NASA adminstrator Charlie Bolden (C) talks with the crew of the space shuttle Discovery STS-133 after their arrival at the Kennedy Space Center in Cape Canaveral, Florida Feb 20, 2011. Space Shuttle Discovery is scheduled to launch on a mission to the international space station on Feb 24, 2011. [Photo/Agencies]

CAPE CANAVERAL, Florida - NASA moved closer Tuesday to sending space shuttle Discovery on its final voyage after a nearly four-month delay.

Shuttle managers said the countdown is going well. What's more, there's an 80 percent chance of good flying weather Thursday. Launch time is 4:50 pm (2150 GMT).

This will be the 39th flight for NASA's oldest surviving shuttle. Discovery first rocketed into orbit in 1984. This time, Discovery is headed back to the International Space Station. It will drop off a humanoid robot as well as an oversize closet full of space station supplies.

A veteran crew of six will be on board. One of the two spacewalkers - Stephen Bowen - joined the team just last month. He's substituting for astronaut Timothy Kopra, who was injured in a bicycle crash.

All six are eager to get going.

"Just landed after some T38 (trainer jet) flying around the Cape. A little g (gravity) loading before launch. Beautiful!" astronaut Nicole Stott said in a Twitter update Tuesday morning.

Discovery should have been finished flying by now and undergoing the necessary preparations for eventual museum display. The delay was caused by cracks in a part of the fuel tank that holds instruments but no fuel. Discovery had to be moved off the launch pad and back into the Vehicle Assembly Building for the unprecedented repairs. A hydrogen gas leak also needed to be plugged.

The fuel tank is now "stronger than ever," NASA test director Steve Payne told reporters Tuesday.

It will be a historic moment when Discovery reaches the space station. For the first time ever, spacecraft from all of the major space station partners will be parked at the orbiting complex: two Russian capsules and cargo ships from Russia, Japan and Europe. Canada has the space station's robot arm.

A European cargo ship launched last week from French Guiana is scheduled to dock Thursday morning. Any problems with that hookup could end up delaying the shuttle launch.

Science feature graphic explaining NASA's Robonaut 2, the first humanoid robot to be sent to space, which is scheduled to be delivered to the International Space Station on Thursday. [Photo/Agencies]

Robonaut 2-  better known as R2 - is the first mechanical humanoid bound for space. He's remained aboard Discovery ever since the first launch attempt back in November.

The robot is taking a one-way trip to the space station. It won't be unpacked until Discovery is gone.

"Countdown's on!" Robonaut said in a Twitter update after the countdown clocks started ticking Monday at Kennedy Space Center.

OK, so it's actually a real person filing the AstroRobonaut tweets.