Some of the most famous space missions, including the two Voyager probes, have been exploring our Solar System for decades. The technology helping to keep them aloft, however, is not what you'd expect.

It took more than 10 years to design and build Europe's four identical Cluster satellites for launch; and just 39 seconds to lose them all in an enormous fireball.

Their remains rained down over the South American jungle as the Ariane 5 rocket veered off course and exploded. VIPs who had been sipping champagne on the outdoor viewing gallery moments earlier were ushered back inside to avoid being injured by the falling debris.

The disaster was one of the European Space Agency's (Esa) most visible and spectacular failures. But within months, work had begun on a replacement mission, Cluster II.

Designed to fly in formation to investigate the interaction between charged particles from the Sun – the solar wind ­– and the magnetic bubble surrounding the Earth, known as the magnetosphere, Cluster II ranks as one of the most successful and long-lasting science missions ever flown. The satellites (named Rumba, Salsa, Samba and Tango, since you ask) have just celebrated 23 years in orbit. 

"The mission was only designed to last three years," says Cluster's mission operations manager at the European Space Operations Centre (Esoc) at Darmstadt in Germany, Bruno Sousa. "It has a very enthusiastic group of scientists working on the mission – some of them are waiting for it to finally end so they can enjoy their retirement.

Cluster is one of many missions still alive today thanks to the skill and ingenuity of the engineering and science teams behind them, problem-solving their way through glitches, malfunctions and near-catastrophic failures. This challenge of maintaining spacecraft long after their original best-before date was highlighted recently when controllers briefly lost contact with Voyager 2.

Launched almost 46 years ago in 1977, the twin Voyager probes continue to send back data from beyond the Solar System.

I checked with Nasa, which has assured me that the spacecraft are still being controlled from the same beige cubicle in an annex of its Jet Propulsion Laboratory (JPL) that I visited in 2017, marked with a homemade cardboard sign reading: "Mission critical hardware – PLEASE DO NOT TOUCH". (You can read the full story of the mission here and listen to a radio programme I produced about it here.)

The set-up will be familiar to the Cluster mission controllers, who have had to battle with 20th-Century ground control software built on an obsolete operating system.

"We've developed a complicated setup where we have modern Linux servers running a virtual environment with an emulator of the old operating system," says Sousa. "The person running the software is part of the original team, he’ll retire when the mission is over."

Power has also been an issue. The Cluster satellites are fitted with solar arrays for electricity but, twice a year, they pass into the Earth's shadow and need batteries to survive.

"The batteries were only designed to last five years and after six we started to lose capacity," says Sousa. "Then we had cracks and eventually leaks and they became completely unusable."

The solution was to power-down the satellites as they approach the eclipse and then send signals to reboot them in an automated sequence. It's like giving Cluster a factory reset twice a year. In fact, when it comes to spacecraft aftercare, the manufacturers are often asked to step in.

Take Esa's 10m-long (33ft) XMM-Newton space telescope, for instance. Built by Airbus and designed to investigate x-ray emissions from distant galaxies, over 24 years the giant telescope has observed black holes, witnessed the birth and death of stars and has helped transform our understanding of the invisible universe. But 10 years into its mission, it was showing its age.

Like many satellites, XMM-Newton has thrusters for manoeuvring and employs reaction wheels to keep it stable. Powered by the solar arrays, these wheels spin rapidly to create a force that causes the orbiter to rotate in the opposite direction. The telescope was designed to operate with three wheels, with a fourth as backup. But, like a struggling computer hard drive, after a decade these moving parts were wearing out.

Rather than wait until the reaction wheels gave up completely, mission controllers had the idea of activating the fourth wheel but running all the wheels at a slower speed.

"We designed the detailed algorithms and software and uploaded it to the spacecraft in 2013," explains UK-based Airbus engineer Patrick Chapman, who had worked on the XMM-Newton since 1995. "It's still healthy and we're even saving fuel."

Developing the fix that saved the telescope played-out over several months but sometimes time is not an option.

At 10.30am on 22 September 2021, the alarms went off in Esa's Integral mission control room at Esoc. A reaction wheel had stopped working on the 19-year-old gamma ray observatory, tripping the satellite into safe mode but, worse, sending it slowly spinning out of control.

"We had a lot of alarms going off and only three hours of power left on the satellite," says mission operations manager Richard Southworth.

"We were running out of power because it was rotating away from the Sun," adds spacecraft operations engineer Greta De Marco. 

Southworth says in some ways they were lucky. "It was morning, we were all at work and, by chance, I was on a Zoom call with around 100 Airbus engineers so I could share the anomaly with them."

The team needed to buy time to work out how to tell the satellite where it was pointing, says De Marco. "We took the decision to power down the instruments."

"The batteries were dying and I knew that if we had to rotate one more time, that would be it," says Southworth. "At that point I was a bit pessimistic."

Eventually, despite intermittent communications caused by the spinning, the team managed to adjust the speed of the other reaction wheels.

"That was the moment when I thought, this has to work or we're done for," says Southworth.

Since then, Integral has been operating well. But what if something goes wrong when your elderly spacecraft is around a distant world 150 million km (93 million miles) away?

Launched in 2003, Esa's Mars Express was designed to orbit Mars for two years. After nearly 20, the mission has investigated the atmosphere and delivered stunning imagery of the Martian surface, as well as acting as a relay for a series of Nasa rovers.

Like their colleagues, controllers have also had to cope with elderly computer systems. Following a glitch with its onboard computer, for instance, engineers worked out a new way of loading commands into the spacecraft's 2MB RAM memory (just by way of comparison, an iPhone 14 starts at 128GB – which is 64,000 times more memory).

They also discovered that the original software for part of the onboard navigation system was running on a Windows 98 PC that no-one could find the password to and ended up using bolt cutters to extract the hard drives. But the biggest challenge came five years ago and, once again, it was a mechanical failure that threatened to bring the mission to an untimely end.

Mars Express is fitted with six gyroscopes to measure rotation. Together with two cameras – known as star trackers – these enable the spacecraft to determine its orientation in space. But by 2017 it was clear the gyros were failing.

"We had this big meeting to discuss the state of the spacecraft and it was concluded that it had two years left to live," says spacecraft operations engineer Simon Wood. "It was a pretty depressing meeting to come out of."

Fortunately, the spacecraft was built to a standard design which was also used for other missions including the cometary explorer, Rosetta. Although Mars Express was never designed to operate without its gyros, Rosetta's software allowed its gyros to be turned off. So, could they hack code designed more than 20 years ago for Rosetta and make it work for Mars Express?

"Most of our colleagues thought we were mad because the very idea that you would go elbow deep into on-board software source code was almost unheard of," says Wood. "But we stuck at it."

In 2018, they prepared to upload the software and reboot the spacecraft. "The spacecraft hadn't been restarted from cold for 12 years, so it wasn't without its risks," Wood says. "It works better than we ever dared hope and we now have a mission extension until 2028."

But another mission in orbit at Mars has lasted even longer. With 95,000 orbits on the clock, Nasa's Odyssey is the oldest operating spacecraft in orbit around another planet.

"I feel very fortunate to have such a robust robot at Mars," says mission project scientist Jeffrey Plaut, who calls the current phase a "golden age" of Mars exploration. "The mission was only expected to last a handful of years and it's gratifying because it allows us to do science we never expected to do, in particular long-term monitoring of the weather and climate at Mars… it's really quite remarkable."

Plaut has been working on Odyssey since it arrived to Mars' orbit in 2001, but the current operations team, based at JPL in Pasadena, has only been on the mission for 18 months after the previous incumbents retired and passed on the baton, alongside decades of wisdom.

Mission manager Jared Call is responsible for keeping the spacecraft going as long as possible. Ideally longer than their Esa rival and, to be fair, partner. "I don't know of a contest [with Mars Express], but if we're in one, I hope to win," laughs Call.

Eventually though, both Mars orbiters will run out of fuel. "The spacecraft doesn't have a fuel gauge," says Call. "The artistry involved in estimating propellant is probably three parts art, two parts math and one part engineering and physics." 

But however much artistry the mission controllers employ, all these missions will eventually come to an end. Except perhaps one. With its messages from Earth on a golden disc strapped their side, the Voyager spacecraft will continue its journey for millions of years – our legacy to the Universe long after we are gone.

BBC

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