NASA's and the European Space Agency's sun satellite, the Solar and Heliospheric Observatory (SOHO), is a satellite launched in 1995 to study the Sun and help predict space weather. SOHO will remain operational at least until 2012, and its original mission profile has been expanded to include a highly successful campaign of spotting previously unknown comets.
- Spacecraft Systems and Orbit -
SOHO is a 1350-pound spacecraft with a total of twelve scientific instruments, designed for regular and long-term study of the Sun and of various solar phenomena. These include modules to study the Sun's corona and magnetic field, and the solar wind. The spacecraft itself consists of a service section, with basic functional equipment and the communications array for maintaining the linkup with Earth; and a payload section, with the scientific instruments. Together the two sections are roughly 15 feet high and 12 feet wide. The communications system is capable of speeds of up to 200 kilobits per second, a fraction of the speed of today's broadband home Internet connections.
On December 2, 1995, the probe blasted into orbit aboard an Atlas launch vehicle. It has remained operational since then. However, the motor which operates its high-gain antenna failed in 2003, forcing NASA and the European Space Agency (ESA) to switch to careful use of its low-gain antennas to transmit data instead. Five years before that, moreover, SOHO was almost written off when its navigation equipment fell out of alignment with the Sun and contact with Earth was lost. After several tense days, in which radar from the Deep Space Network and the Arecibo Observatory frantically tried to pinpoint the location of the wayward satellite, contact was re-established and the spacecraft gradually returned to normal operational status.
SOHO is currently in what is known as a halo orbit close to the Sun-Earth L1 or Lagrange point, a point in space at which an object orbits so that it is directly between the Sun and the Earth at all times. It maintains a continuous communications link with the Deep Space Network dishes here on Earth
- Mission Achievements -
Initially, SOHO's mission was specifically related to study of the Sun, including the first purposeful study of the star's chromosphere and corona (the outer layers), as well as the makeup of the solar wind, the stream of radiation and particles which is steadily emitted into the solar system from the Sun's atmosphere. In addition, some of the spacecraft's instruments were capable of studying the inner layers of the Sun through observations of wave oscillations, a process known as helioseismology.
In addition to its Sun mission, however, SOHO has also been engaged in cataloguing previously unknown comets. This capacity was discovered relatively accidentally, as a result of comets passing in front of instruments' view of the Sun. Through the course of its mission, SOHO has found about 1500 new comets.
SOHO is solar-powered and therefore can continue to operate indefinitely, barring further onboard equipment failures. Its current budget from NASA guarantees continued operation through the end of 2012.
- Further Reading -
NASA. "Solar and Heliospheric Observatory Homepage" (official website).