observed at Mercury were not carried from the Sun by solar wind, but in fact belonged to the planet itself. Simpson had been helping to develop the mission from concept since 1959. Also, in 1980, his detection of a tiny gap in the distribution of energetic particles trapped in the magnetic field of Saturn indicated the presence of a previously undetected small moon orbiting at that position in space and absorbing particles which would otherwise be found. Subsequently, scientists identified Saturn's moon optically. In 1976 and 1982, Simpson also detected bursts of energetic particles associated with the passage of shock waves in the solar wind, and provided evidence that the transition serves as an efficient accelerator of particles. In 1970, Simpson, William Dietrich, and John David Anglin discovered that some impulsive sun flares produce energetic particles, among which helium-3 (a light non-radioactive isotope of helium with two protons and one neutron) is at least ten times more abundant than helium-4 (another light non-radioactive isotope of helium with two protons and two neutrons). On Earth, helium-4 makes up about 99.986% of total helium. In 1973, his instruments on Pioneer 10 and Pioneer 11 indicated a cosmic ray intensity increase of about 1 percent per AU. In 1975, he found that during the low point of activity three years prior, the abundance of cosmic ray helium was strangely enhanced at these very low energies, as opposed to it dropping off with declining energies like protons do toward zero energy. In 1975 and 1977, Simpson discovered that beryllium-10 nuclei are scarce in cosmic rays, and that this scarcity indicates that they have been around for about 2 x 107 years. This led him to conclude that cosmic rays pass freely between the gaseous disk and the extended magnetic halo portion of the galaxy, where the ambient gas density is on the order of 10−2 atoms/cm3 or less. In the 1980s, Simpson and A. J. Tuzzolino developed a dust flux monitor for the Stardust spacecraft. The instrument involved a thin sheet of plastic that was polymerized in the presence of a strong electric field perpendicular to the plane of the plastic, and then electrically polarized carrying a positive electric charge on one of its surfaces and a negative charge on the other. When a dust particle or heavy nucleus penetrated the sheet, it vaporized a small area, releasing the charges on the sheet and