Apollo 15 was the ninth manned mission in the Apollo program and the fourth mission to land on the Moon. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous missions. The mission began on July 26, 1971, and concluded on August 7. NASA called it the most successful manned flight ever achieved.
Commander David Scott and Lunar Module Pilot James Irwin spent three days on the Moon and a total of 18½ hours outside the spacecraft on lunar extra-vehicular activity. The mission was the first not to land in a Lunar mare, instead landing near Hadley rille in an area of the Mare Imbrium called Palus Putredinus (Marsh of Decay). The crew explored the area using the first Lunar Rover allowing them to travel much further from the Lunar Module lander than had previously been possible. They collected a total of 77 kg (170 lbs) of lunar surface material.
At the same time, Command Module Pilot Alfred Worden orbited the Moon, using a Scientific Instrument Module (SIM) to study the lunar surface and environment in great detail using a panoramic camera, gamma ray spectrometer, mapping camera, laser altimeter, mass spectrometer, and lunar sub-satellite that was launched at the end of the mission.