ASTRONOMERS last week found a 鈥渕issing link鈥� between comets and
asteroids. Discovered in the middle of the asteroid belt, 鈥�1996 PW鈥� seemed quite
ordinary until Gareth Williams of the Harvard-Smithsonian Center for
Astrophysics calculated its orbit.
Unlike any other known asteroid, says his colleague Brian Marsden, 1996 PW
goes out to 鈥渟everal hundred times the Earth鈥檚 distance from the Sun, and its
orbital period is at least several thousand years鈥�. He believes it is a dead
comet from the Oort cloud, which extends about one-fifth of the distance from
the Sun to the nearest star. Cosmic events can send comets towards the Sun on a
journey that lasts a million years.
On their first visit to the Solar System, comets shine brightly as sunlight
evaporates gases. The gravitational pull of planets leaves some, like 1996 PW,
in orbits lasting thousands of years.
Advertisement