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Space Extenders Of course – that The telescope can be repaired by astronauts on the space shuttle. But how do you keep one running for years past without being bitten by astronauts? Here is ESA’s XMM-Newton.
ESA’s X-ray Multi Mirror (XMM) telescope, named after the English physicist and mathematician Sir Isaac Newton, was launched on an Ariane 5 on December 10, 1999. Funded for an initial two-year mission, with a design life of ten years. The largest 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 a proposed astronomy mission in 1982. Several task forces were established in 1985, and in 1988 ESA approved XMM as a Cornerstone mission in the Horizon 2000 program.
They do it with mirrors
The spacecraft, which weighs 3.8 tons and is 10 meters long, comprises three X-ray telescopes, each with 58 high-precision concentric mirrors arranged to capture as many X-rays as possible. In addition to telescopes, XMM-Newton is equipped with three European Photon Imaging Cameras (EPIC), a Reflection Grid Spectrometer (RGS), and an Optical / UV (OM) monitor, which observes the same regions as telescopes in X-rays, but in UV and visible wavelengths.
With mirrors among the most sensitive ever developed, XMM-Newton’s mission is to provide the data so that scientists can solve cosmic puzzles; from what happens in and around black holes to the formation of galaxies.
The spacecraft has a limited amount of automation on board and is controlled by the European Center for Space Operations (ESOC) via a live ground station connection.
Register He spoke with the spacecraft’s operations manager, Marcus Kirsch, and with scientist and astronomer Maria Santos Lleo, about keeping the lights on long after the warranty has expired.
Radio mute
XMM-Newton’s first brush with a mission ender came in October 2008, when it was nearing the end of its design life. The spacecraft was approaching the perigee of its highly elliptical 48-hour Earth orbit when ESA controllers lost contact.
After procedures for retrieving the spacecraft failed, there were fears that some sort of catastrophic event had occurred, such as a collision or that a malfunctioning propeller had sent the observatory into a fall. These were mitigated when terrestrial astronomers detected that the sunlit spacecraft was stable in its expected orbit.
ESA’s 35-meter antenna in Australia, using a mode developed for deep-space missions, eventually detected a weak XMM-Newton signal and recovery could begin.
Despite the weak force, controllers theorized that the problem was a jammed radio frequency (RF) switch. After the simulations, the team used NASA’s 34-meter Deep Space Network station in Goldstone, California to send a command to return the RF switch to its last working position. Communications were reestablished and the change has not moved since.
The spacecraft’s deputy director of operations, Dietmar Heger, said at the time, a little underrated: “It was an exciting time for our team.”
They would have more emotions as the mission continued doubling how long XMM-Newton could have reasonably expected to last.
Four-wheel drive at Spaaaaaace
Current XMM-Newton spacecraft operations manager Marcus Kirsch took over the role from the scientific side of the spacecraft around 2008. He was in time for the following problem: fuel. The spacecraft was showing all the signs of enduring years beyond its design lifespan, but fuel would soon present a problem.
XMM-Newton, like many spacecraft, uses reaction wheels to control its orientation. Telescopes look at a given point in space for a specified period of time and then move to another in a maneuver called a “spin.” The thrusters are not used for these orientation changes, as the tanks would soon empty, instead, Kirsch explained, “We have wheels on board and these wheels are sitting in a tetrahedron. So they are getting three-dimensional and angular momentum “Simply by rotating them at different speeds, you can move the spaceship.”
However, the wheels “have to be unloaded from time to time, and for this you need fuel.”
After launch, fuel consumption was on the order of 6 kg per year.
XMM-Newton launched with a lot of fuel for the original mission, but in 2009 there was every danger of running dry before 2018. In addition, ESA had moved from the three-wheel model to four in the later spacecraft: “This it gives them a lot more possibilities, “Kirsch explained. “Imagine you have a three-dimensional problem that you can solve in four spaces. So it makes things a lot easier.”
And, perhaps most importantly, “when you have four wheels instead of three, you need much, much less fuel to unload.”
XMM-Newton actually had a fourth wheel, but it was a spare one, to put into service when one of the others failed.
It was because of the beers, as most of the best brainstorming sessions tend to be, that a contractor put the idea to Kirsch, who recalled being told, “Oh! With four-wheel service we would be much better … and we should try this, but ESA never liked doing this. “
After all, the observatory had a few years to live: why take the risk? It was a working spacecraft, working well and with a decade in orbit under its belt. If something works, for heaven’s sake, don’t play with it, especially the reaction wheels.
“You would never change the wheel of the car while running. Would you?” The river.
Back at the bar, Kirsch replied, “Let’s do a study. And if you can convince me that we would really save fuel, then we can think of implementing some software changes to make this work.”
The result was that “you can save 50 percent on fuel by running a fourth wheel on top,” Kirsch recalled.
“No one thought of this before,” he told us, “and we had a life span of perhaps another eight, nine years.”
The change could add an additional 10 years to the top, which could take the end of the mission beyond 2030.
“So we did some studies, we got the industry involved, and we did several things, and it turned out that we can change the on-board software to activate the fourth wheel and operate on this so-called four-wheel drive.”
It took the team a while to persuade the bigwigs that the idea was a good one: “At first,” Kirsch chuckled, “it took a while, you know, to overcome inertia: ‘We never did it, we’ll never do it. , blah, blah, blah … ‘But then, on the other hand, they said:’ Well, 10 more years? We can’t say no to 10 years’ … “
In fact, the mission had a lot to lose if things went wrong, but much more to gain. The scientific community was interested, the team was interested, and the XMM-Newton gang moved on.
As with all the ESA operations teams we’ve spoken to, Kirsch praised the spacecraft makers, originally Dornier and now Airbus (after the usual round of acquisitions and mergers). However, it was more difficult to put the band back together: “The software boy was already retired,” Kirsch recalled, “and we recovered him from retirement.”
With the mission potentially lasting for nearly another 20 years, the team decided to have a younger member do the work, with the support of the older Brit who had returned from retirement. “It provided the perfect support,” said Kirsch. “He’s one of those computer fanatics, who say, ‘Oh yeah, yeah, this change, it must be on code line 4370 …'”
While it wasn’t a big code change, the modifications were significant. “Most of the effort,” Kirsch recalled, “was tested.”
The team had an emulator and simulation hardware available. Airbus also proved useful once again: “We even reactivated some old simulators at the contractor site at Airbus in Stevenage, UK,” he recalled.
A few years after the idea, the team was ready to upgrade the spacecraft.
Kirsch recalled telling one of the industry team members that he had invited some of those involved to his house to eat paella: “You know, this is really dangerous. If we lose the spacecraft, we no longer have a lightning observatory. X I mean, you are convinced, but I am very nervous.
“And he said, ‘No, you shouldn’t be nervous, it will work fine. It will work.'”
However, Kirsch was still nervous 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 administration side,” he told us, “you must trust people. Not that you can control everything yourself.
“You have to trust people.”
Despite all the documentation, validation plans, and test campaigns, Kirsch told us, “I can tell you that I was nervous until the first moment the wheel spun. And I remained nervous for a week after that.”
He added: “It is not easy to make such a decision.”
As it happened, it was the right decision. With “four-wheel drive,” Kirsch told us, “our extrapolations will continue until December 2030.”
The change, implemented in 2013, cut annual fuel consumption in half.
The thruster could be further depleted if safe mode operations are avoided, where the ship uses its thrusters to orient itself. Each safe mode can cost up to half a year of operations, so it is taken into account in the calculations: “We allow one safe mode per year in terms of fuel budget. Therefore, every time we do not have safe mode in one year , we gain a little “.
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