Next month NASA will launch a pair of spacecraft that will help astronomers study and predict gigantic eruptions from the sun. Currently, the nature and causes of these explosions of high-energy particles and magnetic storms are poorly understood. And being able to predict them farther in advance will help prevent damage to satellites orbiting the Earth.

Coronal mass ejections--giant clouds of high-energy particles and magnetic disturbances that erupt from the sun--can damage satellites when the Earth is in their path. (A pair of them are shown here.) (Credit: SOHO/LASCO C2 consortium)

In the past, astronomers have been able to look at these solar storms, called coronal mass ejections (CMEs), only from the Earth. This limited perspective has made predicting the exact timing and extent of the solar storms difficult, says Janet Luhmann, a researcher at the University of California at Berkeley's Space Science Laboratory, who helped design instruments for the new spacecraft. She compares it to "being run over by a truck: we don't see the full velocity and size of what's coming at us." The two new NASA spacecraft, called STEREO, will orbit the sun in front of and behind the Earth, giving a far better view of the solar storms.

Michael L. Kaiser, a STEREO project scientist at NASA, says a CME currently occurs every few days. The frequency of CMEs is expected to increase substantially, though--to several every day--in about five years when the sun reaches a peak period of activity. (Astronomers don't know why, but the sun's activity varies throughout an 11-year cycle.) Most CMEs don't affect the Earth, but when they do, satellites can be damaged or even wrecked.

The impact of CMEs on Earth can currently be predicted as many as four days in advance, but only within a 12-hour window. "We've got to do a better job," says Kaiser.

Coronal mass ejections, which originate in the sun's outer atmosphere, have two components. One is an enormous cloud of plasma, X rays, radio waves, and visible light. These energetic particles are accompanied by a large, fast-expanding magnetic loop. CMEs cause space weather--conditions that affect altitudes 50 kilometers above the Earth's surface and higher (the upper atmosphere and beyond into the ionosphere and magnetosphere). In these regions, the influx of high-energy charged particles and magnetic storms caused by CMEs can interfere with satellites.

STEREO will carry multiple instruments, including 3-D imagers and particle and magnetic field detectors. Luhmann says that the mission in part is a test of the concept of "parking" a space weather satellite to predict the impact of solar storms on the earth.