On April 24, 1990, the Hubble Space Telescope rode a space shuttle into low Earth orbit to become the most productive observatory in history. A quarter-century on, the universe may be the same but our understanding of it is not, forever transformed by the pristine ultraviolet, visible and near-infrared vistas revealed by the Hubble’s 2.4-meter mirror high above Earth’s atmosphere. Peering across the cosmos, Hubble mapped dark matter and helped discover dark energy, the mysterious force driving our universe’s accelerating expansion. Closer to home, it snapped pictures of giant exoplanets orbiting other stars, found new moons around Pluto and spied watery plumes bursting from the subsurface ocean of Jupiter's moon Europa. Almost everywhere it looked, Hubble made major discoveries. It became NASA’s premiere “flagship” observatory, and the agency supported it by replacing and upgrading obsolete components during five space shuttle servicing missions. But Hubble’s time is running out; the space shuttles are no longer flying, and no more servicing missions are planned. Sooner or later Hubble’s crucial components will degrade and fail. Eventually its orbit will decay, turning the multibillion-dollar telescope into tumbling chunks of slag that burn in the atmosphere and splash into the ocean. Astronomers, ever hopeful, plan to continue using Hubble for several years to come, potentially into the 2020s, but know all too well that its days are numbered. “When Hubble goes, it goes,” says John Mather, a Nobel laureate astrophysicist at NASA Goddard Space Flight Center. “And we don’t have anything else on the books that does what it does.” In particular, Hubble’s ultraviolet observations are crucial because Earth’s atmosphere filters out most of those wavelengths—they are accessible only from space. Mather is also the senior project scientist for the project often billed as Hubble’s successor—NASA’s next flagship observatory, the James Webb Space Telescope. Once launched, Webb will soar some 1.5 million kilometers away from Earth to unfold a giant 6.5-meter segmented mirror. Operating at cryogenic temperatures and using a wealth of newly developed technology, the telescope will gaze out at the universe with exquisitely sensitive infrared eyes designed to study the very first galaxies and stars. That large size and technological complexity have come at a steep price, however. Originally targeted for a 2011 launch and an estimated cost of $1.6 billion, Webb’s price tag has swelled to nearly $9 billion and its launch is slated for no earlier than October 2018. In 2011 the observatory narrowly escaped cancellation when frustrated members of Congress threatened to terminate its funding. Partially in response to those troubles, NASA’s post-Webb plans are to launch a far less ambitious observatory in the 2020s, a repurposed infrared spy satellite called WFIRST. Although Hubble-size, WFIRST will have a field of view a hundred times greater than Hubble’s, and will survey nearly the entire sky to study dark energy. Read More: Scientific American
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