
climate damage on a global scale. The NEA in the Tunguska event was relatively
small,
with a diameter of 50 to 75 meters (150-250 feet). The energy of the impact was
about
40 megaton, or more than 600 times the energy of the Hiroshima bomb.

Even if impacts of large NEAs occur only once every 600,000 years, this does
not mean
that they occur at regularly spaced intervals. This figure comes from looking
at time
spans of millions of years. Likewise, events similar to the Tunguska impact are
not
evenly spaced. Over long time spans, we might expect such events to occur every
2,000
to 3,000 years on average, Stuart said.

REFLECTING SUNLIGHT

When NEAs are first discovered, the only data available are their locations in
the solar
system and their brightness. To estimate their diameters, astronomers assumed
that
NEAs reflect about 11 percent of the sunlight that hits them.

Through new observations and modeling of the reflective properties of the NEAs,
MIT
researchers have found that on average, NEAs actually reflect about 14 percent
of the
sunlight that hits them. This means that NEAs are slightly smaller than
previously
assumed, so the number of NEAs that are larger than 1 kilometer is somewhat
lower
than previously estimated.

To determine the percentage of sunlight that is reflected from NEAs, MIT
researchers
used results from the Small Main-belt Asteroid Spectroscopic Survey (SMASS), a
multi-wavelength observing program that studies the surface properties of NEAs
and
other asteroids. This program comprises visible spectroscopy, near-infrared
spectroscopy and thermal infrared flux measurements.

The visible spectroscopy is collected at Kitt Peak National Observatory and
Palomar
Observatory, and on the Magellan I Telescope at Las Campanas Observatory.
Near-infrared spectroscopy is collected at the NASA Infrared Telescope
Facility. The
W.M. Keck Observatory is used to obtain thermal flux measurements to estimate
reflectivity.

SMASS is directed by MIT’s Richard P. Binzel, professor of earth, atmospheric
and
planetary sciences (EAPS), and operates in collaboration with the University of
Hawaii,
the Space Science Institute in Boulder, Colo., and the Deutschen Zentrum f¸r
Luft- und
Raumfahrt in Berlin and others.

MOON IMPACTS

MIT researchers also calculated the rate at which NEAs collide with the moon to
produce craters. Lunar maria — dark areas that can be seen with the naked eye
—
have been accumulating craters from NEA impacts since their formation more than
3
billion years ago.

The rate of crater formation expected from the NEAs matches the observable
counts of
craters from the lunar maria. Because current NEA population matches the crater
counts
from the lunar maria, the NEA population has been roughly in equilibrium for
billions of
years. In other words, new asteroids are introduced into near-Earth space at
around the
same rate that old NEAs are destroyed by collisions with the planets and the
sun or are
ejected from the solar system.

Stuart, a member of the LINEAR project team, is an EAPS alumnus of MIT.

The LINEAR project is jointly sponsored by NASA and the U.S. Air Force.