[meteorite-list] Scientists Ponder Asteroid Scare Guidelines

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:28 2004
Message-ID: <200402261627.IAA20047_at_zagami.jpl.nasa.gov>

http://www.pasadenastarnews.com/Stories/0,1413,206~22097~1979762

Scientists ponder asteroid scare guidelines

Earth collision incorrectly calculated recently

By Kenneth Chang
New York Times
February 25, 2004

For nine hours last month, a small band of astronomers got an unexpected
scare. Their calculations indicated that a newly discovered asteroid was on
a collision course with Earth, and that it had a one-in-four chance of
striking the planet, somewhere in the Northern Hemisphere, in less than two
days' time.

It did not happen, of course. Further observations in the wee hours of Jan.
14 put the asteroid in a completely different orbit, and no threat to Earth.

But the episode, the latest in a series of false alarms, pointed up the
disquieting prospect of an asteroid's showing up suddenly on Earth's
doorstep with no time for Hollywood heroics. No guidelines exist for who
should have been informed and when and what emergency measures should have
been taken, if the threat had been real.

At least one planetary scientist said he was ready to recommend a public
warning. "I would not have been comfortable with being quiet through the
next morning,' said the scientist, Clark R. Chapman of the Southwest
Research Institute in Boulder Colo., who was involved in discussions of the
meteor that night. "I think the public should be informed of that high a
probability of that big an event occurring.'

Chapman presented a paper on Monday at the Planetary Defense Conference in
Garden Grove, Calif., recapping the sequence of events in the evening of
Jan. 13 through the morning of Jan. 14.

The asteroid, now designated 2004 AS1, was not a planet killer, like the one
that caused the extinction of the dinosaurs 65 million years ago, or even a
city killer. Believed at the time to be about 100 feet across, 2004 AS1
would most likely have exploded with the force of a one- megaton bomb
several miles up in the atmosphere. The shock waves would have set off
hurricane-force winds that could have damaged buildings below.

"It's right at that boundary line, so we don't know how much damage it would
have done,' Chapman said. An object half the size would explode harmlessly.
One twice as wide would be catastrophic, he said.

Donald Yeomans, head of NASA's Near Earth Objects program office, said he
would not have raised an alarm until a second set of observations confirmed
the collision path. He said he hoped the episode would prompt guidelines for
how future warnings should be handled. "Hopefully, policy-makers will take
the ball and run with it,' he said.

For several years, NASA has spent $3.5 million a year on surveys to locate
and map the asteroids zooming through Earth's neighborhood, but the program,
called Spaceguard, focuses on the larger asteroids, and the presumption that
a potential impact would be years away.

"The Spaceguard Survey is not designed to detect objects on their final
approach to Earth,' said David Morrison, a NASA space scientist.

Last month's episode put a spotlight on dedicated but largely ad hoc network
of professional and amateur astronomers. On Jan. 13, the Minor Planet Center
of the Harvard-Smithsonian Center for Astrophysics received the latest
observations from Linear, short for Lincoln Near-Earth Asteroid Research, an
automated telescope survey in New Mexico that is searching for new
asteroids.

Timothy Spahr, an astronomer at the Minor Planet Center, sifted through the
information, finding several interesting, and ran the computer programs that
calculated possible orbits for the new asteroids and posted them on a Web
site so that other astronomers could confirm the locations.

Spahr then went out to dinner with a visiting colleague from Hawaii. He did
not notice that the orbit that he had calculated for one of the new
asteroids went straight through the Earth.

A short time later, an amateur astronomer in Germany noticed on the Minor
Planet Center Web site the odd prediction that 2004 AS1 was to brighten by a
factor of 40 in the next day, a indication like oncoming headlights that the
object was on the express route to Earth. Soon, both amateur and
professional astronomers were buzzing about this object on an Internet
bulletin board devoted to asteroids.

Steven R. Chesley, a senior engineer working in the Near- Earth Objects
office at NASA's Jet Propulsion Laboratory in Pasadena, Calif., called the
Minor Planet Center asking for more details about the asteroid. Spahr was
still out for dinner. Brian Marsden, director of the center, was working
late and sent the details of the Linear observations four images taken over
a one-hour period to Chesley.

The pictures of the asteroid show only its position in the sky; how far away
it is, whether 1 million miles or 50 million, is necessarily a guess based
on sparse data. Marsden then changed the information on the center's Web
site, now saying that the asteroid was actually moving away from Earth an
orbit that still fit the observations.

Spahr did not learn of the worries until he arrived home at 10:30 p.m. "We
never ever imagined that would happen,' he said. "I actually felt pretty
silly.'

He and Chesley exchanged information and calculated. Chesley's calculations
of possible orbits put the chances of impact at somewhere from 10 to 40
percent.

What the astronomers needed was a new spotting of the asteroid to see how
much it had moved since the Linear observations. But with skies cloudy over
most of Europe and North America, asteroid experts scrambled to find amateur
sky watchers with a clear view.

Spahr determined where in the sky the asteroid would be if it were indeed on
the collision course. At 3:30 a.m. Eastern time, an amateur astronomer in
Colorado looked in that swath of the sky and saw nothing. The next night,
Linear spotted 2004 AS1 again, showing that the asteroid was much farther
out and not on a collision course at all. (Because it is farther away, its
size is now estimated to be 1,600 feet wide.)

The errors in the data from the previous night had been somewhat larger than
usual, and that slight deviation in location had led Spahr's software to put
the asteroid on the alarming path.

Subsequent observations showed that 2004 AS1, which came no closer than 7
million miles to Earth, is in no danger of hitting Earth in the next
century.

The Minor Planet Center has updated its software so that it will
automatically warn of trajectories that cross Earth's path.

Since 1998, astronomers have put out several warnings that asteroids may be
on a collision course with Earth, though in each case additional
observations have shown that the asteroids were going to miss. Each time,
astronomers have refined their procedures seeking to cut down their false
alarms.

Marsden is critical of Chapman's suggestion that a public announcement
should have been considered with only preliminary data. But Chapman said: "I
think notifying the public is the appropriate thing to do. I would opt for
us being burnt for crying wolf again in that case.'
Received on Thu 26 Feb 2004 11:27:29 AM PST