The leak was "too close for comfort" said Nasa's engine manager.

"We're very concerned about it, very concerned," said George Hopson, manager of Nasa's shuttle main engine programme.

Nasa engineers inspect the engine nozzles

He added: "We fully intend to find out what happened to this nozzle and make sure it doesn't happen again."

No explosion danger

Officials said there was never any danger of an explosion. However if the leak had been bigger, it could have caused an engine shutdown and led to an unprecedented emergency landing in either Florida or Africa.

The first sign of a problem was that the shuttle ended up 11 kilometres (seven miles) lower than its intended orbit after lift off on 23 July. Nasa checked a video of the launch and saw a bright streak coming from the right main engine nozzle - suggesting a hydrogen leak.

After touchdown on Wednesday, the damage was immediately visible to the inspectors: three holes, about six millimetres (0.25 inch) in size, in three parallel steel tubes. The tubes are used to cool the engines and simultaneously pre-heat the hydrogen fuel.

Mr Hopson said the damage may have been caused by a rock or piece of the launch pad striking the nozzle when the rocket ignited. However, the area is supposed to be clean of debris and it is also possible the tubes were damaged well before the countdown.

The next shuttle mission will see Endeavour blast off in September on a radar-mapping mission. Nasa will double-check all the tubes, Mr Hopson said.

Space pioneer

The fuel leak has overshadowed the shuttle crew's return to Earth, particularly that of Eileen Collins, the first woman to command a shuttle mission.

Commander Collins receives a hero's welcome with her daughter

However, Commander Collins was hailed by US Vice President Al Gore as a bigger hero than Amelia Earhart. Commander Collins had taken with her a scarf worn by Earhart, an early aviation pioneer who vanished over the Pacific in 1937 while attempting to become the first woman to fly around the globe.

"She has not only equalled but surpassed Amelia Earhart in the history of flight," Gore said.

Sick space station

In a separate development, doubts have been cast over the environment aboard the International Space Station. The revelations came in a leaked Nasa documents, posted on the Nasa Watch website.

The astronauts who visited the ISS earlier this year are said to have suffered headaches, burning eyes and one even vomited. The ailments are not thought to be normal space sickness.

A build-up of carbon dioxide might be to blame, due to disruption of the ventilation flow when the wall panels were open. Or, according to the leaked documents, it could be due to chemicals being released from the module components, perhaps the glue used to fix the large amounts of Velcro used.