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Phoenix is to land on icy soil near the north Martian polar ice cap to try to learn more about the history of water on the planet. ((NASA Jet Propulsion Laboratory, University of Arizona))

The Phoenix Mars Lander, a robot explorerwith a Canadian weather station aboard, wassetto lift offfrom Florida Saturday morning, with a 20 per cent chance that bad weather would delay it again, NASA said Friday.

The lander —to be boosted into spaceon a Delta II rocket —was originally scheduled to start its trip Fridayat the beginning of a three-week launch window.If it is not launched in that span, it mustwait two years for another chance.

With missions backed up on its launch pads, the National Aeronautics and Space Administration faces extra pressure to send Phoenix on its way. Further scrubs could delay the agency's next manned flight, a trip to the International Space Station aboard the space shuttle Endeavour.

The shuttle launch, set back by a leaky valve, was postponedanother day on Friday to Aug. 8.

The shuttle crew includesteacher-turned-astronaut Barbara Morgan, who was Christa McAuliffe's backup in 1986 when McAuliffe was killed along with six others when the shuttleChallenger broke up after liftoff.

The crew also includes Canadian astronaut Dave Williams, who is scheduled to take three space walks, one more than any Canadian before him.

Phoenix's chances were "looking good for Saturday," NASA said in a statement on its website. The forecast called for scattered clouds, light ground and upper-level winds and good visibility, it said.

The launch time wasset for 5:26 a.m. ETor, if that attempt failed,6:02 a.m.

After a space flightof about 10 months, Phoenix is to land on icy soil near the north Martian polar ice cap to try to learn more about the history of water on the planet. The ice caps are believed to contain both frozen water and frozen carbon dioxide.

The lander has a U.S.-made robotic arm forcollecting samples.

Canada's contribution is a Meteorological Station, or MET, designed to monitor changes in water abundance, dust, temperature and other variables in the Martian atmosphere.

The Canadian Space Agency and a team headed by York University — and including contributions from the University of Alberta, Dalhousie University, Optech and the Geological Survey of Canada — will oversee the science operations of the station, which wasbuilt by Canadarm maker MacDonald Dettwiler and Associates Ltd. of Richmond, B.C.