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Asteroid capture is the entering by an asteroid into an orbit around a larger planetary body. The larger body is said to have "captured" the asteroid, which thereafter is its natural satellite. Typically, asteroids that approach close to a planet are either thrown out into space or else hit the planet. However, occasionally the asteroid is captured in an orbit around the planet. This is possible with any planetary body given the right conditions.
As of 2014, U.S. engineers were working on methods for telerobotic spacecraft to capture an asteroid. In June 2014, NASA reported that asteroid 2011 MD was a prime candidate for capture by an Asteroid Redirect Mission (ARM), perhaps in the early 2020s. This effort was later cancelled by NASA in 2017.
Asteroid capture happens when an asteroid "misses" a planet when falling towards it, but it no longer has enough velocity to escape from the planet's orbit. In that case, the asteroid is captured, entering a stable, closed elliptic orbit around the planet which does not pass through the planet's atmosphere. This depends on variables such as the asteroid's velocity relative to the planet, the mass of the planet, the trajectory of the asteroid, and perturbations from other bodies.
An approaching asteroid will almost always enter a planet's sphere of influence on a hyperbolic trajectory relative to the planet, because solar orbits within Neptune's orbit have speeds much greater than planets' escape velocities. Put another way, the asteroid's kinetic energy when it encounters the planet is too great for it to be brought into a bounded orbit by the planet's gravity; its kinetic energy is greater than its absolute potential energy with respect to the planet, meaning that the planet's gravity does not constrain its motion. However, an asteroid's trajectory can be perturbed by a third body (e.g. a satellite, or another planet) in a way that reduces its kinetic energy in the planet's reference frame. If this brings the asteroid's velocity below the local escape velocity, its trajectory changes from a hyperbola to an ellipse, and the asteroid is captured. In rare cases, with or without such a perturbation, the asteroid travels on a trajectory that intersects with the planet, resulting in an impact event.
Asteroid Redirect Mission
NASA proposed the Asteroid Redirect Mission (or Asteroid Initiative), an unmanned robotic mission, to "retrieve" a near-Earth asteroid with a size of about 8.2 metres (27 ft) and a mass of around 500 tons (comparable in mass to the ISS). The asteroid would be moved into a high lunar orbit or orbit around EML2 (halo orbit, Lissajous orbit) for research and exploration purposes. Under consideration for moving the asteroid are grabbing the asteroid and using solar electric propulsion to "directly" move it, as well as gravity tractor technology.
Once the asteroid is in lunar or EML2 orbit, at least one crewed mission would rendezvous with it, to collect and return samples. 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. Furthermore, travel times to such a captured asteroid are much shorter, and launch windows are very frequent compared to transferring to near-Earth objects.
The first challenge is to find a suitable asteroid to retrieve – objects of the desired size are very dim and difficult to find. As many larger asteroids are known, an alternative for the retrieval of a small asteroid is to "pick up" a suitable sized rock from a larger asteroid, and move only that rock to a lunar or EML2 orbit.
The timeline in the 2013 mission overview showed a test flight in the 2017 timeframe followed by a rendezvous and capture mission in 2019. The asteroid then would be moved 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).
The technology needed to move a large asteroid into an arbitrary orbit does not yet exist; altering an asteroid's orbit around the Sun would require a large delta-v to be imparted to an object with a mass several orders of magnitude greater than existing spacecraft.
Once an asteroid is on course to encounter a planet with an atmosphere, it is in principle possible to tweak its orbit so that it intercepts the planet's atmosphere, using aerobraking to slow the asteroid at periapsis by dumping some of its kinetic energy into the atmosphere – this technique has however never been used by spacecraft performing rendezvous manoeuvres with other planets such as Mars, but it would reduce the otherwise very large amount of fuel required to decelerate a spacecraft from a Sun-orbital velocity to a planet-orbital velocity. This technique is referred to as aerocapture.
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