An Aging Pioneer Has Halley Comet Show All to Itself
Halley’s comet is streaking around the far side of the sun after putting on a spectacular performance for an aging U.S. spacecraft which had the show all to itself.
The Pioneer Venus Orbiter, launched in 1978 and still functioning long after it was to have expired, was the only spacecraft in position to observe the comet as it passed within 55 million miles of the sun earlier this month.
The 824-pound craft was not designed to study comets, but it turned its instruments on the comet as it went through its most active stage, plunging past the sun at more than 100,000 m.p.h., boiling off its icy surface at astonishing rates as high as 70 tons a second, scientists reported Wednesday at the Ames Research Center here.
The spacecraft is orbiting Venus, which is also on the opposite side of the sun from Earth.
Scientists here also released an ultraviolet image of the comet engulfed in its own immense hydrogen cloud more than 25 million miles across.
The cloud is caused by ice boiling off the surface of the comet as it is heated by solar radiation, and scientists were surprised to find that the rate of burnoff varies dramatically--from a low of 12 tons a second on Jan. 1 to a peak of around 70 tons a second on Feb. 21, said lead scientist Ian Stewart of the University of Colorado.
Scientists will have to cherish the data they get from Pioneer because it has emerged as one of their most important sources on the comet, although no one expected it to turn out that way.
An ambitious effort to send a U.S. spacecraft to intercept the comet was scrapped years ago because of cost, leaving that effort to Japanese, Soviet and European space agencies which have an armada of five spacecraft now on their way to the comet for encounters during the first two weeks of March.
Some other attempts by the United States to study Halley from space have been lost because of the explosion of the space shuttle Challenger, and although several lesser programs are getting under way, including the launching of some instruments aboard small rockets, none ranks nearly as high as the Astro observatory that was to have been flown aboard the space shuttle in March.
Thus the U.S. effort to study Halley from space probes above the Earth’s atmosphere has primarily fallen to the Pioneer Venus Orbiter.
“It’s nice to be the only game in town,” said Richard Fimmel, project manager. “But this wasn’t a good way to get there,” he added in reference to the Jan. 28 Challenger tragedy.
Halley has already emerged from behind the sun, although only trained observers are likely to find it in the early morning sky. From March 10 through 22, however, the comet should be visible just above the horizon in the southeastern sky for about an hour before dawn.
Trick Is to Find It
“The trick will be in finding it,” said Jeff Cuzzi, a comet expert with Ames, which is part of the National Aeronautics and Space Administration. “Use a good pair of binoculars and find a dark southern horizon,” he said.
The “photos” released by Ames Wednesday are far different from what the comet would have actually looked like to an observer aboard the Pioneer spacecraft, because they were made with ultraviolet light emanating from the comet as it was radiated by the sun, and ultraviolet light is invisible to the human eye. The images reveal the size and intensity of the huge hydrogen cloud formed by the atomic decay of the icy water that was blown off the comet by the solar wind.
The nucleus of the comet is hidden by the immense cloud. But the nature of the hydrogen cloud has told scientists much about the “dirty snowball” it conceals.
The fact that the amount of ice boiling off the surface varies enormously from day to day suggests that part of the nucleus is covered by dirt, project scientists said. As the comet revolves, it exposes different areas to the sun--some “insulated” by dirt and others exposed--and the amount of ice released depends on the nature of the surface that is turned toward the sun.
30 Tons of Ice a Second
On most days the comet spewed out about 30 tons of ice per second, enough to reduce the surface of the comet by about “a foot (deep) a day,” said Stewart.
During this passage by the sun, he added, the diameter of the nucleus will be reduced slightly with the top 20 to 30 feet of dirt and ice blown away. Since the nucleus is believed to be about four miles in diameter, and will grow less active as it gets older, it could continue doing that for thousands of orbits, each of which takes about 75 years, added Cuzzi.
He said, however, that the total amount of mass lost by the comet during this visit to the sun is “huge.”
“It’s equivalent to about 10 large icebergs,” he said.
Attention on Europe
Scientists who have been working on the project will spend months analyzing their data, but during the weeks ahead most attention will shift to Europe. The European Halley probe, called Giotto, is to pass within about 300 miles of the head of the comet, providing the first pictures ever of a comet’s nucleus.
But success of the mission is by no means assured. Giotto will have only two hours to photograph Halley, and there is some fear that dust surrounding the comet will wipe out the spacecraft’s instruments or possibly even destroy its ability to communicate with Earth. “There is a tremendous amount riding on the success of Giotto,” said Stewart.
COMET WATCH
The Jet Propulsion Laboratory’s International Halley’s Comet Watch hot lines:
Inexperienced sky observers (818) 354-4300
Amateur astronomers (818) 354-4301
Griffith Park Observatory
Sky Report (213) 663-8171
