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Magsat (Magnetic Field Satellite, Explorer 61, Applications Explorer Mission-3 or AEM-3) spacecraft was launched in the fall of 1979 and ended in the spring of 1980. The mission was to map the Earth's magnetic field, the satellite has two magnetometers. The scalar (Cesium vapor) and vector (fluxgate) magnetometers gave Magsat a capability beyond that of any previous spacecraft. Extended by a telescoping boom, the magnetometers were distanced from the magnetic field created by the satellite and its electronics. The satellite carried two magnetometers, a three-axis fluxgate magnetometer for determining the strength and direction of magnetic fields, and a ion-vapor/vector magnetometer for determining the magnetic field caused by the vector magnetometer itself. MAGSAT is considered to be one of the more important Science/Earth orbiting satellites launched; the data it accumulated is still being used, particularly in linking new satellite data to past observations.

After launch the payload was brought to an orbit of 96.8° facing the sun as the earth rotated underneath. It was kept in a close earth orbit, with vector magnetometers capable of sensing magnetic fields closer to Earth's surface. The data collected by this satellite allowed a 3D-mapping of the Earth's magnetic interior as never seen before. In combination with a later satellite, Ørsted, is has been an essential component for explaining the current declining state of the Earth's magnetic field.


On October 30, 1979 Magsat was launched from pad SLC-5 at Vandenberg AFB in California on a Scout II (101) rocket bearing 97° in a dusk to dawn orbit. The spacecraft was elevated to a height just over 350km in an elliptical orbit with a maximum distance of 550 km. After reaching orbit, its telescoping boom was extended outward by 6 meters. Two star cameras were used to define the position of the spacecraft relative to Earth. The orbit allowed the satellite to map a majority of the Earth's surfaces except the geographic poles. Because of the close proximity of the orbit to earth it was unstable and the spacecraft's orbit decayed on June 11 1980.


Magsat was not without problems. One of the biggest is that the motion of a metallic object tends to create a magnetic field. One study after the mission found a nonlinear fluxgate response when exposed to fields greater than 5000 mT. The applied field had to be transverse to the axis of the magentometer. The design was improved by creating a feedback relay over a spherical design. This was the design used on later spacecraft [See:Ørsted (satellite)].


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