By Irene Klotz
Kennedy Space Center, Florida
The US space agency (Nasa) is growing optimistic of resuming space shuttle flights in May and perhaps squeezing in three missions this year.
Nasa hopes its latest efforts will eliminate critical foam loss
Hope rests on a new fuel tank due to arrive at the launch site on Wednesday.
The key to the programme's future is how well engineers have fixed the foam-shedding problem that led to the loss of shuttle Columbia in 2003.
Foam loss also reoccurred in July 2005 on Nasa's first, and so far only, shuttle launch since the accident.
"I remain optimistic that if we fly this summer, we'll be able to get three flights off this calendar year. But time will tell," shuttle programme manager Wayne Hale said at a press conference on Tuesday.
The primary safety upgrade has centred on the shuttle's 154ft- (47m-) long external fuel tank, which holds the liquid oxygen and liquid hydrogen consumed by the ship's three main engines during the climb to orbit.
Insulating foam on the tank, intended to prevent the formation of ice which could break off and damage the orbiter, became a deadly debris hazard when a briefcase-sized chunk broke off during Columbia's launch and smashed into its wing.
The damage triggered the shuttle's break-up over Texas 16 days later as Columbia flew through the atmosphere in preparation for landing. Seven crew members aboard the shuttle died.
Despite more than two years and hundreds of millions of dollars to fix the tank, the problem reoccurred during the first post-Columbia launch.
Photographs and videos of Discovery's lift-off showed 16 pieces of foam broke off the tank, including a 1lb (0.5kg) chunk from a slab of foam that served as a windshield for critical equipment attached to the outside of the tank.
Managers put shuttle flights on hold and set about redesigning the tank again.
The first of the modified tanks, which omits the protective but troublesome foam slabs, left its New Orleans manufacturing facility on Saturday and was scheduled to arrive at Kennedy Space Center on Wednesday afternoon.
Whether shuttle Discovery, which will fly the next mission, can lift off in May as Nasa hopes, depends largely on the results of wind tunnel tests due to begin this week.
The tests, which will involve precise subscale models of the shuttle, as well as full-size pieces of some tank components, are designed to assess how well the tank withstands the aerodynamic stresses of launch without the protective foam ramps shielding pressurization lines and cable trays.
In addition to clearing the tank for flight, engineers are looking a several other technical issues that could delay Discovery's launch until July.
International Space Station
Even so, Nasa still could have time to fly another two missions this year, provided the tank problem is fixed and no new technical issues arise.
Nasa's goal is to eliminate large pieces of foam from flying off the tank. Hale acknowledged that small pieces of foam, hopefully none bigger than a matchbox, will still shed during launch.
"I wanted to make sure everyone understands we are trying to eliminate critical foam loss, but we will expect to see foam coming off this next tank," Hale said.
"Our task ahead of us is to ensure that all of these very small pieces of foam, most of them less than an ounce, will be of a size that they cannot have enough energy to do damage if they strike the orbiter," Hale added.
"We'll continue to make improvements so we can eliminate even smaller and smaller pieces of foam loss."
Except for a possible servicing mission to the Hubble Space Telescope, Nasa is devoting all remaining shuttle flights to building the International Space Station.
Station trusses and power systems, as well as laboratory modules owned by Europe and Japan, have been stranded on the ground during the shuttle programme's recovery. All equipment was designed to be launched only on the shuttle.
The United States plans to retire the shuttle fleet in 2010.