Discovery Crew Returns Safely, But Concerns Persist About Boosters : Space: NASA vows to ground the four shuttles if it cannot correct leaks in solid-fuel rockets. Inspectors have found soot on an O-ring.

Five astronauts aboard the space shuttle Discovery glided to a homecoming Saturday as NASA studied a booster rocket problem that threatened to delay another shuttle launch on Aug. 5.

Discovery’s landing brought a low-key satellite delivery mission to a safe and successful end, despite new concerns about leaks in the solid-fuel boosters that the shuttles use to begin their 8 1/2-minute climb to orbit.

The National Aeronautics and Space Administration said Saturday it would ground all four shuttles, if necessary, until the problem was understood and solved.

“I have not seen data yet that would lead me to believe that that is going to be a required course of action, but anything is possible,” shuttle program director Brewster Shaw said in a news conference from Mission Control in Houston after Discovery touched down.

“We treat each unusual occurrence, abnormality, unexpected event with the solid motors as very serious,” the former astronaut said.

Space shuttle Endeavour is poised to begin an 11-day science mission. It would be the third shuttle flight in six weeks if it’s not postponed.

Shaw said a decision was expected this week.

While Discovery was aloft, NASA revealed that inspectors found leaks in the two reusable boosters that propelled shuttle Atlantis on its historic mission to dock with Russia’s Mir space station in June. Shaw said the five astronauts and two cosmonauts aboard Atlantis were “in no more danger than any other crew.”

According to NASA, workers at Thiokol Corp.’s Utah rocket plant found problems in the nozzle joints of two of Atlantis’ segmented boosters as the spent casings were being disassembled and refurbished. They also found soot on an O-ring seal in the left nozzle joint.

A booster seal burn-through was identified as the cause of the 1986 Challenger disaster, which killed seven astronauts. After the explosion, the boosters were redesigned with stronger joints that contain primary and backup O-ring seals.

In the 45 shuttle flights since then, a total of 13 small leaks have occurred, but “this is the first time we’ve seen heat effects on the primary O-ring,” Shaw said, adding that the joint “performed as it was designed to perform.”

Discovery commander Terence (Tom) Henricks guided Discovery to a touchdown at 11:02 a.m. PDT at Kennedy Space Center, where the shuttle was launched July 13, after a five-week delay due to damage caused by woodpeckers.

The astronauts passed up an earlier chance to land Saturday because of concerns about visibility on their runway.