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FILE - This combination of artist's renderings released by NASA/JPL-Caltech shows how NASA's Curiosity rover is expected to approach and land on Mars. The second from top photo shows an illustration of signals beamed back to Earth and subsequent images show the descent stage and "sky crane" lowering it to the surface. (AP Photo/NASA/JPL-Caltech) ( Mars Curiosity Photo Gallery )

FILE - In this Thursday, Aug. 2, 2012 file photo, Adam Steltzner, Mars Science Laboratory's entry, descent and landing phase leader at JPL uses a scale model to explain the process during the Mission Engineering Overview news briefing at NASA's Jet Propulsion Laboratory in Pasadena, Calif. (AP Photo/Damian Dovarganes, File) ( Mars Curiosity Photo Gallery )

FILE - In this Monday, April 4, 2011 file photo, NASA engineers stand by Mars Science Laboratory's aeroshell, a conical enclosure that will help protect the rover "Curiosity," a robot the size of a car, from the searing temperatures of atmospheric entry when it lands on Mars, at the Jet Propulsion Laboratory in Pasadena, Calif. (AP Photo/Damian Dovarganes) ( Mars Curiosity Photo Gallery )

FILE - In this Thursday, Aug. 2, 2012 file photo, engineers work on a model of the Mars rover Curiosity at the Spacecraft Assembly Facility at NASA's Jet Propulsion Laboratory in Pasadena, Calif. (AP Photo/Damian Dovarganes) ( Mars Curiosity Photo Gallery )

FILE - In this Saturday, Nov. 26, 2011 file photo, a United Launch Alliance Atlas V rocket carrying NASA's Mars Science Laboratory (MSL) Curiosity rover lifts off from Launch Complex 41at Cape Canaveral Air Force Station in Cape Canaveral, Fla. (AP Photo/Terry Renna) ( Mars Curiosity Photo Gallery )

FILE - This late Sunday, Aug. 5, 2012 PDT photo made available by NASA shows the Curiosity rover, bottom, and its parachute descending to the surface from the vantage point of the Mars Reconnaissance Orbiter. (AP Photo/NASA) ( Mars Curiosity Photo Gallery )

FILE - In this Sunday, Aug. 5, 2012 file photo, Mars Science Laboratory Curiosity team member Miguel San Martin, Chief Engineer, Guidance, Navigation, and Control at Jet Propulsion Laboratory, left, celebrates with Adam Steltzner, MSL entry, descent and landing (EDL) of the Mars Science Laboratory (MSL), right, after the successful landing of Curiosity rover on the surface of Mars at NASA's Jet Propulsion Laboratory in Pasadena, Calif. (AP Photo/Damian Dovarganes) ( Mars Curiosity Photo Gallery )

FILE - This image provided by NASA/JPL-Caltech shows the surroundings of the location where NASA Mars rover Curiosity arrived on Sept. 4, 2012. It is a mosaic of images taken by Curiosity's Navigation Camera (Navcam) following the Sol 29 drive of 100 feet. About 9 feet apart, tracks from the drive are visible in the image. (AP Photo/NASA/JPL-Caltech) ( Mars Curiosity Photo Gallery )

FILE - This image provided by NASA/JPL-Caltech/Univ. of Arizona, shows tracks from the first drives of NASA's Curiosity rover, captured by the High-Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. The image's color has been enhanced to show the surface details better. The two marks seen near the site where the rover landed formed when reddish surface dust was blown away by the rover's descent stage, revealing darker basaltic sands underneath. Similarly, the tracks appear darker where the rover's wheels disturbed the top layer of dust. Observing the tracks over time will provide information on how the surface changes as dust is deposited and eroded. (AP Photo/NASA/JPL-Caltech/Univ. of Arizona) ( Mars Curiosity Photo Gallery )

FILE - This Aug. 18, 2012 image provided by NASA shows the Curiosity rover's landing site and Mount Sharp in the distance. (AP Photo/NASA) ( Mars Curiosity Photo Gallery )

FILE - This Wednesday, Sept. 19, 2012 photo provided by NASA shows a rock about 8 feet (2.5 meters) in front of the Curiosity rover on Mars. The rock is about 10 inches (25 centimeters) tall and 16 inches (40 centimeters) wide. The team has assessed it as a suitable target for the first use of Curiosity's contact instruments on a rock, and named it after the late Jacob Matijevic, who was the surface operations systems chief engineer for the Mars Science Laboratory Project and the project's Curiosity rover. (AP Photo/NASA/JPL-Caltech) ( Mars Curiosity Photo Gallery )

FILE - This image provided by NASA shows a color self-portrait of the Mars rover Curiosity. The rover used the Mars Hand Lens Imager (MAHLI) to capture dozens of high-resolution images to be combined into self-portrait images of the rover. (AP Photo/NASA/JPL-Caltech/MSSS, File) ( Mars Curiosity Photo Gallery )

This March 2013 image released by NASA shows the Curiosity rover holding a scoop of powdered rock on Mars before analyzing its chemical makeup. (AP Photo/NASA) ( Mars Curiosity Photo Gallery )

FILE - In this Thursday, Aug. 9, 2012 file photo, Bobak Ferdowsi, a flight director for the Mars Curiosity rover, known as the "Mohawk Guy," talks with colleagues at his workstation at NASA's JPL in Pasadena, Calif. (AP Photo/Damian Dovarganes) ( Mars Curiosity Photo Gallery )

FILE - In this Wednesday, June 5, 2013 file photo, people look at the "Mars Window," a projection of images taken by NASA's Mars Curiosity Rover at the Visions of the Universe exhibition at The National Maritime Museum in Greenwich, London. (AP Photo/Kirsty Wigglesworth) ( Mars Curiosity Photo Gallery )

FILE - This image provided by NASA shows a rock outcrop in Gale Crater on Mars. (AP Photo/NASA) ( Mars Curiosity Photo Gallery )

FILE - This composite image released by NASA shows a panoramic view of Mount Sharp, made from dozens of photos by NASA's Curiosity rover. (AP Photo/NASA) ( Mars Curiosity Photo Gallery )

FILE - This Dec. 4, 2012 image provided by NASA shows a shadow of NASAís Opportunity rover on the Martian surface. (AP Photo NASA) ( Mars Curiosity Photo Gallery )

WASHINGTON, DC - AUGUST 06: NASA Administrator Charles Bolden speaks about the Curiosity rover during an event at NASA headquarters, August 6, 2013 in Washington, DC. The event was held to observe the first anniversary of NASA's Curiosity rover landing on Mars. (Photo by Mark Wilson/Getty Images) ( 175697781 )

WASHINGTON, DC - AUGUST 06: People listen to NASA Administrator Charles Bolden speak about the Curiosity rover during an event at NASA headquarters, August 6, 2013 in Washington, DC. The event was held to observe the first anniversary of NASA's Curiosity rover landing on Mars. (Photo by Mark Wilson/Getty Images) ( 175697809 )

WASHINGTON, DC - AUGUST 06: NASA Administrator Charles Bolden speaks about the Curiosity rover during an event at NASA headquarters, August 6, 2013 in Washington, DC. The event was held to observe the first anniversary of NASA's Curiosity rover landing on Mars. (Photo by Mark Wilson/Getty Images) ( 175697782 )

WASHINGTON, DC - AUGUST 06: NASA Administrator Charles Bolden speaks about the Curiosity rover during an event at NASA headquarters, August 6, 2013 in Washington, DC. The event was held to observe the first anniversary of NASA's Curiosity rover landing on Mars. (Photo by Mark Wilson/Getty Images) ( 175697811 )