Applying unprecedented refinements to the analysis of
celestial hazards, NASA astronomers have identified a
potential close encounter with Earth more than eight centuries
in the future by an asteroid two-thirds of a mile (one
kilometer) wide.

What will most likely be a miss, even without preventive
measures, will come on March 16, 2880, said Jon Giorgini, a
senior engineer at NASA’s Jet Propulsion Laboratory, Pasadena,
Calif. Odds for a collision are at most one in 300, and
probably even more remote, based on what is known about the
asteroid so far. Still, that makes this space rock, named
1950 DA, a greater hazard than any other known asteroid.

“This is not something to worry about,” said Giorgini,
leader of a team reporting about the asteroid in the April 5
edition of the journal Science. “We’re showing that searches
with optical telescopes and follow-up observations with radar
telescopes can provide us centuries of advance notice about
potential close encounters of asteroids with Earth. That’s
plenty of time to consider the options — 35 generations, in
fact.”

“This report is a success story for our efforts to
identify potential troublemakers,” said JPL’s Dr. Don Yeomans,
manager of the NASA Near Earth Object Program. “Radar
observations are helping us push predictions 5 to 10 times
further into the future.”

This report differs from previous ones about other
asteroids’ Earth-impact potential. Estimates of impact risks
in earlier cases came from a few nights’ optical observations
of newly found asteroids. Astronomers soon ruled out the
possible impacts after a few more observations narrowed
uncertainties about the asteroids’ orbits. The current orbit
of 1950 DA has been mapped with great accuracy using precise
radar data and a 51-year span of optical data. Uncertainty
about how close it will come to Earth in 2880 stems from gaps
in knowing physical details of the asteroid that could subtly
alter its course over the centuries.

“How close 1950 DA will approach Earth turns out to
depend on the asteroid’s physical attributes — it’s size,
shape and mass, and how it spins, reflects light and radiates
heat into space,” Giorgini said. These things are unlikely to
be known any time soon. The way the asteroid radiates energy
absorbed from the Sun back into space has the biggest
potential effect, he said. Releasing heat in one direction
nudges the asteroid in the opposite direction. The resulting
acceleration is tiny, but over the centuries acts like a weak
rocket and could make the difference between a hit and a miss.

Asteroid 1950 DA was discovered from Lick Observatory,
Mount Hamilton, Calif., in 1950. It faded from view for five
decades then was found from Lowell Observatory in Arizona in
2000. Astronomers used large dish antennas of NASA’s Deep
Space Network site at Goldstone, Calif., and the Arecibo
Observatory in Puerto Rico to examine the asteroid with radar
when it passed at a distance 21 times farther away than the
Moon in March 2001.

“Once an asteroid is discovered, radar is the most
powerful way to find its exact orbit and, apart from sending a
spacecraft, the only way to see what it looks like,” said
JPL’s Dr. Steve Ostro, who led the radar observations of 1950
DA.

Giorgini refined calculations of future orbits by
including factors such as the push from sunshine and the
potential gravitational tug from 7,000 other asteroids and
nearby stars. Effects of each small influence on the
asteroid’s movement could be amplified by 15 gravitational
tugs during close approaches to Earth and Mars — none of
which have any chance of an impact — prior to 2880. “It’s
like predicting a 15-bank shot in a pool game,” Giorgini said.
“We know the cue stroke extremely well because it is right now
and we can measure it. But at each future bank, small
variations accumulate and change the next bounce, which
changes the following one and so on. What we’ve done is find
the range of changes possible due to tilt, imperfections and
fuzz on the table, the bounce of the cushions, and wind
blowing across the room. We need to know more about the ‘cue
ball’ to really be sure of how the last three banks in 2809,
2840 and 2860 will line things up for 2880.”

If future generations’ studies of 1950 DA indicate it
ought to be diverted to prevent a collision, the subtle
influences that its physical properties have on its motion
might be manipulated to advantage. For example, Giorgini
suggested, its surface could be dusted with chalk or charcoal
to alter the way it reflects light, or a spacecraft propelled
with a solar sail could collapse its reflective sail around
the asteroid. In any event, determining asteroids’ physical
properties will be important for long-term calculations of
impact hazards.

In addition to Giorgini, Ostro and Yeomans, authors of
the report include Dr. Lance Benner, Dr. Paul Chodas, Dr.
Steven Chesley, Dr. Myles Standish, Dr. Ray Jurgens, Randy
Rose and Dr. Alan Chamberlin, all of JPL; Dr. Scott Hudson,
Washington State University, Pullman; Dr. Michael Nolan,
Arecibo Observatory; Dr. Arnold Klemola, Lick Observatory; and
Dr. Jean-Luc Margot, California Institute of Technology,
Pasadena.

Images are available at http://neo.jpl.nasa.gov and
http://www.jpl.nasa.gov

A videofile will be available on NASA Television April 4
and 5 during the NASA TV feed scheduled for noon, 3 p.m., 6
p.m., 9 p.m., and midnight EST. NASA TV is broadcast on GE-2,
transponder 9C, C-Band, located at 85 degrees West longitude.
The frequency is 3880.0 MHz. Polarization is vertical and
audio is monaural at 6.8 MHz. For NASA TV schedule information
see http://www.nasa.gov/ntv/ .

Arecibo Observatory is operated by the National Astronomy
and Ionosphere Center at Cornell University, Ithaca, N.Y.,
under an agreement with the National Science Foundation.
NASA’s Office of Space Science, Washington, D.C supported the
radar observations. JPL is managed for NASA by the California
Institute of Technology.