Serendipity has been known to catapult science to new heights, but few lucky accidents could compete with the windfall astrophysicists picked up from the biggest, most expensive blunder in the history of spy satellites.

After spending an estimated half billion dollars on two telescopes designed to spy on earthly activities, the federal government gave up and offered them for civilian use. Astronomers quickly realized that turning the scopes from spy craft into science craft could open up a new view of the universe.

Researchers at the University of Pennsylvania are among those eager to use the new scopes.

Physicist Bhuvnesh Jain and colleagues at Penn's Center for Particle Cosmology are studying "dark energy," the force mysteriously pulling the galaxies away from one another in an accelerating riptide.

They had been in line to use the planned space telescope known as WFIRST, but in the current economy, that project's prospects are grim, according to NASA. It looked as if astronomers would have to make do by using telescopes on the ground and partnering with Europeans on a space-based project.

"And then these telescopes pop out and they look amazing," Jain said. "If you were religiously inclined, you might think this was divine timing."

There is a catch, though. The telescopes are estimated to be worth $250 million apiece, but it will take about $1 billion to equip one of them with cameras and turn it into a scientific spacecraft to orbit the Earth.

"It's like you were saving up to buy a Honda Civic and someone gives you just the engine of a Rolls-Royce and says 'Happy birthday,' " said Jonathan McDowell, an astrophysicist from the Harvard-Smithsonian Center for Astrophysics.

When Princeton University astronomer David Spergel got a call from NASA with the news about the scopes, he said, "My jaw dropped." He had been in a funk about the lack of exciting projects on the horizon. There is the successor to the Hubble Space Telescope, called the James Webb Space Telescope, but astronomers are worried because it is way over budget and threatening to suck money from other projects.

NASA asked Spergel for advice on what to do with the surplus spy telescopes. He agreed with Penn's Jain - they should explore the dark energy.

After a conference in late August, NASA put together a "science definition" team of about 20 scientists, headed by Spergel and Neil Gehrels of NASA's Goddard Space Flight Center. Spergel said the team is charged with making a specific plan to use the telescopes for astrophysics.

Over the summer, he said, more details have been declassified about the telescopes, which are stored in a warehouse in Rochester, N.Y. "It really is a superb telescope in terms of optical quality," Spergel said.

One of the first questions they'd like to answer is whether the dark energy is constant in space and time, or whether it is getting stronger or perhaps decaying over the history of the universe. To find out, the scientists say they need to look farther out in space (and hence back in time) than they have ever done before.

The spy telescopes are perfect, the astronomers say, since they see as deep into space as the Hubble but with a much wider view, making them well-suited for taking in the sweep of the universe.

"What you'd see with 200 hours of time on the Hubble you'd see in an hour using one of these telescopes," Spergel said. "This opens up lots of new possibilities."

There have been about 14 big space telescopes looking down on Earth at different times, estimates Harvard-Smithsonian's McDowell. They were classified, he said, but you could see them orbiting with the unaided eye.

He said from what he can tell, the gift telescopes were part of some ambitious, fancier system. "There may have been something [attempted] that was beyond the state of the art," McDowell said, meaning it might not have been technologically feasible. From what astronomers can tell, however, there's nothing wrong with the optics of the telescope mirrors.

There is also the matter of what to do with the second surplus spy telescope. Spergel said it could be equipped to take pictures of large planets orbiting other stars. That mission could be a warm-up for one that would focus on planets the size of Earth and attempt to detect oxygen and other gases in their atmospheres.

Spergel imagines they might look for more planets orbiting the star Alpha Centauri, or nearby. Scientists recently detected an Earth-size planet orbiting that star at about Mercury's distance from our sun. But as they have learned, where there is one planet there are usually more, and what if they found an Earth-size planet orbiting at about the distance of Earth?

And maybe the most exciting things these telescopes will do won't occur to anyone until after the scopes have been launched. That was the case with the Hubble's famous "deep field" images, which showed a patch of space sparkling with galaxies shining from billions of years in the past, said Princeton's Spergel.

"The most important thing you do with these," he said, "may not be what you expected to do."

Contact Faye Flam at 267-443-3504 or fflam@alumni.caltech.edu, or follow on Twitter @fayeflam.