NASA has pushed back the launch of the space shuttle Atlantis to at least Sunday to give engineers more time to troubleshoot a problem with malfunctioning sensors on the shuttle's fuel tanks.

Thursday's scheduled launch was postponed after NASA discovered that two of the four sensors that monitor Atlantis's liquid hydrogen fuel tank indicated that the tank was dry, even though there was fuel inside. At least three sensors must be working for the shuttle to be cleared to fly.

The space shuttle Atlantis and launch pad 39a are seen reflected in a pool of water as workers leave the launch pad late Wednesday night at NASA's Kennedy Space Center in Cape Canaveral, Fla.
(Bill Ingalls/NASA/Associated Press)

Atlantis was scheduled to launch at 4:31 p.m. ET on Thursday from NASA's Florida launch base. The flight was originally rescheduled to Friday afternoon, but late Thursday evening, NASA decided it needed more time to fix the problem.

The new launch time was scheduled for Saturday at 3:43 p.m. ET, according to LeRoy Cain, mission management team chairman.

But shuttle managers needed more time to discuss how to deal with the sensors, and the launch was pushed back again.

NASA weather officials said the weather conditions were less favourable for Saturday's launch, with a 40 per cent chance poor conditions might prohibit the launch.

Problems with the sensors have delayed shuttle missions in the past, most recently in September 2006.

New laboratory

The shuttle was scheduled to carry with it the European-built Columbus space laboratory and attach it to the International Space Station.

Columbus is the European Space Agency's major contribution to the orbiting space platform, the culmination of 25 years and an estimated $2 billion US in funding.

The space laboratory is a seven-metre-long and 4.5-metre-diameter cylindrical living space that will house a number of scientific experiments.

Canada's contribution to the laboratory is the microgravity vibration isolation subsystem (MVIS). It's the latest version of technology designed to allow experiments to be conducted in low gravity without interference from vibrations produced by the space station itself — a high-tech bubble that allows experiments to run more smoothly.

Three spacewalks

It will be the second science laboratory on the space station, joining the U.S.-run Destiny lab.

Japan's contribution to the space station, the Kibo science lab, will be installed over several shuttle missions in 2008.

The 11-day mission is set to include three spacewalks and will also deliver a new crew member — French astronaut Léopold Eyharts — to the space station, replacing U.S. astronaut Daniel Tani.

It will be the fourth shuttle launch of the year, as NASA rushes to complete the space station before the shuttle program expires in 2010.

NASA had originally planned five missions for 2007 before hail damage to Atlantis's fuel tank in February delayed its launch until June and forced the space agency to push back the timing of its flights.

With files from the Associated Press