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What would you do if you were in charge of a billion-dollar satellite that was spinning out of control? In 1998, NASA and ESA engineers had to solve this exact problem. How did they avert this disaster?

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In June of 1998, NASA and the European Space Agency seemed to have a disaster on their hands. A billion-dollar satellite had gone out of control: It couldn’t stop spinning, it wouldn’t talk to Earth, its fuel was starting to freeze, and its batteries were draining by the day.

Scientists around the planet spent months trying to save the satellite—and in the end, well, I’m getting ahead of myself. The spacecraft was known as the Solar and Heliospheric Observatory, or SOHO, and researchers designed it to help us understand the Sun. Partly, we just wanted to get a better idea of how the giant hydrogen bomb at the center of our solar system works on a fundamental level.

But scientists also hoped that this knowledge would help us better protect ourselves from its outbursts, which occasionally mess with our satellites or power grids. SOHO had just the tools for the job: It was outfitted with 12 instruments specially designed to study things like the Sun’s corona, its lower layers, and the steady wind it emits made of high-energy particles. But little did anyone know… it was in for a wild ride.

For one, it had a major close call before it even made it to space. Just an hour before launch, a piece of the rocket that would launch SOHO broke, and the mission was delayed. It just barely missed blowing up with SOHO on board.

But eventually, the spacecraft made it up in one piece in December of 1995. Early on, it spied enormous tornadoes on the Sun’s surface, using its special spectrometer, designed to pick up light from plasma in the solar atmosphere. The tornadoes were made of super-hot plasma and were nearly as wide as the Earth.

This gave us an idea just how turbulent the surface of the Sun is, even at a time when solar activity is relatively low. And what made this even more intriguing was that scientists thought these tornadoes might be linked to holes in the Sun’s magnetic field—although they weren’t sure exactly how. At these spots, charged particles fly out at high speeds, without being held back by the magnetic field.

Although the Sun is always spewing charged particles, some regions of that so-called solar wind move especially fast. Scientists at the time hypothesized that these holes might help explain why, and we now know that they were right. Around the same time, SOHO also helped discover rivers of plasma that help move heat around around the Sun, a lot like jet streams here on Earth.

These rivers help power the cycles of sunspots and other activity that drive space weather and affect us and our satellites—which is exactly what scientists were hoping to learn about. So, things were going great, and scientists were excited to learn more. Even though the original mission was only planned for two years, in 1998, NASA and ESA extended the mission another five years.

But then in June... there was an accident. Engineers were moving quickly through routine tests of SOHO’s gyroscopes, which kept it stable, when they missed a step in the process. That set off a series of problems that eventually broke the gyroscopes and left operators without any way to orient it.

And just like that, the billion-dollar satellite went spinning out of control a million-and-a-half kilometers from home. Without working gyroscopes, engineers just couldn’t steady it, and they actually lost all contact with it for four weeks. Finally they did manage to contact it by bouncing a powerful radar signal off it.

And a few weeks later, they realized there was still one way they could possibly regain control: They could attempt to steady it using its reaction wheels. Reaction wheels are controllable, spinning wheels attached to an axis that are traditionally used for turning. Whenever they’re spun one way, the spacecraft turns the other way.

It’s thanks to the fact that every action has an equal and opposite reaction. NASA. But it turns out engineers could also use them to keep SOHO stable.

See, the wheels wouldn’t only spin when they got instructions to do so. They would also start to spin themselves anytime the spacecraft started rotating, thanks to that same rule about equal and opposite reactions. So any time a reaction wheel started spinning alone, the operators knew the spacecraft had to be rotating, and they could adjust the spinning of the wheels to stabilize it.

And this technique worked really well! By September, SOHO was up and running again. And this new technique turned out to be even more precise than SOHO’s original gyroscopes.

So, for the last 20 years, these reaction wheels have kept SOHO steady. The success even paved the way for some future missions to be designed without gyroscopes at all! As it’s aged, different pieces of the satellite have begun to fail, but overall SOHO has kept on trucking.

And we’ve now gotten over two decades’ worth of science out of it. The results have painted a picture of a much more violent and dynamic Sun than we’d ever imagined. And they’ve also helped inform future exploration of the Sun.

But to our surprise, SOHO didn’t just teach us about the Sun, because it also has a weird, unexpected talent: It’s a pro at finding comets. Comets are tricky to spot, because they usually only get bright when they’re close to the. Sun... except then they get drowned out by sunlight.

But SOHO was designed in part to study the Sun’s corona, the hazy layer of plasma that’s also usually invisible against the bright Sun. {♫Intro♫} It does that by blocking out the rest of the. Sun’s light so that dimmer halo of plasma is visible. And by doing that, it’s not only been able to see the corona—it’s accidentally found thousands of comets, too.

Many just barely graze the Sun, so scientists think they may be the remains of a much larger comet that fell apart thousands of years ago when it came too close to the Sun. So this satellite has taught us more than we ever expected. And even now, SOHO is still going, exploring comets, solar cycles, and solar weather, and helping us protect ourselves and our satellites against our dynamic Sun.

Thanks for watching this episode of SciShow Space! And if you liked it, you might like our December Pin of the Month, which is of SOHO itself! You can preorder it anytime this month over at—but it’s only available this month, so if you want one, check out the link in the description to place your order now. {♫Outro♫}.