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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. Researchers are working on ways for astronauts to capture an asteroid. On June 19, 2014, NASA reported that asteroid 2011 MD was a prime candidate for capture by the Asteroid Redirect Mission (ARM), perhaps in the early 2020s.
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 – elliptical (closed) and not intersecting the planet's surface or atmosphere. 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 enter the planet's sphere of influence with a hyperbolic trajectory relative to the planet, because a typical solar orbital velocity is in most instances equivalent to an escape velocity with respect to a planet – put another way, the asteroid's kinetic energy when it encounters the planet is too large for it to be brought into a bounded orbit by the planet's gravity (its kinetic energy is greater than its absolute potential energy in the planet's gravity well, meaning that its total orbital energy (specific orbital energy) with respect to the planet is positive (by convention, is defined to be negative), and thus the planet's gravity does not constrain its motion). 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 velocity needed to escape from the planet's sphere of influence at the asteroid's distance, its trajectory changes from hyperbolic to an elliptic orbit and the asteroid is captured.
Asteroid Redirect Mission
NASA has proposed the Asteroid Redirect Mission (or Asteroid Initiative), an uncrewed 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 shows 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 is commonly used by spacecraft performing rendezvous manoeuvres with other planets such as Mars, since it reduces 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.
- "Could Earth’s gravity capture an asteroid?". EarthSky. Jan 25, 2010. Retrieved 23 December 2012.
- Borenstein, Seth (June 19, 2014). "Rock that whizzed by Earth may be grabbed by NASA". AP News. Retrieved June 20, 2014.
- This situation arises because the Sun's gravity is much stronger than the gravity of any planet, meaning that orbits around the Sun, broadly speaking, correspond to much higher energies and hence much higher typical orbital speeds. In principle, an asteroid on an extremely elliptical orbit around the Sun which encounters a planet close to the asteroid's solar apoapsis (the orbital position of greatest distance and hence least speed) might be travelling slow enough to be captured by the planet's gravity, but this coincidence is quite unlikely to arise.
- 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.