Satellite gets a kick out of sun's photons.

In 1989, a NASA satellite began to drift subtly from its expected orbit, confounding researchers who used the spacecraft to measure the movement of Earth's crust. Now, a team of physicists has developed an explanation for the drift, a finding expected to keep the spacecraft useful for many more decades.

"Suddenly, there was this mysterious force acting on the satellite," says David P. Rubincam of NASA's Goddard Space Flight Center in Greenbelt, Md., who has worked with the satellite since its launch in 1976.

LAGEOS I, the Laser Geodynamic satellite, looks something like a spinning disco ball orbiting 5,900 kilometers above Earth. By reflecting laser beams off its mirrored surface, geophysicists can make precise measurements of tiny displacements in Earth's surface.

"To get really accurate measurements, though, you need to know the satellite's orbit well," says Douglas G. Currie of the University of Maryland in College Park. He, Rubincam, and John W. Robbins of the Hughes STX Corp. in Greenbelt describe LAGEOS in the Jan. 5 Journal of Geophysical Research.

The researchers suspect that the satellite's drifting stems from sunlight, which falls more heavily on one side of the craft than on the other. The heated side of the satellite radiates this excess energy as infrared photons. The photons' energy provides a small, but persistent, rocket effect, pushing on the spacecraft as the photons spring from its surface. "The net force would be away from the hot end," says Rubincam.

As an additional complication, LAGEOS has developed a wobble, causing the direction of its spin axis to change over the years. The researchers compared their model to a painstakingly constructed record of the spacecraft's decades-long voyage. The results match very well, says Currie.

"[Solar photon force is] a well-known effect but very complicated to measure," says Byron Tapley of the University of Texas at Austin. Gravity is 100 billion times stronger than the force that the photons exert on LAGEOS, but over decades, the light quanta have cumulatively kicked the craft thousands of kilometers off course.

Although the effect influences all spacecraft, it is more noticeable in LAGEOS. The craft orbits far above Earth's atmosphere, which acts as a drag on lower satellites. Moreover, most other satellites have propulsion systems to maintain their positions.

Currie hopes to refine the group's analysis by engaging small telescopes around the world in observing LAGEOS' wobble. "We have an extremely good [explanation]; now we want to improve the model," he says. With a better understanding of the satellite's orbit, NASA expects LAGEOS to be useful to geophysicists for another 50 years.