Apollo 11 Mission
Science Experiments - Solar Wind Composition Experiment
The 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 into 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 mission. These variations were correlated with variations in the intensity of the solar wind as determined from magnetic field measurements.
The Solar Wind Spectrometer was deployed on Apollo 12 and 15. Although the solar wind contains ions of most chemical elements (including the noble gases measured by the Solar Wind Composition Experiment), over 95% of the particles in the solar wind are electrons and protons, in roughly equal numbers. The Solar Wind Spectrometer measured the flux of protons and electrons as a function of particle velocity. The measurements were made in a set of seven detector cups with different orientations in order to determine the direction of particle motion. Most of the measured flux was in the detector that was oriented most directly toward the Sun.
Because the Solar Wind Spectrometer made continuous measurements, it was possible to measure how the Earth's magnetic field affects arriving solar wind particles. For about two-thirds of each orbit, the Moon is outside of the Earth's magnetic field. At these times, a typical proton density was 10 to 20 per cubic centimeter,with most protons having velocities between 400 and 650 kilometers per second. For about five days of each month, the Moon is inside the Earth's geomagnetic tail, and typically no solar wind particles were detectable. For the remainder of each lunar orbit, the Moon is in a transitional region known as the magnetosheath, where the Earth's magnetic field affects the solar wind but does not completely exclude it. In this region, the particle flux is reduced, with typical proton velocities of 250 to 450 kilometers per second. During the lunar night, the spectrometer was shielded from the solar wind by the Moon and no solar wind particles were measured.