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1. Charles Goodwin - July 23, 2009

I don’t wish to be a wet blanket, nor do I wish to belittle the tremendous achievements of the Apollo programme, but why hasn’t there been similar media coverage of the anniversaries of similarly – and in some cases possibly even greater – achievements in space, such as the launch of the first artificial satellite, the first man, and woman, in space, the first space crafts to land on the Moon, Venus and Mars, or the first spacecraft to photograph the far side of the Moon?

Could it be because all of those firsts were achieved by the USSR? Yet without them, there would have been no reason for Kennedy to have committed the USA to landing men on the Moon, in what was almost certainly one of the greatest propaganda exercises of all time. Apollo was also, of course, a huge achievement – but so were the others, conveniently forgotten. I don’t suppose many people even KNOW that the USSR successfully landed a spacecraft on the Moon that dug up rock samples and returned them to Earth.

Speaking of sample return missions, wouldn’t that be a good idea for Mars, rather than sending astronauts – at least in the near future? There are huge problems involved in manned flights to Mars, as detailed in Eugene N Parker’s article “Shielding Space Travellers” in the March 2006 issue of “Scientific American”. A few of the important points:

Cosmic rays pose a large threat to space travellers. These “rays” are actually a mixture of protons and (occasionally) heavier atomic nuclei. They are shielded from us on Earth by the atmosphere, generally hitting an atom at 20-25 kilometres altitude and having the resulting shrapnel gradually reduced on the way down. In space, however, about 5000 ions would pass through your body per second. The heavier nuclei would do most damage – their ability to break chemical bonds is proportional to the square of the charge, so an iron nucleus would do 676 times more damage than a proton. A week or so of this would not be serious, but spending months en route to Mars is another story. One estimate from NASA is that one third of the DNA in an astronaut’s body would be cut by cosmic rays every year. A recent study indicates that Mars astronauts would receive 80 rems of radiation per year – the legal limit for workers in the nuclear industry being 5 rems/year. A rough estimate is that one in ten male astronauts would die of cancer and one in six females. Heavy nuclei could also cause cataracts and brain damage.

The sun can also deliver occasional bursts of lethal radiation – a dose of hundreds of rems over an hour or so. Trips to Mars should be scheduled for periods of minimum solar activity, but there’s always the possibility of an unpleasant surprise…

Various studies into protecting astronauts have suggested shielding them with matter, magnetic fields or by positively charging the space craft (to repel ionised nuclei). None of these looks particularly viable at present, however. To match the shielding of the earth’s atmosphere requires 1 kg of mass per square centimetre (the same as the weight of atmosphere above each square centimetre at the earth’s surface). This could be halved without a significant loss of protection – this would be equivalent to living at an altitude of 5,500 metres above sea level. However even a small capsule would need about 500 tons of shielding (the suggestion is to use water, which the astronauts need in any case). This is simple and guaranteed to work – and way too heavy to lift with current technology (the space shuttle can lift about 30 tons). Polyethylene would be more effective (and solid) but would still need around 400 tons.

A magnetic shield could deflect ions but would need to be around 20 teslas, or 600,000 times the strength of the Earth’s equatorial field. This would require superconducting cables – MIT has devised a system that would weigh 9 tons. The ship would have to be shaped as a torus, and the crew would be subjected to a VERY intense field, with unknown biological effects over long periods. Presumably no magnetic objects would be allowed within the ship! (Anecdotal evidence suggests that a 0.5 tesla field produces flashes of light in the eyes and an acid taste, possibly due to electrolysis of saliva.)

It might be possible to cancel the magnetic field in the ship using a second field, however this bumps up the mass involved – and the possibility of something going wrong.

The third possibility is an electrostatic shield. Charging the ship to two billion volts (relative to the surrounding space) would repel all cosmic rays. Unfortunately, this would attract electrons from the solar wind, which would strike with the same energy as the cosmic rays would have. The electrons would produce gamma rays on impact, giving an even higher radiation flux than before. Also, the power required would be some 2 gigawatts, and the currents in the ship’s hull would exceed 10 million amps…

So perhaps this should be a job for the robots, at least until we can devise a faster way of crossing space.