The Solar and Heliocentric Observatory (SOHO) is a satellite that keeps an eye on the Sun’s activity. It is a joint venture between NASA and the European Space Agency (ESA) and is currently in its 22nd year of operation. SOHO is the longest-lived heliophysics mission still operating and has provided a near continuous record of solar and heliospheric phenomena over a full 22yr magnetic cycle (two 11yr sunspot cycles).
ESA in partnership with NASA and several research institutions, was interested in launching a spacecraft that could keep its eye on the Sun at all times. The goal was to better predict “space weather” and to understand more about how the Sun works. Scientists would place the $1.2 billion satellite in orbit around the Lagrange L1 point about 1.5million Kms from Earth–an area in space where the gravity of the Sun and the Earth cancel each other out.
The Sun is a yellow dwarf star at the heart of our solar system and it is the energy of the Sun that keeps us Earthlings alive, but that security comes with a price. The Sun, although a relatively stable star by celestial standards, is a highly active ball of gas. Eruptions from the Sun’s surface stream out in all directions and are frequently directed towards the Earth causing aurorae, and the more violent eruptions called CME’s (Coronal Mass Ejections) can be a danger to space farers, or knockout satellite communications and power grids. SOHO provided the first ever images of structures and flows below the Sun’s surface and of activity on the far side of the Sun. SOHO also discovered sunquakes and by using its ultraviolet imagers and spectrometers, it was able to reveal an extremely dynamic solar atmosphere and the source regions of the solar wind. The satellite has revolutionized our understanding of solar-terrestrial relations particularly the study of over 28,000 coronal mass ejections (CME’s).
For two solar cycles SOHO has measured the total solar irradiance (the ‘solar constant’) as well as variations in the extreme ultraviolet flux, both of which are important to understand the impact of solar variability on Earth’s climate. Quite unexpectedly SOHO has become the most prolific discoverer of comets in astronomical history: as of late 2017, more than 3400 comets have been found by SOHO most of them by amateur astronomers accessing SOHO real-time data via the internet. SOHO’s observations of recent dynamic activity in the Sun highlights the need to keep a watchful eye on our nearest star and its awesome power, bearing in mind that just one of these CME’s has more power than a billion H-Bombs!