Is it true that it is impossible to survive Van Allen Belt radiation? I mean do humans die on the spot or later in their life with cancer?
A satellite shielded by 3 mm of aluminium in an elliptic orbit (200 by 20,000 miles (320 by 32,190 km)) passing the radiation belts will receive about 2,500 rem (25 Sv) per year (for comparison, a full-body dose of 5 Sv is deadly). Almost all radiation will be received while passing the inner belt.
25/365 = 0.07
5(Sv) / 0.07 = 71.2
So you have to spend a spit balled 71.2 days (with minimal protection) in the inner belt to receive a potentially lethal dose of radiation.
Bear in mind that's with only 3 mm of aluminium shielding. Manned spacecraft have way more protection than that. The more matter radiation has to pass through, the less effective it becomes. So even the atmosphere inside the craft acts as shielding. And indeed proposed procedures for handling radiation spikes in space is to locate to a central most cabin (kinda like waiting out a tornado in a basement).
Astronauts' overall exposure was actually dominated by solar particles once outside Earth's magnetic field. The total radiation received by the astronauts varied from mission to mission but was measured to be between 0.16 and 1.14 rads (1.6 and 11.4 mGy), much less than the standard of 5 rem (50 mSv) per year set by the United States Atomic Energy Commission for people who work with radioactivity.
The real reason everyone's concerned about radiation is because of the nasty long term effects it can have. Kind of hard sending your best and brightest out there with the knowledge they can have increased chances of developing cancer and/or having deformed offspring. However, this aspect is still very much under research as environmental/lifestyle aspects seem to have have equal or greater impacts.
A dose of under 100 rad will typically produce no immediate symptoms other than blood changes. 100 to 200 rad delivered to the entire body in less than a day may cause acute radiation syndrome, (ARS) but is usually not fatal. Doses of 200 to 1,000 rad delivered in a few hours will cause serious illness with poor outlook at the upper end of the range. Whole body doses of more than 1,000 rad are almost invariably fatal.
So in one day you need to subject an astronaut to 10 times the upper threshold of radiation exposure encountered by an astronaut over an entire trip. Just to make them sick.
No, it is not impossible. 9 Apollo missions sent humans through the Van Allen belts, and the astronauts survived just fine.
The radiation levels in the Van Allen belts are high, about 1000 times higher than normal space. Still, so long as one doesn't stay in that region for a long time, one is perfectly okay.