Astrophysics: Taking a Comet’s Temperature

When the comet Ikeya-Seki curved around the sun last October, many a stargazer’s camera was focused on its fiery trail. But two California Institute of Technology researchers were deter mined to record far more than mere surface glow. And the Astrophysical Journal last week reported remarkable success. Eric Becklin, a graduate student in physics, and James Westphal, a senior research fellow in planetary science, not only obtained the first temperature measurements ever made of a comet, they also gained valuable insight into a comet’s composition and behavior.

After fitting a 24-inch telescope with a sensitive infra-red detector, the Caltech scientists trained it on the comet for 19 days during its passage around the sun. Their first temperature measurement—700° F.—was made when Ikeya-Seki came within 45 million miles of the sun and heated up enough for its infra-red emissions to be picked up on earth. As the comet raced closer to the sun, its temperature climbed gradually, reaching 1,200° F. about 20 million miles from the solar surface. Then the comet’s emissions were blotted out by the sun’s own fierce radiation. On the return leg of its journey, Ikeya-Seki displayed identical temperature variations, dropping from 1,200° F. at 20 million miles to 700° F. at 45 million miles.

Solar Heating. To Becklin and Westphal, this consistent temperature behavior suggests that the comet generated no heat, but was warmed entirely by solar radiation. Another set of observa tions seemed to bear them out: temperatures of the comet’s head and tail were always identical. If the comet supplied some of its own heat, its head, or nucleus, should have been warmer.

The infra-red recordings also seem to rule out the theory that comets are composed primarily of ice and dust. Ikeya-Seki’s high temperatures could have occurred only if it contained large amounts of metallic material. Most of the comet’s lighter-weight chemical elements were probably boiled away during a previous close approach to the sun. The scientists also measured the total amount of energy Ikeya-Seki radiated before and after it swung around the sun; they calculated that it lost 65% of its mass and broke into two pieces.

Primordial Matter. Though the new findings add greatly to man’s skimpy knowledge of comets, they will only whet the appetite of astrophysicists, many of whom believe that the fiery-tailed bodies are well preserved condensations of the primordial matter from which the sun and planets were formed. If that is correct, a complete analysis of a comet might provide valuable information about the beginnings of the solar system. To obtain a sample for such a study, some scientists suggest, an unmanned spacecraft should be shot into the orbit of a regularly reappearing comet. The craft would rendezvous with the comet, land and scoop up some surface material. Then, after a brief, blazing ride through the sky, it would blast off for earth, bringing back a sample of the stuff the comet is made of.