Image confirms comet theories

Press, 28 February 1986, Page 6

Image confirms comet theories

NZPA-APMonnUia View, California sUnited States space agency scientists released yesterday a closeup image of the giant cloud surrounding Halley’s Comet and said it had dramatically confirmed theories that the sun’s radiation exerts tremendous pressure on the comet’s tail.

The image was captured by the PioneerVenus orbiter, the only American probe to explore the comet. Sunlight pressing against the atomic hydrogen and dust from the ice that evaporated off the comet’s patchy surface produced the fanlike tail that was the most familiar feature of the comet, said a National Aeronautics and Space Administration researcher, lan Stewart, of the University of Colorado. Chunks of ice had melted off the comet from the sun’s heat during its most active period around perihelion, the closest point to the sun, on February 9. “The most ice was observed turning into gas on February 20, when the comet, which is 6 to Bkm wide, shed 79 tonnes per second, or six million tonnes per day. That would represent a loss of about 30 cm of ice and dust from the surface of the comet,” he said. On each passage around the sun every 76 years, Halley’s nucleus was eroded by about six to nine metres. At that rate, it is believed it will take thousands of orbits before Halley’s falls apart. Scientists have long theorised about the pressure of the sun’s radiation, but this was the first time the effects had been so clearly shown, Mr Stewart said. The false-colour image made from ultra-violet light readings from Pioneer’s spectrometer shows a series of concentric circles that is brightest at, the centre, where the atomic hydrogen is most dense. The nucleus, hidden in the white centre of the image, is surrounded by circles of yellow, green, orange and red, and violet The darkening colours; in the spectrum on the outside show the reduction in the presence of hydrogen. When Pioneer recorded the image over a five-day span this month, the gas cloud was about 15 times larger than the sun, or , about 19.31, million km in diameter. Scientists are ' interested in studying comets because it is believed they hold clues to the origins of the universe.