Humans have been looking at the stars for millenia, but it was just over 30 years ago that the Hubble Space Telescope launched, and we started getting a really good look at what’s out there. Hubble was beset with more than a decade of setbacks before its launch in 1990. Then, just after taking its position orbiting Earth, astronomers realized that something wasn’t right. It took engineers another three years to fix a manufacturing error that had left one of the mirrors misshapen by one-millionth of a meter. Ultimately, that imperfection was enough to render the telescope’s mirrors effectively useless. The long wait was worth it, though. The Hubble enabled dozens of breakthroughs in astronomy. It also took beautiful pictures. A recent version of its famous “Hubble Deep Field” image includes galaxies that are 13 billion lightyears away, making them the farthest objects ever photographed.
NASA is scheduled to soon launch what it calls the “successor” to Hubble: the James Webb Space Telescope. Like the Hubble, the Webb telescope is also designed to take extraordinarily precise measurements of "Ultraviolet and visible light emitted by the very first luminous objects [and which] has been stretched or 'redshifted' by the universe’s continual expansion and arrives today as infrared light."
Webb will also study objects closer to home, such as planets and other bodies in our solar system with the aim of determining more about their origin and evolution. Webb will also observe exoplanets located in their stars’ habitable zones, to search for signatures of habitability, and to learn about their chemical compositions.
In some sense, the Hubble and Webb telescopes share the same general mission. Webb’s “science goals were motivated by results from Hubble,” according to NASA. Insights from those results, combined with technological innovations mean Webb is a very different kind of telescope. In addition to being bigger and far more powerful, the new instrument will occupy a different orbit and use different kinds of instruments to detect different kinds of light (with some overlap). Together, the changes give Webb a remarkable ability — it should be able to see galaxies being born in the early days of the universe.
But there’s a catch: if something goes wrong, there’s nothing anyone can do to help.
Build a Better Time Machine
The night sky contains the history of the universe. That’s because space is so big that even light waves — the fastest thing in the known universe — can take a long time to reach their destination. If an object is close by, an observer on Earth will see it pretty much the way it looks at that moment. Our view of the Moon is less than two seconds out of date. But when we look beyond the solar system, our view is like a time capsule. Take the Big Dipper, for example. The closest star in the constellation connects the cup to the handle. Right now, we see that star as it appeared 58 years ago, in 1963. The farthest star is at the end of the handle, which we see as it appeared 124 years ago, in 1897. The farthest (and oldest) object visible to the naked eye is the collective glow of the trillion-or-so stars that make up the Andromeda Galaxy, the Milky Way’s nearest neighbor. If an alien-astronomer somewhere in that galaxy walked outside right now and used an extremely powerful telescope to look up at Earth, they wouldn’t see any evidence of modern humans (or any humans at all, for that matter). That’s because the lightwaves reaching their mirrors would have spent the last 2.5 million years hurtling through space.
Space is so huge and so empty that some lightwaves that started traveling in the universe’s early days are still going strong. The farthest galaxies in a “Hubble Ultra Deep Field” show up as they would have appeared more than 13 billion years ago when the universe was approximately 800 million years old. If the universe were now 40 years old, Hubble can see objects as they appeared when the universe was one.
Astrophysicists have wildly different theories about what happened during those 800 million or so years after the Big Bang, but Hubble can’t peer back that far, so it can’t provide data to help them sort out the confusion. That’s because Hubble detects light from the ultraviolet through the visible (which our eyes see) and into the near-infrared range. Light from those earliest galaxies probably started off at those wavelengths. But during the many billions of years, it’s been traveling through space, those lightwaves have been elongated and entered a part of the electromagnetic spectrum that Hubble can’t see. The reason for this elongation? “Spacetime itself is stretching apart,” as the University of Iowa notes. As the universe grows bigger, so does everything it contains — including the space between the peaks of lightwaves. The phenomenon is called cosmological redshift because red has the longest wavelength in the visible spectrum. While humans can’t see infrared light, we can feel it as heat. Measuring this "stretching," or loss of energy is one of the main ways that distance is now measured in the Universe.
Bigger, stronger, farther, colder
Webb’s design is significantly different from Hubble’s, and those differences make it extremely powerful. Senior project scientist John Mather put it this way in Astronomy, “If there were a bumblebee hovering in space at the distance of the Moon, the Webb could see both the sunlight it reflects and the heat that it emits.” One key difference is the primary mirror. At 6.5 meters in diameter, Webb’s mirror has more than six times the collecting area as of Hubble's. The new telescope’s mirror is coated with gold because it reflects red light better than alternative surfaces. It’s composed of 18 hexagons arranged like a honeycomb, so it can fold up inside a rocket, according to NASA. It’s the largest mirror ever flown into space, and no rocket currently in service has enough cargo room to carry it in a fully deployed configuration.
Once the telescope is in space, it will spend about three weeks slowly deploying its sunshade and mirror. Each of the hexagons is mounted to a series of actuators that can make extraordinarily subtle adjustments to its individual angle and position. The entire commissioning period will take around six months, and will include deployment of the mirror, cooling down to operating temperatures, mirror alignment, and instrument calibration.
Webb will have four instruments that analyze light collected and focused by the mirror. Three of them detect light with a wavelength of between 0.6 and 5 microns, the near-infrared spectrum. The near-infrared camera (NIRCam) is Webb’s main imaging device. It’s equipped with a series of coronagraphs, which help the camera image dim objects by blocking the light from brighter ones. Thanks to the physics of near-infrared light, NIRCam will be able to see through some particles and reveal objects that have been obscured by dust clouds. The near-infrared spectrograph (NIRSpec) analyzes light by breaking it apart into its constituent colors. While spectrographs are by no means a new technology, NIRSpec has a specially designed “microshutter array” that enables it to analyze up to 100 objects at the same time. The near-infrared slitless spectrograph (NIRISS) is a specialized device meant to take especially crisp pictures of very bright objects. It is equipped with an aperture mask, giving it the ability to capture images of bright objects at a resolution greater than the other imagers.
The new telescope will use its mid-infrared instrument (MIRI) to peer deep into the universe’s past. MIRI is designed to take images and spectrographs of light in the mid-infrared wavelengths, of between 5 and 28 microns. MIRI will see red-shifted light from stars as they form, faraway galaxies, and objects too faint to see with other instruments.
According to researchers at the University of Arizona who are collaborating with NASA, initial surveys for the first stars that formed in the first galaxies — “the 'first light' in the Universe,” as they call it — will come from surveys by NIRCam. That data will indicate if a galaxy formed stars early in its life, but the near-infrared spectrum won’t contain the right data to distinguish between the first stars and stars that appeared. That’s when MIRI comes in. Data contained in the red-shifted light will make the difference between the first stars and the rest “glaringly obvious to the MIRI,” the researchers said.
One reason an infrared telescope is so useful is that practically everything in the universe emits infrared light. That’s good for astronomers because it means an object doesn’t have to burn brightly to be seen, but it’s also a tremendous challenge because the signal Webb is searching for could easily be drowned out by heat from other sources. That’s why it’s especially important that Webb stays cold. This is its first line of defense is its orbit. Instead of circling the Earth-like Hubble, Webb will orbit a point about a million miles from Earth, staying as far from the Sun as possible.
Webb’s orbit follows a special path around the second Lagrange point that keeps it on Earth’s night side and tracks along with the Earth while moving around the Sun. That orbit will keep its biggest sources of nearby infrared radiation — the Sun, Earth, and Moon — on the same side and ensure it stays out of the shadows of both Earth and the Moon. This orbit also allows Webb to be constantly bathed in the sunshine to generate power using a solar array on the Sun-facing side of the spacecraft.
During its journey to that location, Webb will also deploy a sun shield the size of a tennis court that’s designed to protect the “cold side” of the instrument from the Sun’s warmth. Five layers of a material called Kapton will keep the cold side as chilly as 36 °kelvins (-394 °F). The Hubble, by contrast, stays at a surprisingly comfortable 70 °Fahrenheit (21.1 °Celcius).
Webb’s far-out orbit is essential for collecting the data it needs to achieve its scientific goals, such as watching the first stars and galaxies form. But it comes at a cost. As Marina Koren wrote in The Atlantic, “if something goes badly wrong, engineers can only send commands, not a crew to help.” Astronauts have visited the Hubble five times for repairs and updates.
With 14 years of delays already behind it, Webb has had as much trouble getting off the ground as its predecessor did. Its current launch date is the product of more recent problems, including an “incident” that sent vibrations through the entire machine and a “communication issue between the observatory and the launch vehicle system.”
If all goes according to plan, Webb will spend its first six months slowly assembling itself and cooling down. Then it will point its gold honeycomb mirror out into space, away from Earth and Sun, and start detecting well-traveled light waves that contain ancient data about the history of the universe, among other things. Researchers know what they’re looking for, but they don’t know what they’ll find. The Hubble, despite its earlier problems, has turned up many unexpected findings, including evidence of an unexpected element in an ancient star. Just last year, researchers used the instrument to look at one of the earliest galaxies and didn’t find the kind of stars they were expecting. These results suggest that galaxies must have formed much earlier than astronomers thought, and also much earlier than can be probed with the Hubble Space Telescope — but not the Webb.
With its gigantic mirror and state-of-the-art instruments, Webb “will crack open the treasure chest of the magnificent infrared sky, invisible to human eyes,” Mather wrote in the Astronomy piece. “We know where we will look, we can guess what we will find, and there will be surprises.”