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Comment by qazpot | Hacker News Reader

> Data gathered from its scientific instruments, once collected, is stored within the spacecraft’s 68-GB solid-state drive (3 percent is reserved for engineering and telemetry data)

Hope the SSD does not fail after 32768 or 40000 hours of operation.

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All jokes aside, tbere's an article floating around about NASA's software development practices.

They are big TLA+/formal specification fans (the article predates TLA+'s rise) with well resourced and antagonistic Q/A engineers.

That hard drive will have been ordered, custom, and the controller verified by hand I imagine.

Also likely that they are already super experienced with that particular SSD. I read an article a long time ago that talked about a spacecraft with some camera on it. They said the camera was 10 years old when it was installed and although their were better cameras out there, they picked this one specifically because of reliability.

I’m sure the same happened here.

Chances are they or the manufacturer have a room full of those cameras clicking away on a schedule for the last ten years, too, as an early warning system.

Could it perhaps be this? Interesting read about the level of effort and process invested in the Space Shuttle's software (~1996) [0]

For a TLDR, this answer [1] is great.

My favorite excerpt: The Shuttle software consists of ca. 420,000 lines. The total bug count hovers around 1. At one point around 1996, they built 11 versions of the code with a total of 17 bugs.

[0] https://www.fastcompany.com/28121/they-write-right-stuff

[1] https://space.stackexchange.com/questions/9260/how-often-if-...

https://www.fastcompany.com/28121/they-write-right-stuff

Edit: oh you found it

I know this is a joke but...

JWST does not use a typical flash-based SSD. The mass storage is all SDRAM. There are layers of error correction and scrubbing to handle bit flips.

1365 to 1666 days, or 4-5 years of operations.

Maybe they should have chosen a HDD drive? Cosmic particles can flip bits on an SDD, can't they?

I guess hard drives would be dead because of enormous acceleration on launch day.

If they're not on, hdds can handle surprising amounts of acceleration, like 100s of gs

Yeah. Fluid dynamic bearings and parked heads don't seem very vulnerable to shock and vibration.

You could almost invert your logic too. Perhaps the spinning on the HD throws off the telescope enough to be troublesome. I'm not sure how stable the lagrange point orbiting is, but it can't be super stable.

Any moving mechanism is potentially a source for disturbance to the telescope. Not something that affects the stability of the orbit. But the small vibrations can translate to small vibrations in the instruments and secondary mirror which then cause distortion over the integration time of the image.

I don't know that that is the primary reason HDD's have been avoided. But any moving part is another source of failure so my guess is the HDD's life is not as long.

As far as I'm aware I don't know of any spacecraft that has flown a HDD (but there certainly could be). However, some early spacecraft did use tape drives. Hubble originally used tape drives and was replaced with solid state memory during one of the servicing missions.

Since cosmic rays are charged particles I'd expect even more bit flips on the magnetic platter of a HDD.

I am completely guessing here: it is shielded as are most of the computing components. It probably also has error correction of some sort built in.

Shielding adds significant weight, which isn't good for space-bound components.

I would guess they just use RAID, do round-trip data verification before writes succeed, and then reinitialize and scrub any storage module behaving improperly.

For bits stored on SSDs which already rely on error correction, if they were willing to make custom hardware rather than use something off-the-shelf, they could add more chips and scale up the error correcting code to deal with more errors.

Data at-rest in SSDs isn't really at rest. The controller is constantly scrubbing and correcting errors.

The real hazard with SSDs is leaving them unpowered. I know of a story of several systems purchased a decade before they were needed and by the time they were used the boot drives were corrupted.

Huh. Interesting. That explains why my SSD died at boot time. I thought it was just coincidence, and that it had died the night before or something, but that obviously doesn’t make sense.

This thing uses gyroscopes to orient itself. My guess is the less moving parts, the better

Here's an interesting article about the gyroscopes / related tech used on the JWST.

TL;DL — it uses reaction/momentum wheels to change orientation, and it uses non-mechanical gyroscopes to detect changes in orientation.

https://www.universetoday.com/143152/spacecraft-gyroscopes-a...

I just went down a small, but very interesting rabbit hole: I was not aware of non mechanical gyros!

Apparently, light travelling along a fiber optic cable will travel a slightly different distance when the device has angular momentum. That is COOL.

https://en.wikipedia.org/wiki/Sagnac_effect

I believe that the tech you mention is the same tech as used by the flat-earthers in Behind The Curve in the $20k gyroscope they bought, to try and prove that the Earth did not rotate (except they found it picked up a 15-degree-per-hour drift, debunking themselves, so then doubted the technology, heh) [1].

But, aside from the laser/fibre-optic tech, there are also MEMS [2] gyroscopes also — which is almost nanotech IMO — which are used as sensors in modern phones/tablets, and also on drones (to assist with navigation), plus various other robotics uses, and more.

MEMS gyros — often with an accelerometer (aka IMU / inertial measurement unit) and maybe a magnetometer (compass) — can be bought from folk such as Adafruit [3], SparkFun, etc. (I've got a few different ones myself), and hooked up to e.g. an Arduino or similar MCU (or indeed anything else that speaks can speak the appropriate protocol, e.g. I2C/SPI/etc depending on the board in question).

Hmm, just looked for a good image of how a MEMS gyro works, and didn't come up with what I was looking for / recall seeing before (I'm a bit pushed for time), but there's a diagram on this WP page [4].

Edit: ah, here's a better diagram [5]

[1] https://www.triplem.com.au/story/flat-earthers-spend-20-000-...

[2] https://en.wikipedia.org/wiki/Microelectromechanical_systems

[3] https://www.adafruit.com/category/521

[4] https://en.wikipedia.org/wiki/Vibrating_structure_gyroscope#...

[5] https://www.researchgate.net/figure/Block-diagram-of-the-MEM...

The FE link is awesome!

The others are interesting and informative. I am gonna buy me a few parts to play with.

Thanks!

No problem, have fun! — and if you enjoyed that FE article, you will likely enjoy the 'Behind The Curve' movie too, it's really quite amusing :)

I've used the MPU6050 to build a two-wheel self balancing robot (roughly this: https://www.instructables.com/Arduino-Self-Balancing-Robot-1...). Basically, apply PID to the motor angle to keep the robot level with the gravity vector.

I still think MEMS sensors are very cool and extremely convenient/cheap for what you get.

Nice!

I've done some projects myself with MPU6050 and some with its 9-axis "bigger brother" the MPU9250 (same as 6050, but with added 3-axis magnetometer/compass). I've also used the LSM9DS1 — another 9-axis IMU, just a different chip.

— Yeah, definitely amazing tech for the price. Cheap as chips! /me gets coat.

Ingenuity, the Mars helicopter/drone, apparently includes a bunch of off-the-shelf kit like this — IIRC I think a bunch of the parts are made by SparkFun. I don't recall any specifics though.

Yep, ring laser gyro :)

They are pretty awesome. Once got some data that seemed to be bad out of one and realized that the entire error was exactly accounted for by the Coriolis effect from the rotation of the earth.

that means no sandisk or the like

68 GB ? Just ? Shouldn’t that be some 10-15 1-2 TB SSD in parallel ?

Technology for space lags behind consumer technology by 10-20 years. There are a few reasons for this:

- Long lead time to test and certify hardware.

- Higher reliability requirements (e.g. must work non-stop for 10 years)

- Must be able to operate in a higher radiation environment with little to no cooling. In a vacuum, and in zero gravity, cooling works very differently to how it does on Earth.

- These missions often take a decade or more to come together, and changing requirements throughout that process is hard, costly, and risky, so often they stay the same from the beginning.

Notably, SpaceX are bucking this trend a bit with their avionics which just runs on standard Linux machines rather than specialist machines or with a realtime OS, but they have mission lengths measured in minutes to hours, not decades.

They also simply do not store data locally.

The onboard storage is basically a buffer, they beam everything back to earth.

Yep. That means bandwidth is the limiting point, and that maxes out at 28 megabit in ideal conditions. A multi-terabyte array would be a waste.

Sometimes the smartest people in the world still stink at their short game

IIRC the hard part with this thing is the lenses. I wonder how hard it would be for astronauts to swap an SSD if it came to that.

No currently available spacecraft can make the trip and service JWST, it's also not really intended to be serviced.

Spaceship might (Elon time) fly this August.

It's "Starship", and it's unlikely to be able to get to where the JWST is in the next few years due to needing to be refuelled in orbit. Plus they have no payload bay design. Then there's the robotics necessary for doing a repair, or human-rating Starship, either of which is years worth of time.

Oh yea. Much less memorable than BFS.

Spitballing here ... could we de-orbit it back to Earth with what propellant remains? Then fix whatever and refill the propellant?

The sunshield had a complicated unfurling sequence, I very much doubt they could fold it back up while still in orbit.

JWST is not serviceable. If the ssd dies, then it's dead. There's no replacing it.

> If the ssd dies, then it's dead.

Presumably they could operate in a reduced mode where it does live transmission of the data when it's in contact with Earth?

Depends if the sensor readout is slower than 28 Mb/s.

If it generates, at most, 57GB per day[1], assuming 22h operation (2h for transmission), the sensors are generating 2.6GB/h or about 750kB/sec which is just about 6Mb/s (unless my math is wonky.)

[1] "JWST can produce up to 57 GB each day (although that amount is dependent on what observations are scheduled)."

That is the average rate over a day. If storage is not available to buffer it, then a sensor's peak readout rate could easily exceed the transmission rate.

Edit: Which apparently it does, which is why the SSD can ingest up to 48 Mb/s to be read out more slowly later (https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-s...).

But also "The actual data rate depends on the number of detectors simultaneously in use, their exposure parameters, and the precise timing of when their exposure readouts arrive in the ICDH for processing" - in a reduced operation mode, they can turn down the number of sensors, etc., to keep the data rate below the live transmission rate.

It isn't meant to need servicing, but it does have a docking port if it ever does need it and we have the desire to do so.

Yes, but a servicing mission for replacing the memory would still be an unlikely operation to be able to do. The most likely use for the docking port is if the electronics onboard exceed their design life, but JWST is running out fuel (to maintain its position at the Lagrange point) so they basically strap a "jet pack" on to the satellite to keep in operation. This has been the business case of some companies trying to do this for GEO satellites:

https://astroscale.com/astroscale-u-s-enters-the-geo-satelli...

The JWST is about three times further than the furthest a human has ever been from earth, which was on the moon.

So, it would be __really__ hard for astronauts to swap an SSD.

It's much easier to get there and fix JWST than it is to land on the moon though.

Because the landing part was the hardest and take more delta V than actually getting to JWST. Not to mention going back from moon surface.

Didn’t JWST use some kind of grivitational breaking to stop at the LGPoint? A manned mission would require much more fuel to shorten the time of the trip and fuel for returning.

I think the engineering that would go into a mission to do maintenance/repair would be quite valuable and help with many other missions. It’s far enough to be far, but not so far that other factors like communication is extremely delayed. Getting that far out of LEO has a huge set of new challenges, I assume, we should be able to learn about.

Maybe we could send up a data buffer with a high-bandwidth, short-range radio.

Even if a crewed spacecraft could be sent to it (honestly not that crazy given JWST's predicted lifespan of 20 years. Starship should be crew rated in 10 years at most and if not, Orion could be sent with some sort of expanded service module for the trip) the issue would be that docking to or even approaching the telescope is extremely risky.

You don't want to fire any thrusters in its direction to avoid damaging the sunshield or the mirrors, but of course to slow down at it you would need to do that at some point.

If the SSD failed and a constant connection to Earth cannot be maintained, the more realistic solution would probably be to launch a satellite to a high orbit to maintain permanent connectivity with JWST which can act as a store-and-forward relay, effectively replacing the SSD without having to actually go to the telescope.

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