Last week, we saw the first images from NASA's $10B James Webb Space Telescope. The photos are stunning and represent an incredible achievement. The photos are sharper and more detailed than those from the Hubble Space Telescope. They will also unlock key information for researchers working hard to understand the universe's origins, how stars and galaxies form, and if there are planets out there that can support life. The images are spectacular, and the telescope's technology is extremely sophisticated, so it may be surprising to learn that they're being stored aboard the space telescope on a relatively paltry 68GB SSD. The hard drive can hold about a day's worth of Webb images.
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.
Ka-band frequencies can transmit more data than either X-band (7-11.2GHz) or S-band (2-4GHz). Those bands are typical choices for distant spacecraft. Given Webb's high amount of data, these bands weren't a viable option. Further, per Carl Hansen, a flight systems engineer at the Space Telescope Science Institute (STScI), a comparable X-band antenna would have been large enough that it could have hampered Webb's ability to capture sharp images.
Although primary data is transmitted using a Ka-band, Webb includes two S-band channels. A 2.09GHz uplink transmits observation schedules to the telescope at about 16Kb/s. A second 2.27GHz S-band has a 40kb/s downlink that transmits engineering data, including operational status and system health.
At L2, Webb is subject to extreme temperatures of around -223° C (-370° F) and significant radiation. The SSD had to go through extensive testing, of course. Even still, after a decade, the SSD's capacity will be reduced to around 60GB due to radiation and general wear and tear. NASA expects Webb to perform at least 20 years, double its original mission plan. However, at what point will SSD deterioration become an obstacle?