CAPE CANAVERAL,Fla. -- Weather permitting, Discovery will end its 13 days in orbit this morning with the first shuttle landing at the Kennedy Space Center since 2002.
Discovery's six astronauts are scheduled to touch down at 9:14 a.m. to complete a supply flight to the International Space Station that included a critical repair to the outpost and delivery of a crew member. Weather is expected to be acceptable, with the biggest concern a chance of showers near Cape Canaveral.
"My experience is that at the Kennedy Space Center, it [weather] is always a challenge," said Steve Stich, the flight director who will oversee Discovery's return. "I will say that I think we have a pretty good shot."
Discovery's crew packed up yesterday to prepare for landing after checking out the shuttle's flight control systems. The checkout included activation of an auxiliary power unit, or APU, that has been slowly leaking small amounts of either flammable hydrazine fuel or harmless nitrogen gas. Engineers suspect the leak is nitrogen but cannot be certain.
The unit is one of three aboard Discovery that powers the orbiter's hydraulics system and operates the body flaps, steering and brakes during landing. Only one APU is needed to safely return the shuttle.
Mission managers monitored the leaking unit during yesterday's checkout and determined that the rate of the leak did not increase afterward. As a result, the plan is to operate all three APUs as usual during today's landing.
"We're not going to try to start it early or do anything unusual," astronaut Steve Frick told the crew from Mission Control. "We'll be looking at it close like we did" yesterday.
In an interview from orbit yesterday, Discovery pilot Mark Kelly told reporters that the unit wasn't an issue as far as the crew was concerned.
Engineers also cleared Discovery's heat shield yesterday for the return to Earth after reviewing data from two days of late inspections done with a sensor-laden boom held by the shuttle's robot arm. No evidence of significant damage from orbital debris or micrometeoroids was found.
Today, Discovery's astronauts will don their flight suits for the trip home about 6:45 a.m. If the weather looks good for the shuttle to land on its 202nd orbit of the Earth, Mission Control will give the go-ahead an hour later for a three-minute firing of Discovery's twin braking rockets. The firing, scheduled to take place at 8:07 a.m., will slow the shuttle's speed and begin its fiery plunge home through the atmosphere.
If weather is a problem or another issue arises, Discovery has another Kennedy Space Center landing opportunity one orbit later. A 9:43 a.m. engine firing would lead to a 10:50 a.m. touchdown.
Discovery will stay in space for an additional day if bad weather thwarts both Florida landing opportunities. Backup runways at Edwards Air Force Base in Southern California and White Sands Missile Range in New Mexico would be activated tomorrow.
The weather forecast for Edwards predicts optimal conditions today and tomorrow. However, mission managers prefer to return Discovery to Florida to save a week or so in turnaround time and the $1 million cost of ferrying the shuttle back to Florida atop a jumbo jet. Last year, bad weather forced Discovery to land in California to complete the first shuttle flight since the Columbia accident in February 2003.
Discovery can stay in space until Wednesday, when the ship will begin to run low on its supply of liquid oxygen needed to generate electricity that powers the orbiter's systems. The shuttle would land at White Sands only if the weather looked bleak in Florida and California for all three days.
Michael Cabbage writes for the Orlando Sentinel.