The NASA/ESA JWST, named after NASA's second administrator, is the successor to the Hubble Space Telescope (HST). It is scheduled for launch in 2013 onboard an Ariane 5, and, like Herschel, will take up position at the Second Lagrange Point (L2). Unlike HST its great distance from the Earth will mean that it will not be serviceable by shuttle missions, and hence its instrument complement will remain fixed after launch. The mission is planned to last for at least 5 years, extendable to 10 years.
The designed primary mirror size for JWST is 6.5 m in diameter, and so this mirror will have to be folded and stowed before and during launch. The mirror
"petals" will only be commanded to open once the spacecraft reaches the L2 position. The primary mirror will probably comprise three to eight hinged segments that are individually monolithic or segmented in the manner of the Keck telescopes (Section 7.4.2). The whole telescope will be shielded from sunlight by a large sunshade (currently planned to be 22 x 10 m) composed of many layers of lightweight reflecting material, sufficient to sustain a 300 K temperature drop from front to back (Figure 8.6). With a back sunshade temperature of ^90 K, the primary mirror, the optical truss, and the instrument payload can radiate their heat to space and reach cryogenic temperatures of 30 K to 50 K. Combined with the L2 orbit, the large sunshade provides a stable, cold environment with a minimum of background radiation. The mass of the telescope at launch is currently estimated to be ^5,400 kg.
The 6.5 m diameter mirror will provide a diffraction-limited angular resolution of 0.2" at 5 ^m, and the telescope is planned to provide observations between 0.6 ^m and 28 ^m. It is planned that the JWST will carry a near-IR camera, a multi-object spectrometer, and a mid-IR camera/spectrometer, all mounted within an Integrated Science Instrument Module (ISIM), which will include a thermal system capable of passively cooling the near-IR detectors to ~27 K. JWST should help to establish how galaxies first emerged out of the darkness that initially followed the Big Bang since the light from the youngest galaxies is seen in the infrared—not the visible—due to the universe's expansion. JWST will also investigate the formation of planets in disks around young stars, study supermassive black holes in other galaxies, and should also provide high-quality measurements of the giant planets. The current designs for JWST's main instruments will now be reviewed.
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