Bob Strom had begun to lose hope.
A veteran of NASA's Mariner 10 mission to Mercury in the 1970s, he was bursting with questions that the Mariner flybys had raised about the little planet but couldn't answer.
"I've been hoping for another Mercury mission for 30 years, practically," said Strom, an expert on impact craters. But for decades, NASA seemed unable to make it happen.
"I really thought ... I'd never live to see Mercury again," he said.
But he did.
This week, NASA's Messenger spacecraft whizzed past Mercury and sent back more than 1,200 photos and measurements from the sun's nearest neighbor, and Strom was in the thick of it.
At 74, he is the only member of the old Mariner 10 team serving on the Messenger science team. He has been holed up in the mission's Science Operations Center, at the Johns Hopkins University's Applied Physics Lab near Laurel, marveling over the new data from Mercury.
It's no wonder. He's was captivated half a lifetime ago by Mercury, its oddities and mysteries. And he has written two books on the subject while waiting for a new mission to go back there.
"He's jumping for joy," said Sean Solomon of the Carnegie Institution of Washington, principal investigator on the project.
"It's superlative after superlative. With each image, he sees terrain he spent decades wondering if he'd ever get a chance to see. There are grad students 50 years his junior, working side by side to compare interpretations.
"Today, he's like a kid," Solomon said.
Strom, an emeritus professor of planetary science at the University of Arizona at Tucson, is supposed to be retired, but he has so much more to do.
He plans to be back when Messenger flies by Mercury again in October, and again in September 2009 as it uses the planet's gravity as a brake. By the time it finally settles into orbit around the planet in 2011, he'll be 77. And when the primary mission ends, he'll be 78.
"I know I'm getting old," he said. But it doesn't seem to matter. Not this week. "I'm working harder than I did when I was fully employed. But it's fun."
He can hardly believe his good fortune.
"It's so great," he said. "I got very emotional yesterday when I first saw the side [of Mercury] we haven't seen. My God, I waited so many years! So I had a kind of [Sir Edmund] Hillary moment. It was just overwhelming, emotionally, to see this."
The images and data coming from Mercury were scientifically overwhelming as well.
"We got our first look at the high-resolution pictures today, and I was shocked. The quality was unbelievable. ... So much better than Mariner 10," he said. "It's like a new planet. We ... are going to have to go back and look at the entire planet all over again."
"We're gonna make discoveries, that's for sure. We've already made a few," he said.
Strom could not say enough about the Messenger mission's design, execution or the team that's making it happen.
"This is a world-class mission for a Discovery price," he said, a reference to NASA's class of relatively low-cost, tightly focused planetary science missions. Messenger will cost $446 million through its first year in orbit. A similar mission proposed in 1987 by NASA's Jet Propulsion Lab in California was priced - then - at $1.2 billion. It was shelved.
Strom was 40 when he served on the Mariner 10 television science team. The spacecraft sent back television pictures as it made a series of three fast flybys in 1974 and 1975.
The video camera was "fine for the time," he said, but "so inferior compared to the CCD that's on this mission." The CCD is the "charge-coupled device," or solid-state light sensor used in digital cameras.
"I expected to see a difference, but not as much as I've seen today," he said. "It's astounding."
Among the many surprises he has seen in the Messenger data was Caloris Basin, a huge impact basin 1,300 kilometers across. Only half of it was visible to Mariner.
"It has a very peculiar floor structure. We knew that from Mariner 10," Strom said. But the Messenger images reveal that craters inside the bright interior of the basin have dark halos, suggesting that the impacts have excavated a different material from below the surface.
"We'll get to find out what that is when we get into orbit," he said. "It's strange."
The images also show smooth, dark plains all around the basin. "We've never seen anything like that on the moon or any other planet," Strom said. "If they're volcanic, then that impact [that formed the basin] really had an effect on the planet."
He has also spotted craters surrounded not by material tossed aside by the impact, but "big ruts that radiate out from the crater, which is really bizarre. I've not seen that in any other crater in the solar system. It's completely new," he said.
Strom has also been looking at images showing thrust faults - seams in the crust where it appears that one crust plate has ridden over another, perhaps as the planet cooled and shrank.
Some of the faults were seen by Mariner 10, and if the shrinking planet theory is correct, scientists expected that Messenger would reveal more elsewhere on the planet.
"I always worried about that," Strom confessed. If the faults weren't found, they'd need another theory. But the faults seem to be there. "We feel vindicated."
So what is it about Mercury that has so possessed Strom for the past 35 years and spurred him to write two books about it?
"Mercury is a planet of superlatives," he said. It is the densest of the rocky inner planets, with the largest iron core for its size and a magnetic field like Earth's. Scientists can't say for sure yet how that all came about during the formation of the solar system more than 4 billion years ago.
"There are three different hypotheses, and they all have implications for the origins and evolution of the terrestrial planets," including Earth, he said.
Each of those hypotheses predicts a different elemental and mineralogical composition for the surface. But Mariner 10 wasn't equipped to study that composition. Messenger is.
"There are a ton of experiments on there, so I think we can decide among the three composition hypotheses, and then we can start looking at the origins and evolution of the terrestrial planets again," Strom said.
All the excitement this week has cost him some sleep, he says. But he doesn't regret a minute of it.
"When I first asked to be a member of the team in 1997, I saw the [mission] proposal and said, 'Oh, my God.'" It was an ambitious mission, one he felt APL's crew could never manage for the price tag they gave NASA.
But "by God, they did," he said. "I'm elated."