I’ve recently found an interesting article over at MIT’s Technology Review about the James Webb Space Telescope and it inspired me to read up on the differences between the planned next generation telescope and the venerable Hubble.
The image above shows first the Hubble Ultra Deep Field (“It is the deepest image of the universe ever taken in visible light, looking back in time more than 13 billion years. The HUDF contains an estimated 10,000 galaxies.” — See here for a high-resolution picture) and in second a simulation of the performance of the James Webb Space Telescope. There’s a clear difference even to the untrained eye.
Hubble Space Telescope
First lets look at the Hubble Space Telescope.
It was launched April 24, 1990, and NASA celebrated its 17th birthday recently with the following stats: “800,000 observations, 500,000 images and 100,000 trips around the Earth.” The main benefits of putting a telescope in orbit are that it’s images are not distorted by the Earth’s atmosphere, it is not affected by light pollution and it can observe frequencies of light that cannot be observed well on the Earth’s surface (f.ex. Ultra-violet light, because it is absorbed by the ozone layer).
Credit: NASA and the European Space Agency, Public Domain image.
Hubble’s mirror has a collecting area of 4.2 square meters (46 square feet) and a diameter of 2.4 meters (94 inches). It can observe on the following wavelength: Optical, ultraviolet, near-infrared.
Improvement in Hubble images after the first service mission. Credit: NASA/ESA.
The Hubble Space Telescope was designed to be regularly serviced. The first service mission December 1993 allowed the Space Shuttle Endeavour crew to upgrade Hubble as well as boost its orbit. As you can see from the picture above, the upgrade was worth it: The result was much sharper images.
There was at least 3 other servicing missions (see here) but they had less dramatic results than the first one.
The impact of the Hubble Space Telescope on astronomy, physics and science in general is immense: Over 4,000 scientific papers based on Hubble data were published in peer reviewed journals.
Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required whole new theories to explain them. Among its primary mission targets was to measure […] the rate at which the universe is expanding, which is also related to its age. Before the launch of Hubble, estimates of the Hubble constant typically had errors of up to 50% […]
While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. [It] uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating. […]
The high-resolution spectra and images provided by the Hubble have been especially well suited to establishing the prevalence of black holes in the nuclei of nearby galaxies. […]
Other major discoveries made using Hubble data include proto-planetary disks (proplyds) in the Orion Nebula; evidence for the presence of extrasolar planets around sun-like stars; and the optical counterparts of the still-mysterious gamma-ray bursts.
One interesting fact about Hubble: “Anyone can apply for time on the telescope; there are no restrictions on nationality or academic affiliation.” But of course, competition for time on the space telescope is extremely intense.
James Webb Space Telescope
The first thing that jumps at you is how different the design is. For lack of a better technical vocabulary, I’ll say that it is a very “open” space telescope. As you can see on the pictures below, a large part of the space telescope is dedicated to a solar shield. On top of it sits the telescope, and below are solar panels. The telescope will “turn its back” to the sun.
Images credit: NASA.
The James Webb Space Telescope’s mirror will have a collecting area of 25 square meters (269 square feet) and a diameter of about 6.5 meters (about 256 inches). It will observe on the infrared wavelength.
The mirror for the new telescope is made out of beryllium, one of the lightest known metals. The material has exceptional thermal properties that allow its optical performance to remain stable at a wide range of temperatures. It is also thermally conductive: that is, it conducts heat across the whole mirror, eliminating temperature gradients.
So the James Webb telescope will have about 5.8 times more mirror surface area than Hubble, and it will be able to observe on frequencies that Hubble can’t (though, on the other hand, it apparently won’t be able to operate on Hubble’s wavelengths).
The [James Webb Space Telescope]’s primary scientific mission has four main components: to search for light from the first stars and galaxies which formed in the Universe after the Big Bang; to study the formation and evolution of galaxies; to understand the formation of stars and planetary systems; and to study planetary systems and the origins of life.
Due to a combination of redshift, dust obscuration, and the intrinsically low temperatures of many of the sources to be studied, the JWST must operate at infrared wavelengths, spanning the wavelength range from 0.6 to 28 micrometres. In order to ensure that the observations are not hampered by infrared emission from the telescope and instruments themselves, the entire observatory must be cold, well-shielded from the Sun so that it can radiatively cool to roughly 50 kelvin (−220 °C, −370 °F). To this end, JWST will incorporate a large metalized fanfold sunshield, which will unfurl to block infrared radiation from the Sun, as well as from the Earth and Moon.
Unlike Hubble, the James Webb Space Telescope won’t be on an orbit that takes it around the Earth quickly (for Hubble, that a complete orbit takes 96-97 minutes). It will remain stationary with respect to the earth and the sun at about 1.5 million kilometers from the Earth (in the L2 Lagrangian point).
The telescope’s location at the Sun-Earth L2 Lagrange point ensures that the Earth and Sun occupy roughly the same relative position in the telescope’s view, and thus make the operation of this shield possible. Credit: NASA.
Mission length for the James Webb Telescope if 5 years, with possible extensions. In fact, it is designed with these extensions in mind.
The James Webb Space Telescope program is currently in the preliminary design phase (Phase B).
In January 2007 nine of the ten technology development items in the program successfully passed a non-advocate review. These technologies were deemed sufficiently mature to retire significant risks in the program. The remaining technology development item (the MIRI cryocooler) completed its technology maturation milestone in April 2007. This technology review represented the beginning step in the process that will ultimately move the program into its detailed design phase (Phase C).
Full-scale model of the James Webb Space Telescope with some of the people who have been working on it at NASA’s Goddard Space Flight Center in Maryland. Credit: Goddard Space Flight Center/NASA.
I can hardly wait until 2013 to see what marvels of our universe this new tool will be able to uncover.
If you are looking for more, you can read my other Science & Technology posts.