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Free-return trajectory
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<p>In <a href="/facts/Orbital_mechanics/oB3tqNG9">orbital mechanics</a>, a free-return trajectory is a <a href="/facts/Trajectory/STNN33FK">trajectory</a> of a <a href="/facts/Spacecraft/YQqQaUw0">spacecraft</a> traveling away from a primary body (for example, the <a href="/facts/Earth/HLLmDfj0">Earth</a>) where <a href="/facts/Gravity/bqAN7DdQ">gravity</a> due to a secondary body (for example, the <a href="/facts/Moon/zKHqEMPb">Moon</a>) causes the spacecraft to return to the primary body without <a href="/facts/Spacecraft_propulsion/KmpMGU9u">propulsion</a> (hence the term <i>free</i>). </p><p>Many free-return trajectories are designed to intersect the atmosphere; however, periodic versions exist which pass the moon and Earth at constant <a href="/facts/Periapsis/VaL6ow9w">periapsis</a>, which have been proposed for <a href="/facts/Cycler/axBJYucu">cyclers</a>. </p>