NASA's massive James Webb Space Telescope will do nothing less than usher in a revolutionary era of astronomy.

The giant telescope is still being built, but it's nearing completion. The latest milestone was the finishing of the telescope's mirror, which has been under construction at the Goddard Space Flight Center in Greenbelt, Maryland. It resembles a giant honeycomb — 18 golden-hued hexagons that fit together to form a surface measuring more than 21 feet end to end. And it will enable scientists to look more deeply and clearly into space than ever before. The ultimate goal: to see "first light," the stars and galaxies whose births more than 13 billion years ago ended the dark ages of the universe.

"Those are the seeds of everything that we have today, like heavy metals and dust," Jason Kalirai, an astrophysicist with the Space Telescope Science Institute in Baltimore, told Co.Create. The new telescope will also analyze the atmospheres of exoplanets, part of the ongoing search for Earth-like, potentially habitable worlds. It may even detect signs of life.

This has been a long time coming. The tortoise-paced, $8.7 billion project began in 1996 and is slated to conclude with the telescope's launch in October 2018. At that time, the telescope — named for NASA's second administrator, who ran the agency from 1961 to 1968 — will be 80 feet long, 40 feet tall, and weigh about 12,000 pounds. But on the road to a fully assembled instrument, scientists consider their recent achievement to be among the most critical.

"The mirror is the heart of a telescope," Alan Dressler, a senior astronomer at the Carnegie Institute for Science, told Scientific American. Its assembly "really shows we've conquered every possible obstacle we've encountered so far," said Lee Feinberg, an engineer at Goddard who oversees the telescope's optics.

Indeed, the obstacles have been many. As program director Eric Smith put it, the team had to invent 10 new technologies to make the telescope work. Plus they had to develop new methods for testing whether those technologies would survive not only the G-forces and shockwaves that accompany launch, but also hold up in the harsh environment of space.

The latter was one of the lessons of the James Webb Space Telescope's predecessor, the Hubble Space Telescope, which was launched in 1990 and is still in orbit about 350 miles above Earth. A flawed mirror that resulted in fuzzy images could have dealt a lasting blow to the project, but astronauts mended it in a high-wire repair act in 1993. The new telescope, however, will travel to a post nearly 1 million miles away — an impossible distance for a repair mission were something to go wrong.

The outlines of the problem scientists faced in building the James Webb Space Telescope went roughly like this: The new telescope needed to measure light in the infrared range, something Hubble could do only marginally. This is critical because light that's as ancient as scientists want to capture stretches during its journey through the expanding universe — so much so that, by the time it reaches us, its lengthened wavelengths fall outside the visible light spectrum.

What's more, infrared sensors are by their nature more sensitive to radiation. The glow from the telescope itself could swamp a faint signal, plus there's the not-so-small issue of combating the radiation streaming from the sun. So the telescope needed its own form of cosmic air conditioning to bring it to an operating temperature close to absolute zero.

Oh, and to even make it to space in the first place, the entire telescope needed to be sufficiently lightweight and collapsible to the point that it could fit inside a rocket.

Thus, two decades on, NASA's joy at the completion of the 18-part mirror, whose segmentation was among the keys to making it foldable for launch (unlike Hubble's mirror, which was a single plate of glass). The gold color comes from its material, beryllium, chosen for its strength and lighter weight.

Beryllium also behaves more predictably during temperature changes — a benefit given scientists' need to plan for the shape-shifting that each mirror segment will undergo in its transition from Earth to space. They ultimately devised a painstaking process that included cooling the mirror's segments in a cryogenic chamber; measuring the warping that occurred; warming the segments; precisely grinding their surfaces to account for the differences; and then repeat. The final mirrors have imperfections on Earth that will resolve as the material cools in space — to within nanometers of a completely smooth surface.

Another innovation: A sunshield, which looks like a skirt for the mirror, albeit one the size of a tennis court. It has five layers of ultrathin, lightweight material that blocks the sunlight and keeps the telescope at its operating temperature of around minus 370 degrees Fahrenheit, or the average surface temperature of Pluto. (The sun-facing side will be a steamy 185 degrees Fahrenheit.) The sunshield also has a compartmentalized design so that a small rip — say, from a pebble barreling through space — doesn't lead to catastrophic tearing. It also will be stowed for launch.

The telescope's parts still have a battery of tests to go through — individually and in concert with each other — en route to launch. Once in space, the elements will unfold and slowly deploy over "three weeks of terror," said Matt Mountain, a former James Webb Space Telescope scientist and the director of the Association of Universities for Research in Astronomy. But the risk seems to be worth the reward. As Mountain told Science, "If you put something this powerful into space, who knows what we can find?"