Lunar and Planetary Institute






Apollo 15 Mission


Science Experiments - Solar Wind Composition

The Solar Wind Composition Experiment collected samples of the solar wind for analysis on Earth.

Image of aluminum foil sheet that was deployed on a pole facing the SunThe Sun continually emits a flux of electrically charged particles into space. This is termed the solar wind. The Earth's magnetic field prevents these charged particles from reaching the Earth's surface, although in the Earth's polar regions, these particles can reach the upper part of the atmosphere, causing auroras. The Moon is outside the Earth's magnetic field for most of each month and has a negligible atmosphere, allowing solar-wind particles to reach the Moon's surface. Two different experiments, the Solar Wind Composition Experiment and the Solar Wind Spectrometer, were deployed on the Moon to study the solar wind.

The Solar Wind Composition Experiment was performed on Apollo 11, 12, 14, 15, and 16. It consisted of an aluminum foil sheet, 1.4 meters by 0.3 meters, that was deployed on a pole facing the Sun. On Apollo 16, a platinum sheet was also used. This foil was exposed to the Sun for periods ranging from 77 minutes on Apollo 11 to 45 hours on Apollo 16, allowing solar-wind particles to embed themselves in the foil. The foil was then returned to Earth for laboratory analysis. This allowed the chemical composition of the embedded solar wind to be determined more accurately than would be possible if the measurement were made using remotely controlled instruments on the Moon, but limited the periods at which observations could be made. The isotopes of the light noble gases were measured, including helium-3, helium-4, neon-20, neon-21, neon-22, and argon-36. Some variation in the composition of the solar wind was observed in the measurements from the different missions. These variations were correlated with variations in the intensity of the solar wind as determined from magnetic field measurements.

Other experiments were deployed by the crew and then monitored from Earth by radio telemetry after the crew departed. This group of experiments was termed the Apollo Lunar Surface Experiment Package (ALSEP). Each experiment was connected by a cable to the ALSEP central station, which provide radio communication to Earth and electrical power from a radioisotope thermal generator. Some of these experiments continued to return data until September 1977, when the entire ALSEP network was turned off due to lack of funding for the ground support team.