Space agency officials have decided on several ways to redesign the solid-rocket booster joint whose failure led to destruction of the space shuttle Challenger, but they envision extensive tests before selecting a design, a high-ranking engineer said today.

James E. Kingsbury, director of science and engineering at the Marshall Space Flight Center here, said, however, that the new joint will not be completely tested and pronounced operational by February, the date set by the National Aeronautics and Space Administration for resuming shuttle launches.

Kingsbury said he has told NASA officials how much additional time will be required, but he declined to reveal his estimate today.

He said preliminary designs will be ready this summer, noting that "the test program . . . is going to be what takes us a very long time to do."

"We intend to make it a much more forgiving joint," he said.

NASA officials investigating the Jan. 28 accident, in which seven astronauts died, said this week that four problems in the right booster conspired to cause the joint to fail.

Each of the shuttle's two solid-rocket boosters, which provide principal thrust at launch, has four segments. The joint between the two lowest sections of the right booster failed to seal properly Jan. 28, allowing hot gas to escape and ignite a fireball, officials said.

Fewer than 10 new designs are under consideration, Kingsbury said, and they will be winnowed by July or August to a smaller number for testing.

He said these decisions have been made about how to fix the joint:

*The rubber O-rings, which make it airtight, may be eliminated. These must seat perfectly and instantly. Kingsbury said consideration is being given to replacing them with a more reliable pressure seal, perhaps of metal.

*Putty used in the joints to shield the O-rings from heat and fire will be eliminated. Investigators think that the putty prevented the O-rings from instantly forming the required airtight seal. He said the putty may be unnecessary.

*New joints essentially will have interlocking U-shaped metal edges. In the current joint, an upper edge slides into a lower upturned U-shaped edge, but the joint can bulge outward on ignition. And the bulge sometimes prevents proper sealing. The new design, he said, will prevent bulging.

*The joint will be made impervious to rain and cold by providing an internal heating mechanism and external weather seal "something like a big rubber band," he said.

Investigators believe that subfreezing temperatures overnight before Challenger's liftoff stiffened the O-rings and putty, sharply reducing their ability to seal correctly.

Officials have said water may have leaked into the joints during thunderstorms while Challenger sat on the launch pad for a month. That water could have frozen the night before launch, and the ice might have pushed the O-rings out of alignment.

*The boosters will be handled differently, although it is not certain how, so they do not become misshapen during transportation to the launch site or postlaunch recovery from the Atlantic Ocean.

The enormous segments have been moved, on their sides, on rail cars and during transportation, have been slightly squashed, necessitating reshaping so they are perfectly circular for reuse.

Investigators think that the reshaping required for the lower segment of Challenger's right booster may have contributed to the joint failure.

Kingsbury said he has essentially ruled out using unsegmented boosters because at least three years would be needed for design and construction of such a completely new sort of rocket.

He said the redesign project involves about 70 workers here and 200 from Morton Thiokol Inc., which makes the boosters in Brigham City, Utah.

He said he guarantees that the new joint "will mean the failure mode that was experienced on Challenger will never be experienced again in the program."