During its 25 years in orbit, the Hubble Space Telescope has taken thousands of pictures but the iconic, galaxy-filled Ultra Deep Field image stands out as a testament to the observatory's impact on astronomy -- and its limitations.
The original image was the result of two months worth of observations of a small patch of the celestial sky in the constellation Fornax. Astronomers chose a region relatively empty of bright stars, curious about what dimmer objects the telescope could resolve.
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An astonishing 10,000 galaxies turned up, challenging theories about galaxy formation in the universe's early days. The image was later augmented with an even deeper look, courtesy of Hubble's new visible light and infrared cameras, dialing back the youngest galaxies to less than 1 billion years after the Big Bang.
"The Hubble Ultra Deep Field has really transformed understanding of the history of the universe," astronomer Jennifer Lotz, with the Space Telescope Science Institute in Baltimore, said at a Hubble tribute at a recent American Astronomical Society meeting.
Beyond those galaxies, however, the picture goes dark, not because there is nothing else to see, but the light emitted from even more distant objects has shifted from visible and ultraviolet wavelengths into infrared regions beyond Hubble's capabilities.
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That job will fall to Hubble's successor, the James Webb Space Telescope, or JWST, which is due to launch in October 2018. What makes Webb different from other infrared telescopes, such as NASA's Spitzer and Europe's Herschel space observatories, is the size of its mirror, which gives it a greater surface area to collect photons of light.
Hubble's 7.9-foot diameter mirror was state-of-the-art when it was designed and built in the late 1970s. Spitzer's mirror is 2.75 feet across. Webb's is a whopping 21 feet. (For launch, the telescope's 18 gold-coated, hexagonal mirror segments will be folded up, origami-style.)
The ability to see the first light-emitting objects that emerged after the Big Bang is just one of hundreds of studies slated for Webb, which like Hubble is a general purpose observatory, designed to address a wide range of astronomical projects.