The Solar and Heliospheric Observatory launched on December 2, 1995. A joint mission between the European Space Agency and NASA, SOHO’s original operating phase was scheduled for two years – and now, through repeated extensions, it is celebrating a quarter century in orbit. Over the years, its set of groundbreaking instruments became a source for numerous scientific findings, an inspiration for follow-on missions, and an outlet for citizen scientists. SOHO also survived near catastrophe twice and has become the longest-running Sun-surveying spacecraft. What this powerhouse mission has witnessed in its 25 years has changed the way humanity sees the Sun.
The revolution started in its design. SOHO was meant to provide a comprehensive look at the flow of energy and material from the Sun toward Earth. The 12 instruments onboard allowed the spacecraft to return a specialized combination of observations – an asset for solar scientists who wanted to understand how our star worked. At the time, this kind of basic physics research was considered the main goal, but over the last quarter of a century, researchers learned they could, in fact, begin to monitor our Sun in real time, studying and attempting to predict the space weather it sent our way.
“At the time SOHO was designed, very few people talked or thought about space weather,” SOHO Project Scientist Bernhard Fleck at ESA said. “But now, I look at SOHO observations like weather radar. Now it is as normal as opening your weather app and checking when the rain is coming.”
This ability is due to SOHO’s coronagraphs, specialized telescopes that block the bright face of the Sun to allow for better visibility of the faint light extending from the star. SOHO’s Large Angle and Spectrometric Coronagraph, known as LASCO, provides a 360-degree view of the atmosphere around the Sun.
New science came out of LASCO’s ability to image giant eruptions of solar material and magnetic fields, known as coronal mass ejections, or CMEs. Researchers could finally see the shape and structure of CMEs in breathtaking detail. When these storms are aimed at Earth, they can impact the functionality of spacecraft, threaten astronauts on spacewalks and even, when very intense, impact power grids on the ground.
LASCO was especially useful in viewing Earth-bound storms called halo CMEs – so called because when one views a CME barreling toward us on Earth, it appears circular, surrounding the Sun, much like watching a balloon inflate by viewing the top of the balloon. Before SOHO, the scientific community debated whether or not it was even possible to witness a CME coming straight toward us, but today, LASCO images are the backbone of space weather prediction models. They are regularly used in forecasting the impacts of space weather events traveling toward Earth.
“Having a coronagraph observing all around the Sun helped us to see CMEs coming toward us,” said Terry Kucera, astrophysicist in NASA’s Goddard Space Flight Center’s Solar Physics Laboratory in Greenbelt, Maryland. “That’s been really critical in understanding space weather and allowing scientists to study how CMEs affect us here on Earth.”
SOHO Citizen Science
In addition to advancing solar science, SOHO’s LASCO coronagraphs helped it to become the greatest comet hunter in space. The mission has discovered over 4,000 comets to date, many of which were found by citizen scientists.
It began with the availability of LASCO data online. During SOHO’s time in orbit, internet access became common in homes around the world. This made it possible for anyone to carefully scrutinize an 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.
“The Sungrazer Project is one of the oldest citizen science projects,” said Karl Battams, a computational scientist who runs the project at the US Naval Research Laboratory in Washington, D.C.
“The citizen scientists who participate – whether they realize it or not – they are learning so much more than just how to look for little moving dots of light in an image,” Battams continued. “They are having to think about the physics of the situation. They are having to understand a coronagraph image. We aren’t feeding them simplified subsets of the science data. We are saying ‘Here are the science products. Go make some new science from it.’”
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. The star flips magnetic polarity every 22 years. It also ramps up and down in activity every 11 years.
With 25 years under its belt, SOHO has observed full versions of both cycles. EIT, SOHO’s Extreme ultraviolet Imaging Telescope, able to observe in wavelengths of light that are impossible for us to see from the ground as they are blocked by our atmosphere, was poised to see it all. The instrument uncovered solar phenomena like Sun-spanning waves in the corona associated with CMEs.
EIT was the first instrument of its kind to be in orbit, having only previously flown on short rocket flights. The telescope’s continuous view of these processes made it an inspiration for other missions.
“I think SOHO has proven the value of long baseline studies of phenomena that changes on bi-decadal timescales,” said former NASA project scientist Joe Gurman. “Maybe as a result of that success, SOHO has spawned successors.”
SOHO made the case for higher resolution data, inspiring proposals for other missions. Spacecraft like the Solar Dynamics Observatory and Solar TErrestrial RElations Observatory owe their stunning observations in the extreme ultraviolet to their predecessor, EIT. Twenty five years after launch, the technology on those newer missions has been substantially updated from what’s flying on SOHO.
“A one megapixel camera at the time [of SOHO’s launch] was absolutely state-of-the-art,” said Fleck. “You could not sell a cell phone now with a one megapixel camera. When you compare the times, the more amazing it is that we still do really competitive science with that old hardware.”
Despite the fact that newer instruments have more advanced technology, SOHO remains an unmatched trove of continuous data. Six thousand scientific publications to date have made use of SOHO data, and the mission still produces almost 200 papers a year.
There was a chance that we could have lost that long timeline of data, however. All of SOHO’s research potential was nearly lost in June of 1998. During a routine spacecraft maneuver, the operations team lost contact with the spacecraft. With the help of a radio telescope in Arecibo, the team eventually located the spacecraft. SOHO went from cold and spinning through space to awake and productive by November of the same year.
The spacecraft’s good health only held out so long. Complications from the near loss event emerged just weeks later, when all three gyroscopes – which helped 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. It was another chance at life for the mission. SOHO resumed normal operations in February 1999, and it became the first spacecraft of its kind to function without gyroscopes.
The story of SOHO’s recovery and legacy continue to motivate its current US Project Scientist, Jack Ireland, at NASA’s Goddard Space Flight in Greenbelt, Maryland.
“That’s one thing about SOHO that should be emphasized: It is extremely ambitious. You have 12 instruments on a platform which is a million miles away, and we are going to look at the whole kit and kaboodle of the Sun?” said Ireland. “And then to say ‘We are not going to give up. We are going to fight for this thing.’ That takes a degree of ambition that is inspiring.”
As Ireland looks ahead for SOHO, he sees the mission’s prolific past as proof of a bright future.
“Twenty five years should just be the start. From a scientific point of view, we need to keep going, we can’t take our eyes off the Sun.”
By Genna Duberstein
NASA’s Goddard Space Flight Center, Greenbelt, Md.