Solar and Heliospheric Observatory

Lore & Background

SOHO began normal operations in May 1996. It is one of five spacecraft in the vicinity of the Earth–Sun L1 point, a gravitational balance point approximately 0.99 AU from the Sun and 0.01 AU from Earth. The spacecraft is in a halo orbit around L1, not exactly at L1, to avoid radio interference from the Sun and maintain stable communication. It orbits L1 once every six months while L1 orbits the Sun every 12 months. In 1998, an on-board emergency nearly resulted in the loss of the spacecraft, after which SOHO became the first three-axis-stabilized spacecraft to use its reaction wheels as a kind of virtual gyroscope.

Reader's Guide

SOHO's three main scientific objectives are investigating the outer layer of the Sun (chromosphere, transition region, corona), observing solar wind and associated phenomena near L1, and probing the interior structure of the Sun via helioseismology. Its twelve instruments include CDS, EIT, LASCO, SUMER, SWAN, UVCS, CELIAS, COSTEP, GOLF, MDI, VIRGO, and ERNE. The spacecraft transmits a continuous 200 kbit/s data stream via its high-gain antenna to the NASA Deep Space Network. In June 2003, the antenna Y-axis stepper motor failed, but engineers used low-gain antennas and larger ground stations to prevent total data loss. SOHO's data on solar activity are used to predict coronal mass ejection arrival times at Earth, helping protect electrical grids and satellites from geomagnetic storms. The mission has been extended until 2029, subject to review.

Did You Know?

• The spacecraft is in a halo orbit around the Sun–Earth L1 point, not exactly at L1, to avoid radio interference from the Sun.
• In June 2003, the high-gain antenna's Y-axis stepper motor failed, but engineers prevented total data loss using low-gain antennas and larger ground stations.