It's said that a huge asteroid (小行星) smashed (撞击) into the earth 65 million years ago, which certainly sent the earth's ecosystem (生态系统) into a disaster which the earth took thousands of years to recover from. Scientists say that it's probable that such disaster could happen again.
Asteroids are rocky bodies which tend to live in orbits (轨道) between Mars and Jupiter. Comets, on the other hand, are a mix of rock and ice, and they can swing into the earth's orbit from anywhere in the solar system. Comets have an icy tail which reflects the sun and therefore can be seen from the earth. But asteroids have far less visibility (可见度) and have to be at least half a kilometre across to be found. There are millions of such rocky and icy bodies out there which have not been discovered yet.
But did an asteroid kill the dinosaurs—and could another kill us?
Despite what you may have heard, it hasn't been proved that the hitting was the only reason for the dinosaur's dying out. But the asteroid which created the hitting would certainly have had a great effect on the environment, and a mass extinction (灭绝) did take place around the time.
So far we have no idea when the next one will be because we haven't spotted it yet and not many organizations are spending money on asteroid detection, because a deadly hitting is such a faraway possibility.
Task: Answer the following questions.
1．What is the best title of the passage?
A_terrible_disaster_heading_our_way?
2．What are asteroids?
They_are_rocky_bodies_which_tend_to_live_in_orbits_between_Mars_and_Jupiter.
