[单项选择]"My own feelings went from disbelief to excitement to downright fear," says Carl Hergenrother, 23, an Arizona undergraduate who verified a large asteroid barreling toward Earth with a 230em telescope atop nearby Kitt Peak. "It was scary, because there was the possibility that we were confirming the demise of some city somewhere, or some state or small country."
Well, not quite. Early last week, his celestial interloper whizzed by Earth, missing the planet by 450620 km--a hairbreadth in astronomical terms. Perhaps half a kilometer across, it was the largest object ever observed to pass that close to Earth.
Duncan Steel, an Australian astronomer, has calculated that if the asteroid had struck Earth, it would have hit at some 93450 km/h. The resulting explosion, scientists estimate, would have been in the 3000-to-12000-megaton range. That, says astronomer Eugene Shoemaker, a pioneer asteroid and comet hunter, "is like taking all of the U.S. and Soviet nuclear weapons, put
A. are different in nature.
B. serve similar functions.
C. are identical to each other.
D. pose real challenges.

最新试题

• ●    [单选题]竖直力的主要来源是车轮的( )。
• ●    [单选题]按下话音记录器控制板上的测试钮，正常情况下，控制板的测试指示器指针（ ）。
• ●    [判断题] 合环运行时,功率在环路中按阻抗分布的正比分配,称为环路功率自然分布。
• ●    [多选题]社会革命的实质是
• ●    [多选题]救援列车的出发或返回，均应通知( )。
• ●    组合投资管理工作的步骤中，( )是确保实现投资目标的重要手段。