A trillion or more of these ghostly objects are believed to hang out in the deep cold beyond the fringes of the solar system. Periodically, gravity nudges one of them into a long looping voyage past the planets and around the sun.
Because they are so far away, comets are believed to preserve the original gas and dust left over from the formation of the solar system 4.5 billion years ago. Astronomers regard them as virtual time machines that enable scientists to study the raw materials of our sun, planets, and moons.
Furthermore, scientists believe comets brought huge quantities of water and carbon-based molecules to Earth during the era of heavy bombardment, when our planet was young.
"These missions will help us understand where our oceans came from," Morgan said.
Some theorists speculate that the cometary molecules may even explain how life got started on Earth."We may really be the progeny of comets," Veverka wrote in the journal Science last month.
Copyright 2002 Biloxi (Mississippi) Sun Herald