CAPE CANAVERAL, Fla. NASA is looking for new ideas on what to do with two space telescopes left over from a once-secret U.S. spy satellite program.

The U.S. space agency asked the scientific community on Tuesday for its input into possible missions for a pair of space telescopes donated last year to NASA by the National Reconnaissance Office, which operates the nation's spy satellites.

"NRO offered us their leftover hardware if we want it. They've been totally open in allowing us to study whether this hardware would be of advantage to NASA," said Paul Hertz, who oversees NASA's astrophysics programs.

Topping the list of existing proposals is to use one telescope for a mission to learn more about an anti-gravity force known as "dark energy," which is believed to be responsible for speeding up the universe's rate of expansion.

The phenomenon was discovered in the 1990s by two teams of researchers who shared the 2011 Nobel Prize in Physics for their work.

The National Academy of Sciences has made that mission, known as the Wide-Field Infrared Survey Telescope, its top choice for an astrophysics space mission for the next decade.

NASA estimates the WFIRST mission would cost $1.5 billion to $2 billion, but it cannot begin a major new astrophysics project until spending winds down on the over-budget and delayed James Webb Space Telescope, which is a successor to the Hubble Space Telescope and is scheduled for launch in 2018.

The NRO telescopes, which were built to peer down at Earth, each have a primary mirror that is 7.9 feet in diameter, much larger than the 4.3-foot (1.3-meter) observatory originally proposed for the WFIRST mission.

While a larger telescope may allow for more detailed observations, it could be more expensive to outfit with instruments and launch into space.

"There's a whole lot of ways that a larger telescope might benefit you, even if it doesn't save you money," Hertz said.

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NASA ponders missions for old spy telescopes