The James Webb Space Telescope

FAS Astronomers Blog, Volume 30, Number 10.

Those of us in the Forsyth Astronomical Society spend a good deal of time (too much time?) staring at the night sky through our telescopes. There are many things to look at and I’ve summarized them in several previous articles.

• The Night Sky
• Observing the Moon and Planets
• Observing the Stars
• The Deep Sky

However, our view of the heavens pales compared to the view from the many professional telescopes found around the world. Even then, there is a limit to what these telescopes can see. Visible light is only a very small part of the overall electromagnetic spectrum and the Earth’s atmosphere blocks most of this from reaching us on the ground.

To really see out into the universe, NASA (and others) have sent telescopes into orbit. For thirty years, these great observatories have searched the heavens and sent back spectacular images in all wavelengths.

• Spitzer Space (infrared) Telescope (2003-2020)
• Hubble Space Telescope (1993-present)
• Chandra X-Ray Observatory (1999-present)
• Compton Gamma Ray Observatory (1991-2000)

In December 2021, NASA (along with ESA, CSA, and GSFC), took the next step with the launch of the James Webb Space Telescope (JWST). The JWST’s mirror is 2 ½ times wider than Hubble’s at 6.5 meters. Because of its size, it had to be folded up to fit into an Ariane 5 rocket. And, for flexibility and foldability, the mirror is composed of eighteen individual hexagonal segments, each of which can be aligned to achieve the perfect view.

Unlike Hubble, which captures mostly visible light, the JWST will view the universe in infrared light, allowing it to gaze through the dust that obscures much of our view. Because heat radiates in the infrared, JWST must maintain an extremely cold temperature at -233^o C (-388^o F). This is achieved by positioning it at the Earth-Sun L2 point (see below) and deploying a tennis court size sunshield to block the light and heat from the Sun.

The Webb telescope has five scientific instruments, which process the light captured by the telescope.

• Near-Infrared Camera (NIRCam) is the primary camera on JWST covering a wavelength range from 0.6 to 5 microns.
• Near-Infrared Spectrograph (NIRSpec) is a spectrograph covering the same wavelengths as the NIRCam.
• Mid-Infrared Instrument (MIRI) is a camera that extends further into the infrared wavelengths from 5 to 28 microns.
• Near-Infrared Imager and Slitless Spectrograph (NIRISS) / Fine Guidance Sensor (FGS) are two physically linked, but distinct instruments. The NIRISS is a camera and spectrograph covering a range from 0.6 to 5 microns. The FGS is a tool designed to help orient the telescope in space.

Webb’s scientific goals are four-fold (see the NASA/GSFC article).

• The early universe. The JWST will look back in time to around 100 to 200 million years after the big bang when astronomers think the first stars began to form. Because of the expansion of the universe, light from these stars, which was emitted as visible or ultraviolet light, is today shifted to the infrared.
• Evolution of galaxies. The JWST will also look back at early galaxies and help astronomers understand how these galaxies evolve over time.
• Lifecycle of Stars. Webb’s infrared capabilities will peer through the dust and reveal new stars being formed.
• (Exo) planets. Webb will study the outer planets of our solar system as well as exoplanets orbiting distant stars. Its spectroscopy instruments will reveal details about these planets’ atmospheres.

Unlike the other great observatories, JWST will position itself beyond the Moon at a point called the Earth-Sun second Lagrange point (L2). This is where the gravity of the Earth and Sun are balanced allowing the telescope to “sit there” with little need for maneuvering corrections. Although, technically, it will orbit the L2 point while keeping its sun shield between the telescope and the Sun.

The initial plan was to launch the Telescope in 2018. However, it was delayed several times.

Finally, in December 2021, the Webb Telescope launched from French Guiana on an Ariane 5 rocket.

Unlike low Earth orbit, the L2 point is around a million miles from the Earth. It took JWST twenty-nine days to arrive at its desired location, a trip the folks at NASA described as twenty-nine days on the edge. While it was traveling, it had to perfectly perform a series of 50 automated steps including unfolding the telescope’s mirror and deploying the large sunshield. There was no room for error – the JWST was too far out for a repair or rescue mission.

Once Webb arrived at the L2 position, controllers focused on aligning the telescope.

• At the end of February, the 18 individual mirror segments were aligned.
• In mid-March, Webb produced its first test evaluation image of the star 2MASS J17554042+6551277.
• Near the end of April, the last of the alignment steps was completed and soon after, Webb completed the final calibration of the scientific instruments.

Everything was ready to go, but in late May, Webb was hit by a micrometeoroid. Webb was designed and tested to withstand impacts from these micrometeoroids, but the May impact was larger than anticipated. The good news is that Webb is still functioning, and NASA doesn’t anticipate any long-term issues from the strike.

Finally, on July 12, the first official images from the JWST were released to the public.