A、Its surface temperature is above zero. B、It’s a big solid ball. C、It’s gaseous. D、It was formed 4.5 billion years ago. B

A U.S. spacecraft is completing a five-and-a-half-month voyage to a distant comet and could create celestial fireworks when it crashes into it Monday, America’s Independence Day. Scientists want to know what a comet’s nucleus is made of and believe the best way to find out is to blast a hole in one.
    The target is the comet Tempel—1,431 million kilometers away. (7) As NASA puts it, meeting its head on at such a gTeat distance is like threading a needle.
    (8) NASA scientist Tom Morgan describes a comet as a big dirty snowball in space, a glowing collection of ice, rocks, dust, and gases whose interior composition is intriguing.
    Knowing the makeup of a comet would provide information about the chemistry and physics of the solar system as it formed 4.5 billion years ago.