The Hostile Space Environment
Space is a hostile, unfriendly place. There are all sorts of difficulties that AMS has to deal with on-orbit.
- In low-Earth orbit, it's not quite a vacuum. There are a lot of ionized oxygen atoms zipping around (similar to the "free radicals" that you hear about from nutritionists, only worse). Free radicals are the most corrosive things imaginable; they'll eat through a millimeter of plastic in a few months. AMS can't have exposed plastic.
- In low-Earth orbit, it's still a better vacuum than the best experimental chambers on Earth. There will be experiments on/outside ISS that depend on this vacuum - AMS is not permitted to contaminate it. You're familiar with "new car" smell or "new carpet" smell? That smell is due to organic solvents which evporate from all plastics. All plastics on AMS - exposed or not - have to be pretreated so that they don't "outgas" these solvents. For all plastics, this is difficult; for some plastics this is impossible. We have to design with this in mind.
- AMS has to radiate all of the heat it generates. Not only are there 2000 watts of power heating up our computers, but sometimes the sun is shining on us continuously for hours! So AMS's thermal control system is designed to efficiently whisk heat out to AMS's surfaces, and glow it away into the 3 K coldness of space. Unfortunately, sometimes AMS is getting very little sunlight. Since we can't turn off the thermal system, AMS is in danger of getting very cold. We have to strike a careful balance - build the radiators efficient enough that we do not die of heat in the sun, but also not so efficient so we do not get to cold in the dark!!
- Humans have only been flying into space for about 50 years, but we've already left a noticeable trash pile behind. Billions of pieces of junk - bits of abandoned satellites, rocket casings, etc. - are whizzing around up there at thousands of meters per second. In addition, there are natural "micrometeorites" passing by at tens of thousands of meters per second. We have to consider what will happen if we get hit. It may seem like there's not a lot you can do, right? One thing we can do is to try to "localize" damage. If a micrometeorite hits one part of the gas-filled Transition Radiation Detector, valves will close to seal off that part, and the rest of the detector will still work.
- We're going into space to measure high-energy particles, yet these same particles can hurt us. First of all, spending three years in space gives AMS a fairly large dose of radiation. It's not deadly (the astronauts, for example, are OK with it) but it can sort of age semiconductors and lower their performance. A more dangerous radiation effect is the "single event effect". A high energy nucleus really creates a lot of ionization when passing through material; sometimes there is enough ionization to "flip" a bit in memory, or even to momentarily short-circuit a transistor. We've had to pre-certify that our electronics are not subsceptible to this sort of thing - basically by putting it in a particle beam, pounding it with 10 years' worth of radiation, and seeing how well it works. You would be surprised how quickly (and how catastrophically) most transistors die in a particle beam!