The Coast Guard cutter Point Roberts docked at the small Air Force Station here late today to unload 600 pounds of green, white and black debris.

The cutter Dallas later reported finding a cone-shaped object, not immediately identified.

The somber cargo was all that has been found today of the space shuttle Challenger in an exhausting search that has continued since the spacecraft exploded at 11:39 a.m. Tuesday, killing its crew of seven.

The shuttle weighed more than 715,000 pounds, not counting the fuel, at liftoff.

The massive search of Atlantic waters is part of the National Aeronautics and Space Administration's quest for clues to a mystery that seems to have settled like a pall on this nest of ultra-high technology, where people are used to knowing the answers.

Officials at Kennedy Space Center have shifted their focus from a future crowded with ambitious missions to Tuesday's millisecond, when their data screens suddenly froze, each showing "S," for static.

Dozens of pieces of Challenger have been found floating in the ocean, including several large ones. The largest so far, about 15 by 15 feet, looks like aluminum, according to Air Force Master Sgt. Charles J. Miller. "But most of them are small, some are pipe-shaped, some are definitely tiles and a lot of it is a styrofoam-type material."

NASA spokesmen said they "cannot say" what parts of the shuttle assembly the pieces came from because they have not been studied by specialists.

"They've only been handled, so far, by sailors and wharf hands and truck drivers," said Air Force Col. Bob Nicholson.

"Specialists trained in sorting through debris can tell more than you or I can believe," an Air Force spokesman said.

The haul from the sea will help piece together the mystery of the explosion, much as in an airplane crash, he said. It can help pinpoint the areas that may have been distorted or disintegrated when hit first and hardest by the explosion. Fractures can point to places of special stress or to the explosion itself. Distortion of pieces can fill in further detail on the character of the blowout.

Ten Coast Guard, NASA and Navy ships, including two guided missile frigates that happened to be in the area, and 10 aircraft from the Coast Guard, the Navy and the Air Force are searching between Vero Beach and New Smyrna Beach and 60 miles out to sea, an area of about 5,500 square miles, officials said.

The search so far has concentrated on gathering objects floating on the surface, not those that have sunk.

The 82-foot cutter Point Roberts collected the debris found by all those vessels and brought it to the Trident dock at the Cape Canaveral Air Force Station.

Each piece will be assessed by Army and Air Force explosive ordnance experts and divided into categories. Hazardous material will be taken to a disposal range and the rest to a hangar, Nicholson said.

Hours after the explosion, which occurred at an altitude of about 47,000 feet, heat-resistant tiles from the Challenger began washing ashore south of here.

Citizens who found them took them to the south gate of the Air Force Station, officials said.

NASA officials are urging anyone who finds possible shuttle pieces to contact them. "We know there's debris washing up on the beach. We need every piece of that. We don't know where the clue might be," said Dr. Richard G. Smith, space center director. "We do have a few souvenir hunters out there."

The Challenger's sister orbiters, Discovery, Atlantis and Columbia, in hangars here, have been "powered down" and "no work is planned on them for several days," officials said.

Launch pad 39B remains sealed and photographers have not been allowed to go there to retrieve their remote equipment. In the distance at night, the brightly lighted pad resembles a Christmas tree on the beach.

Early this morning, someone finally remembered to turn off the NASA mission countdown clock. Until then, it had kept ticking off the seconds, in digital readouts, as if the Challenger were in orbit, going about its tasks as scheduled.

The ocean search will continue indefinitely, officials said.

To make an efficient search, the Coast Guard has divided the area into two large grids, one for aerial search and one for surface search. Within the aircraft grid, the boxed areas of sea vary with the craft from 20 by 20 miles for helicopters to the 40-by-24-mile areas searched by the C130 aircraft, which can stay aloft for eight hours or more