Neither moon will be in its current orbit forever. Deimos, which whips around Mars every 30 hours or so, is speeding up, while Phobos is slowing down in its eight-hour orbit. Scientists think Mars' gravity will probably destroy Phobos, perhaps in the next 10 to 15 million years or so.
"It will work its way in at some point and get so close that tidal forces from Mars will very likely break it up before it does start grazing the atmosphere and come down," Lemmon said. "So Mars may briefly have a ring system."
The Curiosity team has been doing more than just skywatching since landing the $2.5 billion robot inside Mars' huge Gale Crater on Aug. 5. Researchers have thoroughly vetted Curiosity and its 10 science instruments, which are designed to help the rover determine if the Gale area could ever have supported microbial life.
Curiosity has also hit the road recently, traveling a total of about 950 feet (290 meters) from its landing site so far, researchers said today. The rover now sits about 660 feet (200 m) from its first major science destination, a site called Glenelg where three different types of Martian terrain come together.
But Curiosity will spend the next several days more or less stationary, gearing up to perform its first contact science operations on a pyramidal rock that mission scientists have named "Jake Matijevic," after a rover team member who died shortly after Curiosity landed.
The rover will investigate the 16-inch-high (25 centimeters) rock with its Alpha Particle X-Ray Spectrometer, which measures elemental composition, and its Mars Hand Lens Imager close-up camera. Both APXS and MAHLI sit at the end of Curiosity's 7-foot-long (2.1 m) robotic arm.
Curiosity will also zap "Jake Matijevic" with the laser on its ChemCam instrument, which reads rock composition from the vaporized bits, scientists said.