Erupting volcanoes can cause rivers of boiling lava, giant explosions, and mudslides. But volcanoes aren’t history’s greatest killers – that title goes to asteroids, that is, minor planets. They are rocky objects in space.
Most asteroids in our solar system travel between the orbits of Mars and Jupiter. Sometimes an asteroid will leave that path and hit Earth. These small impacts do little damage. It’s the bigger asteroids that cause problems. Asteroids more than a mile long can create enormous waves called tsunamis (海啸) and send huge amounts of dust into the atmosphere, greatly affecting the climate. Many animals and plans die immediately from the impact and waves. Others die later because they can’t live in the new climate. Scientists say that two giant asteroids have hit the Earth – one 250 million years ago and one 65 million years ago. The asteroid that hit the Earth 250 million years ago may have killed over 90 percent of Earth’s ocean species and 70 percent of its land species. The asteroid that hit 65 million years ago may have caused dinosaurs to go extinct.
What makes scientists draw the conclusions? In Antarctica, a team of scientists found rocks with unusual pieces in them that don’t normally exist on Earth. The team says the pieces show that an asteroid hit the Earth 250 million years ago. “We analyzed them and they seem to be pieces of outer space material.” That means they most likely came from an asteroid. In Mexico, scientists found a giant cave in the ground. The cave is 112 miles (180 kilometers) wide and 3,000 feet (900 meters) deep. Scientists believe that the cave was caused by an asteroid hitting the Earth 65 million years ago.
Many giant asteroids remain in our solar system, but no large asteroids appear on track to hit the Earth.

　　喷发的火山能够产生炽热的熔岩、威力巨大的爆炸和泥石流，然而，火山并非史上最厉害的杀手——这个封号应该赋予小行星，也就是，个头小的行星体，它们是太空中的岩石性物体。

　　我们太阳系中大部分的小行星在火星轨道与木星轨道的中间地带运行。有时，一个小行星会偏离轨道，撞上地球。这些微小的冲撞破害性不大，真正造成麻烦的是个头较大的小行星。超过一英里长的小行星（撞上地球后），能引起称为“海啸”的巨浪，并将极大量的尘埃送入大气，从而严重影响气候。许多动植物在冲撞和海啸发生的瞬间死去。科学家们说，曾有两颗巨大的小行星撞上过地球——其一在2.5亿年前，其二在6500万年前。2.5亿年前撞上地球的那颗小行星可能杀死了地球上90%的海洋生物和70%的陆地生物；6500万年前撞上地球的那颗小行星可能导致了恐龙的灭绝。

　　科学家依据什么得出这些结论呢？在南极洲，一队科学家找到了一些岩石，其中含有不同寻常的碎片，它们通常不会存在于地球之上。这队科学家说这些碎片显示，2.5亿年前有颗小行星撞上了地球。“我们对它们（指碎片）进行了分析，它们似乎是外太空物质的残片。”这意味着它们最有可能来自一颗小行星。在墨西哥，科学家在地面上找到一处巨大的洞穴，它宽112英里（180公里），深3000英尺（900米）。科学家们相信，这个洞穴是由一颗6500万年前撞上地球的小行星造成的。

　　还有许多巨大的小行星存在于我们的太阳系中，但是，还没有哪颗大块头的小行星看来正处于撞向地球的轨道上。