Lunar orbit

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Not to be confused with Orbit of the Moon.
The Moon from lunar orbit, with planet Earth rising over the horizon, taken on the Apollo 8 mission on December 24, 1968.
The Moon from lunar orbit, with planet Earth rising over the horizon, taken on the Apollo 8 mission on December 24, 1968.

In astronomy, lunar orbit (also known as a Selenocentric orbit) refers to the orbit of an object around the Moon.

As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by various manned or unmanned spacecraft around the Moon. The first spacecraft to reach the vicinity of the Moon was the Soviet unmanned vehicle Luna 1 on January 4, 1959.[1] It passed within 5,995 km of the Moon's surface.[1] Luna 3, launched on October 4, 1959 used a figure-8 trajectory, or free return trajectory, to swing around the backside of the Moon, and return the first pictures of the far side of the Lunar surface.[1] The figure-8 trajectory was not a true orbit around the Moon, as the spacecraft only circled a portion of the Moon, and then went on a free return trajectory back to earth.[1]

The Soviet Union's Luna 10 was the first spacecraft to actually orbit the Moon in April 1966.[2] It studied micrometeoroid flux, and lunar environment until May 30, 1966.[2]

The first United States spacecraft to enter a true lunar orbit, in the sense that it was a satellite continuously going around the Moon, was Lunar Orbiter 1 on August 14, 1966.[3] The first orbit was an elliptical orbit, with an apolune of 1,866.8 kilometers and a perilune of 189.1 kilometers.[4] This was the first of five similar spacecraft launched over a period of thirteen months, all of which successfully mapped the Moon. The primary mission of the Lunar Orbiter program was to search for suitable Apollo Project landing sites.[3]

The Apollo Project's Command and Service Module remained in lunar orbit while the Lunar Module landed. Selection of an orbit was constrained on the low side by lunar mountains reaching heights of 20,000 feet (nearly seven kilometers), and on the high side by the need to obtain suitable imagery. Lunar Orbiters were mostly around 200 miles altitude while the Apollo Project used roughly 65 miles. Orbital periods were roughly two hours.

Gravitational anomalies slightly distorting the orbits of the Lunar Orbiters led to the discovery of Mass concentrations ("mascons"), concentrations of mass beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies were too small to be of significance for the short time the Apollo Project's spacecraft were in lunar orbit.

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  1. ^ a b c d Wade, Mark. Luna (HTML). Encyclopedia Astronautica. Retrieved on 2007-02-17.
  2. ^ a b Byers, Bruce K. (1976-12-14). APPENDIX C [367-373 RECORD OF UNMANNED LUNAR PROBES, 1958-1968: Soviet Union] (HTML). DESTINATION MOON: A History of the Lunar Orbiter Program. National Aeronautics and Space Administration. Retrieved on 2007-02-17.
  3. ^ a b Wade, Mark. Lunar Orbiter (HTML). Encyclopedia Astronautica. Retrieved on 2007-02-17.
  4. ^ Byers, Bruce K. (1976-12-14). CHAPTER IX: MISSIONS I, II, III: APOLLO SITE SEARCH AND VERIFICATION, The First Launch (HTML). DESTINATION MOON: A History of the Lunar Orbiter Program. National Aeronautics and Space Administration. Retrieved on 2007-02-17.