Asteroid capture

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 ${\displaystyle T}$ is greater than its absolute potential energy ${\displaystyle |V|}$ 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.

Triton (moon) and Deimos (moon) are suspected to be naturally captured bodies.

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.[4][5] 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.[6]

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,[7] 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).

• 2011 MD
• Asteroid Redirect Mission
• Artemis program

• Stephen D. Covey (May 2011). "Technologies for Asteroid Capture into Earth Orbit". International Space Development Conference. Archived from the original on 12 December 2011. Retrieved 23 December 2012.