WASHINGTON, May 8 (Xinhua) -- A U.S. Air Force C-17 cargo aircraft carrying NASA's Phoenix Mars Lander arrived at Cape Canaveral in Florida on Tuesday.

    A Delta II launch vehicle will start Phoenix on its much longer trip to Mars from Cape Canaveral Air Force Station sometime after Aug. 3, the earliest possible launch date, NASA said in a statement.

    Opportunities for energy-efficient launches to Mars come about every two years, and scientists say this year is particularly favorable for sending a lander to far-northern Mars to arrive when sunshine is at a maximum there.

    "This is a critical milestone for our mission," said Peter Smith of the University of Arizona, principal investigator for Phoenix.

    "Our expert engineering team has completed assembly and testing of the spacecraft. The testing shows our instruments are capable of meeting the high-level requirements for the mission."

    Workers have been assembling and testing the spacecraft for more than a year in Denver, Colorado.

    After launch, Phoenix will land on a Martian arctic plain next spring. It will use a robot digging arm and other instruments to determine whether the soil environment just beneath the surface could have been a favorable habitat for microbial life.

    Phoenix will be prepared for launch in a payload processing facility at NASA's Kennedy Space Center in Florida. The rocket that will launch Phoenix is a Delta II 7925, manufactured by United Launch Alliance, Denver.

    Like NASA's previous Mars spacecraft Polar Lander and Climate Orbiter, Phoenix is a relatively low-cost mission.