Goals: Luna 21's mission was to make the second safe delivery of a Soviet lunar rover (called Lunokhod 2) to the Moon. There, the rover would collect images of the lunar surface, determine whether ambient light levels are conducive to using the Moon as a base for astronomical observations, perform laser ranging experiments from Earth, observe solar X-rays, measure local magnetic fields, and study mechanical properties of the lunar soil.
Accomplishments: Though Lunokhod 2 operated for only four months before it drove into a crater and brought the mission to a premature end, it accomplished a great deal. It covered 37 km of terrain, including hilly upland areas and rilles, transmitted 86 panoramic images and more than 80,000 TV pictures of the lunar surface, and conducted at least 740 mechanical tests of the soil. During the laser ranging experiment, the Earth-Moon distance was measured to within 20 to 30 cm. Surprisingly, the lunar night sky was found to be 13 to 15 times brighter than the Earth night sky, casting doubt upon the Moon's suitability for observatories in visible and UV light (at least on the Earth-facing side).