Soviet scientists are developing an atomic rocket to use in a manned voyage to Mars in about 20 years, a Russian space expert says.

Vasiliy I. Moroz, a scientist in the Soviet Space Research Institute, said an atomic-electric rocket engine will be used to accelerate an explorer craft on a 1 1/2-year mission to Mars.Moroz said the nuclear-powered rocket would be used to accelerate a manned craft toward Mars after the craft had been boosted into Earth orbit by a chemical rocket.

Speaking Thursday at a news conference at the 20th assembly of the International Astronomical Union, Moroz declined to give any details, but he said a Mars mission will require a new type of propulsion.

The current Soviet fleet includes the world's most powerful rocket, the Energia, but Moroz said the chemical rocket could not be used to accelerate a manned craft to Mars.

American space scientists long have recognized the need to develop propulsion systems that did not depend upon the chemical reaction of fuel and oxidizer. NASA has conducted some research on electric and ion drive rockets, but the work never has received development funds.