All seven astronauts gave a thumbs-up as they boarded the van for the ride to the launch pad late Monday. "All right, here we go!" said commander Dominic Gorie.
"Wooo!" shouted his co-pilot, Gregory Johnson.
As the astronauts strapped into their spaceship, Launch Control radioed, "Looks like a great night to go into space."
Earlier in the evening, NASA filled Endeavour's external fuel tank; everything went exactly as planned.
Good weather was forecast for the 2:28 a.m. liftoff. It will be the first shuttle launch in darkness since 2006; only a quarter of all shuttle flights have begun at nighttime. It also will be NASA's longest space station mission: 16 days.
Endeavour's seven-man crew will deliver a new Japanese compartment to the space station - the first of three lab installments - as well as a Canadian robot designed to help with outside maintenance.
The Japanese lab - called Kibo, or Hope - is so big that it will require three shuttle flights to get everything up. A storage compartment for the lab is loaded aboard Endeavour; the lab itself will fly on the next mission in May.
"Our Japanese people have been waiting for a very long, long time," said Yoshiyuki Hasegawa, the Japanese Space Agency's station program manager. Preliminary design work for Kibo (pronounced KEE'-boh) began in 1990; space station construction, however, was stalled over the years for various reasons, most recently the 2003 Columbia tragedy.
Endeavour's launch with the first part of Kibo, as well as a Japanese astronaut, represents "an unforgettable event," Hasegawa said, noting that for the first time the Japanese Space Agency will be a visible partner in the international space station.
The Endeavour astronauts also will try out a caulking gun and high-tech goo on deliberately damaged shuttle thermal tile samples. The test should have been performed by another crew late last year, but was put off because of emergency space station repairs.
It is the second of six planned shuttle missions this year, all but one to the space station. NASA faces a 2010 deadline for finishing the space station and retiring its three shuttles.
To draw attention to the space program and push for increased NASA funding, Rep. Nick Lampson, D-Texas, led a congressional delegation to the launching site. His district includes Johnson Space Center in Houston.
Numerous Japanese and Canadian space representatives also gathered for the liftoff.
Visit NASA's Web Page for the STS-123 Mission for the latest developments, background on mission elements, crewmembers and multimedia.