Thirty Years of SOHO Service: From Crisis Management to Comet Records and Helioseismology

The Solar and Heliospheric Observatory (SOHO), a collaborative venture between NASA and the European Space Agency (ESA), marked a monumental milestone on December 2, 2025: its thirtieth anniversary since launch. Deployed in 1995, the spacecraft was originally slated for just a two-year operational lifespan. Yet, it continues its vital mission from its vantage point at the Sun-Earth Lagrange point L1, providing an almost uninterrupted view of our star. This longevity, exceeding its initial design parameters by fifteenfold, stands as a testament to robust international partnership and superior engineering, as highlighted by Professor Carole Mandell, ESA Director of Science.

SOHO’s operational history has been peppered with significant technical challenges, underscoring the mission’s remarkable resilience. One of the most dramatic episodes occurred in the summer of 1998, just two and a half years after deployment, when the spacecraft lost attitude control, leading to a three-month period of silence. This event necessitated one of the most complex recovery operations ever attempted in space exploration. Shortly after this successful salvage, in December 1998, all three essential gyroscopes failed. In response, engineers at NASA and ESA devised a groundbreaking software solution.

This innovative programming allowed SOHO to maintain three-axis stabilization without relying on the failed gyroscopes. The new code was successfully uploaded to the onboard computer by February 1999, making SOHO the first spacecraft to achieve this level of stabilization without those critical components. This engineering feat ensured the mission could continue delivering crucial solar data long after the initial crises had passed.

A major scientific breakthrough facilitated by SOHO was its pioneering role in helioseismology. This technique probes the Sun's interior by meticulously analyzing its natural oscillations. The mission's findings revealed that the plasma within each solar hemisphere circulates on a single, massive global conveyor belt. The full rotation of this conveyor takes approximately 22 years, aligning perfectly with the solar magnetic cycle. Furthermore, SOHO refined our understanding of solar luminosity, showing that total solar irradiance varies by a mere 0.06% across a cycle, while extreme ultraviolet (EUV) radiation doubles between solar minimum and maximum.

Crucially, SOHO remains indispensable for monitoring space weather, largely due to the indispensable function of its LASCO coronagraph. This instrument detects coronal mass ejections (CMEs), providing up to three days of advance warning regarding potentially devastating geomagnetic storms that could impact Earth. The critical nature of this forecasting capability was formally recognized in the United States with the passage of the PROSWIFT Act in October 2020.

Perhaps the most unexpected, yet prolific, contribution from SOHO has been its transformation into the most prolific comet hunter in astronomical history. As of April 2024, the observatory has cataloged 5,000 comets. A significant portion of these discoveries were made by citizen scientists participating in the NASA-funded Sungrazer project. These objects, often dubbed 'sun-grazers,' pass perilously close to the star, becoming visible only because the coronagraph effectively blocks the Sun's blinding glare.

Looking ahead, SOHO has paved the way for next-generation instrumentation. This includes the NOAA SWFO-L1 satellite, launched on September 24, 2025, from the Kennedy Space Center in Florida. Once positioned at L1, SWFO-L1 will be renamed SOLAR-1. It represents NOAA’s first satellite dedicated entirely to operational space weather observations, ensuring data continuity previously provided by aging assets like SOHO and DSCOVR. Professor Carole Mandell and SOHO project scientist Daniel Müller, who has been a Solar Orbiter staff scientist since 2023, co-authored a review of the mission’s legacy published in Nature Astronomy on December 2, 2025. Both ESA and NASA teams universally regard SOHO as a cornerstone of modern solar physics, whose scientific return and sheer longevity have vastly surpassed its original two-year mandate.