After years of delays and cost overruns, NASA’s James Webb Space Telescope assembly is running smoothly and approaching some critical milestones. We last checked in on the telescopes overall progress last March, and the EU was confident enough of its progress to set a launch date last December. If things continue to go well, the JWST will fly in October, 2018.
NASA is expected to finish assembling the primary mirror surface on the JWST in the near-future, as well as the deep-freeze calibration tests on the telescope’s various instruments. The new telescope’s mirror is far larger than Hubble’s (21 feet in diameter versus 8 feet) and is assembled from 18 separate “panes” of beryllium.
The image below shows the wavelengths that the two telescopes can “see” in:
There’s some overlap between the JWST and the Hubble, but Hubble is primarily designed as a visible-light telescope, with some infrared capabilities. The JWST, in contrast, specializes in infrared, with some ability to observe in the visible spectrum.
The above shows how the JWST’s capabilities compare to common ground-based observatories, as well as various instruments aboard the Hubble. Hubble’s capabilities have evolved considerably over time, and it’s expected that the JWST will follow a similar trajectory. The ultimate goal for the James Webb telescope is to push the boundary of what we can observe back to the so-called “Dark Ages” of the universe. While light existed during this epoch, the universe wasn’t transparent yet. The dark ages mark the practical limit for what we can hope to observe with any telescope.
Why use space telescopes at all?
One question that’s popped up from time to time when we cover astronomy is why we bother building space-based telescopes at all. The Hubble Space Telescope fires the imagination and offers some incredible vistas, but its 8-foot mirror is dwarfed by ground-based hardware. When the European Extremely Large Telescope (E-ELT) achieves first light in 2024, its collecting area will be 978m2 compared with Hubble’s 4.5m2. Ground-based telescopes also have superior angular resolution to space-based counterparts. So why pay the enormous cost of building a space-based telescope in the first place?
First, because the pesky atmosphere gets in the way. The light that reaches us from other stars is attenuated and distorted as it passes through the atmosphere, changing the nature of what we observe. This is why we build telescopes at high altitudes — it’s literally easier to see the universe when we’ve got less atmosphere in the way.
Second, the atmosphere absorbs a great deal of useful spectrum, astronomically speaking. While you can do visible-light and radio astronomy from the ground, infrared, ultraviolet, and X-ray astronomy are varying degrees of impossible when on Earth.
There’s a third factor to consider, as well. Ground-based telescopes can resolve bright objects more accurately than Hubble or the JWST, but space-based telescopes can see dim objects that a ground-based facility could never detect. We use both types of facilities because they serve different missions and needs, and can conduct different types of research.
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