GREENBELT, Md. (AP) _ At 7 a.m. EDT Wednesday, 44 million miles from Earth, an American satellite will slice through the head of a comet, becoming the first human creation to contact one of the most fascinating heavenly bodies.

The encounter, however, is not with the glamorous Halley's, the royalty of comets, which makes its heralded return next March. This space traveler is called Giacobini-Zinner, a comet with a head 45,000 miles in diameter, a third the size of Halley's, and with a mere tail of 435,000 miles - 10 times shorter.

The Giacobini-Zinner mission, with a 7-year-old, well-used satellite, is the U.S. answer and one-upmanship to the far more ambitious and expensive flights the Soviet Union is sending to Halley's.

If all goes well, the 1,034 pound satellite called International Cometary Explorer - or ICE - will pierce ''G-Z'' 4,900 miles behind its mile-wide iceball nucleus to study the interaction between the comet and the ''solar wind.'' The solar wind is a flow of protons and electrons streaming from the sun.

From the returns, scientists hope to learn more about what comets and comet tails are made of, a subject of informed conjecture so far.

''The chances of our surviving the encounter are 50-50,'' said Dr. Robert Farquhar who thought up the idea of diverting the satellite, slinging it around the moon five times for momentum, and placing it in position to intersect the comet.

''Other people working on the thing are far more optimistic than I am but I think the real answer is that no one knows what our chances are,'' said Farquhar, the flight director for the mission and an expert in orbital motion at the Goddard Space Flight Center here.

The comet is traveling at 86,400 mph; the satellite at 45,000 mph. It can't be seen with the naked eye.

Engineers at Goddard, sending radio commands that require four minutes to reach the spacecraft, made a final, 500-mile change in the satellite's path Sunday by firing small steering thrusters shortly after noon on Sunday.

Farquhar worries not that the satellite might be overwhelmed by the ultra- fine dust particles in the comet tail, but that its solar cells might become broken or clouded to the point that there would not be enough electricity to send data back home.

''We are going right through the head of the comet,'' he said. ''we are going on the tail side of the nucleus. The reason we are going 5,000 miles downstream, is we want to get far enough so that the tail has fully formed at that point.''

Because a comet's tail begins well inside the head, ICE must come in close or risk missing the tail because it flutters. The most important part of the encounter - the traverse where the tail is 3,100 miles wide - will take only five to ten minutes.

Engineers will be able to tell from their data immediately if the satellite survived, but it will take a few days or longer to interpret the results.

Farquhar said the mini-fleet of spacecraft going to Halley's comet - two Soviet, one from the European Space Agency and two from Japan - are passing on the sunward side of that comet because they will take pictures. The ICE is not equipped with a camera.

''We are optimizing the thing in a different way,'' he added. ''We are looking at the plasma physics of the mission. What kind of particles are these? What elements are there? What molecules? What different densities? These are things we don't know.''

Six instruments will conduct 10 experiments that will be radioed back and analyzed. The satellite is a 16-sided drum, 63 inches high and 68 inches wide with booms that reach 300 feet from tip to tip.

In the early 1980s, Farquhar suggested a mission in which a U.S. satellite would be launched to Halley's comet, gather material, and return the sample to Earth. When it became apparent the administration would not spend that much money, he came up with the idea of diverting a satellite that had been in space since 1978, and putting it in the path of G-Z through a series of complex maneuvers.

The mission, he said, will cost only $3 million, a bargain price for a $20 million satellite that already paid rich dividends.

The ICE originally was known as the International Sun-Earth Explorer. It first studied the magnetosphere around Earth, a region in which charged particles are trapped. It was diverted in 1982 to study the earth's geomagnetic tail, an invisible field created by the solar wind.

To set it on its path to Giacobini-Zinner, scientists used the gravitational forces of Earth and moon and the spacecraft's hydrazine thrusters for dizzying orbital maneuvers.

If it survives, ICE will return to Earth's vicinity in 2012. Farquhar has worked out a way to retrieve it for display in the Smithsonian Air and Space Museum.

The G-Z comet, a stranger even to much of the scientific community, was first spotted by Michel Giacobini at the Nice Observatory in France in 1900. It was observed again by Ernst Zinner at the Remeis Observatory in Bamberg, Germany in 1913.

It orbits the sun every 61/2 years; Halley's takes 76 years to make the circuit.

Comets, feared by the ancients, still are among the least understood celestial bodies. They are thought to be composed of ice and dust and may contain material from the mixture that formed the solar system 4.5 billion years ago.

Away from the sun, the comet is inactive and there is only the nucleus without all the spectacular baggage. As the orbit brings the iceball to the sun, a hydrogen cloud forms around it and becomes the head. Two tails - one of dust, the other of plasma, trail behind.