From: firstname.lastname@example.org [mailto:email@example.com]
Sent: Monday March 26, 2001 11:59 AM
Subject: Future Missions May Have A Major Impact
Future missions may have a major impact
ESA Science News
March 23, 2001
A 100 metre-wide space rock known as 2001 EC16 paid a passing visit to
Earth's vicinity earlier today. As it swept by at a little over 1.7 million
km from Earth - approximately four and a half lunar distances - the only
people to pay it much attention were a dedicated band of astronomers.
However, this will not always be the case. Although there was no danger of a
collision between the Earth and 2001 EC16, the day will surely come when
luck runs out for our world (and humanity). Our only chance of survival is
to detect the space invader long before a head-on collision occurs. This is
where two of ESA's future missions - GAIA and BepiColombo - may be able to
play a vital role in forewarning us of impending impacts.
GAIA will be ESA's successor to the extremely successful Hipparcos
astrometry mission. As well as measuring the positions and brightness of
stars and galaxies with unprecedented precision, GAIA will be able to detect
all kinds of transient objects in its field of view - supernovae, flaring
stars and ... asteroids.
"GAIA will detect objects brighter than magnitude 20, so we should observe
about one billion objects over the whole sky," said Project Scientist
Michael Perryman. "This means that if there is something there, we will see
it. In the case of near-Earth asteroids (NEOs), we should find objects as
small as 500 metres in diameter."
"The precision optics on GAIA will also give a colossal improvement in
orbital measurements, allowing astronomers to make very precise, long-term
orbit determinations," he added. "This will allow them to work out whether
asteroids measured by GAIA will eventually collide with Earth."
"Although most of the large near-Earth objects will probably have been found
by the time of GAIA's launch around 2010," he explained, "the NEO catalogue,
compiled from ground observations, will not contain all of the class of NEOs
called Atens - asteroids that spend much of their time inside Earth's orbit.
GAIA will be able to observe fairly close to the Sun, so it will carry out a
reasonably comprehensive survey of these objects."
Even GAIA will not be able to detect the entire population of asteroids that
cross Earth's orbit and disappear in the glare of the Sun. However, another
ESA mission should make a significant contribution to the ongoing census of
potentially hazardous asteroids.
Although BepiColombo's prime objective is to explore Mercury, it will also
be able to search for unknown asteroids in the uncharted region of space
between the planet nearest to the Sun and our Earth. These NEOs are
particularly dangerous, since they can approach the Earth unseen against the
brilliance of the Sun.
"The BepiColombo mission will involve two orbiters and a lander," explained
Mission Scientist Rejean Grard. "One of the orbiters will be used for
planet-wide remote sensing and radio science on a polar orbit with periapsis
and apoapsis altitudes of 400 and 1500 km. However we are also planning to
have a telescope on this orbiter."
"This telescope would be able to monitor a strip of sky of 6 degrees by 360
degrees as it looks along the orbiter's direction of motion," he added. "By
detecting asteroids that cross this field of view and comparing measurements
made at different times, we could determine their orbits. Preliminary
evaluations indicate that there could be up to 100 objects in the selected
strip of sky at any one time."
The Cosmic Shooting Gallery.
2001 EC16 belongs to a growing family of space rocks larger than 100 meters
across that can come closer to Earth than 0.05 AU (7.5 million km).
Fortunately, none of the known NEOs are presently on a collision course with
our planet, although astronomers are finding new ones all the time. At the
present time, 291 known potentially hazardous asteroids have been detected.
How many NEOs are out there? No-one knows. Astronomers estimate that there
are between 750 and 1100 near-Earth asteroids bigger than 1 kilometre in
diameter. There are probably millions of smaller objects in orbits that
carry them close to Earth.
Why does it matter? The amount of damage caused by an asteroid impact
depends on its size. Asteroids bigger than 1 kilometre would release energy
equivalent to 100 000 megatonnes of TNT - equivalent to 10 million times the
power of the atomic bomb that flattened Hiroshima. The result would be
devastation on a global scale.
The good news is that such events happen on average only once every 300 000
years. The bad news is that collisions with medium-sized objects are much
more frequent - once in a few thousand years on average.
Even objects the size of 2001 EC16 pack a significant punch. In 1908, an
asteroid or comet just 50 metres across blew up in the atmosphere above
Siberia. If this explosion had occurred over central London, the entire city
would have been flattened.