The ESA/NASA Solar and Heliospheric Observatory (SOHO) has marked 30 years in space since its launch in 1995, far exceeding its original two-year plan. Despite major technical setbacks, it has delivered continuous solar data across nearly three solar cycles and unexpectedly discovered over 5,000 comets. Its enduring success highlights international collaboration in space science.
Launched on December 2, 1995, SOHO was positioned 1.5 million kilometers from Earth toward the Sun, offering an uninterrupted view of solar activity. Intended for just two years, the mission has now spanned nearly three 11-year solar cycles, providing an unbroken record of the Sun's behavior.
Early challenges tested the mission's resilience. About two-and-a-half years after launch, in 1998, SOHO spun out of control, losing communication with Earth for three months. An international team recovered it through tireless efforts. Later that year, in November and December, its gyroscopes failed, but engineers developed new software, enabling operations without them by February 1999.
Prof. Carole Mundell, ESA Director of Science, praised the achievement: "It is testament to the ingenuity of our engineers, operators and scientists, and to international collaboration, that this mission has exceeded all expectations." Nicky Fox, NASA's associate administrator for the Science Mission Directorate, added: "The SOHO mission is a great example of the incredible partnerships between NASA and ESA. Congratulations to the NASA and ESA teams on an amazing thirty years working together."
SOHO's contributions include helioseismology insights revealing a single plasma conveyor belt in each solar hemisphere, circulating over 22 years and linking to sunspot patterns. Its measurements show the Sun's total energy output varies by only 0.06% over a solar cycle, though extreme ultraviolet radiation doubles between minimum and maximum, influencing Earth's upper atmosphere.
The mission's LASCO instrument, a coronagraph blocking direct sunlight to observe the corona, has been crucial for space weather monitoring. It was specifically mentioned in the U.S. PROSWIFT Act of October 2020, aiding forecasts of coronal mass ejections up to three days in advance.
Unexpectedly, SOHO became history's top comet discoverer, spotting its 5,000th in March 2024, many via citizen scientists in the Sungrazer Project. These include sungrazers and others like the Great Comet of 2024.
Daniel Müller, ESA Project Scientist for SOHO, noted: "SOHO pioneered new fields in solar science... its legacy continues to guide future missions." A paper on its legacy appeared in Nature Astronomy on December 2, 2025. SOHO's data still flows daily, informing missions like Solar Orbiter and Parker Solar Probe.