The Endeavour space shuttle with seven astronauts on board has blasted off from the Kennedy Space Center in Florida, following nearly a week's delay.

Sorry we had to keep you here for an additional six days, but everything's coming together now. Good luck and have a great flight

Mike Leinbach, launch director

The orbiter is on a 13-day mission to deliver a relief crew to the International Space Station (ISS). The shuttle is expected to arrive at the platform on Friday.

The US space agency, Nasa, twice delayed the launch. The orbiter first should have gone up last Thursday, but stormy weather kept it on the ground.

The lift-off then got pushed back to Monday, and when a pressure valve had to be replaced, the launch slipped to Wednesday.

"Sorry we had to keep you here for an additional six days, but everything's coming together now," launch director Mike Leinbach told the Endeavour astronauts just before the lift off.

"Good luck and have a great flight."

Duration record

The main purpose of flight is to deliver the new ISS crew - Expedition Five.

The crew will be the fifth for the ISS

The delay means two Americans aboard the ISS, who are due to return home on Endeavour, will set a new space duration record.

Astronauts Daniel Bursch and Carl Walz, who have been on the platform since 7 December, 2001, will now push past the 188-day mark set by Shannon Lucid during a stay aboard the Russian Mir complex in 1996.

As well as switching ISS crews, flight STS-111, as it is called, also has to fix the space station's bulky robot arm and install a base so it can move around the orbiting outpost. Three spacewalks are planned to complete the repair and construction tasks.

The Expedition Five crew comprises Commander Valery Korzun and Flight Engineers Peggy Whitson and Sergei Treschev.

Plenty of experiments

Dr Whitson's involvement is particularly significant. The 42-year-old biochemist will be the first real research scientist to live on the platform.

But with only two colleagues on board with her and a lot of maintenance work to be done as well, she will not have much time to devote to experiments.

Nevertheless, she plans to squeeze in as much science as she can during her four-and-a-half-month stay in orbit.

"Every spare minute that I get, that I can, I will be doing investigations," she said.

Dr Whitson will work on plants, crystals, fluids, liver cells, radiation and vibration monitoring, and a number of medical experiments involving lung function and kidney stones, her specialty.