Project Vanguard’s important contributions to America’s space program were the creation of the minitrack tracking system, the first use of silicon solar cells for electric power in a satellite, as well as the discovery that Earth is not quite round. The Vanguard program drew to a close with the 1959 launch of Vanguard 3. This satellite studied variations in solar and x-ray radiation and the earth’s magnetosphere. It also determined air density in the upper atmosphere. The mysteries of the near-earth space environment drew _Explorer 6_, launched August 7, 1959. _Explorer 6_ instruments measured radiation levels in the Van Allen radiation belts, mapped the earth’s magnetic field, counted micrometeorites, and studied the behavior of radio waves in space. In addition, _Explorer 6_ carried a scanning device which returned the first complete television cloud-cover picture of the earth’s surface. [Illustration: 34. Artist’s concept of IMP-E. This satellite’s primary mission is to study solar wind and the interplanetary magnetic field at lunar distance and their interaction with the Moon.] _Explorer 10_, launched on board a Thor-Delta rocket on March 25, 1961, confirmed the existence of the solar wind—the stream of particles that carries the Sun’s magnetic field beyond the orbit of Earth. During the satellite’s planned 52 hours in orbit, it relayed information on the relationship between terrestrial and interplanetary magnetic fields and the solar wind. To continue the study of solar wind and interplanetary magnetic fields, _Explorer 12_ was orbited by a Delta launch vehicle on August 16, 1961. It was the first in a series of satellites to study energetic particles in space. These electrons and protons constitute the earth’s radiation belts and they affect weather and other phenomena on Earth. _Atmosphere Explorer-A_ was the first of NASA’s aeronomy satellites. It was designed to remain in operation three months, studying the composition, density, pressure, and temperature of the upper atmosphere. This satellite discovered a belt of neutral helium atoms around the Earth. Deriving its name from a spirit in Shakespeare’s play, _The Tempest_, _Ariel 1_ explored the ionosphere, a region of electrically charged air which begins about 40 kilometers (25 miles) above the surface of the Earth. Launched April 26, 1962, _Ariel_ was a cooperative venture between Great Britain and the United States. It was both the first British satellite and NASA’s first international satellite. The Royal Society’s British National Committee on Space Research coordinated the experimental program; NASA scientists and technicians built the craft.