A Lost Spacecraft Came Back and Changed How We See the Sun

For thirty years, SOHO has delivered an unprecedented, nearly unbroken view of the Sun, surviving multiple near-catastrophic failures and becoming a cornerstone of modern solar science.

Its long-term observations unraveled mysteries of the Sun’s internal plasma conveyor belt, revealed subtle variations in solar output, and helped make space-weather forecasting a formal national priority. Along the way, it unexpectedly became the world’s most prolific comet hunter and laid the technological and scientific foundation for a new era of solar-observing missions.

SOHO Celebrates Thirty Years Watching the Sun

On December 2, 1995, the ESA/NASA Solar and Heliospheric Observatory (SOHO) lifted off on a mission originally expected to last only two years.

From its position 1.5 million km from Earth, stationed between our planet and the Sun, SOHO has an uninterrupted vantage point of our star. For nearly three full 11-year solar cycles, the spacecraft has delivered an almost continuous record of solar activity.

“It is testament to the ingenuity of our engineers, operators and scientists, and to international collaboration, that this mission has exceeded all expectations,” says Prof. Carole Mundell, ESA Director of Science. “SOHO has overcome nail-biting challenges to become one of the longest-operating space missions of all time.”

“The SOHO mission is a great example of the incredible partnerships between NASA and ESA,” adds Nicky Fox, associate administrator, Science Mission Directorate at NASA Headquarters in Washington. “Congratulations to the NASA and ESA teams on an amazing thirty years working together.”

Early Challenges and Dramatic Recoveries

The mission’s long history includes several major crises. Two-and-a-half years after launch, a serious fault sent SOHO spinning uncontrollably and cut it off from Earth. A worldwide team worked for three months to locate the spacecraft and bring it back online.

A second major emergency arose in November and December 1998, when SOHO’s stabilising gyroscopes failed. By February 1999, engineers had developed new software that allowed the spacecraft to operate without them, enabling it to continue gathering groundbreaking solar data.

Transforming Solar Science and Space Weather Forecasting

“SOHO pioneered new fields in solar science. It is a game-changer in the study of space weather, providing real-time monitoring of the Sun to forecast potentially dangerous solar storms heading towards Earth, and its legacy continues to guide future missions,” says Daniel Müller, ESA Project Scientist for SOHO and Solar Orbiter.

“SOHO is still producing high-quality data on a daily basis, and with hundreds of papers being published every year, its scientific productivity remains very high.”

Daniel’s new paper, ‘SOHO’s 30-year legacy of observing the Sun,’ is published in Nature Astronomy today, December 2, 2025.

Here are five highlights from the last five years:

1. A single plasma conveyor belt

SOHO led the way in ‘helioseismology’. Akin to studying how seismic waves traverse Earth during an earthquake, helioseismology probes the inside of the Sun by studying how sound waves reverberate through it. Early in its career, SOHO provided the first images of plasma flows (electrically charged material) beneath the Sun’s surface, offering a unique window into its layered interior.

Thanks to SOHO’s long lifetime, scientists have used helioseismology to solve an enduring mystery: plasma flows along a single loop, or cell, in each of the Sun’s hemispheres – not multiple cells as previously thought.

The data show that it takes about 22 years for plasma to complete an entire loop around this single ‘conveyor belt’, flowing from the surface near the equator up to the poles, then traveling back down deep inside towards the equator. This matches the timeline of the Sun’s magnetic cycle, explaining how sunspots – regions where intense magnetic fields break through the Sun’s surface – emerge progressively closer to the equator over the solar cycle.

2. Does the Sun shine steadily?

The amount of energy that floods out of the Sun is a fundamental quantity in understanding the impact of solar heating on Earth’s atmosphere and climate. SOHO’s three decades of data, in combination with older measurements, provide unrivalled measurements spanning nearly fifty years.

The total energy output of the Sun changes very little – on average, by only 0.06% over the solar cycle. By contrast, the variation in extreme ultraviolet radiation is substantial, doubling between solar minimum and maximum. Solar extreme ultraviolet radiation significantly influences the temperature and chemistry in Earth’s upper atmosphere, but is not a direct driver of the global warming trends observed near Earth’s surface.

3. Solar storm monitoring made law

SOHO has played such a pivotal role in the development of real-time space weather monitoring systems that it was signed into United States law in October 2020.

The ‘Promoting Research and Observations of Space Weather to Improve the Forecasting of Tomorrow’ (PROSWIFT) act specifically mentions SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) instrument.

LASCO is a coronagraph, a telescope with a disc masking the center of view. By blocking out the direct light coming from the Sun, the instrument can see light from the surrounding atmosphere, called the corona. This allows us to see coronal mass ejections – large eruptions of solar material and magnetic fields – as they set off from the Sun, providing up to three days warning of potentially disruptive incoming space weather reaching Earth.

4. 5000 comets – and counting!

The telescope’s prowess as a comet hunter was unplanned, but turned out to be an unexpected success. Thanks to the screening effect of SOHO’s coronagraph, ‘sungrazer’ comets – those that approach the Sun at very close distances – also become visible.

Not all comets seen by SOHO are sungrazers. For example, it also beautifully captured Comet Tsuchinshan–ATLAS, also called the Great Comet of 2024, a non-periodic comet from the outer reaches of the Solar System.

SOHO discovered its 5000th comet in March 2024, making it the most prolific comet-discoverer in history. Most of these have been found by citizen scientists worldwide through the Sungrazer Project. The observations have provided valuable data on the movement, composition, and dust production of comets.

5. Enabling future discoveries

SOHO’s success has shaped the next generation of solar observatories, both in terms of their technology and scientific objectives, as well as being a role model for open data policies and international collaboration.

For example, the ESA-led Solar Orbiter mission is imaging the solar poles from higher latitude and flying much closer to the Sun, with many of its instruments being successors of SOHO’s. Similarly, NASA’s Solar Dynamics Observatory carries improved versions of SOHO’s instruments to continue the legacy that SOHO began in areas of full-disc imaging and helioseismology. Moreover, SOHO frequently contributes to ‘multipoint’ measurements, providing essential context for Solar Orbiter and NASA’s Parker Solar Probe as they fly along their own unique orbits around the Sun.

Even more recently, ESA’s Proba-3 took to the skies to open up new views of the Sun’s faint corona, while the Agency’s upcoming Vigil mission will be the first to monitor the Sun from the ‘side’, detecting solar storms before they roll into SOHO’s line-of-sight.

“SOHO is an all-round shining success, thanks to the dedication of the teams keeping this incredible machine flying,” says Daniel. “Its science remains valuable and relevant, serving generations of scientists, and I’m certain that its legacy will continue to guide solar science for decades to come.”

Reference: “SOHO’s 30-year legacy of observing the Sun” by Daniel Müller, Jack Ireland, Anik De Groof, George Dimitoglou and Bernhard Fleck, 2 December 2025, Nature Astronomy.
DOI: 10.1038/s41550-025-02687-4

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