How the James Webb Space Telescope Works

Webb's mission builds upon and expands the work of NASA's Great Observatories, four remarkable space telescopes whose instruments cover the waterfront of electromagnetic spectra. The four overlapping missions have enabled scientists to observe the same astronomical objects in the visible, gamma ray, X-ray and infrared spectra.

The school-bus-sized Hubble, which sees primarily in the visible spectrum with some ultraviolet and near-infrared coverage, kicked off the program in 1990 and, with further servicing, will complement and work with Webb. Appropriately named for Edwin Hubble, the astronomer who discovered many of the events that it was built to investigate, the telescope has since become one of the most productive instruments in scientific history, bringing phenomena like star birth and death, galactic evolution and black holes from theory to observed fact.

Joining the Hubble in the big four are the Compton Gamma Ray Observatory (CGRO), Chandra X-ray Observatory and Spitzer Space Telescope.

What makes Webb different is it has the capability to gaze deeply into the near- and mid-infrared, and it will have four science instruments to capture images and spectra of astronomical objects. Why does that matter? Stars and planets that are just forming are hidden behind dust that soaks up visible light. However, infrared light emitted can pierce this dusty blanket, revealing what's behind. Scientists hope that will allow them to observe the very first stars in the universe; the formation and collision of infant galaxies; and the birth of stars and protoplanetary systems, possibly even those containing the chemical constituents of life.

These first stars could hold the key to understanding the structure of the universe. Theoretically, where and how they formed relates to early patterns of dark matter — unseen, mysterious matter detectable by the gravity it exerts — and their life cycles and deaths caused feedbacks that affected the formation of the first galaxies [source: Bromm et al.]. And as supermassive, short-lived stars, estimated at around 30-300 times the mass (and millions of times the brightness) of our sun, these firstborn stars might well have exploded as supernovae then collapsed to form black holes, later swelling and merging into the huge black holes that occupy the centers of most massive galaxies.

Witnessing any of this is a feat beyond any instrument or telescope built so far.

Webb looks a bit like a diamond-shaped raft sporting a thick, curved mast and sail — if the sail was built by giant, beryllium-chewing honeybees. The "raft" (or sunshield) is made of membrane layers — all as thin as a human hair — of Kapton, a high-performance plastic coated with a reflective metal. Together they protect the main reflector and instruments.

Webb's "keel" is what you'd think of as its unitized pallet structure. That's where the massive sunshield folds up for liftoff. In the center lies the spacecraft bus, which packs all of the support functions that keep Webb running, including electrical power, attitude control, communications, command and data handling, and thermal control. A high-gain antenna adorns the Webb's exterior, as do a set of star trackers that work with the fine guidance sensor to keep everything pointed in the right direction. Finally, at one end of the sunshield, and perpendicular to it, is a momentum trim tab that offsets the pressure that photons exert on the ship, much like a trim flap does on a sailing ship.

Above the sunshield is the "sail," or Webb's giant mirrors. Webb has a primary mirror that is 21.4 feet (6.5 meters) across that measures the light from distant galaxies. (In comparison, the Hubble Space Telescope's mirror is 7.8 feet [2.4 meters]). It's made of 18 hexagonal beryllium sections that unfold after launch, then coordinate to act like one whopping primary mirror. This mirror has a much lighter design and allows the entire structure to fold like a drop-leaf table. The hexagonal shape of the mirrors allows the structure to be roughly circular, without gaps. If the mirror segments were instead circles, there would be gaps between them.

Let's take a closer look at the instruments that will make all of those studies possible.

Although it sees somewhat into the visual range (red and gold light), Webb is fundamentally a large infrared telescope.

But infrared observation is essential to understanding the universe. Dust and gas can block the visible light of stars in stellar nurseries, but infrared passes through. Moreover, as the universe expands and galaxies move apart, their light "stretches out" and becomes redshifted, sliding toward longer electromagnetic (EM) wavelengths such as infrared. The farther away the galaxy, the faster it recedes and the more redshifted its light, hence, the value of a telescope like Webb.

Infrared spectra also can provide a wealth of information on exoplanet atmospheres — and whether they contain molecular ingredients associated with life. On Earth, we call water vapor, methane and carbon dioxide "greenhouse gases" because they absorb thermal infrared (aka heat). Because this tendency holds true everywhere, scientists can use Webb to detect such substances in the atmospheres of distant worlds by looking for telltale absorption patterns in their spectroscopic readings.

The James Webb Space Telescope is the largest, most powerful space telescope ever built. It will be the most complex telescope launched into space. The data it provides during its mission, which is expected to last anywhere between five and 10 years, could change our understanding of the universe.

Why? Because its goal is to examine all of the phases of our cosmic history, including the big bang. But there are four distinct objectives for the Webb Telescope during its mission, and they're grouped into four themes: