Staring at the Sun: Thirty Years of SOHO

Article December 6, 2025
An artist’s illustration of the SOHO spacecraft. Credit: NASA
An artist’s illustration of the SOHO spacecraft. Credit: NASA

How many thirty-year-old machines do you know of? Know anybody driving a thirty-year-old car? Using a thirty-year-old computer? Calling people on a thirty-year-old telephone or catching the news on a thirty-year-old radio? The answers to one or more of these may be yes, but I’m betting we collectively don’t know a lot of 30-year-old tech that still operates smoothly.

Now imagine tech that has spent those thirty years in space, not only being exposed to the normal wear and tear that environment provides but being regularly pummeled by solar storms because this tech’s entire job is to do nothing but stare at the Sun.

It’s the 30th anniversary of the launch of the Solar and Heliospheric Observatory, aka SOHO, and in honor of this spacecraft, designed for a two-year mission, managing to survive for thirty whole years, let’s take a look back at SOHO and its three decades—and look at that time in 1998 when things nearly went totally pear-shaped for this Sun-watcher!

 

The SOHO spacecraft under construction in Europe. Credit: NASA
The SOHO spacecraft under construction in Europe. Credit: NASA

Getting Along with Friends

Proposals for something that resembled the mission that ultimately became SOHO go back as the 1970s, but SOHO itself was first proposed in 1983. It was ultimately developed as a Cornerstone project for the European Space Agency (think of these as the agency’s big investment missions, similar to NASA’s “Flagship” missions like the Webb Telescope or my beloved Cassini).

Although SOHO began as an ESA mission it is operated as a joint mission between NASA and the European Space Agency. Most of the spacecraft was paid for by the ESA, although three of its twelve science instruments came from NASA. But NASA provided an especially key component: the rocket.

This is often how cooperative missions happen, because the launch is such a big part of the total mission expense that covering that even if you cover no other part of the mission you qualify as a partner if you can provide the ride. The Webb Space Telescope had a similar cost breakdown in the other direction: most of the spacecraft came from NASA but the rocket came from the ESA. It’s always good to have friends who can get you where you need to go.

 

Eyes on the Sun

SOHO is a chonky spacecraft, weighing in at over 4,000 lb (1,850 kg). It spans 31 ft (9.5 m) across with its solar arrays fully deployed. If you read the post a couple weeks ago celebrating the Rosetta mission, you might recognize that SOHO’s entire fully-extended body could have fit on one of Rosetta’s solar panels with nearly 15 ft (4.5 m) to spare. Just to give you an idea of how much more densely built SOHO is.

As is logical, all twelve of SOHO’s instruments are on the side of the spacecraft that points towards the Sun. Credit: NASA
As is logical, all twelve of SOHO’s instruments are on the side of the spacecraft that points towards the Sun. Credit: NASA

And SOHO needs all that chonkiness. Twelve instruments is a lot for any spacecraft to carry, and SOHO is a science powerhouse. It can measure mass flux, ion composition, and large scale structure of the solar wind, look at temperature flows, density changes, and alterations in structure of the solar corona, observe surface magnetic fields to map undersurface activity, and map the heliosphere, among many other things. 

Also, while its mission is very much focused on understanding the corona and solar wind, it is also constantly staring at the Sun, which means it’s one of the ways we know when the Sun is about to or is undergoing a solar storm. It’s one of our front-line troops in defending our planet against potentially catastrophic damage from space weather but, you know, just as a fringe benefit.

Another fringe benefit of staring constantly at the Sun is that if something happens to swing by our local star, SOHO is in a good position to see it. You know what kind of previously undiscovered objects swing by the Sun a lot? Comets. On March 25, 2024, SOHO discovered its 5,000th comet. SOHO’s fringe benefits are awesome.

 

The launch of SOHO thirty years ago, on December 2, 1995. Credit: ESA
The launch of SOHO thirty years ago, on December 2, 1995. Credit: ESA

Getting There

SOHO launched on December 2, 1995. In February 1996 the spacecraft settled into its permanent home, orbiting the Sun-Earth L1 point. This is about 1 million miles (about 1.5 million km) closer to the Sun than Earth is but objects at that point circle the Sun in the same amount of time as Earth, despite being closer (Lagrange points are a whole thing, I wrote a post about it if you’re interested).

It was not the first spacecraft that take up residence at L1. That honor, for what it’s worth, goes to the International Sun-Earth Explorer-3 (ISEE-3) spacecraft that was launched in 1978. But as far as I can tell from my research, SOHO was the second to make use of this gravitationally interesting spot in space.

SOHO became fully operational in April of 1996. The spacecraft was designed for a two-year mission. Thirty years later, here we are. Though, of course, there was that time in 1998 when it didn’t look like we’d ever get here.

 

Whoops

SOHO orbits the Sun-Earth L1 Lagrange point, about 1 million miles (1.5 million km) closer to the Sun than Earth. Credit: NOAA/ESA
SOHO orbits the Sun-Earth L1 Lagrange point, about 1 million miles (1.5 million km) closer to the Sun than Earth. Credit: NOAA/ESA

On June 25th, 1998, shortly after SOHO had completed its original two-year mission, somebody screwed up big time. While conducting a series of spacecraft maneuvers, someone sent SOHO a bad command. The spacecraft went into a spin, losing all contact with Earth and steadily losing power as it was no longer firmly pointing its solar panels at the Sun. SOHO was lost, but its team refused to give up on it.

Nearly a month after loss of signal, on July 23rd, the big dish at Arecibo Observatory, then the most powerful radio telescope in the world, sent a loud radar ping into space. When it bounced off of SOHO and back to Earth it was recovered by NASA’s Deep Space Network antenna in Goldstone, CA. Knowing exactly where the spacecraft was and how it was tumbling, on August 3 the team was able to detect faint signals from SOHO at Goldstone and the ESA’s antenna station in Perth, Australia. It was only a carrier signal though—SOHO’s batteries were mostly dead and nearly half of the fuel in its fuel tanks was frozen.

By periodically yelling at the spacecraft to turn off its bare minimum systems to allow the batteries to charge for long periods, interspersed with sessions using those batteries to power the tank heaters, by August 30th the fuel tanks were thawed and by September 16th most of the frozen thrusters and fuel lines were thought to be usable. On that date the spacecraft was sent a command to fire its thrusters to halt its spin and turn itself to once more face the Sun. On September 25th the spacecraft was back to full power.

 

Recovery

Amazingly, all twelve instruments came back online, and the instrument recommissioning process was completed by October 24th. SOHO began its science operations again on November 5th, more than four months after losing contact. There was just one slight problem.

While the instruments had come back online without a hitch, the gyroscopes that stabilize the spacecraft. SOHO launched with three gyroscopes to allow for fine pointing, though its possible to get at least rough pointing with one (this is exactly the dilemma the Hubble Space Telescope crashed into in 2024). So SOHO was down to one functioning gyroscope—until December 1998, when that final gyro died.

Not wanting the spacecraft to start tumbling again, the SOHO team used the thrusters to keep it pointed at the Sun until they could implement another clever fix. SOHO uses reaction wheels (essentially spinning flywheels that impart momentum) to rotate itself without its thrusters when it can. But, of course, if used precisely the same wheels that allow the spacecraft to turn can also be used to hold it steady.  In February 1999 SOHO became the first spacecraft to use its reaction wheels as a kind of gyroscope. It has operated in this mode for over 26 years now.

 

One of the many thousands of comets observed by SOHO while swinging past the Sun. Credit: ESA/NASA
One of the many thousands of comets observed by SOHO while swinging past the Sun. Credit: ESA/NASA

Star in Our Eyes

Talk about the Little Engine That Could! Okay, kind of a big engine in this case, but this is a spacecraft that can do everything but make a souffle that has been operating in a previously untested backup mode for over a quarter of a century after coming back from a screwup that should have scrapped it. And it’s still going! Just recently it was among the suite of spacecraft that observed the interstellar comet 3I/ATLAS from around the solar system, all while still giving daily updates on what the Sun is up to. 

It’s anyone’s guess for how much longer SOHO will operate. It’s currently funded through the end of 2025 when it’s up for review again. But this spacecraft is already the longest-lasting solar mission ever, and as long as it still has fuel in its tanks, who’s going to tell it to stop? Not me, that’s for sure! Keep flyin’, SOHO!