Answers to Why is the James Webb Space Telescope's data storage space so small? have got me thinking.

Solid State Hard Drives (SSDs) are usually made from FLASH memory though alternatives like magnetic memory also have their place in space.

The unit cell of FLASH memory is a very well isolated capacitor on which charge is introduced by tunneling at high(ish) voltage.

The charge can stay there for years or centuries potentially, we rely upon this vanishingly-low discharge rate when we put something important on an SSD or USB stick and expect it to stay there indefinitely. We sometimes even casually toss flash drives from spacecraft and assume they'll get to Earth with data intact!

enter image description here click for larger

Symbolic jettisoning of the video capsule containing messages from last year’s Victory Day celebration. Photo: NASA TV

Flash memory gets smaller and denser by making the devices smaller, and as a consequence, the oxide gaps and voltages and amount of charge on each capacitor get smaller as well.

It gets even smaller by going analog sort-of, by storing eight different levels of charge on one capacitor, so that the difference between one value and the next is a small fraction of the total charge.

My concern:

Ionizing radiation in a space environment will pass through electronic devices and leave trails of charge; electrons separated from their atoms. My concern is that this process can eventually change the amount level in a FLASH memory bit's capacitor enough to change the logical value it is assigned upon readout at some later date.

As SSDs are built larger in storage and smaller in size, stored data integrity may become more susceptible to ionizing radiation in space.

Question: Are Solid State Hard Drives in space made from radiation-hardened FLASH memory? Or do they just use older technology with higher voltage and charge rating that is less susceptible to ionizing radiation? Or is something else done, or is this not actually a problem at all?

  • 3
    $\begingroup$ It would be possible to use FLASH memory chips with the 8 level technology and use only 1 bit for data and the other 2 bits for an error correcting code. Just a different firmware for the solid state drives. The reliability may be even better than possible with one level technology. But less bits for ECC will do it too if multiple error correction is possible. ECC is used for hard drives anyway see, so a space version should just use more bits for ECC than the ground version. $\endgroup$
    – Uwe
    Commented Feb 1, 2021 at 10:13
  • 1
    $\begingroup$ There are so many aspects of the influence of radiation to semiconductors - the active change of charge in a capacitor due to ionization is one of the least issues. There's no way to describe all the aspects in an answer to this question. just to list a few buzz words: Bulk damage, doping changes, parasitic diodes, trapped charges... $\endgroup$
    – asdfex
    Commented Feb 1, 2021 at 13:07
  • 1
    $\begingroup$ Devices not built to meet rad hard requirements can (and do) have highly varied responses to dose. One small process change at the fab can completely alter their sensitivity. Aside from student cube sats, anybody relying on the flash actually working will use a qualified rad hard chip. $\endgroup$
    – Jon Custer
    Commented Feb 1, 2021 at 15:42
  • 2
    $\begingroup$ My comment was meant to say that there are so many factors preventing the use of standard flash cells, it just doesn't matter if charge loss due to introduced charges plays a role or not. $\endgroup$
    – asdfex
    Commented Feb 1, 2021 at 19:10
  • 1
    $\begingroup$ Another possibility here is using redundant storage like RAID 1, where bits can be constantly checked for “bit flips” and fixed within seconds. $\endgroup$ Commented Feb 4, 2021 at 17:21


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.