The phenomenon of collisions in the history of our solar system is very fundamental, having played the major role in forming the planets we observe today. Asteroids and comets may have contributed to the delivery of water and organic materials to the early Earth necessary for the development of life, but later impacts probably played a role in mass extinctions and they currently pose a small but significant threat to the future of our civilization. Collisions of objects with the Earth have taken place frequently over geological history and it is an undeniable fact that major collisions of asteroids and comets with the Earth will continue to occur at irregular, unpredictable intervals in the future.
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As a result of modern observing techniques and directed efforts thousands of near-Earth objects (NEOs) have been discovered over the past 20 years and the reality of the impact hazard has been laid bare. Even relatively small impactors can cause considerable damage: the asteroid that exploded over the Russian city of Chelyabinsk in February 2013 had a diameter of only 18 m yet produced a blast wave that damaged buildings and caused injuries to some 1500 people. The potentially devastating effects of an impact of a large asteroid or comet are now well recognized, although we can only speculate on the complex disruptive effects a major impact would have on today’s technically sophisticated and highly networked world.
The NEOShield project, an international collaboration of 13 partners from research institutions and industry, has been granted FP7 funding to investigate ways of preventing impacts of NEOs by means of space-based deflection techniques.
A prerequisite for the successful deflection of a NEO is accurate knowledge of the physical characteristics of the object, e.g. size, shape, rotation rate, density, composition internal structure, etc. We need to know as much as we can about “the enemy”. An important task is the analysis of existing observational data from the perspective of asteroid defence. We also need to determine the best ways to observe a threatening object to provide the mitigation-relevant information as efficiently as possible, and to study what types of space mission can be used to determine relevant properties.
The types of mitigation mission under investigation by NEOShield include the “kinetic impactor”, in which a spacecraft is used to deflect an asteroid by impacting it, thus transferring momentum so as to change its orbit slightly and cause it to miss the Earth. NEOShield partners are carrying out laboratory experiments in which projectiles are fired at materials thought to be analogous to those in asteroids. The results of such experiments, together with computer modeling and simulations, will provide insight into how asteroids would respond to a kinetic impactor deflection attempt.
Another method is to use a “gravity tractor”, which uses the weak gravitational force between the asteroid and the spacecraft. If a spacecraft is steered into the direct proximity of a NEO, the small but significant attraction between the spacecraft and the asteroid would work like a tow rope. With adequate forewarning, changes in the orbital velocity of the NEO of just a few centimetres per second or less could be enough to prevent a catastrophic impact on the Earth. This method would be relatively slow but would have one big advantage: contact with the surface of the NEO is not necessary thus no information about the interior structure or surface properties of the object is required to predict the outcome.
In the very unlikely case of a very large threatening NEO, or a short warning time, the two methods described above would probably not solve the problem. The greatest force we could apply would be a nuclear explosion. While we obviously have no actual plans to test a mission of this kind, a study of this possibility is being undertaken as part of the NEOShield project.
A major goal of NEOShield is to develop detailed designs of feasible demonstration missions and identify representative potential target asteroids. Testing mitigation techniques, such as the kinetic impactor and gravity tractor, is a vital prerequisite to a reliable international NEO defence system, such as that currently being considered by the United Nations.
The main results from the NEOShield project are expected to be:
- A greater understanding of the mitigation-relevant physical characteristics of NEOs, and the type of objects most representative of those likely to threaten the Earth that could be used as targets in realistic technically and financially feasible deflection test missions.
- A detailed description of the most appropriate instrumentation for the provision of crucial pre-cursor mitigation-relevant data from ground- and space-based reconnaissance observations.
- Detailed designs of deflection test missions to demonstrate our ability to deflect a threatening NEO with current technology. Gaining experience with deflection techniques is crucial in order to maximize the probability of success of a space-borne response to a threatening object that may have to be executed at short notice. While the current NEOShield project does not have sufficient funding to launch a test mission, we expect that such a mission will be carried out in the framework of a subsequent international initiative with European participation.
The events of 15 February, 2013, when a superbolide exploded over Chelyabinsk just hours before the predicted close approach of the ~30-m diameter NEO 2012 DA14, have sharpened public awareness of the dangers of NEOs and led to an avalanche of press interest in the work of NEOShield. The socio-economic impact and the wider societal implications of NEOShield lie in easing public concern over the impact hazard, and demonstrating that the scientific and space-engineering communities are abreast of the problem and have a good chance of successfully deflecting a seriously dangerous NEO should one threaten the Earth in the near future.