NASA has scrubbed tonight's launch of the space shuttle Discovery because of a gas leak.
A gaseous hydrogen leak occurred Wednesday as the shuttle's external fuel tank was being filled.
The seven Discovery astronauts had yet to board the shuttle.
The launch is already a month late because of concerns about hydrogen gas valves in the engine compartment. NASA officials say Wednesday's leak had nothing to do with the previous problems, as the gas leak was on an external tank.
NASA is now hoping to launch Thursday night.
Discovery planned to dock at the International Space Station Friday, though it is likely that will be pushed back a day.
NASA astronaut Scott Kelly told CTV's Canada AM that the Discovery crew will, among other things, undertake a set of four spacewalks to install a final set of power-generating solar arrays at the space station.
"They have several different tasks, they are going to do four spacewalks to get this solar array element...outfitted and deployed so it can provide power to the space station as we go to a crew of six," Kelly, who is not participating in the Discovery mission, said Wednesday morning.
According to Kwatsi Alibaruho, the lead space station flight director for the mission, the installation of the final solar arrays will allow the space station to support a larger crew.
"More crew means that we'll have to run more life support equipment, more crew support equipment -- toilet facilities, water processing equipment and all of that stuff," Alibaruho said in a statement.
"We'll have to run more of all of that, so we need additional power."
Beyond the spacewalks, the crew will also conduct robotics tests and drop off a Japanese astronaut to the station, while bringing a U.S. astronaut back home when the shuttle returns to Earth, Kelly said.
"One of the primary tasks is to exchange Koichi Wakata from the Japanese Space Agency, JAXA, with Sandy Magnus, who is a U.S. astronaut who has been living on the space station for the past three months," Kelly said.
Two teachers will also be taking part in the mission and completing spacewalks, he said.
"Recently, with the last astronaut selection, being an educator whether it's middle school, or high school, or primary school, is a qualifying field to be an astronaut as long as you have the appropriate educational background yourself," Kelly said.
These "educator astronauts" were selected in 2004, he said, and have been waiting patiently for their day in space.
"Just by coincidence, they happened to be assigned to the same flight because they are considered just like any other of the astronauts," he said.
A third educator astronaut flew on a mission with Kelly in August 2007.
Kelly said these were the first teachers to participate in a NASA mission since the death of high school teacher Sharon Christa McAuliffe, who died when the Challenger exploded after liftoff on Jan. 28, 1986.
With files from The Associated Press
