Tales of asteroids or comets in a collision course with the Earth are always good to fill space in newspapers during the quiet summer months. Is there any truth in these tales of impending doom and if so, what is ESA doing about it?

Certainly if a large comet or asteroid were to collide with the Earth the result could be apocalyptic. But, the possibilities of this happening are remote. The latest asteroid scare story to hit the press was that of 2002NT7, believed to be about 2 km in diameter. First reports said that 2002NT7 could collide with the Earth on 1 February 2019 at a speed of 28 km/s. The result would be widespread devastation, if not the end of the world, predicted in some papers.

Fortunately for those of us who will be around in 2019 the possibility of this happening is now reported to be negligible. Although first estimates gave odds of about 1 in 100,000, now that scientists have had more time to study this 'new' asteroid and its orbit - only discovered in July - the odds have lengthened considerably.

Asteroids and comets whose orbits bring them close to the Earth are referred to as Near Earth Objects (NEOs). Those less than 50 m in size burn up on entry into the Earth's atmosphere. It is estimated that around 50 000 fragments of NEOs fall to Earth as meteorites each year, most far too small to do any damage.

Fortunately, the larger the NEO the less likely it is to collide with the Earth, as even a NEO 300 m in diameter could wipe out an entire country if it hit land, and cause even more damage if it struck an ocean, as it would trigger the enormous waves known as tsunamis that could devastate many coastal cities.

According to astronomers, a NEO of around 50 m in diameter collides with the Earth every 100 to 300 years and a NEO with a diameter of 1 km occurs every few hundred thousand years. Very large collisions, that could threaten the existence of all large land species, occur once every hundred million years. To date, astronomers have identified 600 NEOs bigger than 1 km but believe that there are possibly half as many again still waiting to be tracked.

But, even if the odds are on our side, the fact remains that at some time or other the Earth will again be hit by a large NEO, such as that thought to have wiped out the dinosaur population 65 million years ago. So to return to the second question, what is ESA doing about it?

ESA and NEOs
ESA has been supporting activities to monitor and investigate NEOs for a number of years. In ESA's view, these are activities that surpass national boundaries and which it considers to be a service to the international community. ESA's space research institute outside Rome in Italy, ESRIN, also hosts the Spaceguard Central Node. This private non-profit scientific organisation aims to support and coordinate NEO research throughout the world.

Now ESA has launched a new project to seek the best ideas from industry and academia on how to protect the Earth from NEOs and in particular, to learn more about them. The damage an NEO causes depends on the speed at which it hits the Earth, its size and what it is made of. So, the more we know about them the easier it will be to decide which are potentially dangerous and what is the best action to take.

In June a panel of NEO experts met to select the best six proposals. Andres Galvez, one of ESA's representatives on the panel reports, "the six winning proposals were selected because the mission concepts would help to answer essential questions on the NEO threat such as: how many are there, what is their size and mass, are they compact bodies or loose rock aggregates?

This information, as well as other data, is needed before adequate mitigation procedures can be developed."

The winning six are:

• Don Quijote: This proposal is for a spacecraft 'named Hidalgo' to hit a target asteroid at high speed while the other, 'Sancho', observes what happens from a safe distance before, during and after the impact, to gather information on the NEO's internal structure. This will also test possible future mitigation techniques, such as whether 'Hidalgo' could be programmed to hit the asteroid to change its orbit so that it avoids collision with the Earth.
  
  
• Earthguard 1: A spacecraft using propulsion technology such as solar sails or electric propulsion, or 'hitching a ride' on a future launch, would be placed in a heliocentric orbit to observe NEOs from a more favourable viewpoint.
  
  
• ISHTAR: This would probe the interior of an NEO to study its structure and assess the danger with radar tomography, a new technology that uses ground penetrating radar to make images of the interior of a solid body.
  
  
• SIMONE: A fleet of low-cost small satellites would fly by and/or rendezvous with a number of NEOs to characterise the population and obtain first hand information on the hazardous objects.
  
  
• EUNEOS: A space survey would be undertaken from an inner solar system orbit to find the most dangerous NEOs. These are often the most difficult to observe from ground-based observatories as very often these faint objects only appear in the daytime sky or very close to the horizon.
  
  
• 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.

Andrea Carusi, President of the Spaceguard Foundation, believes "it is very important that ESA, one of the largest space agencies, and one that is already deeply involved in support of NEO studies, has decided to take a further step in this direction".

Preliminary studies, funded by the Agency's General Studies Programme, are now under way for the six proposals. Once these are submitted in 2003, ESA will judge whether one or more of the proposed missions is feasible and merits further development.