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: