This marks the completion of one of the most successful missions of interplanetary exploration ever launched, NASA's Jet Propulsion Laboratory (JPL) said.

    "Last night, just after midnight, the last recovery command was sent to Spirit," said John Callas, the Mars Exploration Rover Project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Los Angeles.

    "It would be an understatement to say this was a significant moment. Since the last communication from Spirit on March 22, 2010, as she entered her fourth Martian winter, nothing has been heard from her."NASA has tried in vain to regain communication with Spirit since it got stuck in sand about two years ago. The rover last communicated on March 22, 2010, as Martian winter approached and the rover's solar-energy supply declined.

    NASA checked frequently in recent months for possible reawakening of Spirit as solar energy available to the rover increased during Martian spring, but to no avail.

    A series of additional re-contact attempts ended on Wednesday, designed for various possible combinations of recoverable conditions, according to JPL.

    "Our job was to wear these rovers out exploring, to leave no unutilized capability on the surface of Mars, and for Spirit, we have done that," said Callas.

    With the completion of Spirit's mission, NASA will have to transition the Mars Exploration Rover Project to a single-rover operation focused on Spirit's still-active twin, Opportunity.

    Spirit operated for more than six years after landing in January 2004 for what was planned as a three-month mission. Its twin, Opportunity, landed on the other side of the planet three weeks after Spirit.

    Despite being stuck, the rover performed beyond expectations. It returned more than 124,000 images, ground the surfaces off 15 rock targets and scoured 92 targets with a brush to prepare the targets for inspection with spectrometers and a microscopic imager.

    "What's really important is not only how long Spirit worked or how far Spirit drove, but also how much exploration and scientific discovery Spirit accomplished," Callas said.

    Spirit started out with some problems in its electronic memory, but detected evidence of water-altered rocks and carbonates -- both building blocks of life -- on what scientists call the Gusev site.

    But the rover found evidence that Mars was once like Earth, with water and hot springs. Concentrated deposits of silica led the project's principal investigator to conclude that steam vents or hot springs once existed at a site known as Home Plate. Such an environment could have supported microbial life.

    After its first year on Mars, the rover lost the use of one of six wheels while scrambling over a hilly area, but continued to send data back to Earth for years. It logged 4.8 miles (about 7.7 km) over the Martian surface -- about a dozen times farther than it was expected to go.

    Opportunity is still working. It has logged 12 miles (19.2 km), collected more than 130,000 images and is headed toward a crater called Endeavour.

    NASA is currently preparing for the launch of another Mars rover Curiosity.

    "We're now transitioning assets to support the November launch of our next generation Mars rover, Curiosity," said Dave Lavery, NASA's program executive for solar system exploration.(本文由在线英语听力室整理编辑)