I wondered about the radiation hardening aspect.

At one altitude does that make a difference?

The annoying answer is "it depends." The main drivers are reliability (ie: how much risk of failure are you willing to accept) and mission life (ionizing dose is cumulative, so a 2 year vs. 10 year mission will have different requirements).

I would say you certainly need to start seriously considering at least some radiation hardening at around 600 km, but missions that can accept a large amount of risk to keep costs down still operate at that altitude with non-hardened parts. Likewise, missions with critical reliability requirements like the International Space Station use radiation hardening even down at 400 km.

The "hard" limit is probably around 1000 km, which is where the inner Van Allen Belt starts. At this altitude, hardware that isn't specifically radiation hardened will fail quickly.

The inner Van Allen Belt also has a bulge that goes down as low as 200 km (the South Atlantic Anomaly), so missions in low inclined orbits that spend a lot of time there or missions that need good reliability when flying through the SAA may also need radiation hardening at comparatively low altitudes.

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kulahan 18 hours ago | parent | prev [-]

Always wondered if you could mitigate this somewhat by basically putting your sat in a bag of water and leaving the antenna and solar panels sticking out.

	▲	Sanzig 16 hours ago \| parent [-]
		Not really. Radiation shielding has diminishing returns with thickness as the relationship is logarithmic. A few millimeters of aluminum cuts down most of your ionizing dose by orders of magnitude over unshielded, but doing appreciably better requires impractically thick shields. And that only helps with ionizing dose, which is already not really a problem in LEO. The issue is more high energy particles like cosmic rays, which cause single event effects (SEEs) - things like random bit flips in RAM or CPU registers, or transistor latchup that can cause destructive shorts to ground if not mitigated. These are impractical to shield against, unless you want to fly a few feet of lead. So instead we mitigate them (ECC memory, watchdog timers, latchup supervisor circuits that can quickly power cycle a system to clear a latchup before it can cause damage, etc). If you want to get an idea of how much shielding is effective in a particular orbit, you can use ESA's SPENVIS software (online, free): https://www.spenvis.oma.be/. Despite being free, it's the tool of choice for initial radiation studies for many space missions worldwide.