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.