Space is a erous place, not only because of meteors (流星) but also because of rays from the sun and other stars．The atmosphere again acts 【C1】______ our protective blanket on 【C2】______ ．Light gets through, and this is essential 【C3】______ plants to make the food which we 【C4】______ ．Heat, 【C5】______ , makes our environments tolerable and some ultraviolet rays (紫外线) penetrate the 【C6】______ ．Cosmic (宇宙的) rays of various kinds come 【C7】______ the air from outer space, but 【C8】______ quantities of radiation from the sun are screened off．As 【C9】______ as men leave the atmosphere they are 【C10】______ to this radiation; 【C11】______ their spacesuits or the walls of their spacecraft, if they are inside, 【C12】______ pr a lot of radiation damage.
Radiation is the greatest known er to explorers in 【C13】______ ．Doses of radiation are measured in 【C14】______ called 'rems (雷姆)'．We all 【C15】______ radiation here on earth from the sun, from cosmic rays and from radioactive minerals．The' 【C16】______ 'dose of radiation that we receive each year is about 100 millirems; it 【C17】______ according to where you live, and this is a very rough estimate．Scientists have reason to think 【C18】______ a man can 【C19】______ up with far more radiation
【C20】______ this without being damaged; the figure of (60) rems has been agreed.
【C1】