Lunar orbit

Not to be confused with Orbit of the Moon.

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 altitude at apoapsis (point farthest from the surface) for a lunar orbit is known as apolune (or apocynthion), while the periapsis (point closest to the surface) is known as perilune (or pericynthion).

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Unmanned spacecraft

The Soviet Union sent the first spacecraft to the vicinity of the Moon, the unmanned vehicle Luna 1, on January 4, 1959.[1] It passed within 6,000 kilometres (3,200 nmi; 3,700 mi) of the Moon's surface, but did not achieve lunar orbit.[1] Luna 3, launched on October 4, 1959, was the first unmanned spacecraft to complete a cislunar flight, still not a lunar orbit, but a figure-8 free return trajectory which swung around the far side of the Moon and returned to the Earth. This craft provided the first pictures of the far side of the Lunar surface.[1]

The Soviet Luna 10 became 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 orbit the Moon was Lunar Orbiter 1 on August 14, 1966.[3] The first orbit was an elliptical orbit, with an apolune of 1,008 nautical miles (1,867 km; 1,160 mi) and a perilune of 102.1 nautical miles (189.1 km; 117.5 mi).[4] Then the orbit was circularized at around 170 nautical miles (310 km; 200 mi) to obtain suitable imagery. Five such spacecraft were launched over a period of thirteen months, all of which successfully mapped the Moon, primarily for the purpose of finding suitable Apollo program landing sites.[3]

See also Lunar_mission#US_orbital_missions_(2009)-

Manned spacecraft

The Apollo program's Command/Service Module (CSM) remained in a lunar parking orbit while the Lunar Module (LM) landed. The combined CSM/LM would first enter an elliptical orbit, nominally 170 nautical miles (310 km; 200 mi) by 60 nautical miles (110 km; 69 mi), which was then changed to a circular parking orbit of about 60 nautical miles (110 km; 69 mi). Orbital periods vary according to the sum of apoapsis and periapsis, and for the CSM were about two hours. The LM began its landing sequence with a descent orbit with a perilune of about 50,000 feet (15 km; 8.2 nmi), chosen to avoid hitting lunar mountains reaching heights of 20,000 feet (6.1 km; 3.3 nmi).

Perturbation effects

Gravitational anomalies slightly distorting the orbits of the Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons), beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies are significant enough to cause a lunar orbit to change significantly over the course of several days.

The Apollo 11 first manned landing mission employed the first attempt to correct for this effect. The parking orbit was "circularized" at 66 nautical miles (122 km; 76 mi) by 54 nautical miles (100 km; 62 mi), which was expected to become the nominal circular 60 nautical miles (110 km; 69 mi) when the LM made its return rendezvous with the CSM. But the effect was overestimated by a factor of two; at rendezvous the orbit was calculated to be 63.2 nautical miles (117.0 km; 72.7 mi) by 56.8 nautical miles (105.2 km; 65.4 mi). [5]

See also

References

  1. ^ a b c Wade, Mark. "Luna". Encyclopedia Astronautica. http://www.astronautix.com/project/luna.htm. Retrieved 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"]. DESTINATION MOON: A History of the Lunar Orbiter Program. National Aeronautics and Space Administration. http://www.hq.nasa.gov/office/pao/History/TM-3487/app-c.htm#soviet. Retrieved 2007-02-17. 
  3. ^ a b Wade, Mark. "Lunar Orbiter". Encyclopedia Astronautica. http://www.astronautix.com/craft/lunbiter.htm. Retrieved 2007-02-17. 
  4. ^ Byers, Bruce K. (1976-12-14). "CHAPTER IX: MISSIONS I, II, III: APOLLO SITE SEARCH AND VERIFICATION, The First Launch". DESTINATION MOON: A History of the Lunar Orbiter Program. National Aeronautics and Space Administration. http://www.hq.nasa.gov/office/pao/History/TM-3487/ch9-2.htm. Retrieved 2007-02-17. 
  5. ^ "Apollo 11 Mission Report" (PDF). NASA. pp. 4-3 to 4-4. http://history.nasa.gov/alsj/a11/A11_MissionReport.pdf.