An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time

ARTIST'S CONCEPTION OF AN ASTEROID IMPACT

Artist Don Davis paints a view of what a large asteroid might look like as it hits the Earth. Just such an asteroid is thought to have hit the Earth 65 million years ago, destroying more than half of all living species on our planet. (Courtesy of NASA Ames Research Center)

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Changes in the Solar System