[meteorite-list] European Space Agency To Probe Asteroid Blind Spot

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:26 2004
Message-ID: <200204140531.WAA06912_at_zagami.jpl.nasa.gov>

http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=29787

European Space Agency to probe asteroid blind spot
European Space Agency
April 12, 2002

In the past five weeks two asteroids have passed close by Earth,
at distances of 1.2 and 3 times the distance to the Moon. Another asteroid
has recently been shown to be on course for a collision with Earth in 2880.
Monitoring known asteroids allows astronomers to predict which may collide
with Earth. But that is only true for the asteroids we know of. What about
those that lie in the asteroid blind spot between the Sun and Earth? The
European Space Agency is studying ways in which its missions can assist in
monitoring these unseen but potentially hazardous asteroids.

It is difficult to estimate the danger posed by asteroids. This is, in part,
because astronomers do not yet know how many asteroids there are. A recent
discovery, made using data from ESA's Infrared Space Observatory (ISO),
showed that there could be nearly two million asteroids larger than one
kilometre in the main asteroid belt, between Mars and Jupiter. That is more
than twice as many as previously thought.

In addition, even when an asteroid is identified many observations must be
made before it is known whether or not it will come close to, or even
collide with, Earth.

If the asteroids remained in the main-belt, they would pose no danger to
Earth. However, they can be thrown into different orbits by collisions with
other asteroids or by the influence of Jupiter's gravitational field. If
their new orbits cross the Earth's orbit, they could one day collide with
our planet, inflicting unprecedented devastation.

A number of ground-based searches are already underway to find as many
potentially hazardous asteroids (PHAs) as possible but there is a notorious
'blind spot' that telescopes on Earth can never peer into. It is the region
of space inside Earth's orbit, towards the Sun. From Earth, astronomical
observations close to the Sun are almost impossible because it means
observing during the daytime when only the brightest celestial objects stand
out from the blue sky. That means asteroids lurking in this region of space
can 'sneak up' on the Earth undetected. Asteroid 2002 EM7, which passed
close by the Earth on 8 March this year, was one such object and was only
detected after it crossed Earth's orbit to appear briefly in the night sky,
before it crossed back into the glare of the Sun.

About 550 similar asteroids are known. They are called the Atens and spend
most of their time inside Earth's orbit, close to the Sun. Traditional
estimates suggest there may be several thousand in total and tracking them
from Earth is next to impossible. However, a study performed for ESA has
shown that the Gaia spacecraft will be able to see clearly into this 'blind
spot' and keep precise track of the Aten population.

François Mignard of Observatoire de la Côtes d'Azur, France, conducted the
study. He found that Gaia would be ideal because it is designed to measure
the position of celestial objects with unprecedented accuracy. In addition,
since there is no atmosphere in space to scatter the Sun's rays and create a
blinding blue sky, Gaia can see close to the Sun without disturbance.

Gaia is expected to be launched around 2010. Even if ground-based searches
have spotted more Atens by that time, the mission still has an essential
role to play because it will reveal their orbits to a precision 30 times
better than any observation from the ground, thus identifying whether any
pose a danger to Earth.

"To know how close these objects will come to Earth is very dependent on how
accurately one can measure their orbits. That's the main contribution that
Gaia can be expected to make," says Michael Perryman, project scientist for
Gaia, at ESA's European Space Research and Technology Centre in the
Netherlands.

Gaia's data will also provide astronomers with a first estimate of these
objects' composition. This knowledge could help to determine methods to
divert or destroy asteroids that are set on a collision course with Earth.

Several ESA missions are contributing, or will contribute, to our
understanding of minor bodies of the Solar System: these include ISO, Gaia
and Rosetta, which will study asteroids Siwa and Otawara. ESA is also
considering the addition of an asteroid spotting telescope to its
BepiColombo mission.
Received on Sun 14 Apr 2002 01:31:49 AM PDT