Sky Centaur

What, after all, is Object-Kowal?

When Hale Observatories Astronomer Charles Kowal announced in November that he had spotted a small object circling the sun beyond the orbit of Saturn, some scientists thought that he might have discovered the solar system's tenth planet. Now, Object-Kowal--as it is temporarily called--has been downgraded. The body, which is at least 160 km. (100 miles) in diameter, could be an errant asteroid or an inactive comet.

Re-examining old sky photographs, astronomers have found Qbject-Kowal on plates taken by Harvard astronomers as far back as 1895. In fact, Kowal himself had unknowingly photographed it while making a sky survey in 1969.

From these old plates, Brian Marsden, of the Harvard-Smithsonian Center for Astrophysics, was able to compute Object-Kowal's orbit around the sun. Making a complete revolution once in about 50 years, it swings as close as 1.3 billion km. (790 million miles) to the sun--which brings it to a point just inside the orbit of Saturn--and as far out as 2.8 billion km. (1.75 billion miles), just shy of the orbit of Uranus.

Some astronomers speculate that Object-Kowal is an escaped asteroid that was ejected from the debris-littered asteroid belt between Mars and Jupiter as the result of a collision. Marsden favors another idea: that it is an "inactive" comet so far from the sun that the solar radiation it receives is not strong enough to vaporize its icy surface and thus produce a characteristic comet's head and tail.

Whatever the strange body is, Kowal has suggested what he feels is an appropriate choice for its permanent name: Chiron, after one of the centaurs, the mythological beasts that were half-man, half-horse. His reason: Chiron was a descendant of the gods Saturn and Uranus, for whom the object's closest planetary neighbors were named.

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