21 Astronomers have spent hundreds of years searching for signs of life on other planets, using telescopes. When they saw the geography of Mars (火星), they thought they could see canals and that this might be evidence of intelligent life on the planet. More recently, however, spacecraft have been sent there to analyze (分析) the soil for signs of life. The result were negative and astronomers now feel certain that no life exist on the surface of any craft far beyond our solar system （太陽系）I not realistic because of the huge distances.

Unless life comes and visit us, the only way we likely to know of its existence is from radio message. Listening for intelligent extraterrestrial (外星的) or E.T. life is not a new idea, but the techniques now being used offer a much better chance off success than before. Looking for evidence of life in other solar systems now means using special radio receivers called radio telescopes to listen out for message. The idea is that intelligent forms of life would have discovered radio waves and would, like us, be using them to communicate.

Through its research work, NASA(美國國家航空航天局) has develop two means of systematically searching for E.T. life. The first method is to select the closest 800 stars like our sun and to direct a radio telescope to scan each of one them for electromagnetic (電磁) wave. The second way is to survey the entire sky, listening for continuous signals which may come from extraterrestrial life. Scientists are looking in the 100-10,000 MegaHetz(兆赫) frequency range—a range where there are fewer natural radio signals which could interfere (干擾) with transmission (傳輸). By looking signals in these specific regions, scientists can avoid any confusion that might be caused by receiving a natural radio signal rather than one sent by intelligent life.

NASA’s search in space has already resulted in some discoveries –including new equipment which may help in the development of new communication and medical technologies.

All this, as well as the chance of discovering, has made us feel confident that maybe we are not alone in the universe!

What is the main idea of the passage?

A Astronomers now feel certain that no life exists on the surface of any other planet in our solar system.

B It is realistic to send a spacecraft far beyond our solar system to search for E.T. life.

C There is no E.T. life on Mars.

D Using radio telescopes to listen out for message from E.T. life, scientists have discovered that the new equipment nay help in the development of new communication and medical technologies.

key：D