The National Aeronautics and Space Administration and the European Space Agency have begun to study what promises to be the most ambitious space mission ever tried, landing a probe on a comet to bring a sample of the surface back to Earth.

A brainchild of the Europeans, the joint mission is only in its planning stage and would not take place until 1998 or 2000. Such a mission would cost $2 billion, a record sum for an unmanned spaceflight, and the reason ESA and NASA are planning the mission together.

"The Europeans put the highest priority on this mission when they conceived it a year ago but they quickly realized, in doing their cost analysis for such an audacious mission, that they couldn't do it alone," a NASA source said. "So they proposed a joint mission to NASA last June and NASA took them right up on it."

The first joint "working group" to outline the mission was held at ESA's research center in the Netherlands in September, when both sides agreed that the Europeans would plan the science and the Americans would provide the spacecraft and celestial mechanics involved.

It was agreed that the mission should be flown to an "inactive" comet, or one that has stayed far enough away from the sun that it has not lost much of its pristine outer cover to light and heat.

"If the mission were just a rendezvous, you'd want to go to a comet that's very active because you want to find out what it does when it gets active," the NASA source said. "But for a sample-return mission, you want the most pristine material you can possibly get, because presumably we're talking about the earliest remnants of the solar system's origin that man can possibly get."

The working groups have not identified which comets are candidates for the mission, but there are more than 100 that satisfy requirements. Said Stephen Edberg of California's Jet Propulsion Laboratory, "There's no shortage of inactive comets."

The first mission to a comet took place in September when a U.S. spacecraft named the International Cometary Explorer flew through the tail of active comet Giacobini-Zinner when it was 44 million miles from Earth. The vehicle had been between the Earth and the sun when it was retargeted a year ago.

"One big surprise of that mission was that the passage through the tail was so easy, the spacecraft suffered no damage at all," said the Jet Propulsion Laboratory's Donald K. Yeomans, codirector of the International Halley Watch coordinating worldwide observations of Halley's Comet. "This has got everybody thinking that missions to comets are not as hazardous as people once thought."

A sample-return mission to a comet may not be hazardous but it is considered as bold a mission as any ever undertaken. Most inactive comets that would be mission candidates follow orbits that bring them no closer than 100 million to 200 million miles from the sun, putting them outside the orbit of Mars.