lar & H
bservatory) is a satellite
that was launched as a cooperative effort between the European Space Agency
and National Aeronautics and Space Administration
. SOHO was launched on December 2, 1995 into the Earth Sun Lagrange Point
: L1. From this position about 92 million miles (150 million kilometers) the pull from the earth and the sun is equal and gives the satellite an un-obstructed, constant view of the sun.
Actually, it isn't right at the L1 point, but rather in a halo orbit about the L1 point. This orbit gives it a bit more stability (the L1 point is not 'stable' and moves around as the moon and other planets tug at it slightly). Furthermore, it allows the satellite to get out of the way of the sun and able to send a clear signal back to Earth (the sun is very noisy at radio wavelengths and would overpower the satellite's transmitter).
SOHO is primarily designed to study the internal structure of the sun and the outer atmosphere of the sun (see The Anatomy of the Sun). By being outside of the the Earth's magnetic field it is capable of studying the solar wind and warn scientists of potentially damaging coronal mass ejections (solar flares) that can disrupt satellites, and electrical and communication systems on Earth. SOHO has also been used to measure the solar constant for correlation between the amount of energy the sun puts out to weather on Earth.
The photographs from SOHO are publicly available to all and have allowed many armchair astronomers (both professional and amateur) to discover comets. The vast majority of these comets that are discovered are considered "sungrazers", nearing the sun so close that they either evaporate or break up. The latest images from SOHO are available at http://sohowww.nascom.nasa.gov/data/latestimages.html. People intrested in this should look at http://sungrazer.nascom.nasa.gov/ for information on how to submit a comet find. About 80 comets were found in 2001 by individuals looking at SOHO data and over 360 comets have been found total.