# How can electronics on board JWST survive the low operating temperature while it's difficult to survive lunar nights?

Compared to the temperatures encountered in lunar nights, the desired operating temperature of the James Webb Space Telescope is very low. Lunar nights are typically -170°C while the operating temperature of JWST is -220°C.

If surviving the cold lunar nights is difficult for the electronics, say on lunar rovers, then how does JWST overcome this issue?

And why not make use of the same methods adopted in JWST onboard the lunar rovers to survive lunar nights, avoiding the notorious nuclear heating?

Most of the electronics is on the "hot side" but there needs to be some conventional electronics on the cold side (beside the cooled IR sensor chips).

Small thermal environments on the cold side are equipped with heaters to provide mini-environments at normal operating temperature for these electronic devices.

[...]Thermostatically control heaters are used to maintain equipment above minimum required temperature while under cold conditions. Heater drive electronics (HDE) controlled heaters are used to maintain the +J3 panel, propulsion lines, battery, star tracker, and 1 Hz isolators within the required stability range.

The spacecraft component temperatures are maintained within the required limits by the use of radiators, heat pipes, MLI, and heaters. Thermostat and software controlled heaters are the two types used on this spacecraft. The software control heaters are used to maintain tight temperature control for critical spacecraft components and structures. The heaters are controlled by flight software with temperature feedback control. The flight software enables the ground to modify any TCS mission constants which include on/off heater set-points and failure thresholds.

If surviving lunar nights are difficult to ensure the survival of the electronics, say on lunar rovers, in the low temperatures,

The temperature itself is not the primary reason.

Lunar nights are difficult to survive because you have 14 days of darkness. If you want to design a solar-powered rover that can store enough energy to stay warm for 14 days, the rover becomes large and heavy. Too heavy to launch with current rockets.

So we use nuclear decay heaters instead, which are much smaller and lighter.

The alternative is to power down for the lunar night. But then the entire rover cools down, and will heat up again the next morning. These heat cycles are the usual cause of death: because different materials expand and contract at different rates, it's really difficult to design electronics that stay intact with such large temperature swings.

JWST, on the other hand, is in permanent sunlight. This has several consequences:

• you can heat the electronics directly by putting them on the hot side of the spacecraft
• you can use electric power from the solar panels to run heaters on the cold side, without needing large batteries.

So you can't use the methods from JWST on a lunar rover: their environments are too different.

Nuclear heating isn't "notorious", it just makes the mission a bit more expensive. It's a mature, reliable technology that works, so why not use it?

Source: Me. I currently work as a systems engineer on JWST.

JWST will operating from the 2nd Lagrange Point (aka L2), which is approximately 1.5 million km (or 930,000 miles) past the Earth in Sun-Earth line. This is approximately 4x the distance from the Earth to the Moon. This distance, in addition to its lissajous orbit, ensure it will not encounter any Lunar Night, and its solar array will remain in sun.

Regarding its operational temperature, you have to understand JWST has three (3) clearly defined thermal regions, and a transitional region. Region 3 is the spacecraft compartment. This houses all the main spacecraft subsystems/electronics (main computer, communications system, attitude control, etc, as well as the main instrument computer and cryocoolers). This region is in continuous sun, so operates anywhere from 0-40 C, depending on the location in orbit and efficiency of thermal dissipation. The battery is also housed in Region 1, so will never experience the drastic cold.

With the 5-layer sunshield, there is a transition area to go from the "hot" sun side to the cryogenic cold side. Note that this is a very steep temperature transition, so materials and cabling have to withstand all this, plus the cryocooler lines.

Region 1 is the "cold" part of the observatory, and encompasses the mirrors and actuators and the instruments (meaning all their mechanical parts and their detectors), the support structure, and various heaters, harnesses, and thermal conductors. Here the temperature will passively reduce below 30 K, save for the heaters that keep the instruments at their operating temperatures, and the cryogenic instrument, which goes down to 6 K. The instrument mechanisms are all designed to work at these low temperatures, and the focal plane systems are also specially designed for these temperatures as well, with some immediate/local processing.

Region 2 is located off the back of Region 1. It contains the instrument electronics boxes (outside the main instrument computer), and a couple other support electronics boxes. This is a very-well insulated box with baseplate heaters to maintain it at >0 C, so compared to Region 1, it is quite warm! It also contains specialized, directional radiators that help shed heat from Region 1 into deep space for cooling down and maintaining operational temperature.

Keep in mind that all of these thermal regions are existing at the same time on JWST, although most of the electronics are kept fairly warm. Between 4 cryo-vacuum tests and a thermal vacuum test, the major components of all these have been successfully tested.

@GremlinWranger: Regarding thermal cycling and tin whiskers, NASA (and by extension, its contractors) require usage of specific solders and conformal coatings to mitigate tin whiskers and Paschen discharge. Additionally, once up there and everything has cooled to its operational temperature, JWSTs various regions will remain at their temperature within several kelvin (for heater cycling and seasonal/orbital effects), barring any incidents.

@uhoh: Thanks for the diagram and quoted text!

• Thanks for your detailed and authoritative answer and Welcome to Space! just fyi the question Can the James Webb Space Telescope basically manage its own orbit if necessary? might be in need of an updated answer as well.
– uhoh
Jul 22, 2019 at 23:10
• also fyi, now that you have more than 50 reputation points, you can leave comments under other people's posts. You've formatted the @replies feature (found in FAQ on the main meta site) properly but they don't work in the body of a post, they only work in comments below posts, or in chat, such as the Pod Bay. You can try posting your message to GremlinWrangler as a comment now.
– uhoh
Jul 22, 2019 at 23:13
• Hey! I also wanted to welcome you :)! It's always awesome to hear it from the horses mouth, so to speak. Would love to hear more from you! Thanks for taking the time to talk to us, we all appreciate it. Jul 23, 2019 at 19:17
• Thank you! I can see what I can help out on here and there, but no promises I'll know everything. This agency is a good size and there are many, many people smarter than me working on things! I can try to answer any questions you have that are within my ability and/or capability Jul 24, 2019 at 17:49
• Did you mean the battery is housed in region 3? Apr 12 at 9:55

In terms of the electronics themselves many specific devices can operate at quite low temperatures, though any property depending on the physical dimension (resistance and capacitance) will change slightly due contraction. Some however explicitly cannot.

Batteries depend on chemical reactions that slow down approaching absolute zero and if all the elements are frozen. High capacity batteries also have micro structures that can get disrupted if ice crystals form due to freezing. This disruption can lead to short circuits when thawed again.

Some high value capacitors contain a fluid electrolytic which has similar results to those in batteries when frozen and thawed. They can generally be swapped with lower capacity varieties at a weight penalty.

Semiconductors change properties in various ways with temperature. In particular the resistance in the on state increases at low temperatures. This can handled by changing the doping of the parts and designing so that the part can function with a wide range of parameters but this increases size, weight, power consumption and design complexity. This pretty much rules out high performance processing while at very low temperatures but sensors and basic signal conditioning are possible.

High performance electronic parts involve a number of different materials (with different expansion coefficients) in electrical contact with each other, as noted in Hobbes' answer repeated cycling will produce failures over time if these loose connection. This can be reduced by careful design but involves increase in weight and complexity. Temperature cycling is also believed to be involved in tin whiskers that can create all sorts of complex problems.