Museum-piece satellite goes into space.

Museum-piece satellite goes into space

A former exhibit from the Smithsonian Institution's National Air and Space Museum in Washington, D.C., is now in orbit around the earth. Originally designed as a navigation satellite in the U.S. Navy's Transit series, it was given to the Smithsonian in 1976 instead of being launched, and spent the next eight years on display in the museum's Gallery of Satellites. Now it is in space at last, but with a new mission, new equipment and a name name: Polar BEAR.

Refitted to monitor auroras, magnetic-field changes and other ionospheric effects due to disturbances such as solar flares, Polar BEAR (Polar Beacon Experiment and Auroral Research) was launched Nov. 13 for the Air Force Space Division and the Defense Nuclear Agency. It was initially built for the Navy by the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., but was donated to the museum when the design turned out to be so reliable that other satellites in the series were deemed unlikely to need replacement as often as had been anticipated.

It was called back into service in 1984, when Polar BEAR engineers at APL found that Air Force and Navy satellites would no longer be available for modification, as they had in the past. The idea of making such an unusual request of the Smithsonian occurred to Polar BEAR program manager David Grant at APL, and when the alb offered to trade a similar satellite of an even earlier design, the museum was more than happy to cooperate.

In adapting their windfall for its new role, the engineers kept the device's overall structure or "bus," its solar panels and various electronics packages. Everything was in "extraordinarily good condition," says Grant, and even after extensive mechanical modification and the addition of about 40 more electronics boxes, the use of the craft trimmed more than $2 million from Polar BEAR's projected $12.5 million cost.

Circling the earth in the ionosphere, following a nearly pole-crossing orbit 602 statute miles above the surface, Polar BEAR carries three experiments, whose data will be used to help improve communications over the polar regions. All three had been turned on by Nov. 19, and were undergoing checkout.

One is a scanning device that can produce images of the aurora by both ultraviolet emissions (day or night) and visible light. It is the successor of a similar instrument that was launched in 1983 aboard a Defense Nuclear Agency satellite called HILAT, said to have been the first ever to observe the aurora in full daylight. Though HILAT is still at work, its auroral sensor stopped functioning after only 18 days on the job, but not before elating researchers with the quality of its images (SN: 9/24/83, p. 196).

The Polar BEAR auroral sensor may also shed some light on a controversial hypothesis by the University of Iowa's Louis Frank that "dark spots" in another satellite's ultraviolet (1,304-angstrom) images of the atmosphere may represent water vapor from vast numbers of otherwise undetected comets (SN: 3/29/86, p.199). One other satellite--Sweden's Viking, launched Feb. 22--made 1,304-angstrom images through late June with no such dark spots immediately apparent, though Viking researchers note that additional processing will be necessary to confirm the observations.

Another of Polar BEAR's sensors, meanwhile, is a magnetometer to follow changes in earth's magnetic field over the poles. Besides relating to the shifting aurora as well as other terrestrial responses to varying solar conditions, the device helps determine Polar BEAR's orientation in space. The third experiment, called Beacon, sends signals to the ground at various frequencies as a measure of electron-caused scintillations in the ionosphere, which can affect satellite communications. Beacon, which will also sample the electron spectrum over the poles, handles Polar BEAR's data transmissions.