Orbiting Geophysical Observatory

Orbiting Geophysical Observatory (OGO) Program ^[1] of NASA refers to the six satellites launched by the United States that were in use from September 1964 to 1972, designed to study the Earth's magnetosphere.^[2] The satellites successfully studied the interactions between the Earth and the Sun, despite a number of technical problems. Each satellite had 20 to 25 instruments. The project manager for all 6 OGO projects was Wilfred Scull.

All OGO satellites are built around a common parallelepiped-shaped platform (0.9 × 0.9 × 1.8 m). The satellite's orientation is maintained fixed in space (3-axis stabilized) so that one of the long faces (0.9 × 1.8 m) permanently points towards Earth. On this face, as well as on the opposite face, a surface of 0.6 m² is available for scientific experiments. The attitude control system is also responsible for keeping the solar panels continuously oriented perpendicularly to the solar rays. The cubic SOEP (Solar Oriented Experiment Package) receptacles, attached to the ends of the solar panels on both faces, can accommodate experiments on a surface of 0.1 m². At one end of the satellite's body, two OPEP-1 (Orbital Plane Experiment Package) and OPEP-2 experiment sets are mounted on an adjustable support that keeps them oriented in the direction of the satellite's forward movement. Two booms, 5.7 meters long (EP-5 and EP-6) and four booms, 1.8 meters long (EP-1 to EP-4), hold scientific experiments at their ends that must be kept away from the satellite's body to meet visibility or sensitivity constraints. Additionally, the satellite is equipped with several antennas for telecommunications, the most prominent being an adjustable Yagi antenna. The scientific experiments may have their own antenna, like the one shown in the diagram extending 9 meters from the SOEP-1 experiment on the solar panel. The satellite typically has twelve appendages deployed in orbit in two sequences to avoid any interference.^[3]

The attitude control system relies on horizon sensors, cold gas thrusters, and reaction wheels. It allows the satellite to be stabilized on 3 axes with an accuracy of 2° relative to the local vertical, 5° relative to the Sun's direction, and 5° relative to the forward movement axis. The thermal control system uses louvers that open and close to maintain a temperature of 10 to 24°C within the satellite's body and thermal resistors for scientific experiments mounted outside. Electrical power is provided by solar panels that produce 550 watts, of which 50 watts are available for scientific experiments. The energy is stored in two 28-volt nickel-cadmium batteries. The telecommunications system ensures data transfer at a rate between 1 and 64 kilobits per second. Scientific data can be transmitted in real-time or stored temporarily on one of two magnetic tape recorders with a recording speed of 1 to 4 kilobits per second and a reading speed of 64 to 128 kilobits per second.^[4]

OGO 1, OGO 3, and OGO 5 were in equatorial orbits; OGO 2, OGO 4, and OGO 6 were in lower polar orbits.^[5]

The purpose of the OGO 1 spacecraft, the first of a series of six Orbiting Geophysical Observatories, was to conduct diversified geophysical experiments to obtain a better understanding of the Earth as a planet and to develop and operate a standardized observatory-type satellite. OGO 1 consisted of a main body that was parallelepipedal in form, two solar panels, each with a solar-oriented experiment package (SOEP), two orbital plane experiment packages (OPEP) and six appendages EP-1 through EP-6 supporting the boom experiment packages. One face of the main body was designed to point toward the Earth (+Z axis), and the line connecting the two solar panels (X axis) was intended to be perpendicular to the Earth-Sun-spacecraft plane. The solar panels were able to rotate about the X axis. The OPEPs were mounted on and could rotate about an axis which was parallel to the Z axis and attached to the main body. Due to a boom deployment failure shortly after orbital injection, the spacecraft was put into a permanent spin mode of 5 rpm about the Z axis. This spin axis remained fixed with a declination of about -10 deg and right ascension of about 40 deg at launch. The initial local time of apogee was 2100 h. OGO 1 carried 20 experiments. Twelve of these were particle studies and two were magnetic field studies. In addition, there was one experiment for each of the following types of studies: interplanetary dust, VLF, Lyman-alpha, gegenschein, atmospheric mass, and radio astronomy. Real-time data were transmitted at 1, 8, or 64 kbs depending on the distance of the spacecraft from the Earth. Playback data were tape recorded at 1 kbs and transmitted at 64 kbs. Two wideband transmitters, one feeding into an omnidirectional antenna and the other feeding into a directional antenna, were used to transmit data. A special-purpose telemetry system, feeding into either antenna, was also used to transmit wideband data in real time only. Tracking was accomplished by using radio beacons and a range and range-rate S-band transponder. Because of the boom deployment failure, the best operating mode for the data handling system was the use of one of the wideband transmitters and the directional antenna. All data received from the omnidirectional antenna were noisy. During September 1964, acceptable data were received over 70% of the orbital path. By June 1969, data acquisition was limited to 10% of the orbital path. The spacecraft was placed in a standby status November 25, 1969, and all support was terminated November 1, 1971. By April 1970 the spacecraft perigee had increased to 46,000 km and the inclination had increased to 58.8 deg.^[6]

External videos
Video of reentry

The University of Arizona's Catalina Sky Survey (CSS), funded by NASA’s Planetary Defense Coordination Office (PDCO), detected an object late in the evening of 25 August 2020 which appeared to be on an impact trajectory with Earth.^[7] Two Maui middle school students also observed the 250-pound (110 kg) object. Maui Waena Intermediate School eighth-graders Holden Suzuki and Wilson Chau, with mentor outreach astronomer J.D. Armstrong of the University of Hawaii Institute for Astronomy (IfA), used data from the Las Cumbres Observatory (LCO) Faulkes Telescope North on Haleakala to track OGO-1.^[8] The University of Hawaii's Asteroid Terrestrial-impact Last Alert System (ATLAS), also funded by PDCO, independently observed the object. Further observations were conducted by CSS to confirm the object’s trajectory. Precision orbit calculations were conducted by the Center for Near-Earth Object (NEO) Studies (CNEOS) at NASA’s Jet Propulsion Laboratory, and compared to data from the European Space Agency's NEO Coordination Center. The object was confirmed to be not an asteroid, but in fact Orbiting Geophysical Observatory-1 (OGO-1). OGO-1 reentered Earth's atmosphere and disintegrated on Saturday evening, 29 August 2020 over Southern French Polynesia.^[7][9]

In 1970 OGO-5 used its ultraviolet photometer to observe comets Encke, Tago-Sato-Kosaka (1969 IX) and Bennett (1970 II).^[10][11]

• Hartt, Julian (December 26, 1960). "First 'Streetcar' Satellites To Be Built for NASA". Milwaukee Sentinel. Hearst Headline Service. p. 4M. Retrieved December 6, 2010.
• "Space 'Streetcar'". The Modesto Bee. December 26, 1960. p. A13. Retrieved December 6, 2010.
• "U.S. to Launch Giant Satellite". Spartanburg Herald-Journal. Associated Press. August 30, 1964. p. B8. Retrieved December 6, 2010.
• Biography of Wilfred Scull, OGO project manager

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