Does the U.S. really need to worry about radiation?

Roamy here.  Blame it on the “duck and cover” exercises in the 1950’s or yellow journalism, but many people are afraid of radiation.  I can remember when magnetic resonance imaging (MRI) used to be known as nuclear magnetic resonance, and they changed it because people heard the word “nuclear” and said no to a useful test.  Easy enough to be afraid of something that can kill you and you can’t see it coming.

Discovery has a pretty good article here, and California has a FAQ here but I’ll distill it down further.

California is 5,000 miles away from Japan.  That’s a loooong distance for anything coming from Japan, so any radioactive elements have plenty of time and space to dissipate.  I’m telling my friends and loved ones on the West Coast that they don’t need potassium iodide – the half-life of radioactive iodine is 8 days, and it takes about that long to get here.  Furthermore, you’ll get a bigger headache from the side effects than any extremely slight benefit.  (For any children living with 20 miles of Fukushima, it’s another story.)

I learned my levels in millirem, but I’ll convert it to milliSieverts, since that’s what Japan is reporting.  (For some no-nonsense reporting on what’s going on in Japan, try World’s Only Rational Man.)

  • Radiation from Japan arriving in CA – <0.001 milliSievert (mSv) (they didn’t give a rate with this data – I’d assume per day for now.)
  • Sunbathing on the beach for a day – 0.01 mSv
  • Flying cross-country – 0.04 mSv
  • Extra yearly radiation dose if you live in a brick house instead of wood – 0.07 mSv
  • Chest X-ray – 0.1 mSv (though I’ve seen as low as 0.02 mSv)
  • CT scan – 1 to 2 mSv
  • Eating dinner off “FiestaWare” – 2 mSv 
  • Mammogram – 2 mSv
  • My allowed dose in a year where I work – 5 mSv
  • Barium enema – 3 to 15 mSv
  • Average dose to Ukrainians evacuated from Chernobyl fallout – 17 mSv
  • Average dose to Pripyat (nearest village to Chernobyl) evacuees – 430 mSv
  • What will make you sick, if received in a short amount of time – 1,000 mSv
  • What has a 50% chance of killing you, if received in a short amount of time – 5,000 mSv

If you’re going to squawk about radiation levels, I’d be more concerned with the body scanners at the airport.

Bulletproofing a space station

Roamy here.  So I’ve talked about bulletproof vests and space debris, it’s finally time to talk about shielding.  A common way of testing different shielding designs is a two-stage light gas gun.

Your gun might be bigger, but mine shoots faster.

A high-speed rifle can shoot around 1.5 km/sec; a light gas gun can shoot up to 8 km/sec.  The “light gas” part comes from using either hydrogen or helium in the first stage.  A really good explanation of how this works can be found here. Basically you blow up some gunpowder, which moves a piston, which compresses the light gas behind an aluminum disk machined to burst at a specific pressure. Disk bursts, moving the projectile down the barrel to the target, past X-rays and/or cameras so you can figure out the speed, then impact.

Shielding for space debris is based on the Whipple shield, where you have a sacrificial plate to break up the debris into smaller particles.  (Dr. Fred Whipple, not the “don’t squeeze the Charmin” guy)  For the Space Station, you also have a thermal insulation blanket made up of thin layers of metallized plastic film and netting spacers.  A lot of thought goes into how thick the sacrificial plate should be, the spacing between all the components, and how to replace it on orbit when it’s done its job.

If you have just a sacrificial plate of aluminum and a thermal blanket, you can still get this kind of damage.

Sorry for the poor quality, but it was the best I could find that was cleared.
Add in a blanket like a bulletproof vest – Kevlar and Nextel or other aramids and ceramic cloth, and you get some dimples and craters instead.
L to R, sacrificial plate, protective blanket, simulated pressure wall

That little marble in front of the protective blanket in the picture above?  One of those fired at 6.7 km/sec is what caused that damage.  Remember from your physics class, kinetic energy equals one-half mass times velocity squared.  Only one gram or so, but that velocity squared is a bitch.

Update by XBradTC: Basically, NASA engineers are facing the same problem that every tank, bullet-proof vest, and warship designer faces- the balance between weight, protection and likely risk.

It would be the simplest thing imaginable to protect an infantryman from head to toe against small arms fire. The problem is, the weight needed to protect him would leave him immobile. An infantryman who can’t move isn’t an infantryman anymore. So risk becomes a factor. For a soldier that will spend most of his time in a Humvee, it’s pretty easy to justify adding a groin protector and deltoid armor. But a troop who has to ascend and descend steep mountains all day in Afghanistan can’t really hump that extra armor. The risk of sustaining a non-lethal wound to extremities becomes acceptable in that case.

Same thing with tanks. Tanks are more heavily armored on the front slope than elsewhere, because that is where they are most likely to be hit by the most dangerous projectiles.  You can’t armor something so heavily that it is invulnerable without sacrificing the mobility of the tank to a degree that renders it useless.

In NASA’s case, it costs a lot of money for every pound lifted into orbit. Without any protection, you face an unacceptable risk from small impacts. But with too much mass devoted to protection, you sacrifice lifting up the resources that are the whole point of the project.