No more are planned. The remainder of the shuttle fleet that astronauts used to reach Hubble is scheduled to retire by the year's end. And in 2014, NASA plans to launch Hubble's successor, an infrared observatory called the James Webb Space Telescope, which will probe galaxies even further away and make more measurements of exoplanet atmospheres.

　　According to Grunsfeld, now STScI's deputy director, plans are afoot for a robotic mission to grab Hubble when it reaches the end of its useful life, nudging it into Earth's atmosphere where most of it would be incinerated. Only the mirror is sturdy enough to survive the fall into an empty patch of ocean.

　　But let's not get ahead of ourselves - Hubble is far from finished. The instruments installed in May 2009, including the Wide Field Camera 3, which took this image of the Butterfly nebula, 3800 light years away, have boosted its powers yet again. It might have as much as a decade of life left even without more servicing. "It really is only reaching its full stride now, after 20 years," says Grunsfeld.

　　A key priority for Hubble will be to explore the origin of dark energy by probing for it at earlier times in the universe's history. Hubble scientist Malcolm Niedner of NASA's Goddard Space Flight Center in Greenbelt, Maryland, is not willing to bet on what its most important discovery will be. "More than half of the most amazing textbook-changing science to emerge from this telescope occurred in areas we couldn't even have dreamed of," he says. "Expect the unexpected."