Image: Fuel tank
Jim Grossmann  /  NASA
The external fuel tank for Discovery's next mission has been lowered into a checkout cell within Kennedy Space Center's cavernous Vehicle Assembly Building for further work. One of the issues to be checked involves the tank's low-level fuel sensors, which sparked a delay in Discovery's last launch.
By NBC News space analyst
Special to NBC News
updated 3/7/2006 5:05:14 PM ET 2006-03-07T22:05:14

As NASA prepares the space shuttle Discovery for launch in mid-May, the space agency is dealing with the same type of hardware worry that caused last-minute delays in the shuttle’s "return to flight" mission last year.

The space agency has not yet announced a shift in its launch schedule, and it's not yet clear how serious the hardware problem is. However, if the problem needs correction, Discovery's mission to the international space station could be delayed until near the end of the May 10-23 launch window, if not early July.

The hardware issue involves the low-level fuel gauges on the external fuel tank that's being readied for Discovery's use. These devices — formally known as emergency cutoff sensors or ECO sensors, had been troublesome during tank preparation at NASA's Michoud Assembly Facility in Louisiana. Last week, the tank was brought to Kennedy Space Center in Florida for further preparations.

On Tuesday morning, the independent British-based Weblog quoted sources as saying that there was a "99 percent chance" that the sensors would need replacing, and that the operation would set back the launch until July at the earliest.

In response to inquiries from, NASA spokesman Kyle Herring said that such an assessment was "very premature."

He noted that engineers at Michoud "saw a very slight shift" in one of the sensors during electrical tests, and NASA decided to pursue the issue further while the tank was at Kennedy Space Center's 52-story Vehicle Assembly Building.

"The ET [external tank] project will assess it further and bring it to the program level later," Herring told's Alan Boyle.

Speaking on condition of anonymity, sources in the space program told NBC News' Jay Barbree that even if the sensors needed replacement, the job might require no more than an additional week of work. That would still leave open the possibility of launch in the May 17-23 time frame.

If further delays accumulate, the launch might have to be pushed back into the next available window, from July 1 to 19. The windows are determined by the orbital requirements for reaching the space shuttle as well as the daytime lighting requirements for observing Discovery's ascent.

‘Early in the game’
NASA has acknowledged that the timing for a May launch is tight , but Herring said it was "pretty early in the game" to determine whether or not May was doable. He compared the current state of NASA's planning to the second quarter of a football game.

The concerns over the ECO sensors echo the problems that forced a 13-day delay during the runup to last July's Discovery launch, the first shuttle mission since the loss of the shuttle Columbia in 2003. It took days of intense effort to resolve the subtle causes behind the sensor glitches, but in the end, NASA was able to launch Discovery safely.

If the sensors on Discovery's new fuel tank have to be replaced, the work can be performed while the tank is in the Vehicle Assembly Building. The replacement would have to be finished and cleared before the tank can be mated with the pair of solid rocket boosters and the winged orbiter itself.

Other concerns
The sensors are by no means the only concerns surrounding the preparations for Discovery's mission. A few days ago, a series of missteps took place while engineers were working on Discovery in the Orbiter Processing Facility, the shuttle hangar at Kennedy Space Center.

After a ceiling lamp broke, showering glass onto the shuttle, workers used a boom-mounted platform to position themselves for cleaning up the debris. However, while moving into position, the edge of the work platform made contact with the shuttle’s robot arm, which is stowed along the left sill of the long payload bay.

Inspections are now under way to verify that no mechanical damage was inflicted on the arm or on the special sensors that have been installed for testing on the next mission. has been told privately that such operator errors seem to have become more frequent in recent weeks as workers do double-shifts to meet the schedule for launch processing.

James Oberg, space analyst for NBC News, spent 22 years at the Johnson Space Center as a Mission Control operator and an orbital designer.


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