Key Takeaways:
- A proposed interstellar mission envisions sending a gram-scale nanocraft, propelled by a laser-driven lightsail to a nearby black hole, achieving approximately one-third the speed of light.
- Upon arrival, smaller nanocrafts would be deployed to orbit or fly past the black hole, transmitting data back to Earth to test the predictions of general relativity in extreme gravitational environments.
- Technological hurdles include miniaturizing the nanocraft's components (including communication systems), developing high-energy lasers capable of providing the necessary propulsion, and overcoming the challenges of interstellar radiation and cold.
- The mission's feasibility hinges on the discovery of a sufficiently close black hole within the next decade, leveraging existing and upcoming telescopes like JWST and the SKA to detect faint emissions or gravitational perturbations.
Voyager 2 has traveled farther than any spacecraft. It’s a little more than 12.4 billion miles (20 billion kilometers) away, just beyond the edge of our solar system. But Fudan University astrophysicist Cosimo Bambi is already thinking about how we might study black holes a generation from now: by sending tiny spacecraft on a decades-long interstellar trip to one nearby. In orbit around a black hole, a spacecraft no larger than a paper clip could test our biggest ideas about how the universe works.
Putting general relativity to the ultimate test
If Bambi has his way, then 20 or 30 years from now, a nanocraft — a tiny spacecraft, weighing just a few grams — will unfurl a 33-foot-wide (10 meters) lightsail in low Earth orbit. High-energy lasers on Earth will blast the sail with light, and the pressure of those photons will accelerate the spacecraft to about a third the speed of light in a matter of minutes. For the next 75 years, the tiny craft will cruise across the vastness of space to a rendezvous with one of the most mysterious objects in our universe: a black hole.
Physicists think the super-high-gravity region near black holes may be the one place in the universe where general relativity (the theory that describes the fabric of our universe, spacetime, and how gravity affects it) breaks down. But we won’t know that for sure until we can actually measure what happens near the edge of one. That’s what Bambi hopes his ambitious idea would accomplish.
Here in our solar system, we’ve measured the mass of planets and moons by watching how a spacecraft’s orbit changes under their gravitational pull. Bambi says we can measure a black hole’s gravity the same way — just with much smaller spacecraft.
When the nanocraft finally reaches the black hole, it will release at least one — or maybe several — even tinier nanocraft. The swarm of insect-sized spacecraft would either slow down enough to be captured into orbit (somehow), or else fly past the black hole. Either way, the principle is the same: The mothership tracks the radio signals from its intrepid progeny. Their paths will trace how spacetime bends and warps in the area near a black hole.
When the nanocraft’s signal reaches Earth 25 years later, scientists can compare its data to the predictions of general relativity and other models. And then, a century after launch, we’ll know whether our models of how the universe works hold up even under the tremendous gravitational strain of a singularity.
How to sail to a black hole
“This is just a very vague idea, so there are many things to discuss,” Bambi tells Astronomy about his recent paper. “It is more to stimulate the community to discuss this possibility.” But it’s already possible to see the general shape of the mission.
Picture a sail made of a few square meters of specialized material, unfurled ahead of a minuscule spacecraft that’s basically a computer chip with a tiny radio transceiver attached. That’s the nanocraft Bambi hopes scientists will one day send to probe the gravity of a black hole.
At a bare minimum, interstellar nanocraft would have to be able to keep time (so they know when they’ve reached their destination), as well as send and receive radio signals. But packing much more than that onto a spacecraft designed to be propelled to relativistic speeds by a lightsail would be wildly impractical.
“Propelling a conventional camera with a lightsail is like trying to levitate a brick on tissue paper,” physicist and engineer Kevin Parkin, who runs the Mission Design Center at NASA Ames and formerly worked on NASA’s 100 Year Starship project, told Astronomy. And even that bare-bones setup would need to be miniaturized well beyond what current technology can accomplish — and designed to survive the harsh radiation and deep cold of interstellar space.
Some pieces of the puzzle, like power systems for the nanocraft and maybe even a camera, may be incorporated into the layers of material that make up the lightsail, using something called an optical phased array. But that particular technology is still what Parkin calls “known physics but not yet known engineering.” In other words, we understand how to describe it with equations, and it should be physically possible, but nobody has figured out how to build it yet.
And then there are the lasers. Bambi estimates that if we were to build an array of lasers energetic enough to shove a nanocraft into space at one-third light speed, the price tag would be around 1 trillion euros. Based on the general trend of laser costs (the price per watt gets cut in half every four years), the price could be down to about a billion euros in 30 years. That’s more in line with the budget of today’s large space missions.
All told, Bambi estimates that we’re about 20 to 30 years away from having the technology for his proposed black hole mission — but he thinks it’s a matter of time. Especially if we can actually find a black hole nearby.
First, we have to find one
In his recent paper, Bambi suggests that missions like Breakthrough Starshot, or others designed to visit exoplanets relatively close by, will probably launch before anyone sends nanocraft to a black hole. In part, that’s because the distance (and the necessary speed) is so much greater, and because it’s harder to design a spacecraft for the radiation and gravity around a black hole. And in part, it’s because those missions have something that his black hole concept still doesn’t have: a target.
The nearest black hole that we know of at the moment is 1,560 light-years away. There’s probably a black hole lurking within 25 light-years of Earth, but the problem is that we haven’t found it yet. Bambi says that could change within the next decade.
Astrophysicists estimate that our galaxy contains one black hole and 10 white dwarfs for every 100 “normal” stars. Based on the number of stars in our local neighborhood of the Milky Way, and what astrophysicists know about the life cycle of massive stars, it’s likely that there’s an undiscovered black hole not too far away. However, black holes are notoriously difficult to spot because they’re regions from which no light escapes.
Working together, some of the world’s most advanced telescopes might be able to spot the faint traces of radiation released by material being pulled into a black hole, even one feeding on the sparse material in interstellar space. That astronomical dream team includes the James Webb Space Telescope and the upcoming Square Kilometer Array (two arrays of radio dishes spread across wide swaths of land in Australia and South Africa), along with ALMA (the Atacama Large Millimeter/submillimeter Array) in Chile.
Astronomers might also be able to spot a black hole sharing an orbit with a star in the same way that they spot some exoplanets: by measuring slight wobbles in the star’s orbit, caused by the gravitational tug of its companion. Black holes drifting through the galaxy alone, without a companion star, might be revealed by the way their gargantuan gravity bends the light from stars in the background; that’s how astronomers spotted black hole OGLE-2011-13LG-0462 (a name almost as dense as the object itself) more than a decade ago.
“If we find a black hole which is not too far, I think the scientific community can be interested to discuss, okay, can we really send a probe to this object?” Bambi told Astronomy.
Welcome to the spacefaring Age of Sail
The idea of using a lightsail to catch laser beams and propel a computer chip to interstellar space isn’t new. The Japanese Aerospace Exploration Agency (JAXA) used a lightsail to propel its IKAROS spacecraft to a Venus flyby in 2010, and in 2019 the Planetary Society sent a sail-driven cubesat called LightSail-2 into low-Earth orbit. Both of these projects packed considerably more mass than a nanocraft (LightSail-2 clocked in at around 13 pounds [6 kilograms], and IKAROS at around 121 pounds [55 kg]), and both used sunlight, not lasers, to fill their sails. Of course, neither of these missions was traveling at anywhere near one-third light speed, so they could get away with more mass and less energy.
Since 2016, Breakthrough Starshot, a pet project of physicist-turned-billionaire-businessman Yuri Milner, has been working to develop lightsail-driven nanocraft. The project’s aim is to launch a fleet of the tiny craft to Alpha Centauri by the late 2030s. (The late physicist Stephen Hawking was a board member, as are Meta CEO Mark Zuckerberg and Harvard astrophysicist and alien probe enthusiast Avi Loeb.) But the basic idea has been kicked around in astrophysics and spaceflight circles since the 1970s, according to Bambi. The concept gained traction in the last decade or two thanks to the discovery of exoplanet systems within a few light-years of home.
“Only in the past 10 or 15 years [anyone], especially the exoplanet community, was interested in this kind of spacecraft, because it’s then that we know there are some stellar systems not very far from us,” says Bambi. “If you can do this with exoplanets, why can’t we do this even with black holes?”
Editor’s note: This story has been updated with an additional quote from Bambi.
