Craft take panoramic view of solar eruptions.

Even at its most quiescent, the sun hurls into space a billion-ton cloud of charged particles every 2 days or so. The few particles that strike our planet can disrupt satellites and knock out power systems on the ground.

Twin spacecraft have for the first time tracked these storms, known as coronal mass ejections, from their birth in the lower depths of the sun's atmosphere all the way to Earth's orbit, scientists announced this week. The new capability will enable solar physicists to forecast the terrestrial arrival time of the storms with an uncertainty of just 3 hours, one-quarter the uncertainty of previous estimates, says Russ Howard of the Naval Research Laboratory in Washington, D.C.

The pair of craft makes up the Solar Terrestrial Relations Observatory (STEREO). One of the pair lags Earth in its orbit about the sun, while the other leads Earth (SN: 2/10/07, p. 93). The distance from the Earth-sun line gives each craft a new perspective on storms heading toward Earth.

Other spacecraft, by contrast, have observed solar disturbances only from positions close to the Earth-sun line. That's like watching the evolution of a smoke ring blowing directly at you, notes STEREO scientist Mike Kaiser of NASA's Goddard Space Flight Center in Greenbelt, Md. From that position, it's difficult to measure the shape and velocity of the ring.

Each STEREO craft has a suite of five telescopes designed to observe coronal mass ejections at different points on their journeys toward Earth. By combining images from the telescopes, astronomers can accurately follow the solar storms. At press time, Howard and his colleagues were scheduled to discuss the images during a March 1 telephone briefing.

The newly released STEREO images, taken in late January, show a coronal mass ejection changing from a croissant shape near the sun to a corrugated storm front farther out. Interactions with the solar wind--the breeze of charged particles that blows continuously from the sun--probably altered the storm's shape and speed, says Howard.

Documenting how and when the solar wind slows down or speeds up a solar storm during its journey earthward is critical to improving forecasting, comments Doug Biesecker of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, Colo.

By April, the distance between the STEREO craft, which are drifting apart, will be great enough to begin taking three-dimensional images of the sun and its storms.