Saturday, July 23, 2022

James Webb Telescope Uses a Comically Small Solid State Drive

Do you have a 64 gig USB storage device hanging around; that is, a 64 GB thumbdrive?  You have just a little bit smaller solid state drive than the $10 Billion James Webb Space Telescope, which uses a 68 GB SSD.  That drive holds roughly one day's worth of images and is cycled through daily.

IEEE Spectrum reports Webb generates about 57GB of data each day. In comparison, Hubble generates about 1 to 2GB of daily data. Webb reserves 3 percent of its 68GB SSD for engineering and telemetry data. Scientific data that Webb collects during its mission will also need to be stored on board, as the telescope doesn't maintain constant communication with Earth. However, before the drive fills, which takes about 24 hours, Webb will beam data back to Earth during a pair of four-hour contact windows each day.

Data transmission is a critical component of the Webb mission and has been carefully studied and planned for decades. The telescope is about 1.5 million km (932,000 mi) from Earth at Lagrange point L2. That's a long way to send data. Webb is the first mission to use Ka-band frequencies to send its relatively high amount of data. Specifically, IEEE Spectrum notes that the JWST is transmitting data on a 25.9GHz channel at up to 28Mb/second (0.0035GB/sec). That works out to about 12.6GB per hour.

I've got to say that I was annoyed initially by the statement that the downlink issues had been studied and planned "for decades" but then realized it was correct.  The program planning began in 1997, more than two decades ago, so it's correct.  The originally planned launch date was 2007.  More details on how the schedule slipped over the years is here

My home turf is radio design, so moving over to that I read that the images are sent over a 25.9 GHz Ka-band channel, (pronounced as the letters K and A separately not as "Kah" or "Kay") chosen for its higher available data rates, which translates to higher bandwidths.  There was a design trade of using a more established band, like X-band (7-11.2 GHz) instead of Ka.  For the required antenna gain to get the needed transmitter data rates (bandwidths) required an antenna that was physically larger, and they concluded it was too big for the optical designers to be comfortable around it.  That 12.6 GB/hour rate that IEEE spectrum reports means that the day's 57 GB of image data takes around 4-1/2 hours to transmit.

There are other channels used for technical data exchanges.  Those are in S-band, where many spacecraft operate.  An uplink (ground to Webb) at 2.09GHz transmits observation schedules at about 16Kb/s.  Webb "talks back" (downlinks) at 2.27GHz at 40kb/s.  This is where Webb transmits engineering data, including operational status and system health.

The JWST image released on the 12th of Stephan's Quintet, a famous grouping of five galaxies about 290 million light-years away in the Pegasus constellation. The compact galaxy group was discovered way back in 1787.   Yes, it's five not four - the one in middle, slightly lower right element of an obvious trio, is really two galaxies, NGC7318 A and B.  If you think you've seen it before (and you're not an amateur astronomer) you might remember it from the Christmas classic movie, It's A Wonderful Life.

You might be asking why 68 GB?  That would require access to program records to answer, but my guess is it's really something like a 128 GB drive or bigger that's radiation-hardened and not the consumer-level drives we're used to.  They're expecting Webb to last for 20 years instead of the originally-intended 10, so that drive needs to be more robust and withstand cycling better than the typical ones.  The "brute strength and awkwardness" approach to withstanding cycling better is simply to have tons more memory locations than its rating. 


  1. I understand that we're not robust enough to send a maintenance team to Webb, but does NASA anticipate some maintenance over its 20 year mission life?

    1. At this time NASA is still saying that the Webb is unmaintainable, and has no plans to do any maintenance.

      Give it 5 years after Starship becomes active, wouldn't be surprised if Elon suggests a repair/supplement mission. Then again, who knows, 5 years after Starship becomes active they'll be able to launch a far superior and larger telescope, or even a constellation of large telescopes that form a huge-assed array.

    2. As Beans says, it's considered as having to live or die on its own out there at L2. Almost a million miles from Earth.

      Given SpaceX is working on refueling on-orbit, I can't see why they couldn't get there once Starship is making running as intended.

  2. You are running into the same issues with ruggedness vs OMGblazingspeed that you find in laptops.

    You can buy a really fast with lots of storage gaming laptop with a 17" monitor that weighs practically nothing. Or you can buy a Panasonic toughbook with a smaller monitor, less speed, less storage. Now drop one on the floor from shoulder height.

    Ruggedness works.

    Bullet/Gun version. Yes, you can have a very high velocity small diameter round that will travel forever and... gets deflected by an ant fart. Or you can use a Minie bullet out of a rifle musket and blow the arm off of someone at 500 yards... Slow and heavy, not as much energy but hellava lot more force.

  3. Or in 5 years the marxists and warmongers will have returned society to the stone age.


  4. That photograph of the Quintet is simply awe-inspiring.

    A sky full of galaxies.

    Now imagine handing a wall-poster size high-res print of it to a college astronomy class and professor circa 1920, and watching their heads explode all over the wall.

  5. I was involved in testing some spacecraft components 35 years ago when I first got off Active Duty in the Air Force as a test engineer. When I came on the program, I found that the processor(s) in use were RCA CDP 1802 microprocessors that came out in 1976. Probably they were the radiation hardened Silicon on Sapphire versions. I at first thought it odd with the rate of progress in processor design that they were using such an "ancient" processor for such a leading edge program. After all of the issues were explained to me by a design engineer, I got clued in. When I saw elsewhere that the storage on the JWST was just 68 GB, I understood why especially with the length of time it took to get it into space.

    I have been pleasantly surprised by the functionality of non-space rated digital hardware in amateur radio satellites and the Mars helicopter Ingenuity under conditions which were foretold to make a VERY short life for them. But again, the JWST was designed for an unattended life of 20 years in a very harsh environment.