SOHO and the eclipse
The joint ESA/NASA Solar and Heliospheric Observatory (SOHO) will gather images of both the corona and the Sun surface before, during, and after the eclipse. That will add valuable information to the interpretation of the pictures gathered by ground-based instruments. The satellite will be watching for other phenomena that may occur on the surface of the Sun, such as coronal mass ejections - enormous gas explosions that send shock waves across the solar disk and can even cause magnetic storms on Earth.
The two-tonne spacecraft is the largest and most sophisticated solar observatory ever launched. It is virtually suspended between the Earth and the Sun, in a spot (known as the first Lagrangian point) where the gravity of the two celestial bodies cancel each other out.
SOHO observes the Sun's outermost layer - called the corona - using five instruments:
- the Extreme Ultraviolet Imaging Telescope (EIT)
- the Large Angle Spectroscopic Coronagraph (LASCO)
- the Ultraviolet Coronagraph Spectrometer (UVCS)
- the Solar Ultraviolet Measurement of Emitted Radiation (SUMER)
- the Coronal Diagnostic Spectrometer (CDS).
During total solar eclipses - when the Moon completely obscures the solar disk - observers on the ground get a rare chance of looking directly at the corona, which becomes visible as a white halo.
Scientists using ground telescopes to observe details of the corona during the total eclipse will have the opportunity to compare their pictures with those taken by SOHO.
Despite the eclipse, the spacecraft will have a direct view of the Sun's surface. In short, it will see no eclipse. That's possible because SOHO is four times farther from Earth than the distance between the Earth and the Moon (see diagram above).