Live at KSC: Discovery Countdown Begins

A three-day countdown to shuttle Discovery's scheduled launch at 9:20 p.m. Wednesday is officially under way.

Countdown clocks at Kennedy Space Center began ticking backwards from T minus 43 hours at 7 p.m. The countdown takes longer because of numerous pauses built in along the way.

NASA press officials say no technical issues are being worked that threaten an on-time liftoff.

The countdown started as Discovery's mission commander, Lee Archambault, and pilot, Tony Antonelli, practiced landings on the shuttle landing strip in a modified jet that simulates an orbiter's handling on descent.

Archambault and his crew - which also includes mission specialists Joe Acaba, Ricky Arnold, John Phillips, Steve Swanson and Koichi Wakata - flew into KSC from Houston just before 3 p.m.

Work planned tonight includes final vehicle and facility closeouts for launch, check-out of backup flight systems, configuration of avionics systems for launch, review of flight software stored in mass memory units and display systems and verification of backup flight system software in Discovery's general purpose computer.

On Monday, preparations begin to load liquid hydrogen and liquid oxygen into the fuel cells that powers shuttle systems during flight.

Mission managers are expected to meet Monday morning and hold a pre-launch news conference at 11 a.m. to update the status of the STS-119 mission.

Click here to see a list of significant countdown milestones.

The two-week mission will complete the International Space Station's power supply with the installation of a final set of solar array wings on the starboard side.

The wings are loaded on the 11th and last piece of the station's central truss, a 31,000-pound girder called "S6."

Once docked with the space station, Wakata will replace Sandy Magnus as a member of the station's Expedition 19 crew, becoming the first Japanese astronaut to participate in a long-duration spaceflight. He's tentatively scheduled to return to Earth on shuttle Endeavour in June.

Current forecasts anticipate excellent weather for Wednesday evening's launch, the 125th by a shuttle, the 36th by Discovery and the 28th to the space station.

Today's forecast issued by the Air Force's 45th Space Wing shows a 90 percent chance of good weather on Wednesday, and 80 percent chance of favorable conditions the next two days.

For more about the STS-119 mission, check out the following NASA materials:

- Mission summary
- Mission fact sheet
- Mission press kit

IMAGE NOTE: At 7 p.m. today, a countdown clock at Kennedy Space Center began ticking down from T minus 43 hours to space shuttle Discovery's planned launch at 9:20 p.m. Wednesday. Discovery can be seen in the background on Launch Pad 39A. Photo credit: James Dean, Florida Today (taken by cell phone). Earlier, Discovery's seven astronauts arrived at KSC from Houston on five T-38 training jets. Credit: Craig Rubadoux, Florida Today. Bottom: The STS-119 mission patch, which was designed by the daughter of lead mission spacewalker Steve Swanson. The shape comes from the shape of a solar array viewed at an angle.