LOS ANGELES, July 24 -- NASA has begun fueling the Mars 2020 rover's Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which will power the rover and help keep it warm while exploring the Red Planet, according to a release of NASA's Jet Propulsion Laboratory (JPL) on Wednesday.

"The progression of the Mars 2020 rover project is on schedule," said NASA's Associate Administrator for the Science Mission Directorate Thomas Zurbuchen.

"The decision to begin fueling the MMRTG is another important milestone in keeping to our timetable for a July 2020 launch," he said.

Essentially a nuclear battery, an MMRTG can provide about 110 watts of electrical power to a spacecraft and its science instruments at the beginning of a mission, said the JPL.

The excess heat from the generator can also serve to keep spacecraft systems warm in cold environments.

In all, 27 past U.S. space missions have used radioisotope power -- from the Viking missions on Mars to the Voyager spacecraft entering interplanetary space to, most recently, the Curiosity rover on Mars and the New Horizons spacecraft that sailed past Pluto.

"We are advancing on all fronts -- including completion of the cruise stage that will guide us to Mars and the sky crane descent landing system that will gently lower us to the surface," said Project Manager John McNamee of the JPL, which manages the Mars 2020 mission. "The rover is not only looking more and more like a rover each day, it is acting like one."

With the exception of incorporating the Adaptive Caching Assembly, with its seven motors and more than 3,000 parts, work on the Mars 2020 rover's interior is 100 percent complete, said the JPL.

On the exterior, the most visible additions have been the remote sensing mast, the mobility suspension system, the main robotic arm and the rover's high-gain antenna.

The external components of the Mastcam-Z and SuperCam science instruments have been installed on the high perch of the remote sensing mast, and some key instruments' turret assemblies have been added onto the end of the robotic arm, said the JPL.

Mars 2020 rover is scheduled to launch from Cape Canaveral Air Force Station in Florida in July 2020, and land at Jezero Crater on Feb. 18, 2021.

It will be the first spacecraft in the history of planetary exploration with the ability to accurately retarget its point of touchdown during the landing sequence -- technology that could prove essential to future crewed missions to the Moon and Mars, said the JPL.