NASA Counting Down to Monday's Space Shuttle Launch
The space shuttle Discovery stands ready at Launch Pad 39A at Kennedy Space Center in preparation for its planned April 5, 2010 launch.
Credit: NASA/Troy Cryder

The clock is ticking down toward NASA?s planned Monday morning blastoff of the space shuttle Discovery.

The countdown began early Friday as engineers primed Discovery to launch before sunrise on April 5 at 6:21 a.m. EDT (1021 GMT) from a seaside pad at NASA?s Kennedy Space Center in Cape Canaveral, Fla.

"We're eager to get Discovery flying on Monday morning,? NASA test director Steve Payne told reporters in a morning briefing.

Payne said Discovery has two chances to launch next week before NASA would stand down for three days to top off the vehicle's propellant tanks and avoid a space traffic jam later in April ? when an unmanned Air Force rocket is slated to launch very close to the shuttle?s planned landing day.

Kathy Winters, NASA?s shuttle weather officer, has said Discovery has an 80 percent chance of clear skies for Monday?s shuttle launch, with similar conditions expected on Tuesday. It is the last shuttle flight currently scheduled to lift off in darkness [how to spot the shuttle launch].

Discovery is poised to launch seven astronauts on a 13-day mission to the International Space Station on what is one of NASA?s last few remaining shuttle missions before the orbiter fleet is retired later this year.

Shuttle commander Alan Poindexter commands the four-man, three-woman crew. The astronauts are hauling a nearly 27,000-pound (12,246 -kg) cargo pod packed with scientific experiments, supplies and spare parts for the space station. Three spacewalks are planned to replace an old station gyroscope and deliver a spare ammonia coolant tank ? pieces so big they can only be delivered on NASA?s space shuttles.

Joe Delai, payload manager for Discovery?s spaceflight, said engineers are hard at work loading the shuttle?s middeck with cargo.

The shuttle is also being packed with a few last minute items, including extra seals for the space station?s water filtration system and a spare bracket for a shock absorber used on one of the outpost?s treadmills, Delai said.

Discovery is slated to launch one day after a Russian Soyuz spacecraft arrives at the space station. That Soyuz TMA-18 spacecraft blasted off this morning carrying one American astronaut and two Russian cosmonauts.

The Soyuz?s arrival at the space station will boost the outpost?s population back up to its full six-person crew for the first time since last fall. When Discovery arrives on April 7, that number of people onboard will double yet again ? to 13 astronauts in all.

NASA plans to fly four final shuttle missions this year, including Discovery?s upcoming flight, before retiring the shuttle fleet in the fall. After that, the space agency will rely on Russian spacecraft to ferry astronauts to and from the space station, as well as support the development of commercial spacecraft in the United States for independent access to space, agency officials have said.