Many SF readers will know Arthur C. Clarke’s 1973 novel Rendezvous with Rama, in which an alien starship passing through the solar system is at first mistaken for an asteroid. Twenty years after the novel’s publication, the first nonfictional interstellar objects were found: interstellar dust grains measured by the Ulysses spacecraft. It took another twenty-five years for the first two bigger-than-a-breadbox interstellar objects to be discovered.

The first, 1I/‘Oumuamua (1I means “first interstellar object” while “‘Oumuamua” is a Hawai’ian name translated as “scout” or “first to arrive”) was elongated like Rama and had other unusual properties that led a few scientists to conjecture that it was an alien spaceship. “Rama” was an early contender for its name. The second interstellar object, 2I/Borisov (named after its discoverer), was clearly a comet and puzzlingly looked nothing like 1I/‘Oumuamua. While they’re not alien spaceships, these first known visitors from other solar systems are still very exciting to astronomers. They provide an important window into how planets form and planetary systems evolve.

Like comets and asteroids native to our solar system, interstellar objects like 1I/‘Oumuamua and 2I/Borisov are difficult to discover because they’re small, moving quickly, and relatively rare. 1I/‘Oumuamua was discovered when it was already on its way out of the solar system; it was first noticed by astronomer Rob Weryk in images taken with the Pan-STARRS sky survey in October 2017. 2I/Borisov was discovered by Crimean amateur astronomer G. Borisov in August 2019, a few months before its closest approach to the Sun. Compared to 1I/‘Oumuamua, 2I/Borisov was easier to detect, being brighter than some solar system comets seen at the same distance from Earth.

Observing the sky positions of 1I/‘Oumuamua and 2I/Borisov over multiple nights allowed astronomers to determine the details of their orbits. Everything about the orbits was unusual: both objects were found well away from the plane of the solar system and moving very fast, much faster than the solar system escape speed at their location.

Compared to long-period solar system comets with similar trajectories, the orbits of 1I/‘Oumuamua and 2I/Borisov were highly hyperbolic, meaning that these objects were not gravitationally bound to the Sun. Both entered the inner solar system, did hairpin turns, and headed back out of the solar system, their entire visits having lasted only a few months. While 1I/‘Oumuamua didn’t resemble a typical solar system comet, 2I/Borisov did, developing a tail and coma as it neared the Sun and even undergoing some fragmentation.

Where did 1I/‘Oumuamua and 2I/Borisov come from? Most experts think they are likely “planetesimals”—small lumps of rock and ice left over from the formation of their stellar systems and ejected by gravitational interactions, for example with a planet in the system or with a passing nearby star. Even though we know the directions from which both objects entered the solar system, that doesn’t tell us their points of origin. Although they were both traveling very fast—far from the gravitational influence of the Sun—their speeds were about twenty-six and thirty-two kilometers per second, or about fifty-nine thousand and seventy-two thousand miles per hour—even at those speeds it takes tens of thousands of years to travel the distance to the nearest stars.

Over that time, the Sun and other stars change position within the Milky Way galaxy, moving in orbits that can be quite complex. To date, astronomers haven’t pinpointed definitive origins for either object. 1I/‘Oumuamua’s speed indicates that it likely came from somewhere within about three thousand lightyears, probably from one of the associations of young stars in our Milky Way neighborhood. One attempt to trace 2I/Borisov’s orbit back in time found a close encounter with the nearby red dwarf star Ross 573, about nine hundred and ten thousand years ago. The authors of the paper describing this research are careful to explain that a close encounter doesn’t prove Ross 573 was the comet’s original host: it could have come from somewhere else and been deflected by the star.

Are there more of these interstellar wanderers? Almost certainly. Some may have already visited our solar system and not been recognized as interstellar, if their orbits weren’t measured well enough. Using even the few definitive detections and a knowledge of the efficiency of the detection systems, it’s possible to estimate the size of the potential interstellar visitor population. That size is truly enormous: every cubic lightyear of our galaxy contains something like a hundred trillion (1014) objects, which combined have the mass of four Earths.

To get this many objects over the whole galaxy, every star has to be ejecting material to the tune of about ten quadrillion objects each. These are huge numbers but remember that the solar system is very small compared to the galaxy: estimates for the number of interstellar objects in or near the solar system at any given time range from ten to a hundred. Finding them should be made easier by the soon-to-begin Legacy Survey of Space and Time (LSST) project, which will survey the sky from Chile about once a week, using a large telescope able to detect much fainter objects than Pan-STARRS. Estimates are that LSST will detect about one or two interstellar objects per year.

Is the Earth in danger of being hit by these things? A little, but the danger is pretty small compared to homegrown asteroids that are gravitationally bound to the Sun. According to one study, non-interstellar asteroids are about ten thousand times more likely to collide with the Earth than interstellar ones. Solar system asteroids have a range of sizes, with many more small bodies than larger ones, so the more likely collisions are with smaller objects. Assuming interstellar objects have the same range of sizes, this implies that interstellar objects one hundred meters or larger have struck Earth perhaps twenty-five to fifty times in its four-point-six billion year history.

If they’re (mostly) not going to hit us, could we visit them instead? It’s feasible, but not easy. Since the discovery of 1I/‘Oumuamua and 2I/Borisov, many research papers have laid out designs for space missions to visit an interstellar interloper during its brief sojourn near the solar system. Even getting a small spacecraft to an interstellar object would push the limits of current technology, although at least one paper says it’s not too late to get to 2I/Borisov if a Jupiter gravity assist is used.

The two big problems are getting a spacecraft to go fast enough to catch an interstellar object, and then slowing down to match the interstellar object’s velocity on arrival. (Slowing down isn’t absolutely required—for example, the New Horizons mission didn’t slow down and go into orbit around Pluto—but a flyby means data can only be gathered for a limited time.) Flybys of the Sun, Jupiter, and Saturn can help with the speedup; electric or magnetic sails could help with the slowdown. It also helps a lot to have enough lead time to launch an intercept mission: 1I/‘Oumuamua was detected about a month after its closest approach to the Sun, and 2I/Borisov only a few months before. LSST will help with the issue of lead time: it will improve detection rates enormously, and thus increase the chances that an interstellar visitor will be detected earlier. One study in The Planetary Science Journal predicts that visitable objects will be detected about every ten years.

Could we catch a ride on one of these visitors? There’s no reason why not, if we could build a spacecraft fast enough to rendezvous with one and the object’s surface had properties that would permit attachment. We don’t know a lot about how rough or smooth their surfaces might be. Burying your interstellar spacecraft inside an asteroid or comet could help to protect it from impacts with interstellar dust and meteoroids. The downside of hitchhiking is a lack of control over where the hitchhiker goes and a long wait for it to get anywhere.

Although big enough to catch a ride on, 1I/‘Oumuamua and 2I/Borisov are so small that they only showed up as points of light in images from even the largest telescopes. To get a clue to their true sizes, astronomers used the same technique used for solar system asteroids and comets. These objects shine by reflected sunlight, so by measuring an object’s average brightness, its distances from the Sun and the Earth, and the shininess of its surface (for example, is it covered in dark, rough rock or glittery ice?), astronomers can estimate its size. It’s also possible to use brightness variations with time to estimate shape: a tumbling or elongated object will reflect different amounts of light at different times as it presents different faces to the Sun. 2I/Borisov is estimated to be a few hundred meters across, with only tentative brightness variations indicating that it’s quite round and rotating very slowly.

On the other hand, observations of 1I/‘Oumuamua implied that it was tumbling and nowhere near round or even potato-shaped like many solar system asteroids. Rather, the observations were most consistent with 1I/‘Oumuamua being either long and thin—almost needle-shaped—or flat and pancake-shaped. Such shapes are unheard of among solar system asteroids and caused a lot of speculation about the nature of 1I/‘Oumuamua.

Analyzing the light from 1I/‘Oumuamua and 2I/Borisov with spectrographs allowed astronomers to detect the presence of specific minerals or chemical elements on the objects’ surfaces. The spectroscopic observations of 1I/‘Oumuamua showed no clear detections of specific molecules, just an overall red color fairly typical of the icy Kuiper Belt objects found in the outskirts of our solar system. Spectroscopy of 2I/Borisov showed molecules like cyanide, diatomic carbon, carbon monoxide, and hydroxide, all commonly found in solar system comets.

When molecules escape from a comet’s surface, they can act like tiny rocket thrusters, causing “nongravitational acceleration” and affecting its orbit. 1I/‘Oumuamua’s orbit indicated that it was undergoing nongravitational acceleration but puzzlingly, there was no evidence that it underwent the same kind of outgassing as 2I/Borisov. (This could have been because there wasn’t time to make the most sensitive observations of 1I/’Oumuamua before it got too far from the Sun.) Besides outgassing, another possible cause of nongravitational acceleration in small solar system objects is radiation pressure from the Sun, the same phenomenon that accelerates light sails. If radiation pressure had caused 1I/‘Oumuamua’s measured nongravitational acceleration, the object would have to be something much different from a typical asteroid or comet. Calculations showed it would have to be less than a millimeter in thickness and a few thousand kilograms in mass.

Does this mean that 1I/‘Oumuamua is an alien spacecraft? Astronomer Abraham (Avi) Loeb coauthored the first paper on the idea of 1I/‘Oumuamua as a light sail and recently published a book on the topic: Extraterrestrial: The First Sign of Intelligent Life Beyond Earth. In the book he argues that astrobiologists, in their desire to distance themselves from fringe claims about UFOs and alien abductions, are excessively conservative in their willingness to consider anything other than natural explanations for 1I/‘Oumuamua’s properties.

However, mainstream astronomers have largely dismissed Loeb’s claims. While he is a well-known scientist who has held leadership positions in the astronomical community, his expertise is not in planetary science or in observational searches for asteroids and comets. Loeb is also well-known for advocating that scientists should generate many unorthodox ideas because one such idea might be right; many astronomers point out that Loeb doesn’t always subject his own wacky ideas to peer review.

Arguments against the interstellar spacecraft hypothesis contend that 1I/’Oumuamua’s properties are consistent with it being a natural object. Other configurations than a light sail are consistent with the observations: for example, 1I/‘Oumuamua could be a “dust bunny” made up of extremely porous dust grains that are just barely held together, or a nitrogen icicle ejected by the impact of an asteroid or comet on a Pluto-like dwarf planet in another solar system. While the light sail hypothesis could explain 1I/‘Oumuamua’s elongation and nongravitational acceleration, the orientation that a light sail would need to have is inconsistent with the variations observed in 1I/‘Oumuamua’s light curve. Other authors have argued that there is no good reason for aliens to send a spacecraft to our solar system. With telescope technology not that much more advanced than ours, they could find out everything about our solar system that a spacecraft visit would tell them, and much more quickly. A flyby visit, such as 1I/’Oumuamua’s, doesn’t allow much time to gather data in any case.

So where does this leave us? Still with no undisputed interstellar spaceships. While 2I/Borisov seemed to be a reasonably standard comet, the nature of 1I/‘Oumuamua is still unsettled, and in particular its unusual shape and rotation are still not well-explained. Both are now too far from Earth to be detected but analysis of the previous observations continues. Now that we know these objects exist, substantial work has gone into planning observations of the next ones so that we can make the most of their limited time in our solar system. The universe has come to us in a fascinating way, and we will be able to use that knowledge to better understand how planets form and change throughout our galaxy.

Further Reading:

• “The Natural History of ‘Oumuamua” is a somewhat technical but still accessible summary of what astronomers know (and don’t) about this object. https://arxiv.org/pdf/1907.01910.pdf
• “Sending a Spacecraft to Interstellar Comet 2I/Borisov” describes what it would take to reach the comet with current technology. https://arxiv.org/pdf/1909.06348.pdf
• “Research Programs Arising from ‘Oumuamua Considered as an Alien Craft” describes interesting research questions that come out of the idea of 1/‘Oumuamua as a spaceship; many are still worth investigating even if the hypothesis is disproved. https://arxiv.org/pdf/2111.07895.pdf

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