根据以下材料，回答题
Space is a dangerous place, not only because of meteors but also because of rays from the sun andother stars.The atmosphere again acts as our protective blanket on earth.Light gets through, and this isessential for plants to make the food which we eat.Heat, too, makes our environment endurable.Variouskinds of rays come through the air from outer space, but enormous quantities of radiation from the sunare screened off.As soon as men leave the atmosphere they are exposed to this radiation.But theirspacesuits 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.The unit of radiation is called "rem".Scientists have reason to think that a man can put up with far more radiation than 0.1 rem without beingdamaged; the figure of 60 rems has been agreed on.The trouble is that it is extremely difficult to be sureabout radiation damage--a person may feel perfectly well, but the ceils of his or her sex organs may bedamaged, and this will not be discovered until the birth of deformed (畸形的) children or evengrandchildren.Missions of the Apollo flights have had to cross belts of high radiation and, during theoutward and return journeys, the Apollo crew accumulated a large amount of rems.So far, no dangerousamounts of radiation have been reported, but the Apollo missions have been quite short.We simply donot know yet how men are going to get on when they spend weeks and months outside the protection ofthe atmosphere, working in a space laboratory.Drugs might help to decrease the damage done by radiation,but no really effective ones have been found so far.
According to the first paragraph, the atmosphere is essential to man in that ________.