Peering Beyond: The James Webb Space Telescope

4/22/2017

In the colloquial sense, a telescope is about how far we can see. But for astronomers, or specifically astrophysicists, it is about how far back in time. Cutting edge cosmology is all about studying the universe's past, about how it came into being, and about what will become of it in the far flung future.

This is why the world's scientific community came together to develop what may be our greatest achievement yet as a species: the James Webb Space Telescope.

"It is fitting that Hubble's successor be named in honor of James Webb. Indeed, he laid the foundations at NASA for one of the most successful periods of astronomical discovery. As a result, we're rewriting the textbooks today with the help of the Hubble Space Telescope, the Chandra X-ray Observatory, and the James Webb Telescope."
NASA Administrator Sean O'Keefe upon the naming of the JWST, 2002

Imagine a device that helps us determine the birth of the modern universe and gives us a glimpse at an extraterrestrial future.

When the James Webb Space Telescope launches in 2018, it is expected to signal a new phase of human understanding of the universe. Continuing to probe beyond the limits of the Hubble Space Telescope, it will enable astronomers to determine the point at which visible light first formed some 13.8 billion years ago, and is sensitive enough to detect smaller exoplanets hospitable enough for life.

Twenty years in development, the US$8 billion telescope employed the collective efforts of more than 1,000 scientists and engineers from 24 countries in three space agencies—NASA, the Canadian Space Agency, and the European Space Agency.

What Astronomers Are Looking For

One of astrophysics’ biggest mysteries is the point at which the universe transitioned from the “early universe,” a state of dark plasma, to the formation of light, stars, and galaxies. We don’t know when those first stars and galaxies turned on, and those are the seeds of everything that we have today, like heavy metals and dust. The James Webb Telescope is designed to witness the point in cosmic history when the universe went through this transition. It’s being designed to hit that sweet spot a couple of hundred million years after the universe formed.

In addition to probing the farthest reaches of the universe, the instruments will be able to detect the atmospheric compositions of smaller rocky exoplanets orbiting other stars to gauge whether they might have the potential to sustain life. Towards the end of its life, our sun will balloon to engulf the nearer planets before collapsing, so the human race will inevitably need to leave Earth and the solar system at some point if it is to survive.

How The JWST Works

The Hubble, which orbits about 1,100 kilometers above the Earth, has a 2.4-meter mirror and instruments that collect data mostly within the visible spectrum. By comparison, the JWST has a roughly 6.5-meter mirror, observers in the infrared spectrum, and will reside about 1.6 million kilometers from Earth.

When you measure light in a certain wavelength here, you can calibrate how far away your object is. Infrared enables us to see farther into the universe (thus, farther back in time), because the universe is expanding, creating something called a redshift effect. Objects moving away from us emit longer electromagnetic wavelengths that appear in the red and infrared part of the spectrum. The JWST can peer farther into the infrared spectrum than Hubble, thus sensing farther objects which light has been stretched into the infrared spectrum.

Since infrared cameras are more sensitive to radiation (i.e., light, heat), the telescope has to be far enough away from the sun and its reflected light on the Earth and moon, and have a shield that blocks the sun’s rays to cool it down so as to not be affected by its own heat. But if it’s too far away, its orbit would lag behind Earth’s, interfering with communication. It just so happens, there is a point in space called the second Lagrangian point, or L2, discovered by 18th century Italian astronomer Joseph-Louis Lagrange, that is far enough away from the Earth’s reflected light but where the combined gravitational forces of the Earth and sun enable an object there to rotate the sun at the same pace as Earth. That’s where the JWST will live. The downside is that if the JWST needs repairs, like the Hubble did, there is no way we could reach it. So everything single thing from the shape of the mirrors, to the workings of the instruments, to the deployment of the sunshield have to work PERFECTLY.

Now Comes The Hard Part . . .

When engineers began considering the JWST in 1996, 10 technologies needed to achieve it didn’t exist. They included a giant mirror that had to be broken up into 18 individual parts but work as a unit, the entire telescope had to be folded up to fit inside a rocket that launches it into space, because it’s bigger than the fairing of the rocket, and programmed to unfold to its full size at its destination, which takes a month to reach. It includes a five-layer, tennis-court sized sunshield needed to cool the temperature down and block the radiation from the sun that would otherwise heat up the telescope and new generation detectors that are more sensitive than other instruments we’ve built before in astronomy.

An enormous amount of computer modeling went into exactly how these five membranes will hold up, it’s one thing to fold up one membrane and make sure it’s being held down during a launch. But to do that with five layers, it has to have pins going through all of those layers to anchor them so they don’t loosen or rip. You can’t have a hole going through all five, so that any photon of light from the sun will get through them all. And it has to be tested on the ground, where there’s gravity. But when you launch the telescope in space, there’s no gravity, so you have to account for that in the deployments.

The team had to come up with ways to keep the weight down to a slender 6.6 tonnes. Instead of much heavier glass, the mirror is made of beryllium, a rare metal that is stronger than steel but lighter than aluminum, and coated in a thin layer of gold, which reflects 98 percent of infrared light.

Once In Space

The JWST mirror will collect the photons of light from distant objects and focus them onto the camera detector, which will digitize the information and beam it to Earth. All the images will be archived in the Space Telescope Science Institute for the science investigation team who wrote the proposal for those images. The science community will compete for telescope time to look at objects—whether it’s looking for exoplanets, first light in the universe, distant stars and galaxies, characterizing asteroids, or close-ups of our planets. And a committee of astronomers will pick the best science proposals and build those into the telescope’s schedule.

Even before the JWST launches, next generation telescopes are already in the works to determine the level of life on Earth-sized exoplanets. You can take a rock and evolve it through natural geological processes to create an atmosphere, but there are certain combinations of elements you cannot create if you don’t have life on that planet. A future goal is to detect biosignatures, like the ratio of oxygen and ozone to methane, to validate whether the planet has life.

Ponder this

The telescope is needed to be cooled down to only several Kelvins - a few degrees from absolute zero. Why is this important?

Why wouldn't the JWST be affected by the light reflected from the other celestial bodies?

Discuss

There are wavelengths beyond the infrared. Assuming that a telescope similar to JWST was built to observe along these wavelengths, what would it see? How would it be useful to the astronomical community? What sort of discoveries can be made with it? And how would it be built considering that we were to put as much thought on its design as we did onto the JWST?