17．
LONDON-Scientists have found an unusual way to prevent our planet from overheating：move it to a cooler spot．All you have to do is to send a few comets （彗星） in the direction of Earth，and its orbit （轨道） will be changed．Our world will then be sent moving into a safer older part of the solar system．
This idea for improving our climate comes from a group of US NASA engineers and American astronomers．They say their plan could add another six billion years to the useful lifetime of our planet-hopefully doubling its working life．
The plan put forward by Dr．Laughlin，and his colleagues Don Korycansky and Fred Adams，needs carefully directing a comet or asteroid （小行星） so that it passes close by our planet and sends some of its gravitational energy to Earth．
"Earth's orbital speed would increase as a result and we would move to a higher orbit away from the sun，"Laughlin said．
Engineers would then direct their comet so that it passed close to Jupiter （木星） or Saturn（土星）．The comet would pick up energy from one of these giant planets．Later its orbit would bring it back to Earth，and the process would be repeated．
In the short term，the plan provides an ideal way to worldwide warming，although the team was actually worried about a much greater danger．The sun is certain to heat up in about a billion years and so"seriously compromise"our biosphere （生物圈）-by cooking us．That's why the group decided to try to save Earth．
The plan has one or two worrying sides，however．For a start，space engineers would have to be very careful about how they directed their asteroid or comet towards Earth．The smallest miscalculation （误算） in orbit could fire it straight at Earth-with deadly results．
There is also the question of the moon．As the present issue of Scientific American magazine points out，if Earth was pushed out of its present location，it is"most likely the moon would be stripped （剥离） away from Earth，"it states．This would greatly change our planet's climate．
42．What makes the scientists plan to move Earth？C
A．A few comets are moving to the direction of Earth．
B．Earth's working life is coming to an end soon．
C．Earth will become too hot for mankind to keep alive．
D．The moon is moving farther and farther away from Earth．
43．If the plan is successful，Earth will have a working life ofA years．
A.12billion
B.6billion
C.18billion
D.24billion
44．What serious problems might the plan cause according to the passage？A
A．The comet might hit Earth and man might lose the moon．
B．Earth might be moved too far away and man might die because of the coldness．
C．The comet might hit Jupiter or Saturn and never return to Earth．
D．Earth's working life might be greatly shortened．
45．What does the underlined word"compromise"mean？D
A．provide
B．benefit （受益）
C．share
D．endanger．
