A woman stands near a model of the James Webb Space Telescope (JWST) at NASA Goddard Space Flight Ce...

On deck

What comes after James Webb Space Telescope? Why just one telescope won’t do

Astronomers have outlined their vision for what comes after the James Webb Space Telescope.


It’s nearly the home stretch for the James Webb Space Telescope. If all goes well, the multi-billion dollar scientific instrument will finally launch to space in a matter of days and will stand ready to peer into the deepest recesses of the cosmos sometime next summer.

So naturally, the question that’s already coming up: What comes next?

Not just the next space telescope to be launched — likely NASA’s Nancy Grace Roman telescope in 2027 —but the next Webb-like telescope: a big flagship project for the future, recommended by astronomers today.

It turns out the astronomical community spelled out what they want to see follow the Webb in November, with the publication of the results of the 2020 Decadal Survey: An enormous space telescope, one with a six-meter diameter mirror that can see in the optical and ultraviolet wavelengths. It’s what it will take to see other planets.

“We're at the stage now where our goal should be to be able to characterize the atmospheres of habitable worlds,” Heidi Hammel, astronomer and interdisciplinary scientist on the James Webb Space Telescope team, tells Inverse. “What do you need to do that? Well, basically, you need a very large aperture telescope in space.”

But that’s not all. Hammel points out the Decadal survey also recommended huge ground-based telescopes and newer medium-sized space telescopes viewing the X-ray and other portions of the electromagnetic spectrum. The right question isn’t necessarily, “What happens after Webb?” It’s more like: What set of instruments replaces the current fleet of astronomical tools in the mid-21st century?

And the answer is really big, really cool tools.

What is the Decadal Survey?— To understand what these new tools may look like, you have to understand the Decadal Survey. At this stage, astronomers have yet to set the technical specifications. Instead, they’ve laid down scientific goals for the engineers to backfill over the coming decade.

The Decadal Survey, Hammel says, “is a document by the science community, an assessment of where we are scientifically in astronomy and astrophysics currently and where we want to progress in the next decade.” It’s a stop at the map in the astronomy food court, charting a path to the next stop. The 2000 Decadal Survey recommended what would become the Webb telescope. The 2010 survey recommended the Nancy Grace Roman telescope.

The 2020 Decadal Survey, delayed a year due to Covid-19, framed its specific recommendations within three broad scientific themes:

  1. “Worlds and suns in context,” studying exoplanets and their stars.
  2. “Cosmic ecosystems,” examining the formation and evolution of galaxies.
  3. “New Messengers and New Physics,” studying dark energy and gravitational waves.

The goal of characterizing the atmosphere of Earth-like planets around Sun-like stars falls under the “worlds and suns in context” theme and requires the very large-UV-optical space telescope Hammel described. But what about studying the formation of galaxies? What drives the formation of spiral structures or the evolution of the big elliptical galaxies?

“You’re not going to be surprised to hear,” Hammel says. “You need a greater than six-meter UV-optical tool. You need the same tool to do that science.”

Hubble’s true successor— Just what NASA will name that UV-optical space telescope tool is yet to be determined. Hammel wrote a letter to the National Academies of Sciences, Engineering, and Medicine, which conducts the Decadal Survey, recommending the telescope be named after Carl Sagan. But NASA, she says, is reluctant to name future telescopes after people after some astronomers objected to using former NASA Administrator James Webb’s name following evidence he participated in purges of gay and lesbian people from government in the 1950s and 60s.

“So now I'm calling it a Super Hubble,” Hammel says. The Super Hubble? Not the Super Webb?

“People have sold the James Webb Space Telescope as a successor to Hubble,” Hammel says, “But in fact, they are different kinds of telescopes.”

The Hubble Space Telescope as seen during a 2009 Space Shuttle Atlantis maintenance mission. Stocktrek Images/Stocktrek Images/Getty Images

The Hubble Space Telescope sees in the UV, optical, and some infrared wavelengths, but the Webb is a purely infrared telescope. Webb is “really a successor to the Spitzer telescope,” Hammel says, a 0.8-meter diameter mirrored infrared space telescope launched in 2003. With a 6.5-meter diameter mirror, Webb will be a vast improvement on Spitzer.

“What we’re really talking about is actually building the advanced Hubble,” Hammel says, an instrument with just the capabilities needed to study Earth-like exoplanets and determine if they are habitable worlds. “To truly answer the question, ‘Are we — humanity — alone in the universe?,’ the tool we need is the Super Hubble.”

The new telescope, whatever it’s finally called, will sport a six-meter or larger mirror to Hubble’s 2.4 meters. The Decadal Survey estimated it would cost around $11 billion and take several years to develop (like Hubble and Webb), launching sometime in the 2040s.

The right set of tools for the job— But the Decadal Survey recommended a lot more than the Super Hubble. To characterize the atmospheres of exoplanets around stars cooler than our Sun and fully explore galaxy formation, astronomers also need new ground-based telescopes.

“We’ve been working with eight and 10-meter class telescopes” on the ground, Hammel says. The Gemini observatories in Hawaii and Chile are two eight-meter telescopes, while the Keck observatory in Hawaii consists of two 10-meter telescopes. “You hit a wall in spatial resolution on the sky based on the aperture of these telescopes. and we’ve been at that wall for 20 years.”

The Hawaiian Gemini observatory atop Mauna Kea volcano. 35007/E+/Getty Images

The Decadal Survey recommended $1.8 billion in US federal funding for the previously private Giant Magellan Telescope under construction in Chile and the Thirty Meter Telescope planned for either Hawaii or the Canary Islands. Like the Gemini observatories, these new telescopes will cover the entire sky with unprecedented light collecting power.

And that’s important for the third scientist theme in the Decadal Survey, Hammel says, the study of things like gravity waves that don’t depend on light of any wavelength.

“To actually be able to do real follow-up and to try to connect the gravitational wave observations with the light observations, we need to have the full hemispheric coverage,” she says.

It all ties together. The technical specifications and details of the tools are still being developed, partly because they follow from the scientific questions astronomers want to ask over the next 20 years.

They don’t just want a Super Hubble for its own sake, and they don’t just want a Super Hubble and two new ground-based telescopes. The Decadal survey also recommended more modest instruments, such as a successor to the Chandra X-ray telescope.

That’s the best way to think about what comes after the James Webb Space Telescope. It’s not a single instrument, but a suite of tools conceived to help scientists answer the biggest question they felt they could feasibly pose over the next two or more decades.

“We make the most scientific progress when we have multi-wavelength observations happening simultaneously. All of these telescopes, working together to reveal the universe to us,” Hammel says. “Here are some incredibly deep questions, and we have the capability, if we choose, to build the tools to answer these questions.”