An international armada of spacecraft has begun its rendezvous with Halley's comet, giving scientists their closest view yet of the most fabled wanderer in the solar system.
The first of five probes--the Soviet Union's Vega 1--passed within 6,000 miles of the comet's icy nucleus late Wednesday night, beginning what one astronomer calls "the greatest week cometary science has ever had."
The week's activities, culminating in a razor-close encounter by a European Space Agency probe next Thursday, will spotlight an extraordinary display of international cooperation, with the Soviet Union, Japan, Europe and the United States working closely together to ensure the safety of the fleet.
Success in that cooperative effort is essential if the European probe, called Giotto, is to have safe passage through the eye of a cometary hurricane next Thursday.
Each of the five probes has a specific and different mission. Together, they are carrying about 40 instruments.
Vega 1 took pictures with television cameras as it sailed past Halley late last night, and it checked the temperature of the comet and measured the size and distribution of dust particles in the cloud that surrounds the nucleus. Information from the spacecraft will be analyzed by Soviet scientists and released over the next few days, providing a dramatic scientific boost to Soviet progress in interplanetary exploration.
More than 100 foreign scientists, including about a dozen from the United States, are at Moscow's Institute for Space Research to take part in the historic encounter. Several had high praise for the openness of the Soviet project.
"If you want, you can go into a lab, ask an experimenter what he's doing, you can even twiddle the knobs and the dials, as long as you don't pull the plug," said Louis Friedman of Pasadena, executive director of the Planetary Society and one of the scientists in Moscow.
But despite the early focus on Vega 1, it is the European effort that has captured the most attention from scientists around the world because Giotto is to pass perilously close to the icy core of the comet.
If it survives its plunge next week through the dusty, gaseous cloud that surrounds Halley's nucleus, Giotto will send back the first high-resolution photos ever of the nucleus of an active comet--a treasure some scientists have waited for all their professional lives.
All the participating nations have agreed to make the results of their missions available to scientists throughout the world, promising a storehouse of information that will easily eclipse all that has been learned up to now about comets, which are remnants of the solar system's formation about 4.5 billion years ago.
"It's the greatest week cometary science has ever had and is likely to have for quite some time," said John Brandt, astronomy laboratory chief at the National Aeronautices and Space Administration's Space Flight Center in Greenbelt, Md.
Giotto will approach Halley head on, and the tiny craft--less than 10 feet tall--is to pass within about 300 miles of the dirty snowball at the comet's nucleus, estimated to be about four or five miles in diameter.
The spacecraft and the comet will pass one another at more than 150,000 m.p.h., a speed so great that a speck of debris from the comet could hit Giotto with the same kinetic energy as a small, speeding car, thus possibly destroying the probe.
Soviets Help on Safety
It will be partly up to the Soviets to ensure the safety of Giotto.
Both Soviet probes were launched in December of 1984 and paused to explore the atmosphere of Venus last year on their way to Halley.
Vega 2 is to arrive on the scene Saturday, and it is expected to pass the comet much closer than its twin, Vega 1. But the Soviets will wait until they know what happened to their first probe to decide how close to send their second.
Meanwhile, information relayed back to Earth by the Soviet probes will play a vital role in the effort to guide Giotto through harm's way.
At that distance, about 93 million miles from Earth, the probes are flying blind in the sense that their courses cannot be changed at the last minute if the encounter gets too hazardous. And since comets behave somewhat erratically as they interact with the sun, no one is absolutely certain just where the comet will be at any given moment. Thus navigation becomes the ultimate challenge.
As comets like Halley approach the sun, solar winds activate the nucleus, causing it to radiate ionized gases that spew millions of miles into space from the surface of the comet. These sudden bursts act like small rockets on the surface of the comet, changing its course slightly and causing it to wobble as it whips around the sun. And at that speed, it does not take much of a wobble to wipe out what engineers call the "miss distance" between the spacecraft and the comet.