It wasn’t that long ago that the only known planets in our galaxy were those orbiting our own sun. But over the past few decades, astronomers have discovered thousands of exoplanets and concluded that they outnumber the stars in our galaxy. Many of these alien worlds have fantastic properties, such as planet-wide oceans of lava or clouds that rain iron. Others may have conditions strikingly similar to Earth. We’ll never be able to travel to these distant worlds to see for ourselves, but an audacious mission to interstellar space may allow us to admire them from afar.
Last week, NASA’s Innovative Advanced Concept program announced its new cohort of scientists who will spend the next year developing space mission concepts that sound like they were plucked straight from science fiction. Among this year’s NIAC grants are proposals to turn a lunar crater into a giant radio dish, to develop an antimatter deceleration system, and to map the inside of an asteroid. But the most eye-popping concept of the bunch was advanced by Slava Turyshev, a physicist at NASA’s Jet Propulsion Laboratory who wants to photograph an exoplanet by using the sun as a giant camera lens.
It’s an idea based on a century-old theory first floated by Albert Einstein, who calculated that a star’s gravity would cause light from another star to bend around it, effectively creating a giant lens. If you were to stand at the focal region where the bent light converges, the “solar gravitational lens” would significantly magnify whatever was behind the star. Einstein’s theory about gravitational lensing is now a well-established fact. Observational cosmologists regularly use the gravitational lensing from galaxies and galaxy clusters to study more distant objects.
Turyshev’s plan would take advantage of this effect by sending a telescope on a 60 billion-mile journey to the sun’s focal region to photograph a habitable, Earth-like exoplanet that is up to 100 light years away. He calculates that sending a telescope just one-third the size of the Hubble Space Telescope to the sun’s focal region could produce a megapixel-quality image of an exoplanet after a few years of snapping photos. If the targeted exoplanet is about the size of Earth, each pixel would cover 35 square kilometers. Turyshev says that would be better resolution than the famous “Earthrise” photo taken by Apollo 8 astronauts, and more than enough definition to make out surface features and any signs of life on the exoplanet’s surface.
“The primary motivation for everyone contributing to this project is to move this idea from science fiction to reality, so that the current generation of people living on this planet can enjoy images of an alien world,” says Turyshev. “‘Are we alone?’ is a question we all ask, and we may be able to answer it within our lifetime.”
Snapping photos of our extraterrestrial neighbors is an enticing idea, but the technological challenges involved with this mission are staggering. First, consider the sheer distance: 60 billion miles is about 16 times further from the sun than Pluto. If you were traveling at the speed of light, it would take more than three days to cover this distance. Voyager 1, which has ventured further into interstellar space than any other human-made object, has only traveled about 13 billion miles—and it took the spacecraft 40 years to get there.
Simply getting the spacecraft to the right place is a major challenge. Unlike a camera lens, the sun doesn’t have a single focal point, but a focal line that starts around 50 billion miles away and extends infinitely into space. The image of an exoplanet can be imagined as a tube less than a mile in diameter centered on this focal line and located 60 billion miles away in the vast emptiness of interstellar space. The telescope must align itself perfectly within this tube so that you could draw an imaginary line from the center of the telescope through the center of the sun to a region on the exoplanet.
To image the exoplanet, the telescope moves around within the tube taking a photo at each new position, which represents a new view of the exoplanet’s surface. Since each position corresponds to one pixel in the final image, the telescope must point with extreme accuracy and maintain this accuracy for exposure times ranging anywhere from a few minutes to several hours.
The difficulties don’t end there. When the sun’s gravity magnifies an object, it doesn’t produce a coherent picture like a camera lens. Instead, the image is smeared around the edge of the sun in a halo called an Einstein ring. This halo appears inside the sun’s corona, its fiery outer atmosphere, which both distorts the image and overwhelms it with brightness. Each Einstein ring corresponds to one pixel in the final image and contains a mixture of the reflected light from a small region of the exoplanet’s surface and the rest of the planet. To capture the full image of the exoplanet, the telescope must pick out the faint signal from the Einstein ring against the overwhelming background noise of the sun’s corona, extract this signal, and then use deblurring algorithms to recover the relevant data. To create a megapixel image, it must repeat this process a million times.
Turyshev and his colleagues had to design a unique mission structure to handle these extreme challenges. Traveling 60 billion miles within a human lifetime isn’t possible using conventional propulsion technology like rocket motors. Instead, Turyshev wants to use fleets of small spacecraft outfitted with solar sails, each not much larger than a microwave. The spacecraft would start their journey by passing within about 6 million miles of the sun. The solar gravity assist, plus the boost from sunlight pushing on the solar sails like wind acting on a sailboat, would whip the spacecraft up to 300,000 miles per hour. This is similar to the speeds achieved during a recent solar pass by the Parker Solar Probe, the fastest spacecraft ever built.
At these speeds, it would take the spacecraft about 25 years to reach the beginning of the sun’s focal region in interstellar space. Each spacecraft in the fleet would be carrying a component of the telescope, and along the way they would assemble the telescope. Once the telescope arrives at its destination, it will have to rely on AI systems to do its work; waiting nearly four days for commands from Earth simply won’t cut it. The telescope will also need some beefy onboard processing to perform the signal analysis needed to make sense of the data.
NIAC grants are doled out in phases that range from concepts that are little more than an idea (phase I) to those that are basically ready to become a real mission (phase III). Turyshev’s plan to take a high-resolution photograph of an exoplanet is only the third project to receive a phase III grant in NIAC’s history.
But not everyone shares Turyshev’s optimism about the mission’s prospects. Pontus Brandt is a physicist at Johns Hopkins University Applied Physics Laboratory who is also working on an interstellar mission concept for NASA. Although he acknowledged that Turyshev’s proposal is “theoretically very attractive,” Brandt says there are “a lot of pitfalls that may make this not feasible.” In particular, he has raised concerns about the precision of the telescope, which he says would have to demonstrate a pointing accuracy 300 times greater than that of the Hubble Space Telescope while in the uncharted wilds of deep interstellar space.
Brandt also says he is skeptical that there is solar sail material that can withstand the extreme accelerations and temperatures experienced by the spacecraft as it leaves the solar system. “It will fold backwards like an umbrella,” Brandt says. “I haven’t seen solutions for mechanical structures that can maintain such force.”
There’s also the issue of finding a suitable target, which Turyshev says should be a planet with Earth-like properties. Given the amount of time and material resources that will be needed to make the mission happen, we don’t want to take a photo of a cold, dead world. But of the thousands of exoplanets discovered to date, only a few have properties that make them potentially habitable, meaning these planets are rocky, roughly the size of Earth, and orbit their host star at distances that allow for liquid water to exist on their surfaces. The technological constraints of the mission mean the planet must be located within about 100 light years of our solar system if we want a megapixel-quality photo. In a best-case scenario, our first photo of an exoplanet will reveal signs of life such as vegetation. If intelligent life exists, we might even detect large-scale infrastructure.
But at this point, astronomers have yet to definitively conclude that any of the potentially habitable exoplanets discovered so far are in fact habitable. Even the definition of what constitutes a habitable planet is still an area of active debate, says Nikole Lewis, an astronomer at Cornell University who studies exoplanet atmospheres. She says that a new generation of exoplanet hunting telescopes, such as the recently launched Transiting Exoplanet Survey Satellite and the forthcoming James Webb Space Telescope, will help astronomers discover many more potentially habitable planets, albeit around stars that are smaller than our sun. “The characterization of an Earth-sized planet in the habitable zone of a sun-like star required to dub it ‘habitable’ will likely have to wait for future facilities that employ new technologies,” Lewis says.
As part of the phase III NIAC grant, Turyshev and his colleagues will be working to address many of the technological issues with the proposed mission. Turyshev says one of the goals is to develop a technology demonstration mission and launch it in the next few years. This would involve outfitting a spacecraft with solar sails, whipping it up to extremely high speeds, and then photographing some objects in our solar system. He suggested chasing down an interstellar object as it passes through our inner solar system as an example of a good potential target for the mission.
“By the end of phase III we would like to get commitments from NASA and industry partners for a technology demonstration mission,” says Turyshev. “We would like to get as close to reality as possible.”
There’s no guarantee that the mission to photograph an exoplanet will come to fruition, but Turyshev says it could launch as soon as the early 2030s if NASA decides to pursue it. Given a 25-year travel time and a few years to gather the data, that means we could conceivably have a high-resolution photo of an alien planet as soon as the early 2060s. It would be one of the most ambitious missions ever undertaken, and the odds of success are long. But it also stands to revolutionize our understanding of the universe and our place within it. “It’s through dreamers like Slava that these things actually happen,” says Brandt. “Sometimes it's too crazy to be true, but he’s a dreamer that hasn’t given up.”