Engineers installed the plutonium power source on NASA's Curiosity rover Thursday, adding the final piece to the complex robot before its Nov. 25 blastoff to Mars.

The Mars Science Laboratory's MMRTG power source. Credit: NASA/KSC

The Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG, moved from a preparation building to the Atlas 5 rocket's Vertical Integration Facility early Thursday.

The 99-pound device was lifted inside the building and inserted through an access door on the rocket's bulbous white payload fairing. Technicians placed the power source on Curiosity through an opening on the spacecraft's backshell, which encloses the rover and its landing system during the journey from Earth to Mars.

It's a simple connection, according to David Gruel, manager of the mission's assembly, test and launch operations phase.

"We have four bolts holding it on and we hook up the electrical connections," Gruel said.

Once the MMRTG is all hooked up, the spacecraft will begin receiving its own power. The final power-up is scheduled for Friday morning, and it won't be turned off again.

"It's really cool that when the vehicle is powered on [Friday] with the RTG that it's basically going to stay powered all the way through the mission," Gruel said.

The attachment of the power source is one of the last steps to prepare the $2.5 billion Mars Science Laboratory mission for launch. The 197-foot-tall Atlas 5 rocket will roll to the launch pad Nov. 23 in advance of liftoff Nov. 25 at 10:25 a.m. EST (1525 GMT).

Curiosity will streak into the Martian atmosphere, descend under parachute and touch down with an unflown "sky crane" architecture using a rocket pack and bridle to gently lower the rover to the surface. Landing at Gale crater is scheduled for August 2012.

The mission will seek evidence whether Mars was once habitable for life.

The MMRTG is the final piece to go on the spacecraft because of the hazards with the radioactive power system.

But the unit is built with layers of protective shielding to contain the MMRTG's plutonium fuel through impacts, explosions and an unplanned re-entry back into the atmosphere.

There's no threat of an atomic detonation from the Pu-238 aboard the Curiosity rover.

A glowing pellet of plutonium-238. Credit: U.S. Department of Energy

Steve Brisbin, Kennedy Space Center's associate director for center operations, said there is about a 1-in-400 chance of radioactive material being released in a launch accident.

Even if plutonium does leak into the atmosphere, the health risk is low.

"There would not be expected to be any health effect from a mishap from the launch of the Mars Science Laboratory," Brisbin said, adding someone exposed to released plutonium during launch would only see a small fraction of the annual natural dose of radiation from solar and cosmic rays.

The Mars mission's plutonium generator consists of a nuclear battery that converts decay heat into electricity. It contains 10.6 pounds of radioactive plutonium-238 and solid-state thermocouples that convert the plutonium's heat energy into electricity, according to NASA.

The MMRTG is provided by the Energy Department, which is responsible for procuring, storying and supplying nuclear material to users like NASA. Hamilton Sundstrand Corp. built the device.

Nuclear generators have powered 26 U.S. space missions over the last five decades, enabling exploration of the sun, the moon, Mars, Jupiter, Saturn, Uranus and Neptune. NASA's New Horizons probe, another nuclear-powered mission, is traveling toward Pluto for an encounter in 2015.

The Curiosity rover's MMRTG is a new generation of power source designed for use on planets with atmospheres like Mars, as well as in the vacuum of space. It also operates at power intervals of 110 watts, a smaller increment than previous RTGs.

It measures about 25 inches in diameter and 26 inches long, according to a NASA fact sheet.

Artist's concept of the Curiosity rover at Mars. Credit: NASA/JPL

The Mars rover's generator will produce power over at least 14 years. The rover's baseline mission is expected to last approximately two years on the surface of Mars.

The MMRTG's Pu-238 fuel is a non-weapons-grade isotope of plutonium, but U.S. stockpiles of the material are dwindling, alarming scientists planning missions to the outer solar system. There's currently not enough Pu-238 in the Energy Department's stores to supply a flagship probe to study Jupiter's moon Europa in the 2020s.

The Energy Department does not disclose how much Pu-238 is left in its inventory, but a National Research Council decadal survey report issued earlier this year estimated 16.8 kilograms, or about 37 pounds, of the isotope were left over after the Mars Science Laboratory.

NASA projected in 2010 it needs more than double that amount of Pu-238 through 2027, necessitating purchases from Russia and renewed production in the United States.

Russia, which also no longer produces Pu-238, is demanding a more lucrative contract for the U.S. government to buy its plutonium.

Efforts to restart Pu-238 production in the United States have met resistance in Congress. It would cost between $75 million and $90 million to reach full capacity in five or six years, according to the Energy Department.

The Energy Department is sponsoring development of a more efficient nuclear generator that could fly in space as soon as 2016.

The Advanced Stirling Radioisotope Generators require less Pu-238 to produce electricity, but they are more complicated and contain tiny pistons that could be prone to failure. Traditional RTGs have no moving parts, increasing their reliability.