SOHO: Our Eye on the Sun

Our Sun is the nearest star, the one object that dominates our sky and sustains all life on Earth. And yet, for most of human history, we understood remarkably little about how it actually works. We knew it was hot, we knew it rose and set, and we could sometimes see dark spots on its face through smoked glass. But the Sun’s true nature — a churning ball of plasma with a magnetic personality that drives eruptions visible across the solar system — only came into focus with the advent of space-based solar observatories.

The most enduring of these is SOHO, the Solar and Heliospheric Observatory. Launched in December 1995 as a joint mission between ESA and NASA, SOHO has been staring at the Sun almost continuously for three decades, producing a body of observations that has fundamentally reshaped solar physics. Here is the story of the spacecraft that taught us to understand our star.

Where SOHO Lives: The L1 Lagrange Point

SOHO does not orbit Earth. Instead, it occupies a special location in space called the first Lagrange point, or L1, which sits about 1.5 million kilometers from Earth in the direction of the Sun. At L1, the gravitational pull of the Earth and the Sun balance in such a way that SOHO can maintain a stable position relative to both, orbiting the Sun at the same angular rate as Earth while staying permanently between the two.

This location is ideal for solar observation because SOHO has an uninterrupted view of the Sun — it never passes behind Earth or into Earth’s shadow (as satellites in low Earth orbit regularly do). The L1 vantage point also places SOHO upstream in the solar wind, allowing it to detect incoming solar particles and coronal mass ejections before they reach Earth — a critical advantage for space weather forecasting.

SOHO’s original mission was designed to last just two years, with a possible extension to six. The fact that it is still operating over 30 years later is a testament both to the robustness of its engineering and to the continued scientific value of its observations. SOHO has outlasted most of the scientists who designed it and continues to generate new discoveries.

SOHO’s Instrument Suite

SOHO carries 12 scientific instruments that collectively observe the Sun from its interior (through helioseismology) to its outer atmosphere (the corona) to the solar wind streaming past the spacecraft. This comprehensive coverage is what makes SOHO so valuable — it does not just look at one aspect of the Sun, it monitors the entire system.

LASCO: The Coronagraph

Perhaps SOHO’s most famous instrument is LASCO (Large Angle and Spectrometric Coronagraph), which creates artificial eclipses by blocking the Sun’s bright disk with an occulting disk, revealing the faint outer corona. LASCO has two operational coronagraphs (C2 and C3) that image the corona from about 1.5 to 30 solar radii from the Sun’s center, capturing coronal mass ejections (CMEs), comet passages, and the ambient solar wind in real time.

LASCO images are what most people picture when they think of SOHO — the Sun’s disk blocked by a white or blue circle, with graceful arcs and streamers of coronal gas extending outward. These images have become a cornerstone of space weather monitoring, because they show CMEs — massive eruptions of magnetized plasma — as they leave the Sun and head into the solar system.

EIT and MDI: Seeing the Sun’s Face

The Extreme ultraviolet Imaging Telescope (EIT) produces images of the Sun in four extreme ultraviolet wavelengths, each of which highlights plasma at a different temperature. These images reveal the structure of the Sun’s atmosphere in vivid detail — active regions, coronal holes, prominences, and flares all appear with dramatic clarity. EIT’s images of the Sun in 304 Angstrom light (showing plasma at about 60,000 K) have become some of the most iconic solar images ever produced.

The Michelson Doppler Imager (MDI) measured the velocity of gas on the Sun’s surface with extraordinary precision, enabling helioseismology — the study of the Sun’s interior through the analysis of sound waves that propagate through the solar plasma. Helioseismology has revealed the internal structure and rotation of the Sun in remarkable detail, including the discovery that the Sun’s rotation rate changes with depth and latitude.

SOHO’s Greatest Discoveries

Coronal Mass Ejections and Space Weather

Before SOHO, coronal mass ejections were poorly understood phenomena detected sporadically by earlier coronagraphs. SOHO’s continuous monitoring with LASCO has catalogued tens of thousands of CMEs, revealing them to be a fundamental aspect of the Sun’s behavior. SOHO data showed that CMEs are far more common than previously thought — during solar maximum, multiple CMEs can erupt per day — and that their frequency, speed, and direction follow patterns linked to the 11-year solar cycle.

This observational database has been invaluable for space weather forecasting. When a CME is aimed at Earth, it can trigger geomagnetic storms that affect satellites, communications, power grids, and astronaut safety. SOHO’s LASCO coronagraphs provide early detection of Earth-directed CMEs, typically giving 1 to 3 days of advance warning before the CME arrives. This early-warning capability has become an essential part of modern space weather infrastructure.

Sungrazing Comets

One of SOHO’s most unexpected contributions has been the discovery of comets — over 5,000 of them, making it the most prolific comet-finder in history. Most of these are sungrazing comets of the Kreutz group — tiny fragments of a single large comet that broke apart centuries or millennia ago, now falling into the Sun in an endless stream of cosmic debris.

These comets are discovered primarily in LASCO coronagraph images, where they appear as bright streaks approaching the Sun and, in most cases, not emerging from the other side. Many of these discoveries have been made by citizen scientists and amateur astronomers analyzing SOHO images online — a wonderful example of how public engagement can contribute to real scientific discovery.

Solar Interior and Helioseismology

SOHO’s helioseismology instruments (MDI and its successors) have mapped the interior of the Sun with a precision that was unimaginable before the mission. By analyzing the frequencies and patterns of millions of acoustic oscillations on the solar surface, scientists have determined the Sun’s internal temperature, density, and rotation profile as a function of depth.

One of the most important helioseismology results was the measurement of how fast different layers of the Sun rotate. The Sun’s surface rotates differentially — faster at the equator than at the poles — and helioseismology showed that this differential rotation extends deep into the convective zone. At the base of the convection zone, there is a thin layer called the tachocline where the rotation profile changes abruptly. The tachocline is thought to be the region where the Sun’s magnetic field is generated through a dynamo process, making it a key piece of the puzzle for understanding the solar cycle.

Solar Wind Origins

SOHO’s ultraviolet spectrometers (UVCS and SUMER) have studied the acceleration of the solar wind — the continuous stream of charged particles flowing outward from the Sun at speeds of 300 to 800 km/s. These observations showed that the fast solar wind originates in coronal holes (dark regions in the corona where magnetic field lines open into interplanetary space) and is accelerated by a combination of wave heating and magnetic forces that are still not fully understood.

Understanding the solar wind is important for many reasons, including its effects on Earth’s magnetosphere, its role in shaping planetary atmospheres, and its influence on the environment through which spacecraft travel. SOHO’s data has been fundamental to building the models of solar wind propagation that are now used in space weather forecasting.

SOHO’s Near-Death Experience

In June 1998, SOHO almost died. A series of errors during a routine maintenance procedure caused the spacecraft to lose its orientation, and mission controllers lost contact. Without its solar panels pointing at the Sun, SOHO’s batteries drained, its hydrazine fuel froze, and the spacecraft went completely silent.

What followed was one of the most remarkable rescue operations in the history of space exploration. Using powerful radar from the Arecibo radio telescope, engineers located SOHO tumbling slowly in space. Over several weeks, they sent carefully timed commands during brief windows when SOHO’s antenna happened to point toward Earth. Gradually, they stabilized the spacecraft, thawed its frozen systems, and brought it back to operational status. The recovery was a masterpiece of engineering improvisation and is studied as a case study in spacecraft emergency operations to this day.

SOHO and Amateur Solar Observing

For amateur astronomers interested in the Sun, SOHO provides a wonderful complement to visual solar observing. You can view SOHO’s latest images online in near real-time, comparing what you see through your own solar telescope or solar filter with the ultraviolet and coronagraph views from space.

If you observe sunspots through a white-light solar filter, checking SOHO’s EIT images will show you the active regions associated with those sunspots and the complex magnetic structures in the corona above them. If you have a hydrogen-alpha solar telescope, you can observe prominences on the solar limb and compare them with SOHO’s 304 Angstrom images, which show the same features in extreme ultraviolet light.

If you are curious about observing the night sky beyond our Sun, choosing the right telescope is the essential first step. Our guide to choosing your first telescope covers the considerations for both solar and nighttime observing. And for understanding the broader context of how planets interact with the Sun, our guide to visible planets helps you track what is up in the sky on any given evening.

SOHO’s Legacy and the Future of Solar Observation

SOHO’s legacy is immense. It has produced over 30,000 scientific papers, discovered over 5,000 comets, and created a continuous record of solar activity spanning multiple solar cycles. This long baseline of observations is irreplaceable for understanding long-term solar variability and its effects on Earth’s climate and space environment.

More recent solar missions — including NASA’s Solar Dynamics Observatory (SDO), the Parker Solar Probe, and ESA’s Solar Orbiter — have built on SOHO’s foundation, providing higher resolution, closer approaches to the Sun, and new perspectives from outside the ecliptic plane. But SOHO continues to fill a unique niche with its coronagraphs and its uninterrupted L1 vantage point, and it remains an active part of the solar observation network.

For me, SOHO is a reminder that the most familiar object in the sky — the Sun we see every day — is also one of the most complex and dynamic. Thirty years of continuous observation have revealed a star that is churning, erupting, oscillating, and hurling material into space with a restless energy that affects every planet in the solar system. SOHO gave us the tools to watch this unfold in real time, and in doing so, it made the Sun feel less like a distant furnace and more like a living, breathing neighbor whose moods we are finally learning to read. To explore how space telescopes have transformed our understanding of the universe beyond the Sun, our article on Hubble’s greatest discoveries tells a parallel story of what happens when you put a powerful telescope above the atmosphere.