The National Aeronautics and Space Administration unveiled its latest mission plan yesterday, including proposals to fly by an asteroid and rendezvous with a comet.

Planetary scientists at the Jet Propulsion Laboratory in California put forth two plans; NASA's hierarchy will select one of the $400 million missions for funding after debating their merits.

One would fly by an asteroid named Hedwig in September 1991 and rendezvous with Comet Wild 2 in 1995. The other would whip past the asteroid Freia in 1995 and rendezvous with Comet Tempel 2 in 1996.

Scientists prefer the mission to Hedwig and Wild (pronounced Vilt) 2, in part because Wild 2 is a fresher, newer and brighter comet than Tempel 2. The plan is to send an instrumented spacecraft on a journey that would begin with a space shuttle flight in March 1991 and end in the encounter with Wild 2 on Jan. 8, 1995, when the comet is at the far end of its orbit 465 million miles from Earth.

The spacecraft would be programmed to match the comet's speed as it turns in toward the sun. The sun would do the rest, drawing comet and spacecraft toward it at the same speed. For the next 850 days, the spacecraft and comet would fly together while the spacecraft's instruments measure the comet. WHAT'S ON ICE? . . . What's next for the International Cometary Explorer now that it has become the first spacecraft to fly through a comet's tail?

Fight director Robert Farquhar of the Goddard Space Flight Center would like to maneuver the ICE spacecraft on a roundabout path through space to bring it back into high Earth orbit sometime in 2012. By then, NASA hopes to have amanned space tugboat called the Orbit Transfer Vehicle, which would be piloted out as far as 22,000 miles from Earth to pick up the spacecraft. Scientists expect that it would still be coated with comet dust from its encounter this week with the Giacobini-Zinner comet. "HALLEY HUDDLE . . .

A four-day meeting of the world's major space agencies ended here yesterday, after discussions of coordinating scientific coverage of Halley's comet, which swings around the sun in February on its 30th recorded visit to Earth's neighborhood.

"We've had a very successful meeting," said Burt Edelson of NASA, who chaired the meeting of scientists from the United States, Soviet Union, European Space Agency and Japan. "In fact, there has never been a meeting like this before with so many countries getting together like this."

The Soviets promised that their two Vega spacecraft will help guide the Europeans' Giotto spacecraft to the comet, and NASA agreed to navigate spacecraft belonging to the Soviets, Europeans and Japanese to their targets using its Deep Space Network of Earth-based antennas. FIRE ONE . . .

The newest member of the space shuttle fleet, Atlantis, fired its three hydrogen-fueled engines on its Cape Canaveral launch pad for 22 seconds yesterday, clearing the way for its maiden voyage into space Oct. 3. The spacecraft, the fourth and final member of the shuttle fleet, will undertake the second fully classified American manned space flight.

GOING PLACES . . . NASA announced this week that Air Force Undersecretary Edward (Pete) Aldridge Jr., 47, will ride aboard the shuttle Discovery when it flies a secret Air Force mission inaugurating shuttle service from California's Vandenberg Air Force Base . . . . The White House announced yesterday that President Reagan will nominate William Graham, a senior associate for R&D Associates in Marina Del Ray, Calif., to be deputy administrator of NASA. Graham would succeed Hans Mark. Graham previously worked for the Rand Corp. and the Air Force Weapons Laboratory. -- Thomas O'Toole