_Explorer 1_ measured three phenomena—cosmic ray and radiation levels (data that led to the discovery of the earth’s radiation belts), the temperature in the vehicle (important in the design of future spacecraft), and the frequency of collisions with micrometeorites. There was no provision for data storage, and therefore the satellite transmitted its information continually.
_Explorer 1_ was not the only orbiting American satellite for long. In spite of the early problems, Project Vanguard succeeded in launching the second American earth satellite on March 17, 1958.
The back-up _Explorer 1_ on exhibit is from the National Aeronautics and Space Administration, Jet Propulsion Laboratory. California Institute of Technology.
[Illustration: 7. Artist’s conception of _Mariner 2_ as it flew by Venus.]
The first successful interplanetary spacecraft probed the environment of Venus, Earth’s closest neighbor. _Mariner 2_, working flawlessly, swept by the hot and cloudy planet at a closest approach of 34,834 kilometers (21,645 miles) on December 14, 1962.
The journey began with lift-off on August 27 from Cape Canaveral atop an Atlas Agena-B launch vehicle. During the 109-day trip to the planet, _Mariner_’s on-board instruments sampled the environment of interplanetary space and telemetered information to Earth stations. Ground-based measurements of the Venerian surface temperature were confirmed by the probe to be around 425° C (800° F).
_Mariner 2_ detected no measurable magnetic field or radiation belts, indicating that Venus may have a very different history than has Earth.
_Mariner 2_ passed out of tracking range on January 4, 1963, when the spacecraft was about 87 million kilometers (54 million miles) from Earth. The probe is presently in orbit around the Sun.
The back-up craft on display would have been launched toward Venus if _Mariner 2_ had failed to reach the planet.
Prime contractor for _Mariner 2_ was the Jet Propulsion Laboratory, California Institute of Technology.
_Mariner 2_ is from the National Aeronautics and Space Administration.
[Illustration: 8. Enlarged facsimile of coded _Mariner 2_ tape transmitted December 14, 1962, from the vicinity of Venus. Encircled portions show microwave and infrared coding.]