25 Years in Space for ESA & NASA’s Sun-Watching SOHO
A quarter-century ago, the Solar and Heliospheric Observatory (SOHO) launched to space. Its 25 years of data have changed the way we think about the Sun — illuminating everything from the Sun’s inner workings to the constant changes in its outermost atmosphere.
SOHO — a joint mission of the European Space Agency and NASA — carries 12 instruments to study different aspects of the Sun. One of the gamechangers was SOHO’s coronagraph, a type of instrument that uses a solid disk to block out the bright face of the Sun and reveal the relatively faint outer atmosphere, the corona. With SOHO’s coronagraph, scientists could image giant eruptions of solar material and magnetic fields, called coronal mass ejections, or CMEs. SOHO’s images revealed shape and structure of CMEs in breathtaking detail.
These solar storms can impact robotic spacecraft in their path, or — when intense and aimed at Earth — threaten astronauts on spacewalks and even disrupt power grids on the ground. SOHO is particularly useful in viewing Earth-bound storms, called halo CMEs — so called because when a CME barrels toward us on Earth, it appears circular, surrounding the Sun, much like watching a balloon inflate by looking down on it.
Before SOHO, the scientific community debated whether or not it was even possible to witness a CME coming straight toward us. Today, SOHO images are the backbone of space weather prediction models, regularly used in forecasting the impacts of space weather events traveling toward Earth.
Beyond the day-to-day monitoring of space weather, SOHO has been able to provide insight about our dynamic Sun on longer timescales as well. With 25 years under its belt, SOHO has observed a full magnetic cycle — when the Sun’s magnetic poles switch places and then flip back again, a process that takes about 22 years in total. This trove of data has led to revolutions in solar science: from revelations about the behavior of the solar core to new insight into space weather events that explode from the Sun and travel throughout the solar system.
Data from SOHO, sonified by the Stanford Experimental Physics Lab, captures the Sun’s natural vibrations and provides scientists with a concrete representation of its dynamic movements.
The legacy of SOHO’s instruments — such as the extreme ultraviolet imager, the first of its kind to fly in orbit — also paved the way for the next generation of NASA solar satellites, like the Solar Dynamics Observatory and STEREO. Even with these newer instruments now in orbit, SOHO’s data remains an invaluable part of solar science, producing nearly 200 scientific papers every year.
Relatively early in its mission, SOHO had a brush with catastrophe. During a routine calibration procedure in June 1998, the operations team lost contact with the spacecraft. With the help of a radio telescope in Arecibo, the team eventually located SOHO and brought it back online by November of that year. But luck only held out so long: Complications from the near loss emerged just weeks later, when all three gyroscopes — which help the spacecraft point in the right direction — failed. The spacecraft was no longer stabilized. Undaunted, the team’s software engineers developed a new program that would stabilize the spacecraft without the gyroscopes. SOHO resumed normal operations in February 1999, becoming the first spacecraft of its kind to function without gyroscopes.
SOHO’s coronagraph have also helped the Sun-studying mission become the greatest comet finder of all time. The mission’s data has revealed more than 4,000 comets to date, many of which were found by citizen scientists. SOHO’s online data during the early days of the mission made it possible for anyone to carefully scrutinize a image and potentially spot a comet heading toward the Sun. Amateur astronomers from across the globe joined the hunt and began sending their findings to the SOHO team. To ease the burden on their inboxes, the team created the SOHO Sungrazer Project, where citizen scientists could share their findings.
Keep up with the latest SOHO findings at nasa.gov/soho, and follow along with @NASASun on Twitter and facebook.com/NASASunScience.
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