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Herschel space telescope goes blind, ends mission (earthsky.org)
58 points by jacquesm 1515 days ago | hide | past | web | 33 comments | favorite

We get so used to space missions outlasting their planned duration that it seems surprising when one actually ends on schedule.

It actually went 11 months and 15 days beyond the planned lifespan, that's nearly 30% longer.

My impression is that it's a underpromise and overdeliver thing. For example in a Mars prove, you don't know exactly what the soil will do to your wheels, what the thermal changes will do to your cpu, what the dirt will do to your solar panels, what the wind will do to your camera support and there are even the unknown unknowns. Probably most of them can be simulated, estimated or guessed.

And a 9 years mission that breaks after 8.5 years is a mayor failure that wasted millions of taxpayer's money. A 2 years mission that last 8.5 years is a successful result of good engineering practices.

Probably the Helium evaporation is too easy to simulate and they only put there the right amount plus 25% just in case. The problem is that Helium is a consumable, so the tank has a finite duration. You can't try to continue until something breaks.

The telescope was positioned at the L2 point about 1,500,000 km from Earth [1], which presumably precluded replenishing the liquid helium. Thats rather different to the Hubble telescope's 560km orbit which was (only just) reachable by the space shuttle.

Does anyone know why the Herschel was so far from earth? Easier stationkeeping at L2?

[1] http://en.wikipedia.org/wiki/Herschel_Space_Observatory#Laun...

"By orbiting at L2, some 1.5 million kilometres from Earth, Herschel is not troubled by any atmospheric absorption. In addition, the spacecraft avoids any problems caused by thermal infrared radiation from the Earth interfering with observations. The L2 orbit also prevents the occurrence of temperature changes due to the spacecraft moving in and out of eclipse in an Earth orbit, which are a particular problem for infrared instruments requiring extreme thermal stability."


"Put it over there by itself, in the shade."

Actually, if I read various web sources correctly (IANA rocket scientist, sadly), the Earth-Sun-L2 point is always in the sun. The earth is not big enough to provide shade from that far away, especially since the Hershel's orbit is 700Mm across. Like how a (cruising-altitude) 747 cannot shade (sea-level) me from the sun, no matter where I stand.

So it's always in the sun, which means it has continuous heating, and thus reaches thermal stability, and doesn't expand/contract/whatever as it passes in and out of shade.

Yes, I was being facile for the sake of a one-liner. Sorry about that.

It will be in a certain kind of orbit around the L2 point, not right on top of the L2 point. This means it is not in Earth's shadow, as you note. It's in its own shadow. But if it were in a conventional orbit centered around Earth, it would have thermal issues, observational issues, etc., because pointing to keep the telescope away from the sun would be a big problem.

My friend across the hall uses data from Planck, which is at L2, and I used to use data from SoHO (before its camera was shut down), which is at L1. We used to joke around about how each other's instrument was in a too cold/too hot place.

Here's a nice illustration of Planck's orbit. (Planck was the sister satellite to Herschel.) http://planck.cf.ac.uk/mission/orbit

So here is some interesting math. Estimated cost of the Herschel Space telescope, lets say 850 million dollars [1]

The cost of a SpaceX launch to LEO, 85 million dollars. So we launch three Spacex cargos for a tanker, a refueling facility, and a pre-fueled space craft. And for 250 million dollars we get a fully re-fueled telescope. How cool would that be? Since its in the L2 point it will wait for us to get to it without de-orbiting.

[1] http://esamultimedia.esa.int/docs/herschel/Herschel-Factshee... - 1100 british pounds for total cost of instrument and mission, if half is the cost of the instrument that is 550 million pounds or about 852 million dollars.

It seems like such a waste to scrap have a functional, responsive electronic device hanging at the L2 orbit point, just because it can no longer cool its sensors. Can't it be repurposed?

Sometimes, but often not. Maintaining communications links and operations consumes resources too. If someone can find a solid science case for reusing the instrument, ideas get traction.

Sometimes experiments get handed off as training tools for new aerospace students. When Gravity Probe B ran out of helium, control was transferred to the US Air Force Academy for navigation experiments [1]

It's sad to see good experiments go, but with finite resources, sometimes the young must take precedence over the old.

Edit: Looks like there are alternative repurposing plans, perhaps [2], but a bummer of a use for that mirror.

[1] http://einstein.stanford.edu/content/faqs/faqs.html#sv_contr...

[2] http://www.spaceflightnow.com/news/n1210/26herschel/#.UX_n7n...

Why is the helium exhausted ? is it because of imperfect sealing ?

Temperature of space is around 4K and the instruments were cooled to 1.4K. There is no system to capture and compress the gas after the phase change at these temperatures. The cooling system was designed to boil off helium. to keep the sensors cold. The same way an aerosol can gets cold after being used.

Obviously at 4K there is more thermal noise than at 1.4K - but is there no value at all to the photos it can still take, even with some noise?

The vacuum is an infinite insulator. As the sensors get charged they have no ability to dissipate that energy. Eventually they'll get warmer and noisier, and you'll spend more time trying to clean up the images. It's been operating continuously for over 1000 days .I'm sure they have more data than they can currently process.


From WP:

"The instruments are cooled with over 2,300 litres liquid helium, boiling away in a near vacuum at a temperature of approximately 1.4 K (−272 °C). The 2,300-litre supply of helium on board the spacecraft was a fundamental limit to the operational lifetime of the space observatory; it was originally expected to be operational for at least three years."

more info:


+ "it was originally expected to be operational for at least three years." and it even surpassed that for 11 months.

I hope James Webb will get some extra lifetime as well.

JWST has a closed loop cryocooler. It also doesn't need to get as cold, the coldest part of the JWST is cooled to 7k.

I knew its life is limited by something... It is actually the fuel. As it will be stationed on a L2 point it will have to consume a little of its fuel to keep stable.

They could have used an active refrigeration system, no?

This is right along the lines of 'why didn't they?'.

When you get this close to absolute zero active refrigeration systems are no longer effective, but the phase change of liquid helium to gas still manages to extract a little bit of energy from the system cooling it down just that much further.

Not sure exactly what you mean by "active", but there are plenty of cryocoolers that work great at temperature below 1.4 K, some of which are also qualified for space. So there's no physical reason for Herschel to not use a cryocooler.

But I believe that Planck was the first satellite to cool to below 1 K using only cryocoolers. Planck actually uses a dilution fridge to get to 100 mK, which is kind-of astounding.

I don't know why Hershel used liquid helium instead of a cryocooler, but my guess is that the technology for cooling with liquid helium in space is very well understood and reliable, so it's a risk thing. That's not to say that nothing can go wrong; there was a japanese telescope that lost all its helium within some very short period of time thanks to an engineering mistake.

This is a nice whitepaper about cryocooling in space: http://cmbpol.uchicago.edu/depot/pdf/white-paper_w-holmes.pd...

"I don't know why Hershel used liquid helium instead of a cryocooler...it's a risk thing"

The cryocooler technology (say, to a few kelvins) has proved hard to get ready for space. For example, out of the 10 or so technologies that were judged most risky for JWST, the 6 K cryocooler for the MIRI instrument was the last to be judged ready for space ("at TRL 6" in the jargon) (http://www.stsci.edu/jwst/news/2007/jwst-passes-tnar).

Despite being judged ready, the JWST cryocooler has proved very challenging to build. The effort now has frequent reviews with the director of JPL (and a high-level counterpart at NGST), and tens of engineers are now working on the system.

Part of the problem, as I understand it, is that the heat has to be taken away and radiated at a site distant from the IR detector. This requires a large structure, and a deployable radiator. This large structure can't leak much heat back into the spacecraft bus or instruments, and must not be disturbed by the vibrations of launch. Additionally, vibrations of the cryocooler must not affect the telescope optics (2 micron resolution).

You can tell that these requirements are fundamentally opposed to each other ("be large, don't vibrate, don't touch anything else").

Precisely what I was thinking. Expelling liquid helium doesn't introduce any heat. Any active cooling needs to somehow expel the heat generated while compressing the gas back into a liquid. In the vacuum of space, irradiating heat is the only way to remove it from the system, and that isn't going to be very efficient. Given the choices and the temperatures they needed to reach, liquid helium seems like a rational solution.

There's also a lifetime issue. Say you decide, "forget the whole vibration thing, we'll run the cryocooler half the time and take data half the time while the cooler is powered off"

Then you either get half the mission data, or extend the mission by a factor of two, which means twice the operational labor cost, plus every other part of the craft has to be double lifetime rated, which could get expensive. Hubble used to burn thru gyros on a regular basis, so now you need them to last twice as long or launch with twice as many spares. Or maneuvering propellant if any, now you need twice as much for station keeping.

You can easily get painted into a corner where the cheapest way to run a mission twice as long is to launch two of them. At that point you're better off saying "you know the cryocool half the time and take data half the time idea? Yeah scrap that idea"

This is before we started on energy issues. A heavy helium tank doesn't use much energy. But cryocoolers on earth take quite a bit indeed, well, at least compared to a couple watt transmitter and all that. Whats heavier, a tank big enough to last mission lifetime, or a cooler and a stunning array of solar panels to run the cooler? Or since it only runs half the time as per above to prevent vibration issues, you could put a battery in which is heavy and becomes another exciting point of failure. This makes the existing power system more complicated and less reliable possibly shortening the theoretical craft lifetime to less than you'd get if you just launched a big simple tank.

How about just making a detachable LHe tank and send some replacement helium every few years? Shouldn't be too costly even given the day-to-day cost of running a space observatory.

Given the weight of the LHe tank, I think they'd rather send up a new telescope along with the tank.

Even worse the helium was the limiting consumable in this particular case, but for the next scope the last gyro might burn out in 5 years. So now you need a supply of gyros, more likely an entire nav unit bolted permanently on. And next time the battery croaks so you need a battery and a way to install it and isolate the old one such that you can never end up with neither installed. And next time the transmitter final power amp goes thru too many thermal cycles and cracks so you need a complete telemetry unit.

Yes I know there are good reasons why they put it at a L point, but unscheduled unplanned maint is exactly what the ISS could have been perfect for. Position this dude in the same orbit but at a phase 10 miles away so its not too close, and send a live astronaut with a new battery or whatever anytime "something" fails. To say the ISS project is not managed this way would be an understatement. But someday, an unscheduled repair shop in space will exist, and probably be quite profitable.

Active as in closed loop, which appears to be false!

That Planck figure is extremely impressive, mind-blowing really. That's colder than the space surrounding it.

Even if it were possible, the machinery would undoubtedly have interfered with the functioning of the telescope.

It's probably not feasible to launch a refrigeration system capable of creating liquid helium temperatures.

The helium was a consumable, heat was removed from the instruments by letting the liquid evaporate and dumping the heat into the phase change of the helium.

Congratulations to the Herschel team, and a huge thank-you to the taxpayers that funded it! These experiments are anything but easy.

It is good for great experiments to have an end.

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