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The Phys.org article New NASA radar technique finds lost lunar spacecraft describes the use of radar to relocate two spacecraft that were in orbit around the moon but who's orbit had not been actively tracked for a while.

Finding a derelict spacecraft at lunar distance that has not been tracked for years is tricky because the moon is riddled with mascons (regions with higher-than-average gravitational pull) that can dramatically affect a spacecraft's orbit over time, and even cause it to have crashed into the moon. JPL's orbital calculations indicated that Chandrayaan-1 is still circling some 124 miles (200 kilometers) above the lunar surface, but it was generally considered "lost."

I have not found detailed information on Chandrayaan-1's orbit except that it was polar with an altitude of about 100 km or 200 km. However, the LRO is seems to be in a daring 160 x 20 km orbit.

This Wikipedia article suggests there are "frozen orbits" about the moon with inclinations of 27°, 50°, 76°, and 86°, but the LRO was inserted at about 90°.

Are low, polar lunar orbits in general relatively stable compared to lower inclination orbits, or is it still necessary to maintain station keeping capability? Do stable polar orbits actively avoid mascons, or does the mascon distribution somehow "average out" for polar orbits?

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LRO was inserted into a polar frozen orbit for commissioning, which required no stationkeeping. This orbit was a stable 31.5 km x 199 km polar orbit with periapsis over the South Pole. By "frozen" they mean that the line of apsides and eccentricity remain fixed.

It then moved into a 50km circular polar orbit (+/- 20 km) for its science mission. This orbit required a stationkeeping maneuver once a month, with a budget of about 150 m/s for the year.

Then after the mapping phase, LRO moved again into a polar frozen orbit, where no stationkeeping is required.

references: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070021535.pdf https://arc.aiaa.org/doi/pdfplus/10.2514/6.2010-1985

Circular polar orbits are not stable. The orbit maintenance budget to stay in one varies significantly based on your selected insertion state- a 110 km orbit at 84 deg inclination might stay around for a year without needing a maneuver, but a 100 km orbit at 90 deg inclination can require 100 m/s annually to stay within +/- 30 km.

But there are elliptical polar frozen orbits, like the one that LRO is in, that don't need orbit maintenance. As here: https://arc.aiaa.org/doi/abs/10.2514/6.2006-6749

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  • $\begingroup$ Thanks for the great explanation and well referenced answer! It looks like the question's linked Wikipedia discussion of "frozen" could use some polishing. $\endgroup$ – uhoh Dec 14 '17 at 5:04
  • $\begingroup$ Found that LRO spends merely 2.2 kg/year on its 'Frozen orbit' maintenance youtu.be/d2uaJ2xRYpc?t=450 $\endgroup$ – Ohsin Oct 30 at 12:55

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