The fireball that took seven lives and tragically ended the 25th mission of the space shuttle today cast a doubt over the future of the U.S. space program that cannot yet be measured.

No one in authority today would venture a guess as to how far the space shuttle program will be set back. It could take months to determine the cause of the accident, and more time after that to fix the problems and allow further flights.

One yardstick that can be used to judge the potential impact on the space program is the aftermath of the Jan. 27, 1967, launch pad fire that killed three astronauts. The investigation of that accident took six months. It cost more than $100 million to fix the problem, and the Apollo program to land a man on the moon was set back nine months.

For now, the space program is essentially halted, and will be for some time to come. This is standard practice whenever the National Aeronautics and Space Administration suffers a serious setback. All work stops as the agency sets out to find the cause of the problem and to prevent a recurrence.

The tragedy occurred as the space agency reached a pivotal point in the shuttle program. It had scheduled 15 launches this year, almost twice as many as 1985. Not only is the 1986 schedule now impossible to meet, but it may be impossible to meet any accelerated schedule in the next five years.

Space science suffers the most from today's explosion. Three shuttle launches were planned this year to send the Galileo spacecraft to Jupiter, the Ulysses spacecraft around the north and south poles of the sun and the $1.2 billion Space Telescope into Earth orbit to peer almost to the edge of the universe for the next 10 years.

Ulysses was to be launched May 15 aboard Challenger. Galileo was to be launched May 20 by Atlantis, and the Space Telescope sometime in October, again on Atlantis.

With the loss of Challenger, the shuttle fleet has been reduced to three -- Atlantis, Discovery and Columbia. Besides its flight in May, Challenger was due to fly three more times this year and six times next year.

Sen. Slade Gorton (R-Wash.), chairman of the Senate Commerce subcommittee on science, technology and space, said it probably will be necessary to spend the estimated $2.2 billion to build another shuttle to replace Challenger. He said he has asked his staff to study the matter.

Like several other committee and subcommittee chairmen, Gorton promised a full investigation of the tragedy.

Rep. Don Fuqua (D-Fla.), chairman of the House Committee on Science and Technology, promised a "thorough review" and said he assumed there would be no further shuttle launches until the cause of the explosion is determined.

Numerous House and Senate members, while acknowledging the serious setback to the space program, said the program should continue on as rapid a recovery schedule as possible. But many expressed serious doubts about sending any more nonastronauts into space.

"I don't see a real outstanding purpose to it sending civilians into space ," said Rep. Harold L. Volkmer (D-Mo.), chairman of the investigations and oversight of the House Science and Technology Committee.

Loss of Challenger leaves NASA in a double bind because it decided years ago to forgo its expendable launch rockets for the reusable shuttle. Production of workhorse rockets like the Atlas-Centaur and Titan-Centaur were stopped.

"It's the old don't put your eggs in one basket thing coming back to haunt you," said a source at the Jet Propulsion Laboratory, where the unmanned Voyager 2 spacecraft is being directed past Uranus and toward Neptune. JPL is responsible for this year's Ulysses and Galileo missions. "I don't see how we can get our shuttle missions off on time," said the source.

If Ulysses and Galileo do not leave on time in May, they will have to wait until June 1987 the next favorable period for a launch toward Jupiter, which comes only every 13 months. Although the Ulysses mission is bound for the sun, it must use Jupiter's enormous gravity to fling it toward the sun's polar regions.

There is a chance the upcoming investigation might show that today's explosion was a freak accident that might be relatively easy to fix. If the explosion was caused by a defective part or a failed pump in the spacecraft's main engines, it may necessary only to inspect its other engines.

There was substantial speculation today that the the spaceliner's enormous external fuel tank ruptured during ascent, releasing more than a million pounds of supercold liquid hydrogen and oxygen that then exploded. If that is the case, every external fuel tank may have to be carefully inspected and perhaps redesigned and rebuilt. That would take months.

Similar delays can be expected if the explosion started in the three huge and complex hydrogen engines that power the shuttle into orbit, or in one of the two solid-fueled rockets that help boost the shuttle in the early minutes of flight.

One likely difference between the launch pad fire that killed three astronauts in 1967 and the gruesome explosion that took seven lives today is that today's explosion is not likely to lead to the same time-consuming redesign that was necessary in the Apollo program.

The shuttle has had 24 successful flights in almost five years without a major mishap. Before today, its most serious problems were a small fire that did no serious damage, and a high-speed landing last April that blew out a tire and a brake assembly.