Underpinning tributes this week marking the 25th anniversary of the Hubble Space Telescope's launch is the knowledge that the orbiting observatory has years of cutting edge science projects ahead.
The telescope blasted off aboard the space shuttle Discovery on April 24, 1990, fulfilling a dream dating back to the 1940s to put an astronomical observatory above the blurring and radiation-blocking effects of Earth's atmosphere.
Initially, it appeared that Hubble's mission was over before it began. Shortly after reaching orbit, engineers discovered the telescope's 94-inch primary mirror had a manufacturing flaw that blurred its vision.
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In 1993, NASA launched an unprecedented in-space servicing mission to outfit Hubble with corrective optics. Soon after, Hubble astounded astronomers by catching images of a comet plowing into Jupiter.
Over the next 15 years, astronauts made four more house calls to Hubble, installing new cameras and instruments, upgrading electronics, replacing computers. The repair and servicing missions led to a scientific bounty that has far exceeded Hubble's original goals: measuring how fast the universe is expanding; figuring out how galaxies evolve; and studying the gas that lies between galaxies, astrophysicist Mario Livio, with the Space Telescope Science Institute, noted in an essay in Nature.
Many of Hubble's discoveries are in fields that didn't even exist when the telescope was launched, such as the study of planets beyond the solar system and the detection of a still-unexplained anti-gravity force known as "dark energy" that is speeding up the expansion of the universe.
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NASA aims to keep Hubble operating through at least 2020 so that it can overlap with its successor. The James Webb Space Telescope is due to launch in October 2018 and begin observations in mid-2019.
"Hubble is just doing beautifully. It's as powerful as ever," said Kenneth Sembach, who oversees the Hubble Mission office at the Space Telescope Science Institute in Baltimore.
The institute is reviewing scientists' proposals for telescope time and mulling if some projects merit special attention as Hubble nears its end. Typically, the program receives about five requests for every hour of available telescope time.
"There's clearly there's no lack of things to do with this observatory in its remaining years. The question is what do we do?" Sembach said at a recent American Astronomical Society meeting in Seattle.
One project already underway will continue. Hubble is taking advantage of naturally occurring lenses in space to peer deeper back in time. These so-called "gravitational lenses" are regions where the gravity of massive galaxy clusters distorts the path of light coming from objects beyond them.
In addition to finding out more about the universe's earliest and faintest galaxies, the study is helping astrophysicists map dark matter, which like dark energy, cannot be directly detected. Dark matter, however, leaves its gravitational footprint on nearby, visible matter and is a key component of the shape and power of the gravitational lenses the Hubble team is using.