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As currently conceived, the Next Generation Space Telescope will have a reflecting mirror 6 meters in diameter,with almost three times the diameter and seven times the collecting area of the mirror of the Hubble Space Telescope (HST). Equipped with detectors far more sensitive than those on the HST, the NGST will surpass by 10 times the HST's ability to detect radiation from faint sources. But even more important, the NGST will maintain itself at a temperature much colder than the HST's-only 50 degrees above absolute zero. By doing so,the NGST will observe the cosmos throughout large portions of the infrared spectral domain at sensitivities thousands of times that of any previous telescope. Redshift (the stretching of light by the expansion of the universe) moves the wavelengths of stars at a distance of 10 billion light-years from the visible into the high-sensitivity range of NGST. Thus NGST's capability to make super-sensitive infrared observations will enable it to see galaxies as they were 10 billion years ago in the first moments of their creation.
The NGST will also observe stars and planets as they form in the Milky Way-closer to home by a factor of 100 million. These protostars and protoplanets emit large amounts of infrared radiation but essentially no visible light. The NGST's ability to make high-resolution infrared observations of protostars and protoplanets will allow us to address a number of questions. How does matter orbit a star as it coalesces? How does it evolve into a disk and create planets? How does it disperse once the planet-formation process has ended? By combining observations of the early universe with those of the births of stars and galaxies, NGST will be the premier tool in the quest to understand our origins. The assembly of a space-borne telescope with a mirror 6 meters (about 20 feet) in diameter poses challenges worthy of today's best astronomers and engineers. No rocket is available to launch a mirror of this size in a single piece. Instead, NGST must use a lightweight, segmented mirror that can be folded up into a compact package and that can deploy itself automatically once the package has been launched into space to its final destination. Because this mirror must maintain its surface to a perfection measured in millionths of an inch, its design, construction, and deployment all pose severe tests of skills in many different areas. Telescopes for infrared observations have traditionally suffered from the deleterious effects of their own infrared radiation, which they produce simply because they are warm, and of the infrared radiation from Earth. Both sources of infrared radiation seriously interfere with our attempts to detect infrared radiation from the cosmos. Orbiting the Sun a million miles from Earth, the NGST will carry sunshields to allow its instruments to cool to temperatures hundreds of degrees lower than those of the HST. The HST, by contrast, is in a low orbit only a few hundred miles up, relatively close to Earth and its infrared emissions. The NGST's sunshields will dramatically reduce the telescope's temperature and thus its own infrared radiation. That and its distance from Earth's infrared glare will enable extraordinary sensitivity for astronomical observations. Plans are proceeding for the European Space Agency and the Canadian Space Agency to make substantial contributions to the development, construction, and operation of the NGST, currently scheduled for launch in 2009.
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