An artist's concept of the ISHTAR mission. Credits: ESA/image of Ida courtesy NASA

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 Andres Galvez, 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:

• Don Quijote: This proposal involves the launch of two spacecraft to test technologies required to deflect an asteroid heading towards Earth. The Hidalgo spacecraft will be targeted to impact a 500-metre-diameter asteroid at a relative speed of 10 km/s. Its companion, known as Sancho will deliver a number of sensors to the surface of the asteroid and observe from a safe distance what happens during and after the high speed collision. This will provide valuable information on the NEO's internal structure.
• Earthguard 1: A proposal to mount a 'hitchhiker' telescope on a spacecraft en route to the inner Solar System, e.g. ESA's BepiColombo Mercury orbiter. The telescope would detect Earth-crossing asteroids larger than about 100 metres, which are very difficult or impossible to detect with ground-based telescopes.
• EUNEOS: A medium-sized telescope mounted on a dedicated spacecraft platform that would search for the most dangerous NEOs from inside the orbit of Venus. Its main goal is to detect 80% of the potentially hazardous objects down to a few hundreds of metres in size. It is estimated that this could be attained in 5 years. By systematic re-detection of the objects, their orbits would then be determined with high accuracy.
• ISHTAR: In addition to measuring the mass, density and surface properties of an NEO, this spacecraft would probe the interior of an NEO in order to study its structure and internal strength. This would be done using radar tomography, a new technology that uses ground-penetrating radar to make images of the interior of a solid body.
• SIMONE: A fleet of five low-cost microsatellites that would each fly by and/or rendezvous with a different type of NEO. Each spacecraft would carry a suite of scientific instruments that would provide valuable insights into the nature of large asteroids (400 -- 1 000 metres in diameter) with different physical and compositional properties. Low-thrust ion propulsion would be used to rendezvous with each target.
• Remote observation of NEOs from Space: A space-based observatory to carry out remote sensing and detect physical characteristics of NEOs, such as size, composition and surface properties.

"We now have a number of excellent proposals that are both feasible and affordable," said Franco Ongaro, head of ESA's Advanced Concepts & Studies Office.

"These phase A studies by industry and academia, which were completed in January 2003, provide a valuable framework for developing future missions. They will now be discussed within the Agency and with ESA's international partners in order to determine how best to proceed."