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From all the spacecraft that orbit or orbited the Moon, which one lasted the longest in lunar orbit? By defunct spacecraft includes.

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The comments are slightly out of date. The Lunar Reconnaisance Orbiter, launched in 2009, is still going strong and is expected to last at least until 2026:

As of 2019, LRO has enough fuel to continue operations for at least seven more years, and NASA expects to continue utilizing LRO's reconnaissance capabilities to identify sites for lunar landers well into the 2020s.[1]

Although lunar orbits are inherently unstable because of the Moon's mass inhomogeneities and tidal effects from the Earth and the Sun, spacecraft can be kept in such orbits as long as fuel is available and used for station-keeping.

Cited Reference

  1. Clark, Stephen (June 18, 2019). "10 years since its launch, NASA lunar orbiter remains crucial for moon landings". Spaceflight Now. Retrieved June 20, 2019.
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LRO (Lunar Reconnaisance Orbiter) might be a good candidate from which we have data to know it's exact state.

But since you included "defunct spacecraft" as well there might be something in orbit since more than 50 years. Something quite unexpected, to be honest. Namely the Ascent Stage of Apollo 11 might still be orbiting the moon.

We don't have definite proof but the sumulations that were run from the last data available do not lead to a crash into the lunar surface in most cases.

Discussion about this news here: Could the ascent stage of Apollo 11's Eagle still orbit the Moon?

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