Nasa ready to bend its own safety rules for shuttle launch

Under pressure from its advancing years and questionable weather, the space shuttle Discovery is due to launch today after Nasa managers said that they were prepared to bend their own safety rules.

Officials said that they were ready to approve a waiver if they again encountered a problem with the fuel-gauge monitors as they prepare for the first shuttle launch since the Columbia disaster two-and-a-half years ago.

Discovery was scheduled to have launched on 13 July but, as astronauts were boarding the shuttle, engineers discovered a misreading in one of the four hydrogen sensors located in the external fuel tank. Because of the misreading, they abandoned the launch.

Officials have conceded that they still do not understand what caused the misreading but said that even if they encountered the same problem again, they would most likely go ahead with the launch, scheduled to take place at 10:39am local time (3.39pm BST).

"We've done an extensive degree of troubleshooting and analysis ... to best understand what we've got," Nasa's test director Pete Nickolenko told reporters at the launch site in Cape Canaveral, Florida. "We fully expect that it should work as designed."

The job of the fuel sensors is to prevent the main engines from shutting down too soon or too late during lift-off. The first scenario could result in a never-attempted emergency landing, while the second could rupture the engine turbines. Nasa's launch rule requires that all four sensors work properly, even though only two are needed.

Experts said the admission that Nasa may bend its rules highlights the pressure the agency is under. If Discovery is not launched tomorrow, the next attempt to fly to the international space station will not be made until September.

"It's on a knife-edge," Keith Cowing, who runs the NasaWatch.com website, said. "Is the pressure to fly causing them to bend their own rules? They say there is no pressure to launch but there sure is."

Officials also admitted they were concerned about the weather: forecasters have put the odds of good launch weather today at only 60 per cent.

The problems surrounding the launch has drawn attention to the shuttle's advancing years. The malfunctioning fuel sensor system is reportedly one of 2.5 million parts on the shuttle, most of which were designed in the 1970s. Until recently some of the computers used in testing the shuttle's boosters still contained Intel 8086 microprocessors - the type that powered the first IBM personal computers in the early 1980s.

"I recently turned 50. They were designing the shuttle when I was leaving high school," Mr Cowing said. "It's like an antique car. They did not expect they would be flying this long."

Nasa engineers have said some of the older technologies used in the shuttle may be more reliable than the microprocessors found on newer craft.

"Sometimes hi-tech doesn't go with robust," Jeffrey Carr, a spokesman for United Space Alliance, the main contractor for the space shuttle, told The New York Times.