Space is a dangerous place, not only because of meteors but also because of rays from the sun and other stars. The atmosphere again acts as our protective blanket on the earth. Light gets through, and this is essential for plants to make the food which we eat. Heat, too, makes our environments tolerable and some ultraviolet rays penetrate the atmosphere. Cosmic rays of various kinds come through the air from outer space, but enormous quantities of radiation from the sync are screened off. As soon as men leave the atmosphere they are exposed to this radiation but their spacesuits or the walls of their spacecraft, if they are inside, do prevent a lot of radiation damage.
Radiation is the greatest known danger to explorers in space. Doses of radiation are measured in units called “rems”. 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 millirems (0. 1 rem) ; 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 rems has been agreed. 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 sex organs may be damaged, and this will not be discovered until the birth of ( deformed) 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 ( solar flares), 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 rems per year, others are as high as 5 rems per hour. Missions to the moon (the Apollo flights) 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 millirems per man. It was hoped that there would not be any large solar flares during the times of the Apollo moon walks because the walls of the LEMs (lunar excursion modules) were not thick enough to protect the men inside, though the command modules did give reasonable protection. So far, no dangerous doses of radiation have been reported, but the Gemini 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.
1.Scientists have fixed a safety level of （ ） .
2.The spacemen were worried about solar flares when they were （ ） .
3.When men spend long periods in space how will they protect themselves?
4.Which of the following is true?
