THE APOLLO SWC EXPERIMENT: RESULTS, CONCLUSIONS, CONSEQUENCES

The Apollo Solar Wind Composition (SWC) experiment was designed to measure elemental and isotopic abundances of the light noble gases in the solar wind, and to investigate time variations in the solar-wind composition. The experiment was deployed on the first five Apollo lunar landing missions. The crews exposed a foil at each of the five landing sites, and solar wind particles were collected for time periods ranging from 77 minutes in July 1969 (Apollo 11) to 45:05 hours in April 1972 (Apollo 16). The foils were returned to Earth, where the collected noble gas particles were analysed in ultra-high vacuum mass spectrometer systems. We briefly describe here the flight hardware, and the technical tests and calibrations. The experimental results were published in various scientific journals, PhD theses and NASA science reports, some of them not readily accessible after three decades. In this paper, therefore, the results obtained by the five experiments on the fluxes of the isotopes of He, Ne and Ar are summarized and discussed, so that averages and variations in solar wind composition can be more easily compared with more recent data, particularly those to be obtained by the Genesis mission. The helium flux determined for the five exposure periods varied by a factor of four, with a time-weighted average of 1.2 × 107 cm−2 s−1. Although the composition varied much less than the He-flux, definite variations were found for the 4He/3He and He/Ne ratios. The weighted average solar wind abundance ratios obtained were 4He/3He = 2350 ± 120, 4He/20Ne = 570 ± 70, 20Ne/22Ne = 13.7 ± 0.3, 22Ne/21Ne = 30 ± 3, 20Ne/36Ar = 49 ± 7 and 36Ar/38Ar =5.4 ± 0.3 (errors correspond to the 2σ level). We also measured the flow directions of individual isotopic species. We found that the lunar environment did not significantly affect the solar wind composition measured at the lunar surface, and we conclude that the SWC results are representative of the solar wind prevailing in interplanetary space at the time of the five foil exposures. Finally, we discuss, from today's perspective, some of the implications and conclusions that can be drawn from the SWC results, concerning the Sun and its history, the solar system, the galaxy and the universe.