NASA is facing a mind-boggling challenge: predict the state of astronomy two decades from now and design a telescope for that future. This feat of forecasting is necessary because NASA’s flagship missions, the Hubble Space Telescope–scale observatories that redefine our understanding of the universe, require at least that much advance planning. To that end, the space agency has just embarked on a set of studies to consider four possible major missions—one of which, most likely, will launch around 2035.

In April four “Science and Technology Definition Teams” made up of scientists from around the world will begin to sketch out the various would-be flagships. In 2019 the teams will turn their final reports over to the National Academy of Sciences, whose independent Decadal Survey committee advises NASA on which mission should take top priority. From the beginning of this process to completed construction, almost 20 years will pass. “Space is hard. These things are large,” says Paul Hertz, NASA’s Astrophysics Division director. “It takes a long time to do it right.”

The missions NASA is considering are called the Far-Infrared Surveyor, X-Ray Surveyor, Habitable-Exoplanet Imaging Mission (HabEx), and Large Ultraviolet-Optical-Infrared Surveyor (LUVOIR).

LUVOIR: The Über-Hubble

LUVOIR would boast an eight-to-16-meter-wide mirror, more than three times the size of Hubble’s, even at the smaller end of the range. LUVOIR’s mirror will likely be made of smaller segments pieced together like a mosaic, because constructing one mirror that huge at the precision necessary would be nearly impossible. In the same wavelengths as Hubble observes, LUVOIR could take the same type of eye-catching portraits the former telescope is known for and, like Hubble, it could also split light into constituent colors using a spectrograph. The observatory would be a jack-of-many-trades, able to watch stars, galaxies and black holes form and evolve.

LUVOIR’s mirror would be made of separate segments, like the mirror of the James Webb Space Telescope, shown here, set to launch in 2018.
Credit: NASA

But LUVOIR’s potential to study planets has scientists most excited. The telescope could potentially find Earth-size worlds circling nearby stars and then determine if they are actuallylike Earth. LUVOIR’s spectrometers would parse atmospheres for signs of biology, or at least life-friendliness. “The potential discovery of habitable planets out there, maybe possibly even inhabited ones, really will give birth to whole new fields of science that don’t exist today,” says Aki Roberge, an astrophysicist at NASA Goddard Space Flight Center and lead study scientist for the LUVOIR team. But the telescope would look at all kinds of planets, not just the ones that remind us of home, helping reveal whether Earth is normal or anomalous.

Planets are not easy to image because their stars shine about 10 billion times brighter. To spot those distant worlds, LUVOIR would either need a coronagraph—a disk on the telescope that blocks the light from the stars, much like the moon blocks sunlight during a solar eclipse—or a starshade, a screen positioned in front of the telescope to accomplish the same feat.

Both LUVOIR and HabEx would search for and study Earth-size planets around other stars, such at Kepler-186f, an exoplanet found by the Kepler Space Telescope, shown here in an artist’s illustration.
Credit: NASA Ames/SETI Institute/JPL-Caltech

HabEx: The Planet Hunter

Another telescope under consideration, HabEx, shares many similarities with LUVOIR. Like that observatory, HabEx would also be a “Swiss Army” telescope with the ability to study multiple astronomical phenomena, but it would be designed more narrowly around the goal of planet-watching. It would be optimized to search for and image Earth-size worlds in the habitable zones of their stars, where liquid water can exist. With its four- to eight-meter mirror, HabEx would aim to understand how common terrestrial worlds beyond the solar system may be and the range of their characteristics. Like LUVOIR, it would use spectrographs to study planetary atmospheres and eclipse sunlight with a coronagraph or starshade.

The Far-Infrared Surveyor would be a successor of sorts to the Spitzer Space Telescope, shown here, which launched in 2003 to observe the universe’s long-wavelength infrared light.
Credit: NASA/JPL-Caltech

Far-Infrared Surveyor: The Night-Vision Scope

Both HabEx and LUVOIR collect light around the energies human eyes can see. But the third candidate, the Far-Infrared Surveyor, would see very long wavelengths of invisible light in a range of the electromagnetic spectrum that as of yet has been mostly overlooked by telescopes. With this light, scientists can peer back in time to the earliest galaxies whose light waves have been stretched by the expansion of the universe.

Infrared light reveals parts of the universe that are otherwise undetectable, those objects enshrouded in dust, such as stars and planets in the process of forming. The interstellar compounds that may have led to life and the very first galaxies ever formed also show up in these wavelengths. Such investigations into how we got here, says astronomer Kartik Sheth of NASA Headquarters, the Far-Infrared Surveyor team’s program scientist, can onlyhappen with a far-infrared telescope. And it needs a big mirror, or aperture, and the ability to see huge swaths of sky.

Refrigeration is a problem scientists are still working out with this telescope plan, but it is key: The more infrared radiation (heat) the telescope’s equipment emits, the warmer the instrument gets, masking the weak infrared signals it may pick up from deep space. “If we do launch a larger aperture, how do we cool a larger aperture?” Sheth asks. They will have to freeze the whole thing cryogenically using liquid helium to get the observatory down to a temperature near absolute zero if they want to investigate the universe’s origins.

The X-Ray Surveyor would observe objects bright in X-ray light, such as this flare from the supermassive black hole at the center of the Milky Way observed by the Chandra X-Ray Observatory.
Credit: NASA/CXC/Amherst College/D.Haggard et al

X-Ray Surveyor: Looking back in time

The final telescope choice, the X-Ray Surveyor, would also target parts of the universe invisible to human eyes. “X-ray astrophysics, because it’s so high-energy, allows you to see things you can’t see in any other wavelength,” says Jessica Gaskin of NASA Marshall Space Flight Center, who heads that observatory’s science study team. At those energies, the surveyor could also answer deep “in the beginning” questions—but different ones from an infrared telescope. This instrument would look back to how black holes began, how galaxies formed around them and how the whole structure of the universe shaped up. “They all go back to understanding the evolution of our universe in a really big sense,” Gaskin says.

The team hopes to design a telescope that would be about 50 times more sensitive than the previous x-ray mission, a currently operating scope in orbit called the Chandra X-Ray Observatory, and be able to make maps that are similarly detailed. “It definitely has that above-and-beyond capability to it,” Gaskin says. To create such a telescope, though, scientists will have to figure out how to build a huge-diameter mirror that does not weigh much—a feat that will require developing new technology. “Right now we can make thick optics that can perform very well,” she says. “But the challenge is to make thin optics that can perform consistently.”

To look into the past, NASA must see the future

That challenge and all the others involved in planning these telescopes, not to mention hurdles the teams cannot anticipate, will take awhile to tackle. At the end of April, the teams will give NASA their initial thoughts on the task at hand. Then, in August they must each submit a “study plan,” detailing their timelines and the resources they will need to define the goals, scope and cost of each telescope. After two years of work, in March 2019 the teams will submit their reports, each laying out the best case they can make for each observatory to come to fruition. The Decadal Survey panel will then rate and rank the projects, advising the space agency on which to pursue, at which point NASA will take the first steps toward realizing one of these scopes for launch in the 2030s.

Because of its multigenerational timeline, launching a flagship mission is like building the pyramids—if, from the pyramids’ peaks, you could see back to the beginning of galaxies, peer into planets’ atmospheres and watch the births of supermassive black holes. But only a flagship NASA mission can do that—and in a few years, we will know which future NASA will choose.