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Inspired by answering this question

The Wikipedia entry says this

On 29 April 2013, ESA announced that Herschel's supply of liquid helium, used to cool the instruments and detectors on board, had been depleted, thus ending its mission.

Apparently this lead to the satellite going blind

The "blind" satellite is currently located about 1.5 million km from Earth on the planet's "night side".

Why did it need such coolant?

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2 Answers2

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Imagine your telescope optics looked like this red-hot glass!

enter image description here

Herschel's instruments look at the world in the wavelength range of 55–672 µm. When plotted as a function of wavelength, the thermal spectrum of a black body peaks $ \approx 5 k_\mathrm B T$. The boiling point of liquid helium is 4.2 K. The peak wavelength for something at that temperature would then be given by

$$\lambda_\mathrm{max} = \frac{h c}{5 k_\mathrm B T}.$$

With Boltzman constant $k_\mathrm B$ of about 1.381E-23 J/K and the Planck's constant of 6.626E-34 J s, the spectrum should peak at about 700 µm, so even if the optics were at liquid helium temperature it would be hard to see faint objects at longer wavelengths above the brightly glowing mirrors and optics.

According to Wikipedia:

The light reflected by the mirror was focused onto three instruments, whose detectors were kept at temperatures below 2 K (−271 °C). 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). The 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.

  • So they let the helium boil at very low pressure in order to bring its boiling point down to 1.4 K.

  • The main mirror was highly reflective in the infrared, which means its emissivity was low, probably down near 0.01. This helps reduce the emission even farther. According to the BBC's Herschel space telescope finishes mission the primary mirror was at about 90 K

  • For at least one of the instruments SPIRE internal mirrors and the sensors needed to be kept at 0.3 K. The refrigerator for that needed a cooling mass on its "hot side" and a slow boil-off of liquid helium provided such a sink.

enter image description here

The Herschel telescope had to be kept extremely cold to study its frigid targets

Okay Cold, but Why Helium and not the Cold of Space?

  • Cooling to space is limited by the Cosmic microwave background, and its characteristic temperature is about 2.7 K. That's just not cold enough for the telescope's optics.

  • Also, while radiating into the cold of space theoretically provides a source for cooling besides liquid helium, this is pretty inefficient and all it takes is a short exposure to the hot Earth or a tiny bit of Sunlight to warm everything up dramatically, rendering Herschel at least temporarily blind.

  • Instead, with the current design, the cold optical system can be boxed in and carefully insulated, and the boil-off of the Helium vented to space carries the heat away.

Further reading:

enter image description here

Herschel pictured the "cold cosmos" - places where gas and dust are coming together to form stars. Here, in the Rosette Nebula, in the constellation of Monoceros, a mass of new stars (bright spots) are just firing into life

uhoh
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  • is it boiled off or being used as a refrigerant and slowly escaping? Have worked with various systems using Helium as part of the coolant loop and was part way through writing an answer on Helium as a refrigerant when you posted, but got distracted with why not Neon. – GremlinWranger Jun 04 '19 at 08:50
  • @GremlinWranger As I currently understand it, the helium is use both as a cold storage mass for the 2K stuff, and for boil off on the hot ends of the 0.3 K refrigerators. I'll need to read further to confirm, need a few hours for that. Feel free to edit this, write an additional answer, or we can convert to Wiki. I feel that a better answer than "performance is expected to be not far from background-noise limited" is required but I'm flexible how it's done. – uhoh Jun 04 '19 at 08:55
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    as far as I can tell helium refrigerants get down to 10-15K, well shy of that 3K target so glad I didn't post mine. Used to terrestrial IR Imagers that are working down near Nitrogen boiling point, not absolute zero so made some wrong assumptions. – GremlinWranger Jun 04 '19 at 12:41
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    @GremlinWranger Yes, closed-cycle helium cooling can get down to near the boiling point, as you say. Open cycle systems can get to 0.3K (as in Herschel), but to go lower (or remain closed-cycle) the best terrestrial system we have is the dilution refrigerator, but this critically requires gravity to provide the separation force for the He3/He4 isotopes. Closed-cycle dilution refrigerators for space applications are currently in development, but were not a practical option, presumably, when Herschel was being designed. – J... Jun 04 '19 at 16:57
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    @GremlinWranger An individual pulse tube refrigerator can get to ~10 K, but a multi-stage one can get below 4 K. MIRI on the JWST is planned to use a closed-cycle system including cryocoolers, the wiki page has a decent overview https://en.wikipedia.org/wiki/MIRI_(Mid-Infrared_Instrument)#Cryocooler – llama Jun 04 '19 at 18:11
  • @Loong In what way is using mathrm a "correction"? I'm curious why you believe that k_B is "wrong". Can you cite your source for that? Thanks! – uhoh Jun 04 '19 at 21:52
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    @J... He dilution refrigerators don't need gravity. They flew on the closely related Planck mission (same launch, same bus). One of the "highlights" of that mission was a critical shortage of He-3 right before launch. – user71659 Jun 04 '19 at 23:38
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    @user71659 Traditionally, they do if you close-cycle them, and due to the extreme cost of He-3, in terrestrial applications, they're usually run this way. The Planck cooler ran open-cycle (no gravity), which means it lasted about 30 months with the helium supply on board. That helium-3 was sacrificed to the void of space. Herschel didn't need sub-0.3K temperatures so other open-cycle methods were used that didn't need the very expensive He-3. New designs for closed-cycle He-3/4 separation exist which do not require gravity (capillary separation) but have not yet gone to space, afaik. – J... Jun 04 '19 at 23:50
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    @uhoh Writing $k_B$ is wrong since $\mathrm B$ is neither a physical quantity nor a mathematical variable. There are various sources for the corresponding printing rules; however, instead of giving a list here, I would like to point you to a meta post on another site. –  Jun 05 '19 at 05:43
  • @Loong that's exactly what I needed. Thanks! My voting record shows that I'd up voted that answer a year and a half ago (though I have no recollection of it). btw I've used the Boltzmann constant in several posts here (e.g. 1, 2, 3, 4), feel free to edit them as well if for any reason you may be so inclined! – uhoh Jun 05 '19 at 13:41
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Herschel was an infrared space telescope. According to this paper,

the performance is expected to be not far from background-noise limited, with sensitivities (5σ in 1h) of ∼ 4 mJy or 3 − 20 × $10^{−18}$W/m$^2$, respectively.

At most temperatures, the amount of heat radiated by the spacecraft itself would easily overwhelm the infrared signals it was supposed to detect.

To reduce the thermal noise, the instruments were cooled with liquid helium. This practice is quite typical for other infrared instruments and for radio telescopes.

Uwe
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DrSheldon
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    If you exclude the requirement for liquid He, cooling is used more broadly, Above the entry level, even consumer astrocams are normally equipped with a thermo-electric cooler that lets them operate several dozen degrees below ambient to reduce noise. – Dan Is Fiddling By Firelight Jun 04 '19 at 16:05