The Solar and Heliospheric Observatory (SOHO) spacecraft celebrates its 10th anniversary Dec. 2. The SOHO mission, a collaboration between NASA and the European Space Agency (ESA), has allowed scientists to make significant advances in understanding the closest star, our sun. This includes understanding the violent solar activity that causes stormy space weather, which can disrupt satellites, radio communication and power systems on Earth.

"It's impossible to overstate the importance of SOHO to the worldwide solar science community," said Dr. Joe Gurman, U.S. project scientist for SOHO at NASA's Goddard Space Flight Center, Greenbelt, Md. "In the last ten years, SOHO has revolutionized our ideas about the solar interior and atmosphere and the acceleration of the solar wind."

Some of SOHO's major scientific accomplishments include:

- Allowing space weather forecasters to play a lead role in the early warning system for space weather and give up to three days notice of Earth-directed disturbances.

- Supplying the most detailed and precise measurements beneath the surface of the sun.

- providing the first images of a star's turbulent outer shell (the convection zone) and of the structure of sunspots beneath the solar surface.

- Making the sun transparent by creating images of the sun's far side, including stormy regions there that will turn with the sun and threaten the Earth.

- Discovering a mechanism that releases more than enough energy to heat the sun's atmosphere (corona) to 100 times its surface temperature.

- Discovering that a series of eruptions of ionized gas (coronal mass ejections) from the sun blasts a "highway" through space where solar energetic particles flow. These particles disrupt satellites and are hazardous to astronauts outside the protection of Earth's magnetic field.

- Monitoring the sun's energy output (the "total solar irradiance" or "solar constant") as well as variations in the sun's extreme ultraviolet radiation, both of which are important to understand the impact of solar variability on Earth's climate.

- Identifying the source regions and acceleration mechanisms of the solar wind, a thin stream of ionized gas that constantly flows from the sun and buffets Earth's magnetosphere.

SOHO data are freely available over the Internet, and people all over the world have used images from the observatory to discover more than 1,000 comets.

"I tip my hat to SOHO's engineering and operations teams, whose skills and dedication have overcome multiple technical challenges over the last decade, such as the loss of control of the spacecraft in 1998, the loss of the gyros when we recovered the spacecraft a few months later, and a sticky high gain antenna in 2003," said Dr. Bernhard Fleck, ESA Project Scientist for SOHO.

The observatory was originally designed for a two-year mission, but its scientific insights have proven so valuable that NASA has consistently granted it extensions, the latest of which allows the spacecraft to cover a complete 11-year solar cycle.

For more information about SOHO on the Web, visit: http://www.nasa.gov/vision/universe/solarsystem/soho2005.html