Nobody volunteers a date any more.

While driving his dusty dark blue Ford pickup truck through Flagstaff, some 40 miles southwest of the crater, Turrell wryly addressed whether his life's work would ever be done.

"I firmly said I was going to finish in the year 2000, and I hope to keep to that," he said with a flicker of a grin.

"It's like one of your friends who never finished their thesis. What can I say?

"A lot of art you just stop, you don't obsess over it. But I've had a bit of OCD, so I wanted to use it to my advantage," he added lightly.

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Clinical features of obsessive-compulsive disorder? "That would be my wife's contention, yes," he said, referring to his third wife, artist Kyung-Lim Lee.

But there is now a detailed plan in place so the crater could be finished without the artist if necessary.

This, along with completing the tunnel, is Turrell's biggest accomplishment on the project this decade, said Richard Andrews, who curated the upcoming gallery show and serves as the president of the Skystone Foundation, which supports work on the crater.

"In the last four years since the Great Recession, when fundraising has been so difficult, that's what we have focused on, completing all the design drawings," he said. "His plans are now all buildable and permit-able. You literally could build from them."

And Turrell himself, who moves more slowly than he used to but speaks just as quickly, sounded optimistic. He used to sign emails with a tag line, "Sooner or later … Roden Crater." Now, he has a new slogan: "Sooner than later, Roden Crater." His L.A. gallery is using it as the title of its upcoming show and printing the line on T-shirts for sale.

Turrell was discussing the history and future of the crater while driving to the airport hangar in Flagstaff where he keeps his own planes. He had eight planes there, all antique, which he repairs himself.

"There was a time when I restored antique planes to support my art habit. Now I find that my art habit doesn't quite support my flying habit as well," he says.

The habit dates to childhood. His dad was an aeronautical engineer who ran the technical program at Pasadena Junior College, now Pasadena City College. (His mother was a Quaker, and this blend of science and faith is much discussed in interpretations of his work.) He has early memories of flying with his father, who died when he was 10, and got his pilot's license at age 16.

He continued his hobby during college, as a perceptual psychology student at Pomona College. Later, registered as a conscientious objector, he flew Buddhist monks out of Chinese-controlled Tibet. Some writers have suggested it was a CIA mission; Turrell will say only that it was "a humanitarian mission" — and that he found "some beautiful places to fly."

In any event, he enjoyed flying as a way to escape the maze of streets (pilots call nonpilots "ground-pounders," he said) and see things from "plan view." The cockpit became a sort of surrogate studio — "I do some of my best observing in the plane."

From 1966 to 1974, his other studio was on Main Street in Santa Monica. He transformed the building into an installation in its own right by cutting holes in walls and window coverings to focus and let in light — like the beams from passing cars. He also made his earliest light projection pieces there, creating illusions of solid geometric shapes like cubes. But developers had other plans for the building, and he was sent an eviction notice in 1974. (There is now a Starbucks in its place.)

So when he received a Guggenheim grant that year, he spent months flying from the Pacific to the Rockies searching for an outdoor site where he could make even more ambitious statements with light.

He found the Roden Crater, named after an early owner, William Rhoden, and made an impromptu landing beside it in his single-engine, six-seater Helio Courier. ("It has a very short takeoff and landing. So you can land in the desert without an airstrip," Turrell said.)

He was drawn above all to its form. "It could have been a volcano or butte or raised mesa situation. I just knew I wanted this height and to make a bowl-like shape."