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| Asteroid capture ： ウィキペディア英語版 |
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.== Orbital mechanics ==Asteroid capture happens when an asteroid "misses" the planet when it is falling towards it, but it does not have escape velocity from that planet's orbit. In other words, an asteroid is captured when the asteroid reaches a stable (closed) elliptic orbit around the planet and not intersecting the planet's surface or atmosphere. Capture depends on variables such as the asteroid's velocity relative to the planet, the mass of the planet, and 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 planetThis 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. – 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 T is greater than its absolute potential energy |V| in the planet's gravity well, meaning that its total orbital energy (specific orbital energy) with respect to the planet E=T+V is positive (by convention, V 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 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.
== Orbital mechanics ==
Asteroid capture happens when an asteroid "misses" the planet when it is falling towards it, but it does not have escape velocity from that planet's orbit. In other words, an asteroid is captured when the asteroid reaches a stable (closed) elliptic orbit around the planet and not intersecting the planet's surface or atmosphere. Capture depends on variables such as the asteroid's velocity relative to the planet, the mass of the planet, and 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〔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.〕 – 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
■ウィキペディアで「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.== Orbital mechanics ==Asteroid capture happens when an asteroid "misses" the planet when it is falling towards it, but it does not have escape velocity from that planet's orbit. In other words, an asteroid is captured when the asteroid reaches a stable (closed) elliptic orbit around the planet and not intersecting the planet's surface or atmosphere. Capture depends on variables such as the asteroid's velocity relative to the planet, the mass of the planet, and 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 planetThis 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. – 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 T is greater than its absolute potential energy |V| in the planet's gravity well, meaning that its total orbital energy (specific orbital energy) with respect to the planet E=T+V is positive (by convention, V 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.」の詳細全文を読む
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