The ultimate 4-wheel-drive: How ESA's keeping XMM-Newton alive after 20 years and beyond 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 …
From the terminology (I am not a space engineer), I imagined physically swapping out a reaction wheel, which lead me to wonder "why does it take fuel to do this?" Which had me imagining some physical jostling of the spacecraft to move the wheel out of place - which sounded like something horribly kludgey and risky to be doing with a spacecraft.
Turns out, there is a much more sensible meaning to "unloading" a reaction wheel. It means "unloading the momentum" of the wheel.
I know there will always be an argument over what needs further clarification and what doesn't. For me, this was one of those things that needed that extra bit of clarification.
Like this little gem:-
"The pointing accuracy of the 10-metre long XMM-Newton is 0.25 arcsec over a 10-second interval. This is the equivalent of seeing a melon from a distance of 300 kilometres, using a handheld telescope and seeing it without the slightest wobble!"
With some surfing we find.
"Industrial involvement: The prime contractor was Dornier Satellitensysteme, part of Daimler Chrysler Aerospace (Friedrichshafen, Germany). They led an industrial consortium involving 46 companies from 14 European countries and one in the United States. Media Lario, Como, Italy, developed the X-ray Mirror Modules. About 1000 engineers and 150 scientists were involved in the creation of XMM-Newton.".
But also.
"Prior to June 2013, the SSC was operated by the University of Leicester, but operations were transferred due to a withdrawal of funding by the United Kingdom.".
"the XMM-Newton Survey Science Centre (SSC)"
" It was the UK in this case but they aren't even the worst. "
They have been for space. They're also the lowest spending partner in ESA and lowest spending on space in the EEA of the space-faring countries (there are several who don't bother at all)
Italy gets most of the research contracts for the simple reason that they put in most of the money.
There's some feeling that the UK is nuisance value at best - especially with the anti-science/anti-intellectualism, Gordon Gecko attitudes(*) of governments over the last 20 years or so.
(*) "GREED IS GOOD" - maximise the next quarter and bollocks to long term planning.
"Italy gets most of the research contracts for the simple reason that they put in most of the money.".
It's not that simple.
If we talk about ESA the main contributors are the EU, France and Germany also you don't get any contracts if you have nothing to deliver. On the whole I think there is a tendency to underestimate the quality of Italy and its industry.
""On the whole I think there is a tendency to underestimate the quality of Italy and its industry."
The UK tends to underestimate the quality of Italian science in general. I live near LNGS in Italy and get used to the astonishment of British visitors that there's a world class observatory and the largest underground laboratory (Bond music) in the world on our doorstep. My dream job would be a little further south at Telespazio Fucino. Sadly jobs are very hard to come by and Italian nationals get priority.
It's not just the UK that has anti-science politicians though. A group of local mayors have been rabble-rousing for years now trying to get LNGS shut down because of the perceived threat of radiation. There's also some pitchfork waving by yokels locals who insist that it's causing three headed sheep and that it's all witchcraft.
If fuel is the only real issue:
I know this thing is about the size of a large tourist bus (that "little" scope on the side is 70cm diameter and 4 metres long!) but I wonder if you could fly a Mission Extension Vehicle out to it
Grumman had a couple of concepts which looked like slings that would attach to the side of a large bird (space tug style) rather than attach to the primary thrusters
If you added mass to it, especially off-centre like that, the moment of inertia would change drastically, requiring a complete recalibration to get back to the pointing accuracy needed, if that were even possible. Nowadays, given the capabilities just demonstrated by Intelsat to navigate and mate to a comms sat, it might be worth engineering mounting points and fuel connectors on sats like this so they can be refuelled in flight.
It doesn't matter where you add the mass: the moments of inertia will change, but having it on the side will make repointing a lot easier than if it's on the end jammed into a thruster nozzle as was done with the Intelsat (extra mass has less effect on the moment the closer it is to the centre of mass)
There have been a huge number of proposals to actually refuel satellites on-orbit and they all run into major issues, not just with the fuel connectors but the very real possibility of having something leak during the process - a satellite or probe outside of LEO limits(*) sits in a cloud of its own outgassing "stuff" anyway (which is one of the reasons why cleanrooms are used and everything is baked to hell as part of flight preparation). Having that stuff being hydrazines or other corrosive and sticky shit would be a VERY BAD THING. The current filling procedures on the ground have the filling ports not only capped after filling, but then very thoroughly encased to ensure that they can't be disturbed in flight for this reason.
The MEV system was conceived because it was realised that refilling a comms sat or science probe on-orbit would not only be impractical but the resulting contamination cloud had a very high likllihood of ruining the spacecraft anyway - even if there were grappling and fill ports available.
Refilling a MEV away from the bird to be rescued is a different matter. The MEV is designed to be flown around and can "leave its contamination cloud behind" for the most part. Additionally, anything that settles on the MEV is not going to interfere with the mission of the item to be rescued so it can be manouvered to a parking spot for a while to ensure that it's not leaking, etc before setting off on its next mission - but being a "flying fuel tank" it can be given enough fuel for a 40 year mission in the first place, so why bother?
(*) Satellites low enough to be affected by the atmosphere (anything below about 600 miles) will also have that atmosphere(**) strip away any cloud of contamination that may be flying with them, so that's less of an issue. The tradeoff is needing regular reboosting, or coming out of orbit relatively quickly.
(**) At these altitudes it's still there but the vacuum is still better than anything we can make on the ground. The friction of colliding with a few scattered molecules when travelling at orbital velocities is enough....
I'd sort-of worked out that x-ray telescopes would have to use mirrors where the angle of incidence was tiny, so the x-rays actually reflect, but having lots of nested mirrors and having light (well, x-rays) hit each mirror twice is brilliant. There will be a reason for having three telescopes as well, but I have no idea what it is.
I imagine that conversations between astronomers and other scientists go like this:
Other scientist: 'I had this clever idea that you could do something like [...] only of course the engineering considerations would make it completely impractical: maybe in a century. Obviously you will credit me with the idea'.
Astronomer: 'Oh, yes, we used to do that, but we worked out that [...] would be better. Of course [...] requires [obviously completely absurd engineering even in theory], but we got it all sorted out in the end.'
Other scientist: [gives up science, becomes computer programmer].
I could tell you there are three telescopes because there are three main instruments - EPIC-pn and two EPIC-MOS (EPIC=European Photon Imaging Camera). pn and MOS are two different types of detector. These detectors give imaging capability with lower resolution simultaneous spectra.
In fact, it's a bit more complex than that. Half the light from the two EPIC-MOS cameras is split using a reflection grating and passed to the RGS instruments (RGS=Reflection Grating Spectrometer). The RGS give high resolution spectra of bright X-ray sources. There's also the UK contribution, which is an ultra-violet telescope, the optical monitor (OM), which also observes what XMM looks at. Unfortunately that UV telescope has never been used much because it has a few issues.
That makes sense, except that, say, Hubble has multiple instruments but only one telescope. So there must be a reason why this doesn't work for something like this. I can imagine that being related to the whole x-ray mirror thing: you can't bounce the light around inside the thing to wherever you want it to be the way you can with visible light as all the angles of incidence have to be tiny (or probably I mean, nearly 90 degrees, I can't remember which the zero is).
In case it's not obvious I'm disagreeing: obviously what you say is right! I'm just thinking about it idly to myself and generally wasting time.
You can switch instruments in an X-ray telescope, like in Chandra. There's a cost associated with doing that, however. The fewer moving parts you have in space, the better, because of reliability. It's also a lot cheaper to have a more simple mission. However, switching instruments can be beneficial. You can move the instrument module (like Chandra), or move the mirrors, like the future Athena mission is planning.
For a mission, it may also be cheaper or easier to build multiple telescopes, rather than one big telescope. The choice of number or size of mirrors also affects its performance at different energies, so there is optimisation for the type of science you want to do.
"I'd sort-of worked out that x-ray telescopes would have to use mirrors where the angle of incidence was tiny, so the x-rays actually reflect,"
I used to work next door to the physics department at Leicester. They had one of the old telescopes left outside the back door for a time. It looked like a set of nested drainpipes. Possibly the least exciting looking bit of exciting scientific kit that I have ever seen.
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