While I'm sure it's a technical hurdle in itself, I didn't think that fuel or weight were the big problems with a manned mission to Mars. I'm curious as to what Mr. Musk's plan is for shielding passengers from intersteller radiation that one would experience outside of our magnetosphere.
The Apollo astronauts were exposed to approximately 1140 millirem over a 9 day mission, while the average here on earth is 350 millirem per year. Nuclear workers are limited to about 2000 millirem per year... so the approximately 52,000 millirem per year the astronauts would receive on a Mars mission is a problem.
We could be saved by the square-cube law, or, The Power of Being Big.
In essence, let's assume you've made your own fully-shielded space capsule for one, and that you need, for example, a pound of material for every square inch of its surface, and that you've made it into a sphere to economize on material. Wise choices, all of them. A 2m diameter capsule would come in at 19,500 lbs, or so, for that many square inches.
Congratulations. You've just found out why we don't shield small spacecraft. Now, however, let's get an estimate of what it would cost to shield 200 people, providing them each with a volume-equivalent of your capsule, or 4.2ish cubic meters per person. Really packing them in. This is a 11.6-meter diameter sphere, with roughly 3,300 pounds of shielding per person. At 2000 people, packed in like sardines, we're at 25.2 meters in diameter and 1550 lbs of shielding per person. A 54-meter sphere packs 20,000 sardines, and requires a mere 717 lbs of shielding per person.
A factor of 10-20 better. Obviously, we want to relax the space constraints a bit. However, the form factor of shielding in the larger craft is going to be much better suited to the reutilization of supplies as shielding. In the limiting case, of course, there is effectively no required shielding per occupant. Long before that, magnetic shielding schemes become a viable option, too.
A more practical limit would be to consider a transport module made up of re-entry craft seating 1-10 persons, with the ablative heat-shields and supplies facing outward. In this case, assume we can get down to 5 times your cross-sectional area in shielding, and guess that that's about 2.5 square meters. That's 4000/lbs per person. So, still a factor of 5 better than the solo case.
Additionally, this is a lot of arithmetic to ask of google and http://www.calculatorsoup.com/calculators/geometry-solids/sp..., so the numbers may be off. Also, the Dragon capsule checks in at 5.8 m^2 of heat shield per astronaut, which may be a practical limit. And I just made up the 1lb/in^2 shielding requirement.
Following that math, the 9 day Apollo trip got 0.0114 sieverts. We'll round that to 11 mSv.
So in 9 days, they got 11x more than the EPA's yearly limit to the public, or 1/5th of the EPA's yearly limit to radiation workers.
The 520mSv warpspeed listed as a trip's radiation exposure comes out to 5x the lowest yearly radiation exposure 'clearly linked' to increased cancer risk.
So yeah. A problem, barring adequate shielding, if that number is correct.
Where do you have that 52,000 millirem (i.e. 520 mSv) number from?
Doesn't that completely depend on the thickness and material of the walls of the spacecraft? I.e., isn't it just a question of making the walls thick enough?
I occasionally get to work with the Astronomical Society and have gotten into a few conversations about manned Mars missions with people from NASA, JPL and various educational institutions. The numbers are from a study NASA conducted at Brookhaven National Laboratory.
The problem with shielding is the weight involved to achieve any level of protection.. wrapping the ship in enough lead to make a difference would be hard to get it off the ground. Also while this type of shielding may be effective against normal solar radiation, high energy galactic cosmic rays are far more dangerous and are almost unaffected by conventional shielding.
If you're interested, I recommend this paper. It goes a bit deeper into exactly why radiation is such a problem for any proposed Mars missions (warning, PDF):
Good point, and would that would take care of the gravity well aspect of the issue. You'd still have the same amount of mass to accelerate, though. I suppose that wouldn't be as much of an issue once you weren't fighting Earth's gravity as well as inertia.
That's a silly argument. You could just as well say that the more things you launch that aren't a passenger-loaded vehicle, the more likely you'll have worked out the glitches by the time large numbers of lives depend on it.
A mission to Mars is guaranteed to be the most complex space mission ever attempted. Every additional rocket you need for the mission adds complexity. Complexity is the enemy of reliability.
What happens to your Mars departure window when the rocket carrying your life support system goes off course and is destroyed by the range safety officer?
Unless I'm missing something interesting (maybe a plasma shield?), this only works for radiation with an electric charge - protons, electrons, helium nuceli. Cosmic rays and other EM radiation are not affected by a magnetic field.
Cosmic rays are energetic charged subatomic particles, originating in outer space; i.e. they will be affected by magnetic field. About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. The term ray is historical as cosmic rays were thought to be electromagnetic radiation. [Quoted from Wikipedia]
I didn't read the whole paper, but your own link completely disagrees with your 52,000mrem/520sV-figure:
Whole body doses of 1 to 2 mSv/day accumulate in interplanetary space, and
approximately half of this value accumulates on planetary surfaces (Cucinotta et
al., 2006; NCRP, 2006).
That's millisieverts per day, not sieverts. Am I missing something?
52,000 millirem is 520 mSv, not 520Sv. This number is taken from the Brookhaven National Lab tests. The paper may be talking there about accumulated absorbed radiation, not total exposure.
Practically it really hasn't. Polyethylene is being tested in NASA's Radiation Shielding Program but I don't believe it's been put into service for a manned mission. There are also tests being conducted with adjustable "magnetic bubbles" that can in theory shield a spacecraft from radiation, but the last I heard they were done on a much smaller scale and would require a huge amount of power.
NASA has been making progress in making astronauts less susceptible to radiation damage through diet and vitamins. There's also this paper which talks about a vaccine that's being tested to counteract some of the body's responses to radiation exposure (PDF):
> Nuclear workers are limited to about 2000 millirem per year... so the approximately 52,000 millirem per year the astronauts would receive on a Mars mission is a problem.
The limits for nuclear workers are extraordinarily conservative, so it may not be as much of a problem as you'd think. Given the safety track record of manned spaceflight, the radiation is probably the least of your worries.
This might sound like a stupid question, but here it goes. What if you lined the astronauts in a room with a control unit and really thick lead walls for most of the journey?
Could this theoretically stop radiation, and protect them? Lead blocks most radiation, specifically gamma.
> A better solution is water. You need to carry it anyways.
You certainly won't be drinking that water after a while, though. Some might be used for cooling systems, but you would have a lot of extraneous water.
Why won't you drink it? Just because it's used for shielding doesn't make it radioactive.
Also water basically can't become radioactive. The isotope of oxygen with the longest half life, and more neutrons than the stable ones, has a half life of 26 seconds, so it doesn't stay radioactive.
If you manage to bind a proton (it's virtually impossible for oxygen, but lets say) and make fluorine, the result has a half life of less than an attosecond.
Deuterium is not radioactive, and tritium is hard to make (you need deuterium first, and there barely is any in your water).
The radiation could theoretically fission oxygen into other elements, but that's very unlikely, and the results have extremely short half lives.
In short: Water can't become radioactive, which is why it's so good as shielding. (However water can obviously be contaminated by something else that is radioactive - so don't go drinking the water in a nuclear reactor :)
The Apollo astronauts were exposed to approximately 1140 millirem over a 9 day mission, while the average here on earth is 350 millirem per year. Nuclear workers are limited to about 2000 millirem per year... so the approximately 52,000 millirem per year the astronauts would receive on a Mars mission is a problem.