U-M astronomers anticipate launch of James Webb Space Telescope
Update (Jan. 8, 2022): James Webb Space Telescope more than three quarters through its journey
On Christmas Eve, if every last detail falls into place, a new telescope will sail into the sky, on its way to lodge in a place far beyond the moon.
There, the James Webb Space Telescope will slowly unfurl over the course of a month and cruise toward its parking spot. If all goes well, the telescope will peer so deeply into space it can observe the faintest light from the universe’s first stars and galaxies.
It’s a launch University of Michigan astronomer Michael Meyer has been waiting for his entire career. As a postdoctoral researcher in 1997, Meyer began working on a committee tasked with dreaming up cutting-edge scientific applications of what was then called the Next Generation Space Telescope.
Now, nearly 25 years later, Meyer and the astronomy community are on the cusp of JWST’s launch. The $10 billion instrument is a large, infrared telescope, a kind of telescope that captures different types of infrared light—light much redder than the reddest thing the human eye can see—on the nonvisible electromagnetic spectrum. JWST will complement Hubble, which primarily uses visible light to make observations, but, at 30 years old, is also nearing the end of its life.
“In 1997, there had been a series of high-level panel reports about how important it would be to plan for after Hubble. And because discovering the first galaxies was a key scientific question—and still is today—it was clear this telescope would need to be an infrared telescope,” said Meyer, a professor in the U-M Department of Astronomy.
This is because the earliest born celestial bodies and galaxies reside at extremely great distances from us, and as the universe expands, the wavelength of light from its outer reaches becomes stretched to longer wavelengths—similar to how the pitch of an ambulance’s siren changes as it moves away from you. When light is stretched enough, it shifts from visible light into the infrared.
On Dec. 24, the telescope, which will be launched on an Ariane 5 rocket from French Guiana, will travel one million miles from Earth—Hubble sits about 354 miles from Earth—and nearly 240,000 miles beyond the moon. There, in a position called a Lagrange point, the telescope will orbit the sun in lockstep with the Earth. By the time it has reached this position, its 6.5-meter mirror composed of ultra lightweight beryllium will have slowly unfurled. The telescope’s tennis-court-sized sun shade, which will reduce radiation from the sun by a factor of a million, will also have slowly opened—all over the course of weeks, with the potential for something to go wrong at every step.
“If you remember the Mars Perseverance Rover, it was called the ‘Seven Minutes of Terror’ as they released the rover and sent it down to the planet,” Meyer said. “This is going to be more than four weeks of terror.”
Seeing unseen stars
JWST is not the world’s first infrared telescope—even in space. But it will be far more powerful than the current space-based instruments, and ground-based infrared telescopes have particular challenges: primarily, almost everything on Earth, including Earth, glows. Your body gives off infrared radiation. Instruments glow with radiation. The Earth is bathed in it. Ground-based telescopes fight against this source of noise to make observations.
“Imagine you want to see a star and you’re standing in a parking lot with lots of lights shining down. It’s really hard to see the star against the glare of the parking lights. Similarly, at the wavelengths that we’re talking about, everything on planet Earth glows. And so we can do it from the ground. But we’re fighting against this tremendously bright background,” Meyer said. “An observation that would take an hour in space would take thousands of hours from a ground-based telescope.”
JWST’s position in space means it’s far beyond the radiation emitted by Earth, but it still has to contend with heat from the sun. One of its four instruments, which include cameras and spectrometers, has a cryocooler that will cool one of the instruments to 7 Kelvin, or -447 Fahrenheit.
With this delicate instrumentation, Meyer plans—for one of his projects—to observe a young star cluster, a place within the Flame Nebula where stars like our sun are in their early stages of formation.
“At visible wavelengths, what you can see with your naked eye, the star cluster is not there. It’s blank because all of the dust obscures these nurseries where stars are born,” Meyer said. “In the infrared, the cluster is just a glowing, dynamic place where stars formed, and in particular, we will be able to probe down to objects that form like stars, but that are only a few times the mass of Jupiter.”
This will allow the researchers to see planetary mass objects as the extreme of the star formation process.
“This is something I’ve been studying for almost 30 years now,” Meyer said. “It’ll be very exciting to see this state-of-the-art instrument push down farther than anything we’ve ever been able to do.”
Decoding distant planet atmospheres
Emily Rauscher, U-M associate professor of astronomy, specializes in three-dimensional modeling of the atmospheres of exoplanets, or planets outside of our solar system. What most excites her about JWST is its ability to do something called eclipse mapping.
One space-based telescope, the infrared Spitzer Space Telescope, has been able to do eclipse mapping of one particularly bright Hot Jupiter. Hot Jupiters are gas giant planets similar to our solar system’s Jupiter and Saturn, and doing this kind of mapping allows astronomers to create two-dimensional maps of the daysides of the planet.
But the Spitzer telescope could only measure light at a single wavelength, whereas JWST will be able to take a variety of spectral observations of different wavelengths of light. Each molecule has its own particular set of wavelengths of light that it can interact with, producing recognizable patterns of spectral lines. JWST will be sensitive enough to read these different spectra, allowing astronomers to characterize the atmospheres on the daysides of distant planets.
Using eclipse mapping, Rauscher will also be able to characterize a planet’s dayside atmosphere in all directions: east, west, north, south and even the depth into an atmosphere. To do this, JWST will take a series of measurements as a planet goes behind its star in an eclipse. Each measurement contains the brightness of a slice of the planet. Then, the researchers will be able to compile the slices back into a whole picture of the planet.
“We’re about to go from one map of one planet at one wavelength to a bunch of maps of a bunch of planets at a bunch of wavelengths,” Rauscher said. “And again, these are spectral observations, and at different wavelengths, you see different depths into that atmosphere. These data are going to have three-dimensional information about the day sides of these planets: latitudes, longitudes and depths.”
JWST will also help gather more information about the temperature measurements on the daysides of planets vs. their nightsides. This can help astronomers better understand the distribution of gases throughout a planet’s atmosphere.
For example, astronomers have also used the Spitzer Space Telescope to take orbital phase curves of many planets. These images measure the brightness, which is related to temperature, of the dayside of a planet vs. the nightside of a planet, but the measurements are at a single wavelength of light. With JWST, astronomers will be able to take these measurements at many different wavelengths of light, which will be like going from a black-and-white movie to one in full color, Rauscher says.
“Because of the permanent day time of Hot Jupiters, how bright the dayside of the planet is vs. the nightside has to do with how efficiently winds on the planets can take hot gas from the dayside to the nightside,” Rauscher said.
If a planet’s dayside is the same temperature as the nightside because the winds have efficiently moved gas around the planet, both sides will be the same in terms of brightness.
“But then there’s clouds, and we think clouds form a lot of these planets,” Rauscher said. “And it may be that you have nightsides that are cloudy because they are cooler, and daysides that are clear. That changes how we interpret the brightness differences a little bit.”
With the Spitzer Space Telescope, astronomers can’t tell whether a planet is dim because it’s cool, or dim because it’s cloudy. But with JWST, a cold nightside will have a different spectrum than a cloudy nightside.
“With JWST, if we’re not taking into account all of these messy processes and all of the spatial complexity, we may not really know that we’re measuring the amount of a specific gas in a planet’s atmosphere correctly because we can be tricked if we assume the planet’s atmosphere is the same everywhere,” Rauscher said. “JWST is the most exciting thing for space-based observation—it will change how we understand exoplanet atmospheres.”
The fossils of our solar system
Larissa Markwardt, a graduate student in the U-M Department of Astronomy, will use JWST to examine something a little closer to our home—or at least in our solar system. Markwardt studies Trojan asteroids, which astronomers call the fossils of our solar system. Like JWST itself, these asteroids sit in Lagrange points, those “gravitational parking spots,” Markwardt says, that are stable because they’re getting pulled by the gravity from the sun and from the planet in whose orbit they sit.
Each planet has five such Lagrange points, and two of these five points have the same orbit as the planet. This means Trojan asteroids that occupy these points are either always ahead of the planet or behind the planet, like little companions, Markwardt says.
“The reason we’re interested in them is because we expect that they’re some of the most primordial objects in the solar system, because where they’re at is so stable, we expect nothing has really happened to them since the solar system has formed,” Markwardt said.
Specifically, she will study Trojan asteroids parked in Lagrange points around Neptune. Neptune Trojans are difficult to view, she says: Trying to find these tiny asteroids that emit faint light against a blanket of background stars is challenging. Astronomers have been able to observe the colors of some of these objects, but the Trojans have different colors than they’d expect to see.
“Because we think that Neptune Trojans are probably just captured Kuiper Belt objects, they should have the same colors as those Kuiper Belt objects,” Markwardt said. “But so far, we have found lots of Neptune Trojans that are what we’ll call red, and only one Neptune Trojan that’s what we’ll call very red.”
Many Kuiper Belt objects fall into the “very red” category, she says, “So it’s weird that there’s only one very red Neptune Trojan.”
Markwardt’s JWST proposal will use the telescope’s near infrared spectroscopy instrument to get spectral readings from the surface of the asteroids.
“There’s only so much information you can from the colors of these objects,” she said. “From James Webb’s NIRSPEC instrument, we can get a lot more information about the surface of the asteroids. Beyond color, we’ll know what kind of ices are on the surface—whether there’s water ice or methane ice. We’ll have an idea of how the sun reprocessed these surfaces, because solar wind interacts with the ice on these objects, which is what gives them a kind of red color.”
Markwardt will even be able to see what the grain size of the ices looks like, which will tell her whether an asteroid has had a collision. These observations could tell the researchers about the kind of chemistry present when these objects were forming.
“And since these objects are primordial, this will tell us what was present at the formation of the solar system,” she said. “This is interesting for NASA as well because they are always interested in how Earth got its water, so tracking water ice throughout the solar system is important for them.”
Markwardt says her research largely focuses on our own solar system, but those kinds of projects comprise a small portion of the data JWST will be collecting. Many of the projects deal with the oldest galaxies in the universe, looking back to its birth.
“It’s really going to revolutionize astronomy, and there’s a whole generation of astronomers who are going to be using this data for decades,” she said. “I just got very lucky that I was here at the right time, when it was actually going to launch.”
While JWST’s launch marks the beginning of Markwardt’s research career, for Meyer, the launch is something of a culmination—one beyond which the entire astronomy field hopes to use the instrument’s powerful gaze to peer back to the beginning of our universe, and to find faint objects we know likely exist but haven’t yet been able to see.
“We think we know what we’ll use it for, because that’s how the design process works. But every one of us knows that the most extraordinary discoveries that the James Webb Space Telescope will make will be something we could not have anticipated,” Meyer said.
In all, 19 researchers in the U-M Department of Astronomy are involved in 12 projects that will use JWST observations. This includes work done by Keren Sharon to understand star formation using strong gravitational lensing, a phenomenon in which the gravitational field of a massive structure bends light from objects behind it, allowing it to serve as a natural telescope. Astronomers Kayhan Gultekin and Elena Gallo will use JWST to further their research into the co-evolution of black holes and galaxies.
JWST is an international collaboration between NASA, the European Space Agency and the Canadian Space Agency. The NASA Goddard Space Flight Center is managing the development effort. The Space Telescope Science Institute will operate Webb after launch.