[meteorite-list] Caveat Impactor

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:11 2004
Message-ID: <200207262115.OAA27048_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2002/26jul_nt7.htm

Caveat Impactor
NASA Science News

An asteroid with almost no chance of hitting Earth made big headlines this
week.

July 26, 2002: I slid a dollar bill across the counter, and the cashier
handed back a lottery ticket. The odds for winning: 1-in-250,000. A long
shot, but you never know.

Walking out of the store, ticket in hand, I glance at a newspaper. "Tony
Phillips wins the lottery!" the headline declared. Gosh, I thought, that
seems premature ... not to mention weird.

Indeed, it's fiction. For one thing, I never buy lottery tickets. But
mainly, no one would write such a headline based on such slender odds.

Yet that's what happened this week, in real life, to an asteroid.

On July 9, 2002, MIT astronomers discovered 2002 NT7, a 2 km-wide space rock
in a curious orbit. Unlike most asteroids, which circle the Sun in the plane
of the planets, 2002 NT7 follows a path that is tilted 42 degrees. It spends
most of its time far above or below the rest of the solar system. Every 2.29
years, however, the asteroid plunges through the inner solar system not far
from Earth's orbit.

After a week of follow-up observations, researchers did some calculations.
There was a chance, they concluded, that 2002 NT7 might hit our planet on
February 1, 2019. The odds of impact: 1-in-250,000.

"Space Rock 'on Collision Course'," a headline declared days later.
"Asteroid Could Wipe Out a Continent in 2019," another one warned. Really.

"In fact," says Don Yeomans, the manager of NASA's Near-Earth Object Program
at JPL, "the threat is minimal. One-in-250,000 is a very small number."

The odds are not only low, but also uncertain. Yeomans explains: "We've been
tracking 2002 NT7 for a very short time--only 17 days so far," Meanwhile,
the asteroid takes 2.29 years to orbit the Sun. Predictions based on such a
small fraction of an orbit are seldom trustworthy.

It's becoming a familiar routine: Astronomers discover a near-Earth
asteroid. With only meager data at hand, they can't rule out a collision in
the distant future. Headlines trumpet the danger. Finally, the alarm
subsides when more data lead to a better orbit--one that rules out an
impact.

"As far as the public is concerned," says Jon Giorgini of JPL's Solar System
Dynamics Group, "it just isn't worth getting worked up about an object with
a couple weeks of data showing a possible Earth encounter many years from
now. Additional measurements will shrink the uncertainty by a large
amount--and Earth will (almost certainly) fall out of the risk zone."

Already this is happening for 2002 NT7. The calculated probability of a
collision with Earth is shrinking as astronomers add new data each day. "I
suspect it will take only a few more weeks (or maybe months) to completely
rule out an impact in 2019," says Yeomans.

Giorgini explains further: "When we calculate an asteroid's position (based
on measurements made at a telescope), the result isn't a single point in
space. Instead, it's a volume of space where the asteroid could be with some
probability. We deal with probabilities, not absolute answers, because the
measurements contain errors." For example, optical data can be corrupted by
twinkling and refraction in Earth's atmosphere. (Radar is better, notes
Giorgini, but no radar data have yet been obtained for 2002 NT7.)

"When you project this initial probability region years into the future, it
naturally expands. For a newly discovered object with only a few days
tracking, the uncertainty region can easily grow to cover a big part of the
inner solar system. Because Earth is in the inner solar system, and can
potentially cut through this volume of smeared out probability, we end up
with finite impact probabilities."

"A finite probability, however, is not really a prediction of
impact," he cautions, "but a statement that one is possible." Of
course, many things are possible. Like the newspaper headline "Tony Phillips
wins the Lottery!" But most of them do not happen.

JPL lists asteroids like 2002 NT7 on their Internet "risk page" not to raise
an alarm, says Yeomans, but to alert astronomers when new discoveries merit
attention. "It's important that we continue tracking these asteroids to
refine their orbits," he says. The more observers, the better.

What's an ordinary person to do?

The next time you see a headline "Killer asteroid threatens Earth!" ask
yourself two questions: Have we known about this space rock for more than a
week or so? (If not, check again in a month. It probably won't be considered
a killer then.) And what are the odds of impact?

If you're more likely to win the lottery, there's probably nothing to worry
about.

Editor's note: Big asteroids have hit Earth before and it's only a matter of
time before one threatens us again. Will it be years, decades, millions of
years? No one knows. The point of this article is not that we are safe from
asteroid strikes. We are not safe. Rather, we hope to give readers some of
the information they might need to evaluate popular reports of impending
collisions.
Received on Fri 26 Jul 2002 05:15:00 PM PDT