By Corey S. Powell

The interstellar visitors have arrived.

Two years ago, an automated telescope in Hawaii detected ‘Oumuamua, the first known object from interstellar space observed passing through our solar system. Then in August, an amateur astronomer in Crimea found a second interstellar visitor, 2I/Borisov, suggesting that such objects come our way on a regular basis.

Now, scientists are eagerly developing plans to explore these messengers from afar and to learn their secrets.

“Imagine something that’s traveled for hundreds of thousands or even millions of years through space, now reaching us,” says Andreas Hein, an aerospace engineer at the Initiative for Interstellar Studies in Charfield, England. “What will it tell us about its origin? What composition do planets have that orbit this alien sun? Does life propagate between stars?”

The path of interstellar comet 21/Borisov now passing through our solar system.

The path of interstellar comet 21/Borisov now passing through our solar system.
NASA, ESA, J. Olmsted, F. Summers (STScI). / ESA/Hubble

For now, answers are trickling in slowly from observatories on the ground and in orbit around Earth, including an intriguing new image of Borisov just taken by the Hubble telescope. Come 2028, though, a European spacecraft called Comet Interceptor could be on its way to a brief encounter with another, as-yet-undetected interstellar object.

Hein and his colleagues have an even grander project in mind. They’ve drawn up plans for Project Lyra, a space probe that could travel fast enough to overtake either ‘Oumuamua or Borisov (or another interstellar object) as it speeds its way out of the solar system, reaching either one by the mid-2040s.

To Hein, launching Project Lyra would be akin to building humanity’s first starship — and doing it on the cheap. “We won't have the capability of reaching another star system within the next few decades at least, but having the opportunity to study a big chunk of material from another star is a bit like flying to another star,” he says. “It is a literal version of, ‘If you can't go to the mountain, let the mountain come to you’.”

Inching toward the interstellar visitors

The many mysteries surrounding ‘Oumuamua (“oh moo-uh moo-uh”) have further boosted the desire to get to see one of these interstellar visitors up close.

Researchers anticipated that the vast majority of interstellar objects would be comets, but ‘Oumuamua sure didn’t look like one. It was strangely elongated, like a half-mile-long cigar, tumbling as it went. It also displayed no comet-like tail, and it sped up slightly as it moved away from the sun. These oddities led Harvard astronomer Avi Loeb to float the controversial idea that ‘Oumuamua might not be a comet at all, but an alien spacecraft.

In contrast, Borisov seems to closely resemble the comets in our own solar system. “It appears to be of a very different nature than `Oumuamua,” Loeb says. “But when you walk down the street and notice a weird person, the fact that later on you encounter many normal people does not take away the weirdness of that first unusual person.”

Alas, the European Space Agency’s Comet Interceptor will be unable to reach either of these objects. In fact, it wasn't designed to go after an interstellar object at all. The original plan was that, after its launch in 2028, the 1-ton spacecraft would park itself in orbit around the sun, waiting for a comet to arrive from the outskirts of our own solar system. Then it would fire up its engines, making an encounter and deploying smaller probes that could come within a few hundred miles of its target.

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The discovery of ‘Oumuamua and Borisov has the Comet Interceptor team reconsidering its plans. By one estimate, at any time there are about 100 interstellar objects within Jupiter’s orbit around the sun — and there’s a decent chance that at least one of them will come within range of the spacecraft.

“If an interstellar object happens to turn up with the right trajectory at the right time, it would be too good an opportunity to turn down,” says mission co-leader Colin Snodgrass, an astronomer at the University of Edinburgh.

Although Comet Interceptor’s flight past an interstellar visitor would be fast and brief, it could prove highly revealing. “It would be fascinating to see one up close,” Snodgrass says. “Either it would look like nothing we ever saw before, and would require new theories, or it would look surprisingly familiar, implying that there are some universal similarities in planetary systems.”

Hit the space accelerator

If you want to chase down ‘Oumuamua or Borisov to determine their true nature, you’ll need a spacecraft with a lot more oomph than Comet Interceptor. ‘Oumuamua is now more than 1 billion miles from Earth and speeding away from us at nearly 18 miles per second. Borisov will make its closest approach to the sun Dec. 7, and then it too will head back to interstellar space at high speed.

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That’s where Project Lyra comes in. The engineers at the nonprofit Initiative for Interstellar Studies came up with this concept based on their research into technologies for advanced space travel. The team concluded that the most promising way to reach an interstellar object is with a so-called Oberth maneuver, in which a spacecraft swoops to within 2 million miles of the sun and then fires its rockets full-blast to slingshot toward its target. Using a giant rocket like NASA’s long-awaited Space Launch System, combined with solid-rocket boosters, Hein and his group calculate they could get a speed boost of about 12 miles per second, just enough to do the job.

With a 2033 launch, this scheme could deliver a small but capable probe weighing perhaps 100 pounds to ‘Oumuamua by 2048. Slowing down at the other end would be another challenge. “Maybe we can use electric or magnetic sails,” Hein suggests, referring to a giant metal web that would create drag against the magnetic field and charged particles found in deep space.

Laser sail spacecraft arriving at 'Oumuamua, the interstellar asteroid.

A fleet of laser sail spacecraft arriving at 'Oumuamua.
Maciej Rebisz / Project Lyra

Surviving the sun’s intense heat may be an easier problem to solve. NASA’s current Parker Solar Probe, which will repeatedly swing within 4 million miles of the sun during the 2020s, is testing a carbon-composite heat shield that can effectively withstand temperatures of 2,500 degrees Fahrenheit.

Aliens closer to home

But there is another way to accelerate a mission to an interstellar object: Instead of going faster, aim closer. Loeb notes that not all of the incoming objects head back to the stars. He calculates that some might get deflected by Jupiter’s gravity and trapped into an orbit around the sun. If so, a conventional spacecraft could reach them in a matter of years.

“There are already a number of candidate objects known,” Loeb says. He cites in particular 2015 BZ509, a recently discovered asteroid that orbits near Jupiter but in the opposite direction of all the planets — a likely sign of an interstellar origin.

Loeb also proposes scouring the surface of the moon for debris left by other interstellar objects that might have crashed there over billions of years of lunar history. That’s a timely suggestion, since NASA’s Artemis program could send astronauts back to the moon as early as 2024.

The realization that so much interstellar material has passed through our solar system over the eons, with more coming all the time, provides another powerful motivation to study the incoming objects up close. “It is possible that a non-negligible fraction of them carry evidence for life,” Loeb says.

He’s talking not about derelict alien spacecraft but about possible extraterrestrial microorganisms or their remains, which in many ways would be just as shocking. Finding such evidence would open the possibility that alien life is raining down on us from afar — “between the stars and across the galaxy,” Loeb says — and that humans could be descended from extraterrestrial microbes that landed on Earth billions of years ago.

It’s a Hollywood-worthy twist: We’re looking far off for the interstellar visitors, but it could turn out that they’ve been here all along.

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