Solar and Heliospheric Observatory Marks Three Decades of Solar Science and Comet Discoveries

The Solar and Heliospheric Observatory (SOHO), a collaborative mission between the European Space Agency (ESA) and NASA, marked its 30th anniversary in space on December 2, 2025. Launched on December 2, 1995, from Cape Canaveral, LC-36B, aboard a Lockheed Martin Atlas IIAS vehicle, the observatory was initially designed for a two-year operational period. The mission has received multiple extensions and is currently slated to continue operations through the end of 2025, pending confirmation from ESA's Science Programme Committee.

Positioned approximately 1.5 million kilometers from Earth at the Sun-Earth Lagrange point L1, SOHO maintains an uninterrupted view of the Sun. This sustained platform has provided nearly continuous observational data across almost three full 11-year solar cycles, enabling scientists to investigate the Sun from its deep interior using helioseismology to its outer corona. The observatory’s Large Angle Spectrometric Coronagraph (LASCO) instrument, designed to occult the Sun’s glare for coronal study, has inadvertently established SOHO as the most prolific comet-discoverer in astronomical history.

As of April 1, 2025, the mission has cataloged 5,189 confirmed comet discoveries, a figure that has significantly advanced the understanding of comet composition and orbital mechanics. This success in cometary detection is largely driven by citizen scientists participating in the NASA-funded Sungrazer Project, which began around 2000 and is managed by the U.S. Naval Research Laboratory in Washington D.C. The 5,000th comet confirmation was officially recorded in March 2024, underscoring the value of this collaborative model. These sungrazers, which pass extremely close to the Sun, offer unique data as they traverse the solar atmosphere.

SOHO's longevity and comprehensive dataset now provide essential context for newer solar observatories. For example, ESA's Solar Orbiter, which adjusted its orbit in June 2025 to image the Sun's south pole, utilizes SOHO's measurements for 'multipoint' context alongside data from NASA's Parker Solar Probe, which concluded its primary mission's final perihelion in June 2025. Furthermore, instruments on NASA's Solar Dynamics Observatory (SDO) incorporate successor technologies for full-disc imaging and helioseismology, building directly upon SOHO's foundational work.

The mission's legacy also informs future endeavors, such as ESA's Vigil mission, scheduled for a 2031 launch to the Sun-Earth Lagrange point L5. Vigil is designed to monitor the Sun from a novel 'side' perspective, potentially increasing solar storm warning times by up to five days—a capability bolstered by significant funding secured at the 2025 ESA Ministerial Council. The continuous, near-real-time data stream from SOHO remains indispensable for space weather forecasting, which protects terrestrial electrical grids and orbital assets from coronal mass ejections. Despite historical operational challenges, including a major crisis in 1998 following gyroscope failures, SOHO's continued function highlights the importance of sustained international cooperation and open data policies in heliophysics, as recognized by figures such as ESA Director of Science Prof. Carole Mundell and NASA's Nicky Fox.