Nasa: 'Ice impact risk acceptable'

By Irene Mona Klotz
at the Kennedy Space Center, in Florida

Discovery is looking good for a July launch

Ice striking the shuttle's heat shield remains a possibility, but the risk is low enough to safely resume flights, top officials with the US space agency have decided.

"At the end of the day, the recommendation... was that we're in an acceptable risk posture and that we proceed on with the launch," shuttle programme manager Bill Parsons said.

"I accepted that recommendation; and that's where we're headed now."

The decision, which followed a day-long meeting at the Florida spaceport on Friday, clears the last major hurdle from Nasa's plan to launch shuttle Discovery as early as 13 July on what will be the first flight since the 2003 Columbia accident.

A final decision about whether to proceed with the flight will be made next week following a wide-ranging readiness review and the findings of a panel overseeing Nasa's return-to-flight efforts.

Managers have spent most of the past 2 1/2 years focusing on how to stop foam from flying off the shuttle's tank during launch and striking the ship's delicate heat shield.

Columbia was lost due to an impact on its wing during launch by a piece of falling foam insulation.

Earlier this year, however, engineers became more and more concerned that ice, which can build up on the outside of the tank once it is filled with cryogenic fuels, could pose just as great a threat to the shuttle heat shield as foam.

Numbers game

The analysis showed that ice striking the shuttle's heat-resistant tiles was the greatest debris impact threat, managers said on Friday, but more information would be needed to quantify the risk.

"This is going to require constant vigilance," said John Muratore, who manages the shuttle systems engineering and integration office.

Despite the uncertainty, Nasa said it was ready to proceed with the launch.

"As far as (Discovery) is concerned, I believe our concerns are put to bed," Muratore said. "We're ready to fly."

The officials declined to put a number on the chance of a heat-shield strike, saying there are too many variables in play to have any statistic be meaningful.

"It doesn't lend itself to a single number," Muratore said. "You can't say, like in blackjack, 'If I have a 15, these are my odds of winning.' "

The single greatest threat of heat-shield damage from an ice impact was reduced by an order of magnitude when an additional heater was added near the top of the shuttle's tank to thwart ice formations around the liquid oxygen feedline bellows, he added.

That work prompted Nasa to delay Discovery's lift-off from May until July.