Scientists use telescopes to observe the light given off by distant astronomical objects, and from that light, create a picture of space. Because light takes time to travel, the farther away an object is observed in space, the further back in time it is.

Since its launch in 1990, the images produced by the Hubble Space Telescope (HST) have given scientists insight into the early universe and its basic structure; the birth and evolution of stars, solar systems, and galaxies; and the first convincing evidence for supermassive black holes. But the HST has an important limitation: it can only "see" so far back in time. With many scientists interested in understanding the early universe, engineers are developing a telescope that observes light in the part of the spectrum where the oldest astronomical objects are visible—ones that date back not just millions of years, but several billions of years, when the first galaxies formed. Enter the James Webb Space Telescope (JWST), named after the NASA administrator who oversaw the Apollo space program.

Whereas the HST makes its observations primarily in the visible light spectrum, the JWST will be optimized to observe some of the oldest galaxies' earliest stars in infrared wavelengths. Infrared light allows astronomers to see objects that are hidden from view by stellar gas and dust or that are otherwise too cool to be visible in other wavelengths. Infrared also allows astronomers to look back in time because short wavelengths emitted by distant objects are stretched into the infrared range as the universe expands. Astronomers are able to use infrared light to study in greater detail the formation and evolution of planets, stars, and galaxies as well as the distribution of matter throughout the universe.

A telescope's sensitivity, or how much detail it can see, is directly related to the size of its mirror area. A telescope’s mirror collects the light from objects being observed, and a larger mirror area can collect more light. The JWST design features a primary mirror with a diameter of 6.5 meters (21.4 feet). This will give it a light-collecting area about seven times larger than the HST’s mirror, which has a diameter of 2.4 meters (8 feet). The ability of the JWST to collect more light means that it will see deeper into space than the HST can.

Funding for astrophysics relative to other NASA science divisions has plunged since 2008. Despite the success of the HST, the future of the JWST is still in doubt. The telescope's price tag—now estimated at $8.7 billion—has risen considerably from original estimates of $3.5 billion, due largely to technical setbacks. NASA has not yet determined how it will cover the extra costs. This will stir up further congressional debate over whether to pursue this program. As it is, the launch date has been pushed to 2018, at the earliest.

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