"We need to retain possession and control," NASA's astrophysics division director Paul Hertz told Discovery News. "That doesn't preclude us from partnering (with other countries). It just sets boundaries on the nature of the partnership." NASA also isn't allowed to use the telescopes for any Earth-observing missions. Topping the list of possible missions for the donor hardware is a remake of NASA's planned Wide-Field Infrared Survey Telescope, known as WFIRST. The mission, estimated to cost between $1.5 billion and $2 billion, is intended to answer questions about dark energy, a relatively recently discovered phenomenon that is believed to be speeding up the universe's rate of expansion."
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