WASHINGTON -- The only scientist to explore the lunar surface believes the best way to get humans to Mars is to first send them back to the moon.

That's where people will need to go to live and work for long stretches in preparation for an eventual human landing on Mars, said former Apollo 17-astronaut Harrison "Jack" Schmitt.

Only 12 humans have landed on the moon on six Apollo missions from 1969 to 1972.

On the last of those missions, Schmitt, a geologist, spent three days at the Taurus-Littrow region in December 1972 with fellow moonwalker Eugene Cernan while their crew mate Ronald Evans circled overhead in the command module. Schmitt later served as a U.S. senator from New Mexico.

Schmitt, Apollo 17 lunar module pilot, uses an adjustable sampling scoop to retrieve lunar samples during Apollo 17.

"But I don't think we'll go there until we go back to the moon and develop a technology base for living and working and transporting ourselves through space," he said in a telephone interview. "That's a much faster way to get to Mars than just to launch a single purpose, "let's-get-to-Mars" mission, which I don't think can be done politically."

Schmitt plans to speak on the moon's origin and evolution on April 25 at the Smithsonian's National Air and Space Museum in Washington. He is this month's featured speaker in the Exploring Space Lecture Series, which is sponsored by SPACE.com.

Getting back to the moon "occupies half my time today," Schmitt said. He heads a private company, Inter-Lune Inter-Mars Initiative, that hopes to mount commercial space missions to further explore the moon.

NASA has not ruled out sending humans back to the moon, said Kirsten Williams, a NASA spokeswoman.

Schmitt with his adjustable sampling scoop, heads for a selected rock on the lunar surface to retrieve the sample for study.

"It's one possible location for longer-duration missions," she said. "There's a possibility we would return to the moon as a precursor mission if both the moon and Mars are selected as places we want to go for future exploration. But nothing's been decided."

Schmitt's biggest regret is that he couldn't have stayed on the moon longer.

"Another day would have been nice," he said. "It's a fascinating place…but it's the sort of experience that's not readily transferable. It's like trying to describe what you feel when you're standing on the rim of the Grand Canyon or remembering your first love or the birth of your child. You have to be there to really know what it's like."