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Helium Depleted, Herschel Space Telescope Mission Ends
AmiMoJo writes "The billion-euro Herschel observatory has run out of the liquid helium needed to keep its instruments and detectors at their ultra-low functioning temperature. This equipment has now warmed, meaning the telescope cannot see the sky. Its 3.5m mirror and three state-of-the-art instruments made it the most powerful observatory of its kind ever put in space, but astronomers always knew the helium store onboard would be a time-limiting factor."
Reader etash points to a collection of some infrared imagery that Herschel collected.
If only we had a plan for recurring orbital missions... A "space pickup" that would launch on a regular basis to make pit stops for things like extra helium.
To think how many multi-decade projects like this will "rot on the vine".
I'll bet you feel stupid for filling all those party balloons last week.
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That's my nitpick of the day.
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It's not exactly an efficient sink, is it? Your only option for heat transfer "outside" is infrared radiation, since vacuum does not exactly support conduction/convection.
Another problem with the system you mention is that heat doesn't radiate away efficiently in space. While such a system may be possible I'm sure that the up-time of the scope would suffer greatly from it.
Do we have any thermal dynamic geeks here with something a bit more insightful?
Actually, strangely the inverse is true.
In space, there are very few particles, which means that heat transfer is almost non-existant when away from the atmosphere. This causes a problem in that if you generate any heat, it dissipates extremely slowly, which was why the Helium was important. If this piece of equipment was in the sun, it would have been even worse.
SpaceX should go after it and salvage it robotically for use as a solar thermal concentrator. 3.5M mirrors that are already in space don't exactly grow on trees. A simple high-efficiency Ion engine (Dawn-class)and a robonaut should be able to handle the job. They can then lease the asset to Planetary Resources or whoever wants to do industrial experiments. Doesn't have to be quick. Cheap and slow is the way to go here.
They are in deep space, so they have an infinite sink at nearly zero deg kelvin.
What exactly could it 'sink' that heat into? While we consider space to be 'cold' the reality is that it is less 'cold' and more 'generally won't make things warm.'
The vacuum is both a benefit and a problem. When you want to keep things a certain temperature, the vacuum is great as you don't have to sorry about convection/conduction altering the temperature. But when you want to cool things off, that vacuum is a problem because you can't use convection/conduction to remove that heat from your system. You can certainly move the heat from one part of your system to another part of your system, but it takes a long time to take that heat OUT of your system.
You would have to move the heat to a massive radiator and wait a long time for it to cool due to radiation. Whatever you are using to move that heat will have to work the entire time, (and may have to be cooled as well!). Even then, the temperatures involved mean that such a process would take a very long time to get as low as they needed to conduct the experiments.
Don't think of space as cold, think of space as very effective insulation.
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The forthcoming ASTRO-H X-ray observatory mission will have a cooling system that will be able to run without coolent. The X-ray microcalorimeter detectors must be cooled down to 50 mK in temperature. ASTRO-H should be launched in 2014.
They knew at some point helium will be gone and the telescope will become useless. It ran for four years more or less. Not as bad as the summary made it sound like.
They are in deep space, so they have an infinite sink at nearly zero deg kelvin. It should be possible to design a closed circuit cooling system that just uses energy from solar panels to pump the refrigerant. But in space applications the weight of such a system of compressors, radiators and pumps might prove to be prohibitive.
Still feel sad such a fine piece of machinery is rotting away. Well, may be a better design next time.
No, they have near perfect insulation. The only heat they can get rid of has to happen by radiating it away.
Go step outside.
Notice how warm it is in the sun?
There's no way you can radiate much heat if you're in direct sunlight -- that's why the space shuttle flew upside down in orbit. It kept the heat shield towards the sun, so it had a chance to radiate heat away from the other side.
"So, put a big sun shade and block the sun", you might say... well that's easier said than done, the solar wind would apply a lot of pressure to it, and (for that matter) the solar wind itself is well above the operating temperature of the telescope.
But by all means, I'm sure you're smarter than the experts to designed it.
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the preceding comment is my own and in no way reflects the opinion of the Joint Chiefs of Staff
That reminds me of a puzzle!
Some parts of Herschel's detectors had to be chilled to 0.3 K, others to 1.7 K. There's no way to get that low with radiative cooling; indeed, it's below the temperature of the cosmic microwave background. Virtually all known materials except for helium freeze solid at those temperatures; no standard refrigerant can do it.
The only technologies we have that can get that cold are all based on liquid helium, and they inevitably lose trace amounts of it over time. They could have given it a bigger dewar vessel, but that would have been heavier, and therefore needing a bigger rocket, and therefore more expensive.
(Ref: http://herschel.esac.esa.int/Docs/Herschel/html/ch02.html )
They did think about that.
But it's a million and a half kilometres away. A robotic service ship to catch and refill it after four years would cost more than just sending up a second, newer-generation telescope.
Radiating heat goes to the 4th power. So at 273K (0C) a panel in space radiates 314 watts per m2. However at 4K we radiate a mere 14.5 micro watts. So to radiate 1 watt we would need a square panel 262 meters a side (69000m2). Even worse space is radiating the same amount of heat back at you. So you in fact would not get rid of any heat. In fact i think this particular system needed to be colder than 4K. So no passive system can do it.
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Thanks, I did not realize things are different in space. So how would one design an active cooling system to dissipate heat in space?
I am not a rocket scientist; but my understanding is that the space-equivalent of a 'heatsink' is a fin, with a surface that approximates a black body as closely as engineering constraints allow, aligned so that as much surface area as possible(the flat faces) receives as little incoming light as possible, with as little as possible exposed to the sun(so, in practice, the alignment is pretty much the opposite of a solar panel, where you want as much surface area getting sunlight as you can and as little being wasted by facing into deep space as you can). Depending on the orbit, and whether your thermal load is constant or can accept variations, this may or may not require the fins to move.
If you need active cooling(as you probably would here, since ultrasensitive IR hardware generates some heat on its own and works less well for every additional kelvin) you use a heat pump of some sort, just as on earth; but your 'sink' is thermal radiation from the fins, rather than conduction from the fins into the atmosphere or coolant water.
The real problem(in addition to the fact that solid-state heat pumps are miserably inefficient, and ones with moving parts have mechanical levels of reliability in an area where you can't just schedule a tech visit), is that thermal radiation alone is miserable compared to conduction/convection into air, which is weak compared to conduction into forced air.
If you have a large enough payload budget, it isn't necessarily insurmountable, all it takes is more surface area radiating heat; but the engineering challenges of having a cryogenic heat pump capable of keeping the instruments at liquid helium temperatures and enough fin surface area to dump the waste heat from both the instruments and the heat pump's own inefficiencies are significant.
Liquid helium isn't cheap, and relying on a consumable cuts mission lifespan; but "just let the helium boil off where you need things to be colder" simplifies the engineering considerably.
There are two problems with your approach: one, the near vacuum of space does not allow for effective cooling via convection. Two, compressors only displace heat, and in doing so they actually generate more heat overall. A good example of this is the coils on the back of your refrigerator, which get quite warm during operation. Your kitchen warms up slightly while the interior of the fridge cools. In space, this heat does not dissipate readily and would build up until the system overheats.
I'm sure that an active cooling system wouldn't have been vibration-free either. Telescopes don't work so well when you keep bumping them around.
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According to NASA it will still last just three years.
"The instrument utilizes a multi-stage cooling system that will maintain the ultra-low temperature of the calorimeter array for more than 3 years in space."
You're limited to radiation, and the cosmic background temp, but that's the only limit. Although inefficient, peltier coolers can be used - the advantage is there is no fluid. Heat pipes are the most common form of heat transport, allowing the evaporation of a liquid in a sealed tube to migrate to the radiator end.
One challenge is the temperatures you're trying to work with. Remember that the temperature of the universe isn't actually 0K, but more like 3K. Liquid helium needs to be 4K or less. That's a slim margin, and at those temps the heat transfer rate is very, very low.
I clicked on the story because I was an engineer involved in the Superfluid Helium On Orbit Transfer (http://istd.gsfc.nasa.gov/cryo/SHOOT/shoot.html) research project back in the early 90s. If you get Helium just above absolute zero, it loses it's viscosity (like a superconductor loses it's resistance). That makes it far easier to transfer the fluid from a storage container/refueling dewar to a spacecraft in service.
I actually like radiative heat transfer - it's very straight forward, much like conduction. Convection problems make me cry.
Is it just my observation, or are there way too many stupid people in the world?
Well, put, but one other major killer ... There are no "good" ways to get rid of vibrations on a spacecraft. There's no atmospheric drag (see the mythbusters on the flag on the moon). You basically have to have a damper attached to a mass that kind of sort of slowly adsorbs the energy, re-radiating it as heat. However, most materials are very linear in compression and tension at their minimum range, so it just doesn't work well. Bad enough trying to point a terestrial comms satellite. Absolutely mission killer for aiming a telescope.
I doubt that's the main reason why the shuttle flies upside down. The bottom of the shuttle is also black, while the top is white. From a simple light-absorption-radiation point of view, this configuration would lead to heating of the shuttle as a whole. The heat shield is designed to shield from heat conduction due to superheated compressed air in contact with the shuttle during reentry. Shielding from radiative heating makes use of reflective surfaces like what satellites are coated in.
It seems the shuttle would fly upside down to aid in radio communication with the earth, allow viewing of the earth through the windows (a human concern, but still an important one), and to protect the shuttle from earthbound debris (though I'd think the heat shield is the last thing you'd want to damage before attempting reentry).
Your doubt is misplaced -- that is precisely why it flew that way. The shuttle's radiators were on the inside of the cargo bay doors. The shuttle had a limited time, once on orbit, to get positioned and get the doors open because of the heat build-up.
I do know how it works and all, but still, I find it kind of ironic that the Herschel Space Telescope is bricked for lack of the second most abundant element in the universe.
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It's not exactly an efficient sink, is it? Your only option for heat transfer "outside" is infrared radiation, since vacuum does not exactly support conduction/convection.
If you really want liquid-He temps, then you can't really radiate heat to lose it. At 1 atm it is almost as cold as the cosmic microwave background, and probably colder than the inner solar system. If they're running below 1atm then it is probably colder than the microwave background itself. This means that your radiator will only serve to warm up the spacecraft, not cool it off.
For an IT analogy - how large a heat sink do you need to cool your PC in an oven? The only way to cool under such conditions is using active technologies, like phase change, or maybe Peltier. Since you're fighting entropy, this will ultimately require some source of energy, which will always be depleted eventually in a closed system.
In 1993, NASA proved the basic technology for resupply of helium on the Space Shuttle. The project was called Superfluid Helium On-Orbit Transfer (SHOOT) and flew on STS-57, Endeavour. It was also the first use of an AI system in space (to automate long running transfers while diagnosing and recovering from issues).
Plans to use the SHOOT technology in SIRTF and other telescopes never materialized. There is a tradeoff in enabling a telescope for resupply. Versus a non-refillable telescope, a telescope designed for resupply will provide less science (resupply forces a low earth orbit which is a poorer vantage point for most missions and a given supply of helium will be consumed faster). In an era of expensive space transport, resupply missions were not cost effective.
http://istd.gsfc.nasa.gov/cryo/SHOOT/shoot.html