As shown on the temperature plots, the D-Instrument Radiator temperature was slowly dropping after sunshield deployment. Around day 34, however, the temperature started to decrease faster than before.

JWST temperature plot

The sunshield deployment started on day 6 and was fully deployed around day 10. This is clear to see on the plots. The mirrors were fully deployed around day 14. The MCC2 burn to orbit L2 was on day 30, but that seemed like a fairly small burn that didn't change the telescope's distance much. It seems like the only other deployments between that and the amazing cryocooler started up on day 87 were mirror segment alignment.

What happened on day 34 to cause the temperatures to start dropping faster?

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    $\begingroup$ My theories: disabled heaters, deploying/enabling additional radiators, change in telescope orientation, the last stowaway life form finally died :) $\endgroup$
    – Nate Lowry
    Commented Apr 11, 2022 at 21:28

2 Answers 2


That is very likely the day the instrument went to active cooling. For a number of reasons they wanted to cool the instrument slowly, mostly to avoid any ice freezing on it. In fact, there's an article dated Feb 10, so only 2 weeks after the event, that tells exactly what happened.

Meanwhile, the NIR instruments are also cooling. Early in the cooldown process, the Webb team used heaters to keep the instruments warmer than the cold-side structures, to prevent water ice from forming on the optical surfaces. But that is all done now, and the instruments and their detectors are cooling nicely.

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    $\begingroup$ Where would the water (that could be frozen) come from? $\endgroup$ Commented Apr 12, 2022 at 8:34
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    $\begingroup$ @SteveMelnikoff the attitude thruster exhaust contains water. Of course most of it is directed away from the instruments, but not all, in particular before the sun shield was deployed (in vacuum the plume extends in all directions) and some water may get adsorbed to cold parts and only slowly outgas again. That's normally not too much of a problem, but if there are really cold parts in the vicinity, the outgassed vapour would systematically resublimate on those parts, and thus form significant (i.e. more than a few nanometres) ice crystals. $\endgroup$ Commented Apr 12, 2022 at 9:58
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    $\begingroup$ @SteveMelnikoff When you assemble a complicated piece of machinery on Earth, water vapor gets into everything. Then, when you put it into a really good vacuum, some of that ad- and ab- sorbed water takes a long time to outgas. $\endgroup$
    – John Doty
    Commented Apr 12, 2022 at 12:57
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    $\begingroup$ How would the "D" curve staying a while near 70K instead of 50K really make a difference? $\endgroup$ Commented Apr 12, 2022 at 21:37
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    $\begingroup$ @Hagen von Eitzen I'm guessing this was 100% expected. The ADIR radiator temperature rate of drop (D curve) between day 10 and day 34 DOES NOT not in and of itself 'really make a difference'. It is simply an expected measurable result of heat being generated by the anti-water heaters on the instruments being being conducted to the radiator. On the other temp graph showing the various benches look at far left day 34... see those also drop at day 34. Also see FSM (fine steering mirror) drop at day 36. Its rapid temp drop is because its optical path means is radiating to deep space directly. $\endgroup$
    – BradV
    Commented Apr 12, 2022 at 23:33

Short and sweet... instrument heaters were switched off on day 34. Fine Steering Mirror heater turned off on day 36.

I've seen quite a few heavily thermocoupled equipment cooldown plots from performing avionics qualification testing in environmental chambers. The plotline discussed to for the aft deployed instrument radiator (ADIR). Comparing ALL of the temp plotlines from both graphics it can be seen that there is a fairly concerted and harmonious trending... with the exception of the ADIR between day 10 and day 34 where the ADIR line slope appears to be artificially held up. This to me is very consistent with the designed in heaters being used on all the various instruments to keep residual moisture from hanging around the instruments, giving the moisture additional time to migrate to open space rather than foul the instruments. This does not appear to me to be associated with active cooling. The only active cooling is the cryocooler for MIRI. (The cryocooler radiators are on the "hot" side of sunshade!)

this shows instrument bench temps drop at day 34 when ADIR begins to drop. Looks like heaters turn off (except for MIRI) on day 34

enter image description here

WOW... the fine steering mirror sure did chill out quickly when its heaters turned off on day 36!


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