Lens-Grinders for the Webb
|The temperatures in the planet-forming band around the star HD 113766A range between 805 and 195 K (990 and minus 110 F), with the hottest material closer to the star. For these temperatures, the dust bands spread over a distance comparable in our own solar system between the orbits of Mercury and Jupiter, or between 0.35 and 5.8 times Earth-distance from the Sun. Artist schematic of proto-planetary disk spiral around a star Credit: Pat Rawlings, NASA|
Balancing its position between the Sun and Earth’s gravity, a next generation of Space Observatory is beginning to come off the drawing boards and onto the lathes and lens grinders of machine shops. Scheduled for launch in 2010, the Webb Space Telescope was officially named today after James E. Webb, NASA’s second administrator. Particularly sensitive to the infrared range of the universe, the telescope is part of large-scale international collaborations which also may enable imaging some kinds of extrasolar planets.
The L2 Point
Webb will differ in key ways from how the Hubble Telescope works now. There will be no rendezvous, since like the current Chandra X-ray telescope, the Webb will reside far distant from the Earth. There will be a much simplified refrigeration system to enable ultra-cool visualizations and more control to target the hotter spots that aren’t currently viewable. Among those targets will be the burgeoning list of planet candidates.
When deployed, the Webb telescope will take about three months to reach its destination, an orbit 940,000 miles or 1.5 million kilometers in space, called the second Lagrange Point or L2, where the spacecraft is balanced between the gravity of the Sun and the Earth.
The most important advantage of this L2 orbit is that a single-sided sun shield on only one side of the observatory can protect Webb from the light and heat of both the Sun and Earth. As a result, the observatory can be cooled to very low temperatures without the use of complicated refrigeration equipment. These low temperatures are required to prevent the Webb’s own heat radiation from exceeding the brightness of the distant cool astronomical objects.
Before and during launch, the mirror will be folded up. Once the telescope is placed in its orbit, ground controllers will send a message telling the telescope to unfold its high-tech mirror petals.
Infrared Astronomy to Get Hot Views
To see into the depths of space, the James Webb Space Telescope is currently planned to carry instruments that are sensitive to the infrared wavelengths of the electromagnetic spectrum. The new telescope will carry a near-infrared camera, a multi-object spectrometer and a mid-infrared camera/spectrometer.
The James Webb Space Telescope will be able to look deeper into the universe than Hubble because of the increased light-collecting power of its larger mirror and the extraordinary sensitivity of its instruments to infrared light.
Hubble to Even Cooler Webb
Webb’s primary mirror will be at least 20 feet in diameter, providing much more light gathering capability than Hubble’s eight-foot primary mirror.
The telescope’s infrared capabilities are required to help astronomers understand how galaxies first emerged out of the darkness that followed the rapid expansion and cooling of the universe just a few hundred million years after the big bang. The light from the youngest galaxies is seen in the infrared due to the universe’s expansion.
Looking closer to home, the James Webb Space Telescope will probe the formation of planets in disks around young stars, and study supermassive black holes in other galaxies.
Proto-planet Discovery: Case Study for the Infrared Signature
Even with the current generation of 6-10 meter ground-based telescopes- and if using adaptive optics to adjust for viewing distortions– "direct detection of extrasolar giant planets may soon be possible," says Michael Meyer of the University of Arizona. But "the detection of Earth-mass planets around solar type stars is the goal of future space missions and is currently beyond the limits of observation."
For instance, pinpointing a relatively dark, Earth-like planet orbiting around a star likely to be millions to billions of times brighter remains one of the great challenges in modern astronomy across all wavelengths. But secondary signatures in the infrared dust glow that the Webb Space Telescope will examine, present some of the better opportunities for inferring the presence of planets.
Michael Meyer of the University of Arizona and his colleagues announced earlier this year, some of the best earth-based views of regions of infrared glow that are likely hatching planetary embryos. Their findings, reported at the 199th National Meeting of the American Astronomical Society in Washington, D.C., highlighted an area in the direction of the constellation Centaurus.
What the infrared astronomers found in the hot debris disk around the star classified as HD 113766A proved surprising (HD is the Henry Draper classification system). Only the first of the binary star (A) had a hot dust cloud, a remnant of planetary formation, and the radiation temperatures of the cloud were similar or slightly hotter than those expected for Earth-like planets. Looking for such infrared planetary dust around such star candidates "is by far the easiest way to identify these systems", according to Meyer.
Meyer concluded: "We have not yet been able to conduct the systematic survey of solar type stars needed to understand which systems are the exception rather than the rule." While a direct image of a giant planet around a distant star may soon be within reach of very large, ground-based telescopes, going to space remains the most promising way to image the much smaller Earth-like planets directly.
The Goddard Space Flight Center, Greenbelt, Md., manages the James Webb Space Telescope for the Office of Space Science at NASA Headquarters in Washington. The program has a number of industry, academic and government partners, as well as the European Space Agency and the Canadian Space Agency.
Collaborators on the study with Arizona’s Meyer included: Eric Mamajek, Philip Hinz, and William Hoffman of the University of Arizona (Tucson, AZ), Dana Backman and Victor Herrera of Franklin and Marshall College (Lancaster, PA), John Carpenter and Sebastian Wolf of Caltech (Pasadena, CA), and Joseph Hora of the Harvard/Smithsonian Center for Astrophysics (Cambridge, MA).