[meteorite-list] Asteroid Risk to Earth Lowered (Asteroid 99942 Apophis)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri May 19 11:59:07 2006
Message-ID: <200605191557.IAA05531_at_zagami.jpl.nasa.gov>

http://www.msnbc.msn.com/id/12859900/

Asteroid risk to Earth lowered, scientists say

Chances of impact move 'in right direction,' toward odds of 1-in-24,000
By Alan Boyle
MSNBC
May 18, 2006

After a fresh round of radar observations, astronomers said Thursday
that the chances of a catastrophic asteroid impact in the year 2036 are
lower than previously thought - and they're hoping the threat will be
completely ruled out once more readings are made.

The chances of collision with the asteroid Apophis in 2036 now stand at
1 in 24,000, said Steve Chesley, an expert on near-Earth objects at
NASA's Jet Propulsion Laboratory. That's a significant advance from the
previous orbital predictions, which set the odds of impact at 1 out of
about 6,000.

Chesley told MSNBC.com that there's still a slim chance of catastrophe,
due to the remaining uncertainties about Apophis' orbital path, but
"things are moving in the right direction."

Apophis, also known as 2004 MN4, was detected in 2004 and has been a
source of concern since late that year. Astronomers
estimate its diameter at about 1,300 feet (400 meters) - which means a
collision would spark a regional catastrophe, though not a global
extinction event on the scale of the dinosaurs' demise.

A strike in the Pacific Ocean would set off a "cosmic Katrina"
capable of doing an estimated $400 billion worth of
damage, according to the B612 Foundation. For that reason, astronomers
are keen to reduce the projected risk to zero if they can- and devise a
mission to divert the space rock if they can't.

Apophis is a special case because it spends most of its time within
Earth's orbit, making observations difficult. The May 6-7 window for
radar observations represented the best opportunity until the end of
2012. Jon Giorgini, a senior analyst for the Solar Systems Dynamics
Group at JPL, said the Arecibo team got a good fix on Apophis.

"We were able to improve our understanding of its motion by about 6
millimeters per second," Giorgini.

Those readings underwent analysis for more than a week, and on Thursday,
the researchers felt confident enough to share the results.

Will it hit the 'keyhole'?

The key calculations actually have to do with an encounter with Earth in
the year 2029. If Apophis passes through a well-defined area of space no
wider than a half-mile (1 kilometer), known as a "keyhole," the
gravitational deflection will put the rock on a course to hit Earth
seven years later. "You go through that little hole, and you're going to
hit the earth," Chesley explained.

The uncertainty has to do with whether Apophis' path will take it
through the keyhole in 2029. Although this month's observations reduced
its chances of hitting the keyhole, there's still a margin of error that
needs to be accounted for.

Asteroid-watchers have emphasized that there's nothing to worry about
yet. In fact, Apophis appears to be following the pattern set by
previous asteroid alerts, Chesley said.

"We've seen this happen very often," he said, "where [the perceived
risk] rises to some level and starts to steady, then drops off. ... It
goes up, and then it goes down. We're very much on a steady downward
trend."

More chances for observations

Previously, NASA had said that no more observations could be made until
2013. But on Thursday, Chesley said more may be known before then.
"Actually, we're going to probably get some observations, though they'll
be difficult to obtain - maybe by next spring," he said. Astronomers at
the Mauna Kea Observatory in Hawaii could spot the asteroid in 2007 as
well as in 2011, he said.

In the 2012-2013 time frame, Arecibo could come into play again.
Giorgini said those observations would be even more accurate because
Apophis would come within 8.3 million miles (13 million kilometers) of
Earth - much closer than this month's pass at a distance of 26 million
miles (42 million kilometers).

If Apophis is still perceived to pose a threat after the 2013
observations, NASA has said it would draw up plans for
missions to monitor the asteroid even more closely and divert it if
necessary.

Asteroid alerts have been on the rise over the past decade, due to the
greatly improved capability for detection of near-Earth objects. Chesley
said "we're likely to see more" alerts as NASA and other agencies watch
the skies more vigilantly.

"But I have to say my sense is that Apophis is absolutely extraordinary
- a once-in-a-lifetime event," Chesley added. "I hope we don't have any
more cases like this one. This is going to be memorable."
Received on Fri 19 May 2006 11:57:02 AM PDT


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