Before dawn on August 30, an amateur astronomer named Gennady Borisov spotted an alien visitor: a comet from beyond the confines of our solar system. That object, now named 2I/Borisov, is just the second confirmed interstellar object ever detected inside our cosmic neighborhood.
Now, as large telescopes around the world turn their gaze toward Borisov, astronomers are beginning to glean tantalizing clues about the comet’s composition and its track through the heavens. Each new data point reveals a bit more about the chemistry of Borisov’s home star system and, in turn, how unique our own solar system is within the cosmos.
The first known interstellar interloper, a comet called ‘Oumuamua discovered in 2017, wasn’t found until it was already headed out of the solar system. Studying it put astronomers through their paces as they raced to decipher some of the comet’s more enigmatic traits, including its bizarrely oblong shape. But ultimately, ‘Oumuamua left more questions than answers in its wake.
By contrast, Borisov is currently incoming, and views should only get better toward the end of the year: “We’ll be able to do a hell of a lot more, in terms of compositional studies,” says Queen’s University Belfast astronomer Michele Bannister.
Still, the timing is tight, and astronomers are now scrambling to file proposals with some of the world’s biggest telescopes to ensure that they can gather the most possible detail.
“Man, this was frustrating—we were not ready, we had not done our homework yet,” says Karen Meech, an astronomer at the University of Hawaii’s Institute for Astronomy. “I guess that’s nature’s way of saying, You better not be unprepared!”
That’s why astronomers can’t resist sifting through the early data and getting their first good look at a fragment from another star system.
“Basically my whole career we’ve been waiting for this, and it’s exciting to see it finally happen,” says Luke Dones, an astrodynamicist at the Southwest Research Institute who studies comets.
Perhaps the most extraordinary thing is just how ordinary Borisov appears. Early results collected by astronomers in Spain, Poland, and the Netherlands show that visually, the object looks a lot like our solar system’s own comets, with a reddish surface made of a carbon-rich schmutz.
Borisov also resembles our comets in terms of composition. In a study submitted to Astrophysical Journal Letters on September 27, a group including Meech and Queen’s University Belfast astronomer Alan Fitzsimmons reported that Borisov is jettisoning about 170 grams of cyanide every second as sunlight warms up the comet’s icy surface.
The cyanide is no surprise; it’s one of the first things that astronomers typically detect in more locally sourced comets. And Borisov isn’t unusual in its gassiness. It’s less than half as effusive as the comet Siding Spring was during its 2014 flight through the inner solar system.
Researchers can also place some rough constraints on the size of the cometary core, or nucleus, embedded within its halo of gas and dust. Right now, it seems that the nucleus is no bigger than five miles wide, and it’s more likely between half a mile and two miles wide.
“It seems ironic that ‘Oumuamua was so weird, and this object seems pretty much like a typical solar system comet,” Dones says.
In addition, a team of Polish astronomers has taken a first crack at trying to trace Borisov’s path through interstellar space. As they describe in a paper uploaded to the preprint platform arXiv, the team suggests that Borisov passed within 5.4 light-years of a star system named Kruger 60 about a million years ago.
At that time, they calculate that Borisov was moving at the fairly pedestrian velocity of 7,700 miles an hour, and though the researchers stress that the analysis will change with more data, that might mean Kruger 60 is a plausible home system for Borisov, they say.
The Polish team is already well-respected for doing this kind of work, publishing similar looks at comets that formed in the solar system’s deep outskirts. But at this point, any effort to retrace Borisov’s steps will run into intense skepticism.
“I think it’s totally premature,” Meech says. For starters, while we know Borisov’s trajectory well enough to confirm that it’s interstellar, there’s still some uncertainty about its exact path through space. In part, this is because we’ve observed the object for only a short time. It’s also because Borisov is shrouded in dust and gas, which makes its center hard to pinpoint.
Borisov is also still in the low night sky, near the horizon. This means light from the comet passes through more air before it reaches our telescopes, letting the atmosphere layer on even more potential error.
“What [my colleagues] are seeing ... is huge chunks of the data that have terrible accuracy,” Meech says. “It’s not because everyone’s being sloppy, but it’s really hard to measure the exact center of a fuzzy object.”
As sunlight heats up Borisov, the dust and gas flying off its surface can also act like rocket exhaust and nudge the comet’s trajectory. Astronomers don’t yet have a good handle on these non-gravitational forces, so they can’t account for them in their calculations.
On top of that, backtracking Borisov’s path requires rewinding the clock and determining where stars in the Milky Way used to be. But right now, the world’s best mapping dataset for the Milky Way—courtesy of the European Space Agency’s Gaia satellite—gives 3D motion data for only a small fraction of the galaxy’s total stars.
“You have to make the three-dimensional movie of the galaxy and play it backward, with gravitational perturbations, and even then, you don’t get the origin star—you get the last port of call,” Bannister says.
As it stands, these errors are stymieing far shorter-term projections. Marc Buie, an astronomer at the Southwest Research Institute, would love to watch Borisov pass directly in front of several stars, which would let him discern the comet’s size and shape more exactly. But to plan out those observations, Buie says that Borisov’s orbit would need to be known thousands of times more precisely than it is now.
“If you don’t know where it is, and we don’t know where it’s going on the timescale of a year in our solar system, what hope do you have of knowing exactly which star it came from a million years ago?” he says. “I’m very, very skeptical of our ability to know where it’s come from at this point.”
Best is yet to come
Still, efforts to study Borisov are only just beginning. The comet will remain visible for the next year or so, and the best opportunities to see it are still ahead of us. The comet will make its closest approach to the sun around December 8, and proposals are piling up to keep an eye on the show using the world’s biggest telescopes, including the Hubble Space Telescope, the European Southern Observatory’s Very Large Telescope, and the ALMA telescope array.
For practical reasons, humans won’t be sending a last-ditch flyby mission to visit the careening comet. But researchers with the Initiative for Interstellar Studies, a U.K.-based nonprofit, did calculate that for a spacecraft to reach Borisov, the robotic emissary would need to launch in 2030 atop NASA’s not yet operational Space Launch System, which will be one of Earth’s most powerful rockets when completed.
The good news is that the next time objects like Borisov and ‘Oumuamua pass through our neighborhood, we’ll be ready. Astronomers expect that the Large Synoptic Survey Telescope, a huge new facility being built in Chile, will detect many more interstellar objects.
“The estimates before Borisov were that LSST, when it’s up and running ... is gonna be able to find one [interstellar object] a year,” Buie says. “But if this is really telling us that these are far more prevalent and far more varied in their appearance and attributes—wow, we could be looking at a really, really exciting time.”
Buie adds that LSST might detect interstellar objects even sooner on their approach, buying us more time to observe—and maybe even visit—them. Already, concepts for interceptors are getting off the ground. In June, the European Space Agency announced its Comet Interceptor mission, which will launch in 2028 and park in a gravitational neutral point off Earth. It will then lie in wait, available to fly by a newly incoming local comet—or maybe even an interstellar sequel.
“I think it’d be wonderful to have a spacecraft ready to go to one of these objects,” Dones says. “They’re just completely unexplored.”