A U.S. spacecraft put into space more than seven years ago to study the solar wind streaming off the sun was on target yesterday to become the first ever to fly through the tail of a comet in a possibly suicidal encounter.
The historic encounter between the International Cometary Explorer (ICE) and a comet known as Giacobini-Zinner is to take place 44 million miles from Earth at 7:02 a.m. EDT today, and will presage the encounters between other spacecraft and Halley's comet next March.
"This is a very risky mission," flight director Robert Farquhar said at the Goddard Space Flight Center in Greenbelt, where the comet encounter is being directed. "We are putting the spacecraft in harm's way, and there is a distinct possibility the spacecraft will not survive the encounter."
The most likely damage to the spacecraft would be to the solar cells that power it. A hail of dust or a rock the size of a bowling ball could come tumbling out of the comet's head and strike the spacecraft at high speed, cutting off all its power. The spacecraft has no protective dust shield and no cameras to see whether it is facing unintentional suicide.
Not everyone at Goddard was as pessimistic about the encounter as Farquhar. Project scientist Tycho von Rosenvinge said, "We are optimistic that we'll survive our flight through the dust tail. We don't think the dust tail is as dense as some people think."
Politics had a lot to do with the comet encounter. The mission to Giacobini-Zinner has its roots in the Reagan administration's 1981 decision not to mount a mission to Halley's comet, which is racing toward its 30th recorded encounter with Earth. The Soviet Union, the European Space Agency and Japan have instrumented spacecraft on their way to meet Halley's comet next March after the comet swings round the sun.
"By the summer of 1981, it was obvious that the United States would not be able to send a space probe to Halley's comet," Farquhar said. "It appeared that the United States would be the only major nonparticipant in the Halley sweepstakes."
Out of the gloom that there would be no U.S. mission to Halley's comet came a plan by Farquhar to use what was then called the International Sun-Earth Explorer 3 to intercept Giacobini-Zinner and save face. Even though the spacecraft has no cameras to photograph the comet and no instruments to analyze its dust tail, it has at least six instruments capable of measuring certain data about the dust and plasma tails.
It also was expendable and could serve as a "kamikaze" pathfinder for spacecraft flying missions to Halley's comet.
Farquhar said, "The spacecraft was launched in 1978 and had already completed the majority of its primary mission objectives. In addition, we found that the surcharge for sending the spacecraft to the comet would be less than $3 million."
Farquhar and his Goddard team devised a way to maneuver the spacecraft away from its position about 1 million miles from Earth onto a path that would take it around the moon and back through the earth's geomagnetic tail. The spacecraft was maneuvered around the moon four times at a distance of 12,000 miles and a risky fifth time at a distance of only 75 miles.
The encounter with the moon gave the spacecraft a "slingshot" effect that flung it at high speed onto a path that would take it out of the gravitational pull of the moon, the Earth and the sun. It also sent it on a trajectory to intercept Giacobini-Zinner, needing only a few course corrections to target it right for the comet's tail.
Moving at almost 46,000 miles an hour toward the comet, the ICE spacecraft was less than a half-million miles from the comet at 6 p.m. yesterday. The spacecraft's heaters were turned off to save power so its 10 working instruments can send as much data as possible during the encounter.
The spacecraft also made a course correction over the weekend to aim it at a spot in the tail 5,000 miles behind the comet's head.
"We expect to spend a minimum of four to five minutes inside the comet's tail," von Rosenwinge said. "Our time in the tail could be as long as 12 to 20 minutes, depending on whether the tail widens or shrinks. Each comet has a personality all its own."
Giacobini-Zinner is no different. Discovered in 1900 by Michel Giacobini at the Nice Observatory in France and found again in 1913 by Ernst Zinner at the Remeis Observatory in Germany, the comet visits the Earth's environs every 6.5 years and is easily observed by astronomers every 13 years when it swings close to the Earth on its way around the sun.
As comets go, it is not as spectacular as Halley's comet but is far from being burned out.
Its nucleus of primeval rock, ice and snow is about a mile across and the "coma" of exotic gas and dust that surrounds its nucleus is 50,000 miles across. Its yellow dust tail is about 300,000 miles long and its second tail of electrified gas, the plasma tail, is at least 1 million miles long.
Flight directors at Goddard have already witnessed some erratic behavior in Giacobini-Zinner. The comet wandered 1,000 miles off its predicted course last week.
To understand why a comet wanders, a comet can be looked at as if it were a speeding spaceship equipped with its own jet-like engines. In this case, the dust and gas being boiled off the comet as it swings close to the sun.
Comets blow off gas toward the sun, generating a thrust in the opposite direction like a balloon whose escaping air blows it across a room. The tiny thrusting motions that the escaping dust and gas give the comet's own rotation though space either force the comet to spiral away from the nearest celestial body or in toward it.
"All this time, snow and ice are subliming off the comet's surface," John C. Brandt, chief of Goddard's Laboratory for Astronomy and Solar Physics, said. "The effect is like an upside-down snowstorm coming off the comet."
Goddard is going all out to communicate with the ICE spacecraft as it moves toward the comet's tail, enlisting the help of tracking networks in Australia, California, Spain, Puerto Rico and Japan to keep in touch with the spacecraft through the critical hours of encounter.
The tracking antennas were also equipped with new low-noise amplifiers to pick up the weak signals expected from the spacecraft's 5 watt transmitter at a distance of 44 million miles.
"The spacecraft was designed to work at an Earth distance of 1 million miles," said Raymond J. Ambrose, manager of tracking and data acquisition for the U.S. Deep Space Network. "Giacobini-Zinner is almost 50 times that distance, which gives us 2,500 times less signal."