What has a sunshade the size of a tennis court, an 18-segment mirror, and a great view of the early universe? The James Webb space telescope — or just Webb for short.
After more than two decades of design, construction, and travel to the far side of the Earth-Moon system, the James Webb Space Telescope has finally seen the light. Webb left its final assembly point at a Northrop Grumman facility near Los Angeles and traveled to French Guiana on the Atlantic coast of South America for launch at the tail end of 2021, and the world’s space scientists watched as our next generation space telescope cruised more than a million kilometers past the moon. Currently stationed 1.5 million km (about 932,000 miles) from Earth, this super-cool space telescope is singularly positioned to look back at the bleeding edge of space and time — to when the first stars opened their radiant eyes and the first galaxies formed.
But first, all 18 of those mirrors must align. The process of getting more than a dozen curved pieces of gold-coated glass to behave like a single, giant light collector will take several months to complete. It will take substantially less time to recount how many pieces had to fall into place for the telescope to become the biggest eye to ever sail into our sky.
On the Shoulders of a Space Giant
It’s nigh-impossible to launch into a discussion of the James Webb Telescope without first mentioning the telescope that set the very, very, very high bar for astrophotography. The Hubble Space Telescope (HST) is a workhorse in the sky. After being ferried into low Earth orbit on the space shuttle, HST had to be serviced several times by astronauts before it could rise to the task of capturing images of galaxies forming and plumes erupting from Europa. But indeed, it has seen these sights and more. HST brought us the deep field — then the ultra deep field. It found four moons around Pluto. With HST, we tracked asteroids and other objects in the universe. We hunted the cosmos for dark matter and energy. Our understanding of the universe exploded, and it continues to expand at light speed, thanks to this school-bus-sized satellite and its 4.5 m2 (5.38 square yard) mirror.
After more than two decades in service, Hubble’s gaze is still sharp. It can see objects the width of a human hair a mile away. But HST’s sight is largely limited to visible wavelengths of light (wavelength 2.5 to 0.1 microns). There’s far more cosmos to see than what meets Hubble’s telescopic eye — for instance, heat emitted by exoplanets as they form. Unfortunately, signals such as the first rays of light shed by the earliest stars — still cooling and stretching across space 13 billion years later — lie outside HST’s capacity to detect.
It is exactly those kinds of signals that Webb was designed to pick up. Using technology similar to what firefighters use to see through smoke, James Webb will look past what Hubble would see in the visible light spectrum to collect far fainter infrared (IR) signals. Using IR, the Webb will see giant jets of gas in nebulae. It will see through giant jets of gas surrounding newly born stars and recently exploded ones. The planets forming in dusty disks around distant suns will be clearer than ever thanks to Webb’s primary mirror — which, at 6.5 meters (21 feet, 4 inches) across, is more than twice the size of Hubble’s .
Webb’s vast capacity to collect IR signals is secondary to its careful engineering. All 18 segments of Webb’s mirror are coated in a microscopically thin layer of gold. As it turns out, gold is an excellent reflector of infrared light. Once the segments have been brought into alignment, they will send the signals they collect to Webb’s similarly coated secondary mirror. Finally, the information will pass to Webb’s infrared camera, which itself has an image-collecting area roughly six times greater than Hubble’s. All of this means clearer pictures of the Universe and all it contains, further out in space and farther back in time, with up to 100 times more sensitivity than Hubble can see now, but in a slightly different wavelength.
Keeping It Cool
Calling Webb a “super cool” satellite isn’t just a turn of phrase. When it comes to detecting IR, super cool is the key to success. IR radiation is, simply put, heat. The cooler an IR detector is, the better it can differentiate between heat signals coming from the depths of the asteroid belt, the Oort cloud, or distant stars and heat lifting off its own body or from nearby bodies, such as the Sun. When it comes to IR signals, the Sun is a non-stop source of heat-based noise.
Hiding from the biggest source of infrared radiation in several light-years is no mean trick. The Spitzer space telescope used active cooling to keep itself hot on the trail of IR signals. Webb stays cool by staying behind a giant reflective sunshade. One of its instruments, the MIRI, hides behind a flying refrigerator. The MIRI — or mid-infrared instrument — receives super cold helium gas from a cryogenic cooler and compressor that live on the spacecraft’s warm side near the shade. Webb is so big that the MIRI is 10 meters away from both the sun side and the compressor. The helium gas pumped away from the cooler toward MIRI keeps the instrument 10 degrees Kelvin colder than the other Webb instruments, which allows it to pick up very small signals: between 5 and 28 microns. For reference, a human hair is between about 50 and 90 microns thick.
With the Webb launch on December 25, 2021, not only did humankind put a larger telescope into space than ever before — it also expanded our Earth-Moon-Sun system. The Webb telescope now lives more or less permanently in L2: a gravitational balance point that keeps Webb roughly 930,000 miles away from the Earth. There, it faces away from the Sun, the biggest source of infrared energy in our neighborhood. Seeing back to the beginning of time requires that Webb keep its back turned to the Sun and the Moon and the Earth, facing away from a generation of engineers and scientists who laid eyes or hands on it.
The last human to lay eyes on Webb in person saw the biggest space telescope in history folded up like origami. HST was only the size of a school bus; Webb is many times bigger. It had to be accordioned like a fan the height of a three-story building into the rocket that carried it away. No one who watched it be packed away, loaded onto the launch pad, and sent into space knew for certain if it would unfold. If it unfolded, would all the pieces deploy as they should? The mirrors alone took 10 days to fully deploy. Now that the pieces are out, will they work together? Unlike Hubble, which was rescued by human hands, Webb sits far beyond our physical reach.
The Webb telescope will face away from us forever. The sensors are online. The systems are warming up (while keeping it cool). On Earth, the world waits to see if the earliest parts of the universe will emerge from the darkness to light up our screens, introducing us, image by image, to a place we’ve lived in forever but have never fully seen.
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