SAN FRANCISCO — When NASA's next Mars rover touches down in 2021, its six-wheeled sister, Curiosity, may still be chugging around the Red Planet.

NASA announced Tuesday (Dec. 4) that it plans to launch an unmanned rover toward the Red Planet in 2020, to help pave the way for an eventual Martian sample-return mission. The new rover will be based heavily on Curiosity, whose Aug. 5 landing kicked off a mission to determine if the Red Planet could ever have supported microbial life.

Curiosity's mission was originally planned to last two years. It has now been extended indefinitely.

"We've already decided with this plan that we will continue to operate Curiosity as long as it's scientifically viable," John Grunsfled, NASA's associate administrator for science, said here Tuesday at the annual fall meeting of the American Geophysical Union. "And that could be a long time."

Just how long Curiosity could keep roving is an open question. The $2.5 billion robot is powered by a radioisotope thermoelectric generator (RTG), which should be able to continue converting the heat of plutonium-238's radioactive decay into electricity for a long time to come.

"I never get a straight answer on this, but I think it has 55 years of positive power margin," Grunsfeld said.