When the space shuttle Challenger returned to Earth with a cracked windshield in June 1983, engineers assumed the culprit was a micrometeorite -- a stray piece of cosmic dust that could have hit the windshield at 44,000 miles an hour.

But after examining the fracture pattern and trace elements in the crack, scientists concluded that whatever Challenger ran into was man-made.

The case of Challenger's windshield illustrates a serious concern among people who put spacecraft into orbit. So much debris litters the spacelanes that it poses a major collision hazard.

Experts suspect that space collisions have destroyed at least two satellites -- one American and one Soviet -- and possibly a second American craft, all of which had been in good condition.

Even now, the world can expect a major collision in Earth orbit every 15 years, according to Donald J. Kessler, a specialist in orbital debris at Houston's Johnson Space Center.

"If the debris keeps accumulating, the chances of collision are greater," Kessler said yesterday. "We might get to where we see a collision that breaks up an operating satellite once every 10 years."

Now being tracked in space are 5,400 objects the size of a baseball or larger, each circling Earth at 17,500 miles an hour. Only 200 to 300 are operating satellites. The others are old rockets, payloads, shrouds, fuel tanks or remnants of previous explosions and collisions.

A more serious threat is golfball-sized space junk, whose number is estimated at 40,000 based on counts by earthbound telescopes.

Even the third category of space garbage -- tiny orbiting flakes, estimated to number in the billions -- are potentially hazardous. They are the prime suspect in the case of the shuttle windshield.

The windshield crack was the first proof engineers had that space debris was a growing problem. A more convincing case came when a later astronaut crew returned to Earth with parts of the Solar Maximum satellite they repaired in orbit last April. Kessler and his team found 160 small craters in the layered plastic that insulated Solar Max from the extreme cold and heat of space.

"Most of the holes we found in the plastic had been put there by man-made objects, either particles of paint or tiny pieces of metal that had punctured the plastic at anywhere from 15,000 to 18,000 miles an hour," Kessler said. "The number of man-made craters was two to five times what we would expect from meteorite hits."

Kessler said much of the space debris came from satellite explosions. No fewer than 80 objects have exploded in space since June 29, 1961. On that day the second-stage rocket of a Air Force payload blew up into 261 trackable fragments, 199 of which still circle Earth.

In addition, the second stage engines of nine U.S. Delta rockets have exploded, scattering more than 1,400 fragments into orbits that most still follow.

Most space explosions have been of Soviet origin, including 19 explosions during tests of an antisatellite (ASAT) weapon. The Soviet ASAT works by maneuvering a "killer" satellite into the path of a target spacecraft and blowing up the killer in a hail of pellets. The 19 Soviet tests have produced almost 1,000 pieces of debris.

Between 1975 and 1983, the Soviets deliberately destroyed 11 electronic surveillance satellites, leaving behind almost 600 "large" fragments. The Pentagon won't disclose the reason for their destruction but all the explosions have taken place over the Soviet Union or the United States, suggesting a pattern.

At least two and perhaps three catastrophic collisions have occurred in Earth orbit in the last 10 years. A U.S. balloon satellite named PAGEOS put into orbit in 1966 to make a geodetic survey of Earth broke up for no apparent reason in 1975. The sun's strong ultraviolet light was a suspect, but careful analysis ruled that out. The prime suspects are any of the 1.2 billion metal needles put into orbit by the Air Force in 1962 and 1963 to see if radar signals could be bounced off them and back to Earth.

The needles were supposed to be released as single objects but entered orbit in clumps. They're in orbit at approximately the same altitude and inclination (88 degrees) to the equator as the destroyed PAGEOS satellite.

A second collision apparently took place on July 24, 1981, when the Soviet satellite Cosmos 1275 broke up -- an event seen on the radar screens of the North American Air Defense Command. Kessler said Cosmos 1275 was in an orbit of "high collision probability" and disintegrated into 10 or 12 large pieces. "We now believe it struck another large object in space of unidentified origin," he said.

A satellite named Landsat 4 was in the same "Bermuda Triangle" of space when it failed in 1982, leaving fragments in its wake that suggested a sideswiping collision.

The most dangerous highways in space are those over or near the poles. Most weather and reconnaissance satellites and some scientific satellites are put into polar orbits so they will travel over every spot on the globe every two weeks or so. Not only do these orbits converge on single points above the poles, but they contain a substantial share of Earth's satellite traffic and hence much of the debris from space collisions.

"The collisions and explosions have unfortunately taken place at fairly high altitudes, which means that most of the leftover debris will stay in orbit instead of coming down and burning up in Earth's atmosphere," Kessler said.

Kessler is concerned that a collision could befall the occupants of the space shuttle or the permanent space station to be put into orbit in 1993.

"I worry about the space station because it's going to be so big, 10 times the size of the shuttle," he said. "But the shuttle worries me, too. Not a day goes by when the shuttle is in flight that it doesn't miss something up there by 25 miles or less.

"We warn our shuttle astronauts on spacewalking missions now that a dropped wrench or even a dropped pencil could spell catastrophe to them or a crew following them in the same orbit."