Solar and Heliospheric Observatory, better known as SOHO, left Earth in December 1995. Back then, no one imagined it would become a permanent fixture of modern solar science. It was a compact spacecraft with a tightly focused purpose: to observe the Sun continuously from the Sun-Earth L1 point, 1.5 million kilometres away, and deliver the kind of uninterrupted dataset that ground-based telescopes could never achieve. Engineers planned for a two-year operational period. Scientists hoped for perhaps a few more. What SOHO ultimately delivered is something far greater: a 30-year record of the Sun’s changing atmosphere, its magnetic cycles, its violent outbursts, and the invisible streams of particles that fill near-Earth space.
Over those decades, SOHO evolved into a benchmark for solar missions, a reference for space-weather forecasters, and a workhorse whose archive is still used today to calibrate new spacecraft. The mission’s longevity has reshaped how researchers study the Sun. It has also shown why stable, continuous observations matter for every field that depends on understanding solar activity, from climate modelling to the safety of satellites and astronaut missions.
Three decades of watching a restless star
SOHO’s position at L1 gives it an advantage no Earth-based facility can match: an uninterrupted, full-disc view of the Sun every hour of every day. That steady watch has captured the rise and fall of nearly three full solar cycles. With each cycle lasting about eleven years, the three-cycle record created by SOHO allows scientists to study long-term behaviour without gaps or instrument shifts that normally complicate comparisons across decades.
Researchers used SOHO’s helioseismic instruments to map the Sun’s interior flows with an accuracy that had not been possible before. The spacecraft revealed the presence of large-scale circulation streams deep below the photosphere, flows that help transport magnetic fields across the solar interior and shape how sunspot belts drift over time. These insights are essential because they feed into solar-dynamo models that try to predict the amplitude and character of future cycles.
SOHO’s Extreme Ultraviolet Imaging Telescope (EIT) and LASCO coronagraph extended that picture into the outer atmosphere. LASCO’s ability to record faint coronal structures made it the most productive coronal mass ejection observer ever flown. Its continuous data record became the foundation of modern space-weather forecasting. These images are now used to estimate the speed, direction, and potential impact of solar storms that can disturb GPS systems, radio communication, and power grids on Earth.
The near-losses in the mission
SOHO’s 30-year run is even more remarkable given the crises that threatened it early in life. In 1998, the spacecraft entered an uncontrolled spin after a sequence of altitude-control mistakes. For weeks, SOHO was silent, tumbling, and possibly lost. Engineers on both sides of the Atlantic worked around the clock to recover it. They re-established contact, stabilized the spacecraft, and brought its instruments back online, a recovery that has since become a case study in spacecraft rescue.
The triumph was short-lived. Months later, SOHO lost all three of its gyroscopes. Those were essential for fine-pointing, and their failure could have ended the mission. Instead, the operations team wrote new software that allowed SOHO to fly without gyros, relying instead on reaction wheels and solar sensor data. The solution was bold and untested, yet it worked. That improvisation is the reason the mission survived long enough to reach its 30th anniversary.
Comet discoveries and the rise of public science
One of SOHO’s most unexpected legacies is its impact on comet discovery. LASCO’s coronagraphs often capture small objects plunging toward the Sun. Amateur astronomers began scanning the images, identifying moving points of light that turned out to be new comets. Over time, this citizen-science activity became organized, forming what is now known as the Sungrazer Project.
SOHO has discovered more than 5,000 comets through this effort, far more than any telescope in history. While this is not central to the mission’s scientific goals, it demonstrates the value of open data and public participation. It also highlights how a mission designed for one purpose can still produce major contributions in other fields.
A mission that still sets the baseline
Even today, new spacecraft rely on SOHO’s long archive for calibration and comparison. ESA’s Solar Orbiter and NASA’s Parker Solar Probe both observe the Sun from new vantage points and at unprecedented proximity. But their data must be understood relative to long-term trends, and SOHO provides those trends with unmatched continuity. Where Solar Orbiter captures high-resolution views of the polar regions, and Parker Solar Probe samples the solar wind from inside the corona, SOHO remains the reference frame that ties these missions together.
SOHO’s longevity has reshaped expectations for solar missions. Engineers now plan with recovery options in mind. Scientists understand the value of uninterrupted datasets not as luxuries but as necessities. Space-weather forecasters rely on a heritage that started with SOHO and evolved through generations of missions.
Thirty years after launch, the spacecraft remains operational. Its images still appear in daily space-weather reports. Its archive is still mined for research on irradiance, solar wind structure, and CME dynamics. It holds a place in space science not because it was the newest or largest observatory, but because it stayed in the sky long enough to reveal patterns that only time can show.
Clear skies!
