Thanks to one of the most productive spacecraft ever built, scientists are far better acquainted with the star that lights our world and gives us life. Built for ESA by European industry, the Solar and Heliospheric Observatory (SOHO) went into space on December 2, 1995.
The tenth anniversary of SOHO's launch is a time for celebration for the scientists and engineers in Europe and the USA who conceived, created and still operate this unprecedented solar spacecraft - and who have rescued it from oblivion three times.
Four months after its flawless launch by a NASA rocket, SOHO was at its special vantage point 1.5 million kilometres away from the sunward side of Earth. There it watches the Sun unblinkingly 24 hours every day, and sends home a stream of images of the ceaseless frenzy in the solar atmosphere. Apart from unmasking the Sun, and teaching us how it works, SOHO's pictures give early warning of storms in space that can affect astronauts, satellites and power and communications systems on Earth.
Originally planned for a nominal life of just two years, SOHO has performed so well and delivered such important data that operations are now to set to continue at least until 2007. That corresponds to a full 11-year cycle of magnetic storms on the Sun, and a further extension is under discussion. But the going has not always been easy.
All contact was lost with the spacecraft in June 1998. Dramatic efforts by ESA and NASA engineers, supported by Matra Marconi Space (now Astrium) who built SOHO, restored the spacecraft to full operation in November 1998. Shortly afterwards, the spacecraft's last gyroscope failed, but the teams developed new software that controls the spacecraft without a gyroscope. A third crisis occurred in June 2003, when SOHO's main antenna became stuck. Using the secondary antenna and software for intermittent recording, observations continued.
"I tip my hat to SOHO's engineering and operations teams, whose skills and dedication let us overcome all these challenges," says Bernhard Fleck, ESA's Project Scientist for SOHO.
More than 3200 scientists from around the world have been involved with SOHO, which is a project of international collaboration between ESA and NASA. SOHO's telescopes probe the Sun from deep in its interior and all the way out to Earth's orbit and beyond, where the magnetised solar wind of atomic particles sweeps through interplanetary space.
"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:
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.
- 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.