Future Missions May Have A Major Impact

SPACEGUARD

Detecting Planet Killers as a Sideline

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.

Paris (ESA) March 27, 2001
A 100 metre-wide space rock known as 2001 EC16 paid a passing visit to Earth's vicinity last Friday. 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 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.

SPACEGUARD

The Comet/Asteroid Impact Hazard: A Systems Approach

Boulder - February 24, 2001
The threat of impact on Earth of an asteroid or comet, while of very low probability, has the potential to create public panic and -- should an impact happen -- be sufficiently destructive (perhaps on a global scale) that an integrated approach to the science, technology, and public policy aspects of the impact hazard is warranted.

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