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How did the Planck satellite lose its helium?

Spitzer, Herschel had the same problem.

How was it consumed?

How come other satellites, like Hubble, don't have this problem?

And basic logic says that starting with 7 times more hydrogen makes them last 7 times more than they lasted.

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    $\begingroup$ All of these used open-loop cooling. A good question would be why not closed loop. $\endgroup$
    – ikrase
    Nov 27 '20 at 0:53
  • $\begingroup$ What is the weight of the smallest closed loop helium refrigerator? $\endgroup$
    – Joe Jobs
    Nov 27 '20 at 1:33
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    $\begingroup$ Good question. I'm under the impression that cryocoolers can be made with satellite-suitable weights. $\endgroup$
    – ikrase
    Nov 27 '20 at 3:08
  • $\begingroup$ Collect the helium in a container. The disconnect container from telescope. Then connect the cooler to container. Make liquid helium. Then connect the cooler to the telescope and pour in the liquid helium. This way the cooler can stay disconnected from the telescope and prevent vibrations. $\endgroup$
    – Joe Jobs
    Nov 27 '20 at 4:21
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    $\begingroup$ That sounds mechanically complicated. More likely they would just put the cryo cooler on mechanical isolators $\endgroup$
    – ikrase
    Nov 27 '20 at 5:16
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The helium used for cooling was liquid helium, it slowly evaporated, the resulting gas has to be vented to space to avoid a very high pressure and to optimize cooling.

Herschel used liquid helium cooling, too, but Hubble did not use helium.

The instruments were cooled with over 2,300 litres (510 imp gal; 610 US gal) of liquid helium, boiling away in a near vacuum at a temperature of approximately 1.4 K (−272 °C)

Source Wikipedia article about Herschel

Hydrogen was not used for cooling, it is less cold than helium.

The Wikipedia article explains the use of helium for cooling.

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    $\begingroup$ How much helium they had when they started? Kilograms or tens of kilograms? $\endgroup$
    – Joe Jobs
    Nov 26 '20 at 23:36
  • $\begingroup$ ditto comment $\endgroup$
    – uhoh
    Nov 26 '20 at 23:56
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    $\begingroup$ Neither Kilograms nor tens of kilograms but hundreds of kilograms, 2300 litres are about 287,5 kg $\endgroup$
    – Uwe
    Nov 26 '20 at 23:57
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The helium evaporated away, as it was intended to do, thereby helping to cool the critical parts of the telescopes' detectors.

The difference between these telescopes and Hubble are the frequencies of "light" which they detect. Hubble operates mainly with visible and ultraviolet light (occasionally near IR) while the others you mention operate with deep infrared or short microwave radiation. As a result, Hubble does not need any of its components cooled to extremely low temperatures and does not need liquid helium at all.

Starting the others with more liquid helium would have meant using a bigger (more expensive) rocket and/or leaving out some of the scientific instruments.

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    $\begingroup$ Why they dont replace the helium to increase lifetime of these super expensive telescopes? $\endgroup$
    – Joe Jobs
    Nov 26 '20 at 23:42
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    $\begingroup$ The helium was just in a (slightly) open-ended thermos and just boiled away? It wasn't a closed cycle refrigerator? Oh, maybe there are arguments against complexity and/or vibrations for this, but I never stopped to think about it. $\endgroup$
    – uhoh
    Nov 26 '20 at 23:54
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    $\begingroup$ @JoeJobs None of the three telescopes you mention is in Low Earth Orbit (unlike Hubble), Planck and Herschel were at L2 a million kilometers or more away and Spitzer orbits the Sun. This is for scientific reasons -- not having a warm planet filling half the sky makes it easier to observe things. So a manned mission couldn't have reached them. Robotic resupply has been talked out, but it always seems to work out that you get more science per dollar by just launching the next telescope, $\endgroup$ Nov 27 '20 at 8:05
  • $\begingroup$ Yet robot resupply seems to be the future. There were already a few tests done - too few though imho $\endgroup$
    – Joe Jobs
    Nov 27 '20 at 12:28

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