4 ms·

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

yardie 14y ago
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.
- ars 14y ago
  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?
  - yardie 14y ago
    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.
jacquesm 14y ago
Evaporation. 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: http://en.wikipedia.org/wiki/Herschel_Space_Observatory http://en.wikipedia.org/wiki/Herschel_Space_Observatory
- deletes 14y ago
  + "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.
  - Mvandenbergh 14y ago
    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.
    - deletes 14y ago
      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.
- iwwr 14y ago
  They could have used an active refrigeration system, no?
  - jacquesm 14y ago
    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.
    - weichi 14y ago
      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.pdf http://cmbpol.uchicago.edu/depot/pdf/white-paper_w-holmes.pd...
      - jacquesm 14y ago
        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.
      - mturmon 14y ago
        "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 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").
Mvandenbergh 14y ago
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.

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.

ars 14y ago
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?
- yardie 14y ago
  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.

jacquesm 14y ago

Evaporation. 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: http://en.wikipedia.org/wiki/Herschel_Space_Observatory http://en.wikipedia.org/wiki/Herschel_Space_Observatory

deletes 14y ago
+ "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.
- Mvandenbergh 14y ago
  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.
  - deletes 14y ago
    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.
iwwr 14y ago
They could have used an active refrigeration system, no?
- jacquesm 14y ago
  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.
  - weichi 14y ago
    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.pdf http://cmbpol.uchicago.edu/depot/pdf/white-paper_w-holmes.pd...
    - jacquesm 14y ago
      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.
    - mturmon 14y ago
      "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 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").

Mvandenbergh 14y ago

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.