A file photo from STS-1 shows the point where the solid rocket booster is attached to the launch pad. Photo: NASA TV/Spaceflight Now

Two of eight bolts that held the space shuttle Discovery to its launch pad failed to separate cleanly during last week's blast off, NASA officials said.

Although slight damage was caused to the base of Discovery's right-hand solid rocket booster, the mishap did not endanger the astronauts and is not considered serious, NASA spokesman Joel Wells said.

"There was not any threat to the shuttle or the booster," he said.

The problem, referred to by NASA engineers as a "stud hang," has occurred on 11 of the previous 99 space shuttle launches. But this is only the second time that there has been a problem with two bolts on the same mission. The first time was on STS-2 in November 1981.

NASA has begun an investigation into the incident, but unless anything untoward is revealed no further action is thought necessary, Wells said. The only long-term concern with a stud hang is possible damage to the booster's launch pad attach points.

The space shuttle is held to its mobile launch platform by eight massive bolts, four at the base of each solid-fuel booster. Each bolt is 28 inches long and 3.5 inches in diameter.

The bolt is supposed to fall away at liftoff as shown here in a file photo from STS-1. Photo: NASA TV/Spaceflight Now

At booster ignition, small explosive charges shatter the four nuts on each booster, releasing the bolts. They are designed to immediately slide out of the booster attach point and fall into a housing inside the launch pad posts the shuttle sits on.

In the case of Discovery's launch, the explosive charges fired, but the two right-most bolts on the right-hand booster did not drop away as planned.

As Discovery leapt away from its launch pad, the stuck bolts shaved metal from inside their attach points on the booster's aft skirt. The scrape was immediately detected by sensitive accelerometers aboard the shuttle, but the crew would not have been aware of the problem, Wells said.

High-speed film of the launch confirmed that the two bolts had not fallen away. The protruding bolts were seen to flex as the shuttle climbed off the pad.

An examination of the recovered solid rocket boosters also found that metal inside the attach points had been shaved away after coming into contact with the problem bolts.

Discovery's launch was held up by concerns about a bolt that attaches the shuttle to its fuel tank. That bolt failed to retract on previous missions, but is unrelated to the launch pad system.