[meteorite-list] Search for Large Asteroids Nears Completion, Experts Ponder Gaps in Program

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:07 2004
Message-ID: <200307211523.IAA20408_at_zagami.jpl.nasa.gov>

http://space.com/scienceastronomy/asteroid_report_030721.html

Search for Large Asteroids Nears Completion, Experts Ponder Gaps in Program
By Michael Paine
space.com
21 July 2003

SYDNEY, Australia -- A stated goal of finding 90 percent of all large Near
Earth Asteroids (NEAs) by 2008 is more or less on target, leading experts
said last week at the General Assembly of the International Astronomy Union
(IAU) in Sydney, Australia.

The goal, originally outlined by NASA and mandated by the U.S. Congress, is
designed to insure that space rocks in the vicinity of Earth's orbit, and
larger than 1 kilometer (0.62 miles), are found and tracked. An object of
this size could cause global destruction if one were to hit Earth.

An international affiliation of groups, collectively called Spaceguard,
carries out the search and follow-up observations needed to purse the
targets. NASA funds much of the work.

No asteroids are presently known to be heading toward the planet.

The global experts gathered here reiterated a common concern among asteroid
scientists: When the current goal is reached, there will still be many
unfound smaller objects that are easily capable of destroying a large city.

Yet present technology is somewhat limited in its ability to find the
thousands and perhaps millions of smaller asteroids that could potentially
threaten the planet.

The search for Earth-threatening asteroids was described by three
international experts at a public forum Thursday: David Morrison from NASA's
Ames Research Center; Alan Harris of the Space Science Institute in
Colorado, and Andrea Milani from the University Pisa, Italy.

Bigger, better telescopes are on the drawing board for finding these smaller
asteroids, the scientists said. Meanwhile, images of the sky taken by
current programs could be crucial to future attempts to find and track small
asteroids using the new telescopes. Determining an asteroids exact
trajectory and potential future danger requires observations across time, so
the object's movement can be plotted.

Importantly, there are no large telescopes devoted to the task of hunting
asteroids from the Southern Hemisphere. A major search program down under
would help in this task, the panelists said.

The forum, "The danger from space: Are Near Earth Objects a catastrophe
waiting to happen?" was moderated by Australian science celebrity Karl
Kruszelnicki.

NASA's Morrison described the consequences of an asteroid colliding with
Earth. The larger NEAs, bigger than a half mile (1 km), would cause global
environmental disaster, he said. Smaller objects are still a danger to
cities and regions and Spaceguard will eventually be expanded to cover these
smaller objects.

Harris, from the Space Science Institute, showed the progress made with
Spaceguard and described new telescopes that, within the next decade, could
be used for detecting and tracking NEAs. He cautioned, however, that these
new telescopes would have competing demands and most would be devoted to
Spaceguard on a part-time basis.

Harris said the lack of a major asteroid search program in the Southern
Hemisphere was limiting the pace of new discoveries.

A specialized Schmidt 1.2-meter (4-foot) telescope at Siding Spring in
Australia is under-utilized and could be upgraded to search for near-Earth
asteroids, he said. This would involve replacing a large photographic plate
holder with a modern CCD camera (like those in video cameras).

With new technology involving multiple CCDs, the Siding Spring telescope
could be better at searching for asteroids than any current Spaceguard
telescopes, Harris said. Australian astronomer Duncan Steel pointed this out
in a science paper written in 1995, shortly before the Australian Spaceguard
program was terminated as government funding was dropped.

Milani, from the University Pisa, described the computations necessary to
establish an asteroid's orbit around the Sun and to calculate whether it
would hit the Earth in the future.

He suggested that it was not very useful searching for new near-Earth
asteroids if the extra work was not also done to check whether there was a
risk of a collision. This usually involves alerting astronomers around the
world to conduct follow-up observations. Much of the asteroid tracking
effort involves amateurs who volunteer their time.

Asteroids are sometimes initially found to have remote risks of hitting
Earth. The additional observations are included in revised computations
that, so far, have resulted in an impact risk being downgraded to zero in
all cases that have been studied over time.

Milani and his colleagues recently developed computer programs to generate
numerous possible paths for a newly discovered asteroid. If any of these
"virtual impactors" is found, by computation, to be on a collision course,
then mathematicians can describe its predicted orbit and astronomers can
carefully check whether the actual asteroid is on the same path.

Just as important in this process is the checking of old sky images. The
mathematicians can run a predicted orbit back in time and old images can be
checked for signs of the object.

Harris explained that not finding an object in old images could be used to
eliminate the possibility an impact. The approach illustrates the importance
of archiving astronomical observations -- a blank area on a picture could be
just as important as a small white dot.

It is also a good reason to immediately start taking images of southern
skies, researchers say.

After the public forum a discussion ensued about ways to deflect an incoming
asteroid.

The work by Milani and others has demonstrated that, if an asteroid is
determined to be on a collision course, the most likely scenario is that
over several decades it will perform multiple close approaches to the Earth
before the impact. During these close encounters the asteroid must pass
through a window, or "keyhole" in space if it is to continue on its
collision course.

In these cases it only takes a deflection of a few hundred kilometers
(miles) to force a rock to miss the keyhole and therefore avoid Earth impact
in a subsequent orbit.

This compares with an object heading directly for an impact that must be
deflected by thousands of kilometres (miles) so that it misses the Earth.
Because they pass close to the planet on several occasions before impact, it
should be possible to detect quite small keyhole asteroids with current
telescope technology -- but only if scientists are effectively looking for
them.

A keyhole deflection is only possible if the threatening asteroid can be
detected decades ahead of the impact and if sufficient sightings have been
made to pin down its orbit. This was cited as another good reason for
stepping up the Spaceguard effort and adding a southern sky search.

On July 25, the Australian Minor Planet Workshop will take up discussion of
how to jump-start asteroid searches in Australia and New Zealand.
Received on Mon 21 Jul 2003 11:23:18 AM PDT