Chemical collisions cause shuttle's halo.

An experiment to test a 50-year-old theory of astrophysics has solved the myster of why orbiting spacecraft sometimes develop a golden glow.

The halo along a satellit's leading edges results when nitric oxide combines with oxygen atoms to form nitrogen dioxide, reports Edmond Murad of the Philips Laboratory at the Hanscom Air Force Base in Massachusetts. After this reaction occurs, electrons in the molecules become temporarily excited and the molecules leave the shuttle surface, Murad and his colleagues explain in the Nov. 7 NATURE. Then, as the electrons return to their normal state, the molecules emit light, creating an aura that extends about 20 centimeters out from the shuttle surface.

Shuttle astronauts first noticed this glow in 1982. Also, during two flights of the Atmospheric Explorer satellites, these unmanned spacecraft had picked up signals that differed from what researchers expected. At first, scientists blamed the instruments, but as a result of the early shuttle flights, "it became clear there was a reason for these anomalies," says Murad. The mysterious glow - rather than poorly calibrated instruments - could be the cause.

Scientists then tried to reproduce the glow in their laboratories, but they got a different color than what shuttle astronauts saw outside their windows, says Murad. And for good reason, he adds: The researchers needed air filled with oxygen atoms, not the two-atom oxygen molecules found in air at Earth's surface. Also, since the shuttle orbits Earth at about 7.8 kilometers per second, the free-floating oxygen atoms that collide with the shuttle hit their target quite fast. "It's not easy to make a very fast neutral oxygen in the lab," Murad says.

During last April's flight of the shuttle Discovery, Murad and his colleagues were testing whether accerated atoms or molecules can reach high enough speeds to become charged briefly and then release energy as they return to a neutral state. This phenomenon, called the critical ionization velocity, can help explain the formation of the solar system; it was first suggested by Hannes Alfven, a Nobel-winning plasma physicist.

To study the phenomenon, the Discovery astronauts conducted four experiments. In each one, they used a nozzle with 27 tiny holes to shoot a different gas outside the shuttle bay for 10 seconds. A videocamera recorded the event, picking up any energy emissions -- in the form of light -- from the gases. In addition, a detector on the outside of the shuttle monitored for the discharge of any electrons.

Neither the astronauts nor the ground-based researchers expected what they saw when they ejected nitrix oxide into space.

"They were startled by the light; it lit up the shuttle," says Murad. The plume of nitrix oxide created an intense blue-white glow. Then the orange halo along the shuttle's tail got much brighter for about 3 seconds as the nitric oxide settled back onto the shuttle surface and reacted with the oxygen that collided with the craft.

Now that scientists know how the glow originates, Murad says, they should try to mount space-bound instruments to face away from the craft's leading edge to present these instruments from "seeing" the glow. Otherwise, they should adjust their measurements to account for any artifacts caused by the halo.