Thirty years after launch, the SOHO space observatory remains humanity's “eye” on the Sun. From December 2, 1995, to December 3, 2025, the joint mission of the European Space Agency (ESA) and NASA has evolved from a planned short experiment into a leading platform for long-term, uninterrupted observation of our stellar neighbor. Located approximately 1.5 million kilometers from Earth in the vicinity of the L1 point of the Sun–Earth system, the Solar and Heliospheric Observatory (SOHO) spacecraft provides a unique, uninterrupted view of solar storms, magnetic cycles, and changes in solar irradiance that shape the space environment around our planet.

SOHO has become synonymous with perseverance, engineering ingenuity, and international cooperation. After losing orientation and contact with Earth in 1998, the spacecraft was—thanks to a complex international search and precise interventions—restored to function. Subsequently, in just a few months, it “learned” to fly without gyroscopes: engineering upgrades enabled it to stabilize using reaction wheels, star trackers, and Sun sensors. Thus, SOHO became the first three-axis stabilized spacecraft to routinely operate without gyroscopes—a solution that changed management standards for similar missions and extended the mission's life by decades.

Why L1: The stage for a solar “non-stop broadcast”

The L1 point provides a gravitational balance between the Sun and Earth. A spacecraft in a halo orbit around L1 does not pass through terrestrial nights or eclipses and has a constant “frontal” view of the Sun. Because of this, SOHO has been a source of continuous images and real-time data from the very beginning. From total solar irradiance (TSI) radiometry by the VIRGO instrument, through helioseismological measurements (MDI) that “listen” to solar oscillations, to spectacular sequences from the LASCO coronagraph revealing the crown (corona) and the departures of coronal mass ejections (CMEs)—insights from SOHO enter scientific papers, operational space weather forecasts, and educational materials worldwide.

Scientific Milestones: From the Sun's Interior to an Alarm System for Earth

Helioseismology: First time “under the skin” of a star

SOHO opened the door to helioseismology—a discipline that studies the Sun through the travel of sound waves through its interior. The MDI (Michelson Doppler Imager) instrument enabled the first detailed maps of plasma flows beneath the photosphere, revealing jet-stream-like structures and global “winds” in the convection zone. Combined series from MDI (SOHO) and HMI (SDO) also resolved a long-standing dilemma regarding large meridional flows: instead of multiple cells, each solar hemisphere is dominated by one vast “conveyor belt” that guides plasma from the equator toward the poles and deep back toward the equator, in a cycle close to 22 years in length. Such dynamics naturally link internal flows and the solar magnetic cycle and explain why sunspot belts migrate toward the equator over time.

How stably the Sun shines: TSI versus variable EUV

Total Solar Irradiance (TSI)—energy per square meter at the top of Earth's atmosphere—changes relatively little: on the order of about 0.1% during a typical eleven-year cycle. A long, carefully calibrated series of VIRGO measurements from the SOHO spacecraft enabled comparisons with other radiometric instruments and solidified reference values for climate studies. Unlike TSI, extreme ultraviolet (EUV) and soft X-ray radiation vary much more strongly: across most of the EUV spectrum, changes are approximately double between activity minimum and maximum, and in shorter wavelengths and X-rays, they reach even greater factors. This variability directly heats and ionizes the upper atmosphere, affecting satellite orbit expansion, radio communications, and GNSS accuracy.

LASCO and space weather forecasting: From science to operations

LASCO is a coronagraph—a telescope with a disk mask that blocks the glaring photosphere, revealing the extremely faint corona. In its frames, CMEs are seen as vast “caps” of plasma and magnetic field detaching from the Sun. These sequences serve as input for operational models like WSA-Enlil, which determine if and when a frontal wave impact and change in solar wind will reach Earth. The usual warning window ranges from 1 to 4 days, depending on the speed and geometry of the ejection and interplanetary conditions. The US Space Weather Prediction Center (NOAA/SWPC) has systematically used LASCO in operations since 2011, and a new generation of operational coronagraphs (e.g., CCOR-1) further strengthens this link between science and daily monitoring.

Unexpected hit: 5000 comets—and counting

SOHO has also become the most prolific “comet hunter” in history. Thanks to the Sungrazer citizen science project, volunteers around the world systematically review LASCO images and report discoveries. In March 2024, the catalog reached the 5,000th comet, solidifying SOHO's title as the most successful comet discoverer of all time. And 2024 was further marked by the spectacular passage of comet C/2023 A3 (Tsuchinshan–ATLAS) through LASCO's field of view—during strong eruptions on the Sun—when a unique, narrow dust trail visible across the entire frame was recorded.

Open data and instrumentation legacy

SOHO also shaped the philosophy of open data and the instrumentation “matrix” for the generation that follows. Solar Orbiter images the Sun's poles from higher heliographic latitudes and flies much closer to the Sun than SOHO, while the Solar Dynamics Observatory (SDO) continues the tradition of full-disk images and high-resolution helioseismology. In “multipoint” configurations, together with NASA's Parker Solar Probe, missions create a bridge between eruption sources and measurements in the solar wind—a synergy that enables comparisons, model verification, and a deeper understanding of solar-terrestrial connections.

Five highlights of recent years

1) One “conveyor belt” per hemisphere

Cumulative records from MDI (SOHO) and HMI (SDO) showed that meridional circulation in each hemisphere on average forms one large cell with a full circuit on the order of 22 years. This image elegantly explains the migration of the sunspot belt toward the equator throughout the cycle and strongly links internal flows, magnetic field regeneration, and surface manifestations of activity.

2) TSI: small change, great significance; EUV: big “swing”

VIRGO measurements showed that TSI varies about 0.1% throughout the cycle—little, but consistent and climatically relevant. In contrast, EUV radiation typically doubles between minimum and maximum, and in the shortest waves and X-ray region, fluctuations are even greater. These changes are directly reflected in the thermosphere and ionosphere, with consequences for satellite dynamics, radio links, and positioning.

3) From laboratory to law

In 2020, the PROSWIFT Act was adopted in the USA, a law strengthening the national space weather observation and forecasting system. Although a general framework, in operational practice LASCO images have long been a key input for NOAA forecasts and WSA-Enlil simulations providing 1–4 days of warning for possible geomagnetic storms.

4) 5000th comet—the crown of citizen science

March 2024 brought the 5,000th comet in the SOHO catalog—a historic threshold achieved thanks to thousands of volunteers. Additionally, autumn 2024 delivered a visual spectacle: Tsuchinshan–ATLAS in the same frame with powerful eruptions, which LASCO recorded with exceptional detail.

5) Proba-3 and Vigil: looking beyond today's horizon

ESA's Proba-3 demonstrated in 2025 the first controlled “artificial eclipses”—two satellites in precise formation creating an artificial occultation for hours and opening a view to the innermost corona. The next step for the operational community is Vigil, the first European mission envisaged for constant “side” monitoring of the Sun from the vicinity of Lagrange point L5, which will enable earlier detections of eruptions just emerging in Earth's direction.

SOHO today: Infrastructure that is hard to do without

Thirty years after launch, SOHO remains the foundation of solar “situational awareness.” LASCO remains the only space instrument continuously providing high-frequency images of the corona from Earth's direction; other instruments (e.g., SEM, CELIAS, ERNE) provide real-time radiation and particle data. Public animations and fast GIF/MPEG clips, available to everyone, allow professionals and enthusiasts to follow events almost as if in a control room.

What's next

As the formal end of 2025 approaches, SOHO continues to deliver valuable data and context without which it would be difficult to understand the records of Solar Orbiter and Parker Solar Probe. Even when the spacecraft eventually falls silent, its archive will remain the golden standard: from TSI calibrated by VIRGO and long series of CMEs to the statistics of thousands of comets. In an operational sense, a new generation of instruments (such as CCOR-1) and Vigil from a “side” angle will complement the picture—building on the legacy of a mission that showed the most enduring results arise when science, engineering, and open data work together.