Can a hand-me-down be better than new? NASA hopes so, after being given two Hubble-quality telescopes by the US Department of Defense – boosting plans to study the mysterious dark energy that is accelerating the Universe’s expansion.

The telescopes were designed for spying down on the Earth, but the Pentagon decided they weren’t needed after all and in January 2011 offered them to NASA. The space agency has since been contemplating its windfall, and on 4 June unveiled a plan to use one of the telescopes for the Wide-Field Infrared Survey Telescope (WFIRST) mission. Its wide field of view is ideal for WFIRST’s task of surveying the sky for type-Ia supernovae – exploding stars that can reveal the influence of dark energy by how much their light has been stretched by the expansion of the Universe.

In 2010, WFIRST was named the top space telescope priority for the coming decade by the US National Research Council. But the spiralling costs of the James Webb Space Telescope had placed its future in doubt, and pushed the launch back to 2024 at the earliest.

NASA officials hope that date can now be pulled forward to 2020, but it’s not yet clear how much the windfall would reduce WFIRST’s costs, previously estimated at $1.6 billion. The repurposed telescope would be put into geosynchronous orbit, rather than the original plan of a more expensive launch to a stable gravitational point some 1.5 million kilometres from the Earth. But instruments to collect data would still need to be built.

So far, only a few researchers have examined the telescopes, currently in storage in Rochester, New York. But excitement is growing. “Just the existence of these assets makes your head spin, and makes you wonder what else you can do,” says Adam Riess of Johns Hopkins University in Baltimore, Maryland, who shared the 2011 physics Nobel for the discovery of dark energy.