The ultimate 4-wheel-drive: How ESA’s keeping XMM-Newton alive after 20 years and beyond

You thought that yoghurt in the back of fridge was time-expired? Behold X-ray boffinry YEARS past its design-life

Space Extenders Sure – that telescope can be serviced by Space Shuttle astronauts. But how do you keep one running for years past expiration without a prodding by spacewalkers? Behold ESA’s XMM-Newton.

ESA’s X-ray Multi Mirror (XMM) telescope, named for English physicist and mathematician Sir Isaac Newton, was launched on an Ariane 5 back on 10 December 1999. Funded for an initial two-year mission, with a ten-year design-life, the biggest science satellite built in Europe (at the time) is now entering its third decade of service.

The XMM-Newton story, however, begins a little earlier with an astronomy mission proposed in 1982. A number of working groups were established in 1985, and by 1988 XMM was approved by ESA as a Cornerstone mission in the Horizon 2000 programme.

They do it with mirrors

The spacecraft itself, weighing in at 3.8 tonnes and measuring 10 metres in length, comprises three X-ray telescopes, each with 58 high-precision concentric mirrors arranged to capture the greatest amount of X-rays possible. In addition to the telescopes, XMM-Newton is equipped with three European Photon Imaging Cameras (EPIC), a Reflection Grating Spectrometer (RGS) and an Optical/UV Monitor (OM), which observes the same regions as the X-ray telescopes, but in UV and visible wavelengths.

With mirrors among the most sensitive ever developed, XMM-Newton’s mission is to provide the data to enable scientists to solve cosmic conundrums; from what happens in and around black holes to the formation of galaxies.

The spacecraft has a limited amount of onboard automation and is controlled by the European Space Operations Centre (ESOC) via a live ground station connection.

The Register spoke to the spacecraft’s operations manager, Marcus Kirsch, and scientist and astronomer Maria Santos Lleo, about keeping the lights on long after the warranty has expired.

Radio silence

XMM-Newton’s first brush with a mission-ender came in October 2008, as it approached the end of its design-life. The spacecraft was approaching the perigee of its highly elliptical, 48-hour orbit of Earth when ESA controllers lost contact.

After procedures to recover the spacecraft failed, there were fears that some sort of catastrophic event, such as a collision, had occurred or a malfunctioning thruster had sent the observatory into a tumble. These were assuaged when ground-based astronomers spotted that the sunlit spacecraft was stable in its expected orbit.

ESA’s 35-metre antenna in Australia – using a mode developed for deep space missions – eventually detected a weak signal from XMM-Newton and recovery could begin.

Despite the feeble strength, controllers theorised that the issue was a stuck Radio Frequency (RF) switch. Following simulations, the team used NASA’s 34-metre Deep Space Network station in Goldstone, California to send a command to set the RF switch back to its last working position. Communications were re-established, and the switch has not moved since.

The spacecraft’s deputy ops manager, Dietmar Heger, said at the time, in a bit of an understatement: “It’s been a thrilling moment for our team.”

More thrills would be in store as the mission continued to a doubling of how long anyone might have reasonably expected XMM-Newton to last.

Four-wheel drive in Spaaaaaace

XMM-Newton’s current spacecraft operations manager, Marcus Kirsch, moved into the role from the science side of the spacecraft around 2008. He arrived in time for the next issue: fuel. The spacecraft was showing every sign of enduring years beyond its design-life, but fuel would soon present a problem.

XMM-Newton, like many spacecraft, uses reaction wheels to control its orientation. The telescopes peer at a given point in space for a given period of time and then move to another in a manoeuvre called a “slew”. The thrusters are not used for these orientation changes since the tanks would soon be emptied, instead, explained Kirsch, “we have wheels on board and these wheels are sitting on a tetrahedron. So, they’re getting three dimension and angular momentum. By just spinning them in different speeds, you can move the spacecraft.”

However, the wheels “have to be unloaded every now and again, and for this you need fuel.”

After launch, the fuel usage was in the order of 6 kg per year.

XMM-Newton was launched with plenty of fuel for the original mission, but by 2009 there was every danger of running dry before 2018. In addition, ESA had moved on from the three-wheel model in favour of four in subsequent spacecraft: “This gives them much more possibilities,” explained Kirsch. “Imagine that you have a three-dimensional problem which you can solve in four spaces. So, it makes things much easier.”

And, perhaps more importantly, “When you have four wheels instead of three, you need much, much less fuel for unloading.”

XMM-Newton actually had a fourth wheel, but it was a spare, to be brought into service when one of the others failed.

It was over beers, as most of the best brainstorming sessions often are, that a contractor put the idea to Kirsch, who recalled being told “Ah! With four-wheel service we would be much better… and we should try this, but ESA never liked to do this.”

After all, the observatory had a good few years of life left in it – why take the risk? It was a running spacecraft, working well and with a decade in orbit under its belt. If something is working, for goodness sake don’t fiddle with it, especially not the reaction wheels.

“You would never change your wheel on the car while running. Right?” he laughed.

Back at the bar, Kirsch replied, “Let’s do a study. And if you can convince me that we really would save fuel, then we can think about implementing some software changes to get this going.”

The result was that “you can save 50 per cent of the fuel if you operate a fourth wheel on top,” remembered Kirsch.

“Nobody thought about this before,” he told us, “and we had a life horizon of maybe another eight, nine years.”

The change could add a possible 10 more years on top, potentially moving the end of the mission to beyond 2030.

“So we did some studies, we involved industry and we did various things and it came out that we can change the onboard software to activate the fourth wheel and to operate in this so called four wheel drive.”

It took some time for the team to persuade the bigwigs that the idea was a good one: “In the beginning,” laughed Kirsch, “it took a while, you know, to overcome the inertia: ‘we never did it, we will never do it, blah, blah, blah…’ But then, on the other hand, they said, ‘Well, 10 years more? We can’t say no to 10 years’…”

Indeed, the mission had a lot to lose if things went wrong, but so much more to gain. The science community was keen, the team was keen and so the XMM-Newton gang went ahead.

As with all the ESA operations teams we’ve spoken to, Kirsch was full of praise for the spacecraft manufacturers, originally Dornier and now Airbus (after the usual round of acquisitions and mergers.) It was trickier, however, to put the band back together: “The software guy was already retired,” Kirsch recalled, “and we got him back from retirement.”

With the mission potentially lasting for almost another 20 years, the team decided to have a younger member do the work, with support from the older Brit who had been returned from retirement. “He provided the perfect support,” said Kirsch. “He’s one of these computer freaks, who say, ‘Oh, yeah, yeah, this change, it must be in code line 4370…'”

While not a huge change to the code, the modifications were significant. “Most of the effort,” recalled Kirsch, “went into testing.”

The team had an emulator and simulation hardware available. Airbus also proved helpful once again: “We even reactivated some old simulators at the contractor site at Airbus in Stevenage in UK,” he remembered.

A few short years after the idea was had, the team was ready to update the spacecraft.

Kirsch remembered saying to one of the team from industry, having invited some of those involved over to his home for paella: “You know, this is really dangerous. If we lose the spacecraft, we don’t have an X-ray observatory anymore. I mean, you are convinced, but I’m very nervous.

“And he said, ‘No, you should not be nervous, it will work fine. It will work’.”

Kirsch was still, however, twitched about the whole thing. His team was a little more relaxed, having been closer to the actual code and testing.

“If you’re more on the management side,” he told us, “you have to trust the people. It’s not that you can control everything yourself.

“You have to trust the people.”

Despite all the documentation, validation plans and test campaigns, Kirsch told us: “I can tell you I was nervous until the first moment where the wheel was spinning up. And I was still nervous for a week after that.”

He added: “It’s not easy to make such a decision.”

As it transpired, it was the right decision. With “four-wheel drive,” Kirsch told us, “our extrapolations go until December 2030.”

The change, implemented in 2013, saw the annual fuel consumption halved.

The propellant could be eked out even longer if safe mode operations, where the spacecraft uses its thrusters for orientation, are avoided. Each safe mode can cost up to half a year of operations, and so is factored into calculations: “We allow for one safe mode per year in terms of fuel budget. So, every time where we have no safe mode in one year, we gain a little bit.”

About the fuel

Kirsch does not have the same battery worries as the Cluster team, and the solar cells on XMM-Newton have not seen the degradation as those of its sister spacecraft, Integral, thanks to its orbit. While XMM-Newton has a comfortable (in spacecraft terms) power margin of 500 watts, the fuel is most definitely running low.

Cluster II (pic: ESA)

20 years deep into a ‘2-year’ mission: How ESA keeps Cluster flying

READ MORE

Kirsch recalled a similar over-a-beer conversation with one of the old spacecraft engineers where he was told “by the way, if you want to operate so long you will enter this regime where you need fuel replenishment…”

Replenishment? The team had never heard of such a thing, and there were certainly no procedures on the matter.

The original engineers had, of course, already considered it: “Oh, yes, we thought about this already. But we never delivered this to ESA because it was not foreseen that you operate so long.”

Without a pump, the ESA boffins learned everything they could about fuel migration and replenishment. The auxiliary tanks of XMM-Newton are connected to the main tank, which provides fuel for the thrusters. “Whenever you take something out of the main tank,” Kirsch explained, “there is some fuel floating from the aux tanks into the main tank.”

However, the design meant that eventually the main tank would run dry with some fuel still lurking in the other tanks. After all, the spacecraft was never supposed to have been operated for this long – the expectation was that something would have broken long before the fuel ran out, but here we are.

The first solution was migration. The team realised that by heating the aux tanks by just one or two degrees, and making the main tank colder, some fuel would move over. “That’s what we call migration,” explained Kirsch. And it bought the team a few more years.

However, the main tank will still eventually run dry. “So, now the magic word ‘replenishment’ comes into the game,” said Kirsch.

This time it is the main tank that is heated, pushing fuel back into the other tanks. “But you push, as well, gas on top back from tank one”, said Kirsch. “But,” he added, “there are some devices on the other tanks and the main tank which will not allow the gas to come back so the gas has to stay in the aux tanks.”

The temperature is then changed again to push things back.

“So, every year we will empty the tank for some days, and then we push a little bit of gas more into the aux tanks, then we change the temperature again: up, down, up, down. And like this, we are always able to push maybe two or three litres back into tank one. And then we have to do the same exercise again. So, it is juggling around the temperatures.”

“We’ve never done this,” he added, “we’re doing it the first time in June now.”

“We are planning a simulation in May. And I think on the 15th of June, we are planning this operation. And again, industry tells us there’s no problem… but I’m nervous!”

So much science

It is fair to say that XMM-Newton has blown past the science goals laid out before its launch and continues to prove a hugely valuable resource for scientists, not least because of its impressive longevity.

Astronomer and scientist, Maria Santos Lleo, who joined the XMM-Newton mission a year before launch, told us that “some of the discoveries have required a long period of time because the X-ray sources are variable and it’s very important to monitor how they change over time.”

For example, she said: “We have been able to monitor X-ray binaries and observe a black-hole capture a nearby star and swallow it.”

XMM-Newton has contributed an enormous amount to the field, and its instruments? “They are functioning very well after 20 years,” said Santos LLeo, modestly. Part of this is thanks to international collaboration and working on the experiences of the likes of NASA and its own observatories, and partly down to good design in the first place.

Santos Lleo is justifiably proud of the scientific achievements of the instruments aboard XMM-Newton, which have proven remarkably reliable. The science has continued rolling in despite a change from a team solely dedicated to monitoring the instruments 24 hours a day in the early part of the mission to procedures that required less resource as time went on.

However, “we have discovered,” she told us, “with time that contamination is building up on some of our detectors.”

“This has been another big challenge,” she explained, “to monitor how the instruments evolve over time and to take in account [the contamination] in the calibration… we monitor that and we see it increasing, if it increases at the rate it is doing right now, we can extrapolate and we lose a little sensitivity, but if it continues at this rate, it won’t be a problem.”

Handy, because the desire for telescope time from the large scientific community has continued unabated: “The community,” she laughed, “is requesting more than seven times the time available!”

It’s unsurprising – even in the last few months, XMM-Newton has spotted (with NASA’s Chandra) what looks like a star surviving a close call with a black hole, supplied data to support a theory that the universe’s growth may not be uniform and spotted the aftermath of the most powerful explosion ever seen in the Universe.

Scientific collaboration, for the veteran XMM-Newton, is the name of the game.

10 more years

XMM-Newton has been out of warranty for years, the builders only designed it for an initial two-year mission with an extension of eight.

Twenty years on from launch, it continues doing hugely useful science.

Part of the longevity can be attributed to a side effect of the innovative four-wheel approach.

The team had seen a “caging” effect, where the wheels “scratch” a little over time due too much or too little oil and require more power to drive them at the same speed. However, “with our four wheel drive,” explained Kirsch, “we can not only save fuel, but we can as well define the wheel speeds much better, so we don’t need to run them at such high speed.”

The low speed running has been in place since 2013, according to Kirsch, with a little caging showing up previously. Wheel one, which according to Kirsch, was suffering most from the problem, saw a rapid drop in caging friction as the speed was dropped. For wheel two, he said, “we cured it by re-lubricating it.”

(A small reservoir of oil is built into the wheel, and by increasing the temperature a drop of oil can be squeezed out. The cure “worked fine,” he said.)

“So that [the caging or scratching] went away and since we are running on low speeds for wheel one, this is as well fine.

“So, of course, if we lose one wheel, then everything cascades down. If we lose one wheel, we use more fuel, we get more caging you get more problems on the system, so that would kill us as well eventually. Losing a wheel is critical for us.”

But despite the spacecraft being long past limits set before launch for the likes of radiation exposure and the like, it continues to perform admirably.

And a “warranty”? “We are far, far out of that,” laughed Kirsch, “But the systems are still working fine. There were everywhere big margins built in. So, everything which we have is super-duper like a good old Mercedes Benz, you know, yes – and it just works and runs.

“Of course, tomorrow something may break, but they all tell us that you have margins, it will work for another eight, nine years.”

The hope is that XMM-Newton will manage to overlap ESA’s next X-ray observatory, the Advanced Telescope for High Energy Astrophysics (ATHENA), which is expected to launch in 2031.

A miracle

“This spacecraft is built in a way that… we had never done something like this before. It’s a unique observatory,” said Kirsch, “And everything is built tremendously well. And we have so much redundancy everywhere in such a well-defined way.

“And I think the good thing with XMM is that we can still touch everything. So, it is built quite dumb: all the intelligence is down on ground. And we can still reconfigure things, we can really change a lot of things.” The downside of that, of course, is that ground stations are needed to maintain a connection.

“But I think what has been done there is a masterpiece of engineering.”

“My dream,” he added, “would be to operate it a year or two in parallel [with ATHENA] to do cross calibration.”

But the remarkably long-lived XMM-Newton? “To those people who had this idea, thank you very much!” said Santos Lleo.

And Kirsch? “We think it is a miracle, to be honest.” ®

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