The in-flight explosion of the space shuttle Challenger yesterday was the worst disaster in the 25 years that the United States has rocketed humans into space. It came 19 years and one day after the only other fatal accident in the U.S. space program, a fire that killed three astronauts during a launch pad rehearsal.

Never before in 56 U.S. manned missions had an astronaut been killed or injured during a space flight.

In a launch pad tragedy on Jan. 27, 1967, astronauts Virgil (Gus) Grissom, Roger Chaffee and Edward White were killed when fire rapidly engulfed their Apollo 1 spacecraft as they rehearsed for a flight that was to be one of the preliminary steps to man's first landing on the moon 2 1/2 years later.

In addition to the 10 U.S. space-related deaths, four Soviet cosmonauts are known to have been killed in two separate accidents.

One, the first death of a man in space, killed Vladimir Komarov in 1967 when his Soyuz 1 capsule parachute failed to open on landing and the spacecraft crashed in Russia's Ural Mountains. Four years later, three cosmonauts died when a hatch seal failed and the Soyuz 11 spacecraft experienced a rapid decompression that killed Georgy Dobrovolsky, Vladislav Volkov and Viktor Patsayev. They were found dead after the spacecraft made an automatic landing.

Death in space was on the minds of America's space planners from the day the United States decided to put a man in space.

James McDivitt, one of the second group of U.S. astronauts and a veteran of a 97-hour 1965 flight in which Edward White made America's first spacewalk, faced the question in an interview in the early days of space exploration.

"Eventually," McDivitt said, "we will have a flight that will not be successful. The astronauts know it, and I hope the United States realizes it. It will happen just as you have accidents with regular-type aircraft."

"We were afraid of losing people," Julian Scheer said in an interview yesterday. Scheer headed public affairs for the National Aeronautics and Space Administration during the height of America's manned space program, when the United States was racing the Soviets to the moon.

"We place a great value on human life. So we just scrubbed a lot of launches. When in doubt, we didn't fly," he said.

As in yesterday's explosion, none of the three Apollo 1 astronauts in 1967 had a chance of escaping the oxygen-fed fire that roared through their spacecraft. Grissom, White and Chaffee were strapped inside the command module, which was perched atop a giant Saturn rocket on a launch pad at Cape Canaveral, and workers trying to break open the hatches were driven back by dense smoke. The astronauts suffocated.

NASA hit its lowest point with that fire. Despite an avowed concern for safety, the space agency's unusually frank and harshly worded investigation laid the blame for the deaths on the agency's faulty spacecraft design and construction, and lack of attention to crew safety. Sloppy wiring caused the fire, a poor choice of materials fed it and a lack of attention to safety made it impossible to get the astronauts out, the report concluded.

From that day on, NASA's space flight record reached such a high level of safety that a citizens' committee two years ago found the shuttle safe and reliable enough to fly non-astronauts into space. It was that recommendation that led to the presence on yesterday's flight of New Hampshire schoolteacher Christa McAuliffe. She was the self-described first "ordinary citizen" on a shuttle mission.

"I felt, and I still feel, that the shuttle is indeed a transportation system and that NASA had demonstrated, in flight after flight after flight, incredible reliability. But nothing is certain in this world," said Scheer, a member of the citizens' committee who is now a senior vice president for the LTV Corp.

Despite NASA's precautions, there have been a number of close calls since Alan B. Shepard Jr. roared off from Cape Canaveral and became the first American in space on May 5, 1961.

Perhaps the most hair-raising experience came in April 1970 when Apollo 13, planned as America's third landing on the moon, survived an explosion 250,000 miles from Earth and provided four days of drama as astronauts and flight controllers in Houston steered the crippled spacecraft home.

The safe return of Apollo 13 and its three crewmen gave NASA added confidence in the reliability of its spacecraft and the skill of astronauts and flight controllers. Speaking to Congress shortly after the Apollo 13 landing, Thomas Paine, then NASA administrator, said, "I see no reason why this setback should be the occasion for major change in the course of the nation's space program."

NASA's success in surmounting a serious accident in deep space gained praise from Congress and the American people. As a result, the Apollo moon landing program was completed on schedule and the U.S. manned flight program continued with the Skylab orbiting space station and the space shuttle.

Yesterday's explosion came as NASA scheduled an ambitious shuttle program to demonstrate the spacecraft's reliability as a space transportation system and its usefulness for more accurate and less expensive launches of satellites.

Although it was considered safe enough to carry two lawmakers -- Sen. Jake Garn (R-Utah) and Rep. Bill Nelson (D-Fla.) -- into orbit, the shuttle has been dogged by minor but nagging problems from the start.

Even before the shuttle's first journey to space, two workers were killed and four others nearly asphyxiated in a nitrogen-filled compartment of the Challenger during a launch rehearsal in March 1981.

Its first test launch, in April 1981, was halted in the final seconds of countdown and delayed for two days because of a software problem. Once it left the ground, everything worked but the toilet, which became a chronic problem.

Two years later, more serious problems developed as a shuttle headed back to Earth. A computer and navigation device failed as the astronauts were prepared to leave space, but after an eight-hour delay, astronaut John Young headed for a landing at Edwards Air Force Base. As he approached the landing site in the California desert, two of three power units that control the landing gear and flight surfaces failed. Nonetheless, Young made a smooth landing.

Last July, sensors showed that one of the three main engines was overheating, causing NASA computers to shut it down minutes before the spacecraft was to go into orbit. The other two engines boosted the shuttle into a lower-than-planned orbit. That spaceship suffered a launch abort earlier in July when the engine started firing early for some unexplained reason.

A November Challenger flight was disturbed by a smoke alarm that sounded six times, but checks by the astronauts failed to find a fire. Later that month, the launch of the newest shuttle, Atlantis, was delayed for a day because of a bad valve in the engine's hydraulic system.

Earlier this month, a combination of foul weather at Cape Canaveral and mechanical glitches caused a record seven launch postponements for the shuttle Columbia, which was supposed to lift off Dec. 18 but didn't succeed until 25 days later.