Astronomy

Midcourse Sensor Experiment



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After more than 12 years of successful operations and contributions to two diverse defense missions, the Midcourse Space Experiment satellite, known as MSX, was retired in 2008, having operated well beyond its designed four-year life span.

Launched atop a Delta II rocket, the Midcourse Space Experiment (MSX) was placed into a circular orbit with an altitude of 900 km in 1996. Built by the Johns Hopkins Applied Physics Laboratory for the Ballistic Missile Defense Organization (BMDO), MSX was designed to demonstrate various multispectral imaging technologies for identifying and tracking ballistic missiles during flight. It also studied Earth's atmosphere with a special view toward understanding climate change by measuring the distribution of various greenhouse gases.

The MSX spacecraft weighed just under 3 tons, and was roughly 5 meters tall and 2 meters in diameter. It was powered by 1200 watts provided by 120 square feet of solar panels which were deployed after reaching orbit. MSX also had a nickel hydride battery with a capacity of 1400 watt-hours to power the satellite while in Earth's shadow and to support brief periods of high power usage. Command and data communications with Earth were carried out using S and X band radio equipment.

MSX had 11 instruments which produce time-resolved optical images using light wavelengths ranging from 110 nanometers (far ultraviolet) to 28 microns (far infrared). These instruments included the Spatial Infrared Imaging Telescope (SPIRIT III) and a set of Ultraviolet and Visible Imagers and Spectrographic Imagers (UVISI), including separate narrow field-of-view visible and ultraviolet imagers, as well as wide field-of-view visible and ultraviolet imagers. There was also an S-band beacon receiver for initial detection of incoming experimental targets.

These instruments were pointed by control torques generated using balance wheels to rotate the spacecraft, and the orientation was determined using GPS positioning technology and ring laser gyroscopes. The MSX also incorporated another function not normally seen in spacecraft: closed loop tracking on targets other than stars.

MSX had a compartmented structure, driven largely by special temperature requirements of the Spatial Infrared Imaging Telescope (SPIRIT III), which was also the largest instrument on board. SPIRIT III produced infrared images with a resolution of about 20 seconds of arc (about the size of Saturn's disk) in the spectral range of 2.5-28 microns. Most of the optical sensors on MSX shared this level of optical resolution.

The longest wavelength of 28 microns corresponds to a blackbody temperature of about 100 degrees Kelvin. Accordingly, the operating temperature of the imaging sensors, and optics had to be maintained at a temperature much colder than 100K. In fact, an operating temperature of 8.5 K was provided by a solid hydrogen cryostat. Use of this cryostat drained an expendable cryogenic liquid, with enough supplied to keep MSX working at super-low temperatures for 18 months with a 10% duty cycle. Although cryogenic operations were not possible after this period, all other instrumentation continued to function normally.

The electronics for SPIRIT III and the other optical sensors on MSX required a much higher operating temperature, and were held roughly at room temperature. To accommodate this wide range of operating temperatures, MSX was divided into three sections: the instrument section, structural skeleton, and electronics section.

The structural skeleton was required to provide a very stable mount for the optical instruments, together with a low coefficient of thermal expansion, so that the alignment of the instruments would not change as, for example, the spacecraft passed through the Earth's shadow. To accommodate these requirements, the skeleton was made of a composite graphite epoxy material having extremely low thermal expansion and conductivity.

The MSX spacecraft provided new and unique equipment for hyperspectral observation, and tested those capabilities for BMDO applications as well as for a wide range of scientific studies, in particular those with relevance for climate change research. Maintaining an active and productive life for three times the design lifetime, MSX represented a remarkably successful mission for NASA and JPL.



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