December 13, 2005
Spacecraft Stares at the Sun to Stand Guard for the Earth
By Warren E. Leary
Battered and somewhat broken, a sentinel stands between the Earth and the Sun, continually watching for impending solar storms and other activities. For a decade, the spacecraft - the Solar and Heliospheric Observatory, or SOHO - has not only peeled back many mysteries of the Sun but also revolutionized studies of the space weather that bathes every corner of the solar system.
Scientists and engineers around the world are celebrating SOHO's 10th anniversary and heralding it as one of the most productive spacecraft ever flown. Built by the European Space Agency and operated by NASA's Goddard Space Flight Center in Greenbelt, MD, it stares at the Sun 24 hours every day and sends back a steady stream of data and images documenting its influence on everything in its realm.
"SOHO brought home the point that we are living in the extended atmosphere of an active star," says Bernard Fleck, the solar physicist who is European project scientist for the spacecraft. "Aside from its scientific accomplishments, it really has revolutionized the whole space weather business and provided a practical early warning system for solar disruptions."
SOHO was launched on Dec. 2, 1995, and four months later took up a position about 930,000 miles from Earth at a point where the planet's gravity and that of the Sun hold it in a line between the two. There, the spacecraft's instruments can provide early warning of mass ejections of solar material that can affect astronauts in space, satellites and distant spacecraft, and power and communications systems on Earth.
Before SOHO, Earth had little or no warning before being hit by a shockwave of high-energy, radioactive particles from the Sun. The spacecraft now provides space weather forecasters up to three days' notice of Earth-directed disturbances, scientists say.
SOHO's suite of 12 instruments has also changed solar science, including understanding of the steady stream of particles, called solar wind, that are emitted by the Sun and permeate the solar system, said Joseph Gurman, NASA project scientist for the mission.
"In the last 10 years," Dr. Gurman said, "SOHO has revolutionized our ideas about the solar interior and atmosphere, and the acceleration of the solar wind."
Keeping SOHO operating past its original two-year mission time has not been easy. Dr. Fleck said, "Everything that can break has broken."
All three stabilizing gyroscopes aboard have failed, but European and American engineers developed software to retain control. All contact with the craft was lost for five months in 1998, but heroic efforts revived it. And in 2003, its main antenna got stuck, but operators found a way to use a secondary antenna to retrieve data.
SOHO's mission has been extended to 2007 in hope of studying a full 11-year cycle of solar activity. After that, scientists expect that a new NASA spacecraft called the Solar Dynamics Observatory, set for launching in 2008, will assume many of its duties.
Questions:
1. What is the SOHO and what is it designed to do?
2. How does Bernard Fleck define its successes?
3. When was SOHO launched?
4. Where is it situated?
5. What effect does SOHO have on weather forecasting and why?
6. In what ways has information gathered by SOHO influenced solar science?
7. What obstacles have SOHO scientists faced?
8. What mission are scientists planning next for SOHO, and what subsequent goals do they have for 2008?