Everyone is going gaga over the images from the James Web Space Telescope (JWST).
But for nerds like me, the telescope itself is the interesting part. It’s a long way away (3000 times as far as the Hubble!), and it’s a pretty slick bit of kit, that’s for sure. We’ve already noted it’s origami inspired sun shade
Like my garage, the JWST is solar powered [1]. In fact, it is roughly the same size and capacity as my garage roof. : – ) In space the sunlight is a lot more predictable. No clouds or trees, and the orbit is simpler than on the surface of a precessing planet. On the other hand, L2 is literally twice as far from the sun as Earth, so the light is a quarter as intense.
Sunlight is great, indeed vital, but the actual telescope itself needs to be shielded from the sun, both to see in the dark and to be cold [3]. The Webb is an infrared telescope—we’re trying to image really faint objects—which means that it has to be cold to operate. It needs to be 40 degrees K, which is really, really cold.
This is accomplished with a sun shade, five layers of reflective foil that keeps the telescope in complete darkness. The telescope is exposed to space, which is about 2.5 degrees K, so any heat from the machinery radiates away. This is a simple, elegant passive cooling system. Honestly, It seems unlikely that anything more complicated would not work, at least for long.
And finally, closer to my own career, the telescope downloads data. A lot of data. Up to 57 GB per day, which is more than 25 times the capacity of the Hubble [2].
As an old data mover, I know that getting data from this kind of remote (1.5 M km!) instrument is a waltz of time and storage space.
The radio downlink isn’t fast by surface standards (a few MB per second), and has to share the DSN on the ground. So gigabytes of data has to be marshalled on board to be compressed and downloaded in scheduled time slots. Observations are complicated “programs” which choreograph the capture of photons and the movement of data into storage for download.
The JWST has 68GB of storage, which isn’t much by surface standards, but is huge by L2 standards. This storage can hold about 24 hours of data, which should encompass several opportunities to download it. This design is intended so that if communications fail on a single download, there should be at least one second chance before the observation is lost forever.
With the first pictures this month, we see that this whole thing is actually working. As an engineer, that’s amazing to me. : – )
Running at low power near absolute zero. Executing complicated workflows by (extremely) remote control. Downloading gigabytes of data. This shouldn’t work—but it does!
Nice work, all.
- Ryan Kennedy, Solar panels power the James Webb telescope, in PV Magazine, July 13, 2022. https://pv-magazine-usa.com/2022/07/13/solar-panels-power-the-james-webb-telescope/
- Michael Koziol, The Webb Space Telescope’s Profound Data Challenges, in IEEE Spectrum – aerospace, July 8, 2022. https://spectrum.ieee.org/james-webb-telescope-communications
- Ned Potter, Inside the Universe Machine: The Webb Space Telescope’s Chilly Sun Shield, in IEEE Spectrum – Aerospace, July 7, 2022. https://spectrum.ieee.org/james-webb-telescope-sunshield
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