SOHO and Its Eyrie

On December 2, 1995, the Solar and Heliospheric Observatory (SOHO) spacecraft was launched onboard an Atlas rocket from Cape Canaveral in Florida. It’s the size of a minivan, or, with its solar panels extended, about the size a school bus, and it weighs roughly two tons. SOHO was conceived by the European Space Agency; fourteen countries and more than three hundred engineers were involved in its design and construction. NASA was respon­sible for the launch and ground operations. It’s a testament to the power of collaboration that so many nationalities can work to­gether to produce a state-of-the-art scientific experiment.

SOHO is like a Swiss army knife, reminiscent of Cassini in its size and complexity. Its available space is crammed with twelve instruments that can measure everything from magnetic fields to X-rays. Nine instruments are led by European scientists and three are led by scientists from the United States, but all of them have teams that are a patchwork quilt of nationalities. Unnoticed by the fractious world of politics and tribalism, science is one field of en­deavor where national distinctions and borders are almost mean­ingless. Some of the instruments have intimidating names, like the “Comprehensive Suprathermal and Energetic Particle Analyzer” built by the University of Kiel in Germany, but generally they all measure the location, intensity, and spectrum of either high-energy X-ray and ultraviolet radiation or cosmic rays. Multinational har­mony does not, however, extend to gender parity. Space astronomy is still male-dominated; all twelve instrument principle investiga­tors and 80 percent of the science team members are men.17

SOHO moves around the Sun in step with the Earth, by slowly rotating around a point in space called the first Lagrangian point (L1), where the sum of the Sun and the Earth’s gravity combine to keep the satellite locked onto the Sun-Earth line.18 This position is about a million miles away from the Earth in the direction of the Sun, about four times the distance to the Moon. SOHO’s eyrie gives it a ringside seat for watching solar activity. Other space mis­sions are using or plan to use L1, but it’s perfect for solar observa­tions since a spacecraft in this orbit is never shadowed by the Earth or the Moon.

SOHO has been beaming data to the Earth from twelve scien­tific instruments at a gigabyte, or two CD’s worth, per day. Analy­sis of this data has yielded some exciting insights into the Sun, including the first images of a star’s convection zone, where energy is carried from the fusion core to regions near the surface, and the structure of sunspots just below the photosphere. SOHO has also provided the best measurements to date of the temperature, rota­tion, and gas flow patterns within the Sun, and it has revealed new types of solar activity, such as waves in the corona and tornadoes on the surface. As of late 2012, SOHO data had been used to dis­cover over 2,350 comets.19 The spacecraft had a nominal lifetime of two years. In 1997, it was extended for five years due to its great success. In 2002, it got another five-year extension, and in 2009 it got a third extension, until the end of 2012. SOHO is now well into its second solar cycle of observations. However, it was not always smooth sailing.