NASA officials, worried that a buildup of ice outside the shuttle Discovery could cause damage on its return to Earth, tonight listed three options for removing the ice, the last of which is a spacewalk that would delay reentry by at least a day until Thursday.

Two pieces of ice were discovered late Sunday under a folding door on the port side near the nose, out of sight of the six-member crew. One mass estimated at 18 inches and about 10 pounds covers an excess water-dump nozzle, while a shorter icicle blocks a waste nozzle just below it.

About 5 a.m. Tuesday, officials said, the astronauts will be told to try, in sequence until something works, pulsing heated water through the clogged line from inside while rocking the shuttle with rocket thrusts, knocking the ice loose with the shuttle's 50-foot mechanical arm and a two-man spacewalk utilizing the arm and assorted tools to chip away the blockage.

Astronauts Henry W. Hartsfield Jr., Richard M. Mullane, Steven A. Hawley, Michael L. Coats, Judith A. Resnik and Charles D. Walker are scheduled to land at 9:39 a.m. EDT Wednesday at Edwards Air Force Base in California's Mojave Desert.

Officials emphasized that the ice does not endanger the crew but expressed concern late today, after a report by Rockwell International Corp., Discovery's builder, indicated that, on reentry to Earth's atmosphere, such an ice chunk could strike the shuttle's tail and damage protective black tiles there.

If damaged, the tiles might not be able to prevent the tail from being burned in the extreme temperatures of reentry, they said.

"I guess I've been concerned all along about the size of that thing and its implications for entry," shuttle commander Hartsfield told Mission Control earlier today after being informed of options under consideration. "Apparently, you've been worried about it, too."

The crew had tried aiming Discovery at the sun to melt the ice, but officials determined that melting occurred at a rate of only one-fourth of an inch a day because the ice has adhered very tightly to a new plastic insulation on Discovery than it would have to the fiberglass composite used on shuttles Challenger and Columbia.

Officials said the astronauts would try the first option repeatedly to clear the ice, hoping that the combination of heated water pulsed at high pressure and the firing of engine thrusters would shake the ice loose as the water moves over it.

Hartsfield suggested that he fire the ship's big control jets to give Discovery "a few bangs" back and forth and up and down to try to dislodge the ice. "Give it a few whacks," he said. "It can't hurt anything. Then . . . we'll see what it looks like."

If that fails, officials said, Resnik will be directed by ground controllers in trying to manipulate the robot arm to knock off the ice even though it cannot be seen from inside the shuttle. They said the arm is long enough to reach the blockage but too long for an astronaut to see its end.

Astronaut Sally K. Ride, Hawley's wife and a robot-arm expert who tried the procedure in a simulator, said today that visibility with it is poor and that she hoped something else would be tried.

A spacewalk, described as a "last-resort" option, would be attempted Wednesday by Hawley and Mullane, although officials said it has not been decided exactly what each man would do. Ground rules call for a two-man "buddy" system whenever a spacewalk is attempted.

One plan involves placing a spacewalker on the end of the robot arm and letting him use a hammer, chisel or pry bar to bang the ice away, officials said.

After first being told that a spacewalk might be ordered Tuesday, Hawley and Mullane donned helmets to begin breathing pure oxygen to eliminate nitrogen bubbles in their blood in case they have to venture outside in low-pressure spacesuits. The presence of nitrogen could cause decompression sickness, known as the bends. The ship's cabin pressure also was being lowered.

Two members of each shuttle crew are trained for emergency spacewalks, and the proper equipment for one is stored aboard Discovery.

The astronauts wrapped up their final scheduled major task today when they pulled into the cargo bay the 10-story experimental solar wing they had flown above the spaceliner for three days.

"The tests of the experimental solar wing went much better than expected. We're delighted with what we got back . . . ," said Gary Turner, solar panel experiment program manager for the Lockheed Corp., which built the panel for NASA at a cost of $7.6 million.