Explorer 7

With the success of America's first artificial satellite, Explorer 1 in January/February 1958, planning began almost immediately thereafter (March 1958) for a heavier, second-generation American satellite. Jet Propulsion Laboratory (JPL) and the Army Ballistic Missile Agency (ABMA), the entities managing the Explorer program and the Juno 1 rocket that launched it, envisioned the utilization of a bigger rocket, the Juno 2, which would share the same upper stages, but use a Jupiter missile as its first stage rather than the smaller Redstone. Appropriately, the satellite was initially known as the International Geophysical Year (IGY) Heavy Payload. Such an advanced satellite with the capacity to carry a multitude of experiments offered the opportunity not only to learn more about the Van Allen Belts discovered by Explorers 1 and 3, but also an unprecedented chance to observe the Sun in wavelengths of light, such as X-ray and ultraviolet, that are blocked from ground observation by Earth's atmosphere. Although sub-orbital sounding rockets had made previously short observations, solar output is unpredictable and fluctuates rapidly, making long-term, continuous study preferable. A satellite in orbit can make those kinds of observations. Prior to the establishment of NASA, primary support for the satellite was provided by the National Academy of Sciences. Originally planned for launch in Mid-1958, this schedule slipped as ABMA and JPL focused their attention on Explorer 4, Explorer 5, Pioneer 3, and Pioneer 4. The State University of Iowa (SUI), whose science team under James Van Allen had provided the Geiger counter for Explorer 1, was also occupied developing equipment for and processing the data returned by prior Explorer satellites. It was not until 16 July 1959 when the first of the JPL heavy satellites, now called S-1 by NASA, was ready for launch. Unfortunately, the power supply for the Juno 2 rocket's guidance system failed upon lift-off, and 5.5 seconds later, the range safety officer exploded the nearly-full rocket just off the launch pad. It took firefighters more than an hour to put out the resulting conflagration, which kept the science and engineering crew trapped in their protective blockhouses. Three months later, however, a duplicate satellite was ready for launch. == Satellite description ==

Explorer 7 (like its S-1 predecessor) was made of two truncated conical fiberglass shells joined by a cylindrical aluminum center section, wide at its equator and about high. The payload massed about . Spin stabilized, the spacecraft was powered by approximately 3000 solar cells mounted on both the upper and lower shells. Additional power was provided by 15 rechargeable nickel-cadmium batteries positioned on its equator near the outer skin to help maintain a proper spin rate. Two crossed dipole (1 W, 20-MHz) telemetry antennas projected outward from the center section, and a 108-MHz antenna was mounted on the bottom of the lower shell. This beacon transmitter was designed for cut-off after one year as solar-powered satellites can operate indefinitely and it was feared that too many operating in orbit would crowd the commonly used 20-MHz frequency. For this purpose, ABMA included a Bulova timer powered by a one-billionth horsepower battery. == Experiments ==

Aside from the beacon ionospheric experiment, Explorer 7 was equipped with a number of scientific packages provided by several commercial and educational agencies: Radiation and Heat Balance experiment A University of Wisconsin team, Dr. Verner E. Suomi and Robert Parent, Its primary instrumentation consisted of five bolometers in the form of hollow silver hemispheres that were thermally insulated from, but in close proximity to specially aluminized mirrors. The hemispheres thereby behaved very much like isolated spheres in space. Two of the hemispheres had black coatings and responded about equally to solar and terrestrial radiation. A third hemisphere, coated white, was more sensitive to terrestrial radiation than to solar radiation. A fourth, which had a gold metal surface, was more sensitive to solar radiation than to terrestrial radiation. The fifth hemisphere, protected from direct sunlight, was used to measure the reflected sunlight. A glass-coated bead thermistor was mounted on the top of each hemisphere to measure the temperature. A complete set of four temperature observations and one reference sample required 30 seconds. Thus, in each orbit, about 180 temperature measurements could be obtained. George H. Ludwig was the principal designer, assisted by Bill Whelpley, with graduate student John W. Freeman calibrating the counters, and Brian O'Brian joined the team in August 1959 as an assistant professor to assist in data analysis. Telemetry would be broadcast on two frequencies: 108.00 and 19.994 MHz; the low frequency and the relatively high power output of .6 watt was to make it easy for amateur radio enthusiasts to help collect data. Solar X-Ray and Lyman-Alpha Radiation A Naval Research Laboratory team led by Herbert Friedman and Talbot Chubb provided a solar X-ray and Lyman-alpha radiation experiment. Micrometeorite Marshall Space Flight Center's Herman E. Laggow, project manager for Explorer 7, sponsored the satellite's micrometeorite detector. == Launch and flight ==

The launch of Explorer 7 was originally scheduled for late September 1959, but the mission was delayed for two weeks after a Jupiter IRBM test on an adjacent pad failed shortly after liftoff, causing flying debris to hit and damage Explorer 7's launch vehicle. But on 13 October 1959 at 15:31 GMT, Explorer 7, the last of the satellites developed for launch during the International Geophysical Year, was launched ten months after the end of the IGY from the Atlantic Missile Range into a orbit and began operations. == Scientific results ==

Heat Balance The University of Wisconsin's heat balance instrument worked perfectly, Radiation in orbit The SUI Trapped Radiation and Solar Protons experiment operated satisfactorily from launch, except for a brief period in September and October 1960, Solar X-Ray and Lyman-Alpha Radiation The data collected by the Solar X-Ray and Lyman-Alpha Radiation were impossible to interpret in terms of incident solar radiation due to both the saturation of detector circuits by Van Allen radiation (150 keV electrons) (as had also happened on Vanguard 3 ) and electronic difficulties in the feedback amplifier. Successful measurement of solar X-rays would have to wait for next year's SOLRAD 1, which also carried an NRL payload, this one magnetically shielded. Micrometeorite The onboard micrometeorite experiment was exposed to the space environment of micrometeorites, trapped radiation, and sputtering for 38 days of active life. One of the three cells was damaged on launch, another returned no data. The third, however, was penetrated on the 13th day by a particle (either a micrometeoroid or the result of sputtering) approximately in diameter. Ionosphere Explorer 7's ionospheric experiment returned data over a period of 16 months. Interpretation of the changes of beacon signal characteristics between Explorer 7 and the ground stations showed irregularities in the Earth's ionosphere with horizontal dimensions of to to be present at all times of the day. == See also ==