Space is a erous place, not only because of meteors but also because of rays from the Sun and other stars.
Radiation is the greatest known er to explorers in space. Doses of radiation are measured in units called "rem". We all receive radiation here on the Earth from the Sun, from cosmic rays and from radioactive minerals. The "normal" dose of radiation that we receive each year is about 100 millirem; it varies according to where you live, and this is a very rough estimate. Scientists have reason to think that a man can put up with far more radiation than this without being damaged; the figure of 60 rein has been agreed on. The trouble is that it is extremely difficult to be sure about radiation damage—a person may feel perfectly well, but the cells of his or her organs may be damaged, and this will not be discovered until the birth of children or even grandchildren.
Early space probes showed that radiation varies in different parts of space around the Earth. It also varies in time because, when great spurts of gas shoot out of the Sun, they are accompanied by a lot of extra radiation. Some estimates of the amount of radiation in space, based on various measurements and calculations, are as low as 10 rem per year, while others are as high as 5 rem per hour! Mission to the Moon have had to cross the Van Allen belts of high radiation and, during the outward and return journeys, the Apollo 8 crew accumulated a total dose of about 200 millirem per man. It was hoped that there would not be any large solar flares during the times of Apollo moon walks because the walls of the lunar excursion modules (LEMs) were not thick enough to protect the men inside, though the command modules did give reasonable protection. So far, no erous doses of radiation have been reported, but the Gemini (双子座) orbits and the Apollo missions have been quite short. We simply do not know yet how men are going to get on when they spend weeks and months outside the protection of the atmosphere, working in a space laboratory or in a base on the Moon. Drugs might help to decrease the damage done by radiation, but no really effective ones have been found so far. At present, radiation seems to be the greatest physical hazard to space travelers, but it is impossible to say just how serious the hazard will turn out to be in the future. According to the passage, which of the following is NOT true