The image above shows the nucleus of Halley's Comet --an
orbiting iceberg visible from Earth every 75 years. Stephen
Hawking believes that one of the major factors in the possible
scarcity of intelligent life in our galaxy is the high
probability of an asteroid or comet colliding with inhabited
planets. Through Earth's history such collisions occur, on the
average every one million years. If this figure is correct, it
would mean that intelligent life on Earth has developed only
because of the lucky chance that there have been no major
collisions in the last 70 million years. Other planets in the
galaxy, Hawking believes, on which life has developed, may not
have had a long enough collision free period to evolve
intelligent beings. We have observed, Hawking points out in Life
in the Universe, the collision of a comet, Schumacher-Levi, with
Jupiter, which produced a series of enormous fireballs, plumes
many thousands of kilometers high, hot "bubbles" of gas in the
atmosphere, and large dark "scars" on the atmos