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Asteroid capture can happen when an asteroid approaches a large planetary body. Typically asteroids that approach close to a planet are thrown out into space or impact the body. In rarer instances, the asteroid is captured in orbit around the planet. This is possible with any planetary body given the right conditions.
Asteroid capture happens when an asteroid has enough velocity to keep "missing" the planet itself when it is falling towards it, but it does not have enough velocity to escape that planet's orbit. In other words, an asteroid is captured when the asteroid reaches a stable orbit around the planet that it was heading towards. The properties of an asteroid that are most significant to this process are its mass and its relative velocity toward the planet in question. The mass of the planet in question is also a key variable, as is the trajectory of the asteroid.
An approaching asteroid will usually pass by the planet with a hyperbolic trajectory. In rare cases, the asteroid travels on a trajectory that intersects the planet, resulting in an impact event. Even more rarely, the asteroid receives a nudge from a third body (e.g. a satellite) that slows it down. If the velocity of the asteroid dips below the escape velocity, its trajectory changes from hyperbolic to an elliptic orbit and the asteroid is captured.
Human capture of an asteroid
NASA has proposed the Asteroid Initiative, an unmanned mission to drag a near-Earth asteroid with a size of about 8.2 metres (27 ft) and a mass of around 500 tons (similar to that of the ISS) into a high lunar orbit or orbit around EML2 (halo orbit, Lissajous orbit) for research and exploration purposes. Subsequently there could be manned missions to it. One of the advantages of a lunar orbit compared with an Earth orbit would be the safety: even at the end of the mission the natural perturbations of the trajectory would cause an eventual impact on the Moon, not on Earth. The first challenge would be to find a suitable asteroid. An alternative for the "Get a Whole One" approach could be the "Pick Up a Rock" approach[clarification needed]. The current timeline in the mission overview shows a test flight in the 2017 timeframe followed by a rendezvous and capture mission in 2019. The asteroid then would be hauled back to cislunar space by around 2021.
The mission could provide a relatively low-cost route to satisfying Barack Obama's goal of sending astronauts to a near-Earth asteroid by 2025, and help develop knowledge and skills useful for future asteroid impact avoidance, In-situ resource utilization (including water for astronauts and for producing fuel, and material for bulk shielding against cosmic rays, for use there or elsewhere in space) and other asteroid mining (as well as providing a first target for the latter two).
There are not yet technologies to move a large asteroid and put it wherever we want. Aerocapture relates to a method of slowing down an object with the atmosphere, which could theoretically be an asteroid.
- "Could Earth’s gravity capture an asteroid?". EarthSky. Jan 25, 2010. Retrieved 23 December 2012.
- Stephen D. Covey (May 2011). "Technologies for Asteroid Capture into Earth Orbit". International Space Development Conference. Retrieved 23 December 2012.
- Sean O'Hare (23 December 2012). "Revealed: Nasa plan to 'lasso' asteroid the size of two buses and turn it into a 'space station' to orbit the moon". The Daily Mail. Retrieved 23 December 2012.