Environmental Research Satellite
The Environmental Research Satellite (ERS, alternatively Earth Resources Satellite) program was a series of small satellites initially operated by the United States Air Force Office of Aerospace Research. Designed to be launched "piggyback" to other satellites during launch, detaching once in orbit, they were the smallest satellites launched to date—what would today be classified as microsatellites. 33 ERS satellites in six different series were launched between 1962 and 1971, conducting scientific research and serving as test beds to investigate the reliability of new spacecraft components. TRS Mk. 1. The TRS (Tetrahedral Research Satellite) Mk. 1 was  developed by Space Technology Laboratories, a subdivision of TRW Inc., as an inexpensive, miniaturized "off-the-shelf" satellite that customers could use to perform simple experiments in orbit. The Mk. 1 was a regular tetrahedron measuring on a side, each face mounted with sufficient solar cells to operate the experiments and the telemetry system when the satellite was in the sun. Transmission of data was constant at that time as the spacecraft included neither internal battery nor command system. A transistor-based system provided eight channels of data: five for experiments, two for telemetry calibration, and one for the spacecraft  temperature. A antenna transmitted data. on 136.771 Mhz. Each TRS satellite was estimated to cost only $25,000 () to build, excluding development, launch and mission operations costs. The Air Force Space Systems Division (AFSSD), then headed by Col. T. O. Wear, was the first and only customer for STL's TRS Mk. 1 satellites, initially purchasing six for its Environmental Research Satellite (ERS) program. Ten TRS Mk. 1 satellites were ultimately produced, designed to research radiation and micrometeoroid flux in Earth orbit. All were launched from Vandenberg Air Force Base attached to primary payloads. Significant flights. ERS 2 (TRS 1 ). At the time it was launched, TRS 1 was the smallest satellite ever placed into orbit. The aluminum spacecraft carried 140 solar cells, producing 600 milliwatts of power, and five radiation-detecting cells. Though the satellite did not separate from its primary satellite, it returned eight minutes of data per orbit to tracking stations below. It was designed to turn off after 90 days of operation. TRS 1 circled the Earth in a Low Earth orbit. ERS 5, 6, and 9 (TRS 2-4). Data returned from these three fully successful TRS satellites, circling the Earth in Medium Earth orbits  (within the Van Allen Belts), returned valuable data on the effects of orbital radiation on solar cells. Of significance, it was determined that p-on-n silicon cells deteriorated five times more quickly than n-on-p cells. While protective covers did not affect n-on-p degradation, they were shown to be helpful for the more sensitive p-on-n cells. An unexpected result of the solar cell experiment was that, while it had been observed in ground tests that exposure to radiation of the cells' quartz covers and the epoxy adhesive that held them to the cells reduced  the light they transmitted (and thus the power generated) to the cells by 15%, such was not observed in orbit. TRS 2 and 3 marked the first time two satellites were deployed into orbit simultaneously. Data was obtained by NASA's Minitrack communications network in cooperation with the USAF. TRS Mk. 2. The TRS Mk. 2 design was a tetrahedron measuring 21 cm on a side. Four were produced: ERS-11 through ERS-14, though only two were launched, both pick-a-back with primary USAF payloads. They were designed to operate for one year, at which point, an onboard timer would  shut the satellites off. Flights. ERS 12 (TRS 5). Launched into a highly elliptical orbit that took the satellite as high as above the Earth and as close as at perigee, ERS 12 measured the intensity of charged particles in the magnetosphere. Its experiment package detected radiation from all directions, measuring electrons at levels greater than 0.5 and 5 MeV and protons between 10 and 20 eV and 50 to 100 eV. The spacecraft returned data until 1963-10-30. ERS 13 (TRS 6, "Pygmy"). ERS 13's orbit was similar to that of ERS 12, with a perigee of  , but an even higher apogee: . Spinning once every six seconds, the satellite measured electron and proton levels in the Van Allen Belts with omni-directional radiation detectors—a scintillation counter and a solid-state detector. The onboard transmitter, with a power of 100 mW could only reach ground stations when ERS 13 was within of Earth. The satellite functioned normally until 1964-10-20, when transmission became erratic. The satellite went silent on 1965-01-25. ORS Mk. 1. There is no evidence that any Octahedral Research Satellite Mark 1 ever flew. ORS Mk. 2. The ORS Mk. 2 design  was an octahedron measuring 23 cm on a side. Five were produced: ORS-1 and ORS-2 (ERS-15 through ERS-16), which carried out cold welding experiments in space on a variety of metal samples, and ERS-23 through ERS-25, classified satellites whose flights may have been cancelled. Both ERS 15 and 16 used actuators for their cold welding experiments, the first (16) making five metal to metal tests, and the second (15) making eight. ORS Mk. 3. The ORS Mk. 3 design was an octahedron measuring 28 cm on a side. Two were produced: ORS-3 and ORS-4 (ERS-17 and ERS-18). OV5. The design  of the OV5 (Orbiting Vehicle 5) series was based on the ORS Mk. 3 design and its vehicles were also given ERS numbers. Nine were produced: OV5-1 through OV5-9. TETR. TETR (also known as TTR and TATS, all standing for "Test and Training Satellite") was a series of octahedral ERS satellites which were built to train Apollo program ground station crews for the Manned Space Flight Network. Four were produced: TTR-1 through TTR-4 (ERS-30 through ERS-33) TETR 2 supported training for Apollo missions 8 through 13 despite a faulty battery pack. TETR C failed to orbit due to  a failure in the launch vehicle.