SOHO marks three decades of groundbreaking solar research and discovery

On December 2, 1995, the Solar and Heliospheric Observatory (SOHO), a collaborative mission between the European Space Agency (ESA) and NASA, was launched into space with the ambition of conducting a two-year exploration of the Sun. Fast forward 30 years, and this unassuming spacecraft, stationed 1.5 million kilometers from Earth, has become a powerhouse of solar research, providing an almost uninterrupted record of solar activity through three complete solar cycles.

Distinguished figures from the ESA and NASA praised the mission’s longevity and success. “This mission has defied expectations, showcasing the brilliance of our engineers, operators, and scientists, as well as the effectiveness of international collaboration,” stated Prof. Carole Mundell, ESA’s Director of Science. Nicky Fox, NASA’s associate administrator for the Science Mission Directorate, echoed this sentiment, celebrating the remarkable partnership that has marked SOHO’s operations over the decades.

However, the journey has not been without its challenges. Just two and a half years post-launch, SOHO experienced a critical malfunction that resulted in the spacecraft spinning uncontrollably, losing communications with Earth. It took a dedicated international team three months to regain control. Further troubles struck in late 1998 when the spacecraft’s gyroscopes failed, forcing another frantic mission to save it. In response, engineers developed new software that allowed SOHO to operate without gyroscopes, helping it continue its groundbreaking work in solar science.

SOHO has redefined the field of solar research, particularly in the monitoring of space weather. According to Daniel Müller, ESA Project Scientist for SOHO and the Solar Orbiter mission, it has revolutionized how solar storms are predicted, enabling timely forecasts that protect Earth from potential disruptions. Müller noted that SOHO continues to generate high-quality scientific data daily, with hundreds of research papers based on its findings published each year. His recent work, titled “SOHO’s 30-year legacy of observing the Sun,” published in Nature Astronomy, highlights these incredible contributions.

Among its many achievements, SOHO made significant strides in helioseismology, allowing scientists to examine the Sun’s interior like never before. Use of this technique revealed that plasma inside the Sun moves through a single conveyor-belt-like loop, a discovery that extends our understanding of solar cycles and the movement of sunspots.

Two key findings emerged over SOHO’s tenure regarding the Sun’s energy output and its impact on Earth. Researchers found that the Sun’s total energy output varies minimally—only by 0.06%—over its solar cycles. However, extreme ultraviolet radiation fluctuates significantly, doubling between solar minimum and maximum phases. This radiation plays an important role in affecting the Earth’s upper atmosphere’s temperature and chemistry, although it does not directly contribute to global warming trends observed at the surface.

SOHO has also become an indispensable tool for the timely monitoring of solar storms. Its Large Angle and Spectrometric Coronagraph (LASCO) was incorporated into US law in a 2020 act aimed at enhancing space weather forecasting. LASCO’s unique design enables it to observe coronal mass ejections as they emerge from the Sun, offering critical alerts about incoming solar disturbances.

Unexpectedly, SOHO has also become a champion comet discoverer, detecting over 5,000 comets since its launch, a feat achieved largely through contributions from citizen scientists via the Sungrazer Project. This capability is a byproduct of its coronagraph, which enables visibility of comets as they approach the Sun.

The legacy of SOHO has laid the foundation for future solar observatories, influencing both their technological advancements and scientific missions. The Solar Orbiter, for example, aims to provide images of the solar poles and includes many instruments that trace their lineage back to SOHO’s technologies. As space missions continue to evolve, SOHO remains a linchpin in the global effort to improve our understanding of the Sun and its profound influences.

With its ongoing contributions to solar science, SOHO’s three-decade journey exemplifies perseverance and innovation, serving as a beacon for future exploration in the field. As Daniel Müller aptly summarized, “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.”