Communications satellites can be grouped into two broad categories. Passive vehicles reflect signals from one ground station to another. Active satellites accept ground signals and either amplify and rebroadcast them immediately or record messages for later transmission. The Echo satellite balloons typified the passive category of communications spacecraft. These satellites “bounced” radio signals from one ground station to another. Uninflated Echo payloads were carried into orbit packed in special storage containers. When released in space, the balloon was inflated by chemicals packed inside which subliminated to produce inflating gas. The mylar plastic skin of the satellite was sandwiched between two layers of aluminum foil. _Echo 2_—on display—included a system for releasing gas over a long period of time to maintain the satellite’s spherical shape. Launched January 25, 1964, _Echo 2_ was the first satellite used for communication experiments between the United States and the Soviet Union. Project West Ford, launched May 9, 1963, was a unique experiment in passive satellite communications. It was not a solid vehicle, but a series of 400-million tiny individual copper filaments called dipoles. The dipoles formed a reflective layer some 64,300 kilometers (40,000 miles) long, 32 kilometers (20 miles) thick, and 32 kilometers (20 miles) wide. The distance between the individual dipoles averaged 536 meters (one-third mile). The West Ford experiment proved disappointing, and advances in the design of active communications satellites made further experiments of this nature unnecessary. _Oscar 1_ (Orbital Satellite Carrying Amateur Radio) was conceived, designed, and constructed by American amateur radio “hams.” Launched as a “piggyback” satellite on December 12, 1963, Oscar transmitted a series of Morse code dots spelling “hi.” The message was picked up by 5000 operators in 28 nations during the 18 days of transmission. Oscar investigated radio propagation phenomena in space on that portion of the radio frequency spectrum allocated to amateur radio (144-146 megaherz). Testing the use of a “delayed-repeater” satellite in global military communications, _Courier 1-B_ was placed in a high-altitude orbit on October 4, 1960. The craft accepted and stored messages as it passed over one ground station, then replayed them on command. _Relay_, another active repeater satellite, was placed in orbit on December 13, 1962. _Relay_ carried communications experiments to test a variety of relay equipment—including that for photofacsimile, teleprinter, and data transmission. During its 25-month lifespan, _Relay 1_ introduced the nations of the world to satellite communication. A second, improved _Relay_ was launched in 1964. [Illustration: 39. The exterior of eight-sided _Relay_ is composed of honeycomb aluminum panels studded with 8215 solar cells.]