ESA Studies Missions to Safeguard the Earth

Early on the morning of 30 June 1908, the vast forest of western Siberia
was illuminated by a strange apparition: an alien object streaking across
the cloudless sky. White hot from its headlong plunge into the Earth’s
atmosphere, the intruder exploded about 8 km above the ground, flattening
trees over an area of 2000 square kilometres.

Despite the huge detonation, equivalent to a 10 megaton nuclear warhead
(about 500 times the energy of the Hiroshima atomic bomb), there were few
if any casualties in the sparsely populated taiga. If the Tunguska object
– probably an asteroid about twice the size of a tennis court – had
exploded over London or Paris, the list of casualties would have run into
millions.

Fortunately, cataclysmic events caused by incoming near-earth objects
(NEOs) are few and far between. Current estimates suggest that a 50 metre
Tunguska-like object is likely to collide with the Earth once every
100-300 years. A 1 km object, which typically arrives every few hundred
thousand years, could wipe out an entire country. An impact in the ocean
would be no better, generating enormous waves (known as tsunamis) that
would devastate coastal areas thousands of kilometres away.

An increasing awareness of the potentially disastrous consequences of such
impacts has driven recent efforts to detect and categorise the larger
Earth-threatening objects. However, much more needs to be done if the
millions of Tunguska-like objects are to be found and catalogued. Only
then can advance warning of pending impacts be provided and measures be
taken to reduce the threat.

Despite the introduction of increasingly sophisticated search programmes
in various parts of the world, the search for objects heading our way
needs to expand into space. Only space-based observatories can provide
the all-sky coverage required and detect Earth-crossing objects that would
normally be hidden in the glare of the Sun.

In July 2002 the general studies programme of the European Space Agency
(ESA) provided funding for preliminary studies on six space missions that
could make significant contributions to our knowledge of NEOs.

“The six proposals were selected because the mission concepts would help
to answer essential questions on the NEO threat, such as how many there
are, their size and mass, and whether they are compact bodies or loose
rock aggregates,” said Andrés Gálvez, head of the Advanced Concepts Team
at ESA’s European Space Research Technology Centre (ESTEC) in the
Netherlands.

“This information, as well as other data, is needed before appropriate
mitigation procedures can be developed,” he said.

“There are two broad categories. The observatory missions are able to
detect and track many more NEOs than can be seen from the ground. This
enables astronomers to calculate their orbits and predict whether they
will offer a threat to the Earth far into the future.”

“The flyby/rendezvous missions are designed to look at a small number of
NEOs in great detail, sending back information on their size, composition,
density, internal structure and so on. This is important because we need
to know as much as possible about how they will behave if we try to divert
them from a collision course with Earth.”

The six missions under study were: