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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

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.

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."

Cluster II (pic: ESA)

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

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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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