Visitors from the Vast Beyond

This past week the interstellar comet 3I/ATLAS buzzed past Mars. At the end of this month it will make its closest approach to the Sun before continuing the cosmic journey on which our solar system is a mere waypoint (zipping by Jupiter on its way out). This seems like a good time to talk about interstellar objects, what we know about them, and what we can learn from them!

So let’s take a look at these visitors from the vast reaches of space—the few we know about so far, anyway! 

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Identifying the Tourists

In October 2017 an observatory in Hawaii spotted a previously unknown object about 21 million miles (33 million km) from Earth. Originally thought to be a comet, it was given the designation C/2017 U1, which was changed to A/2017 U1 when it didn’t show normal comet-like activity (C to A = comet to asteroid). A perfectly normal designation for a perfectly normal object. 

Within a week of that first observation though, it became apparent that A/2017 U1 was not that at all. As with any newly discovered object, a series of observations were made to determine its speed and trajectory and to use these to create an orbit model. When the numbers were plugged in, a jaw-dropping discovery was made—A/2017 U1’s path reached way out into the vastness of space beyond the realm of the Sun’s control.

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This meant A/2017 U1 had come flying in from outside the solar system, got its path radically bent by the Sun, and was, even when first discovered, already on its way back out with a speed that guaranteed it would make its eventual return to interstellar space and leave our solar system far behind. A/2017 U1 was an interstellar object.

An interstellar object is anything that is drifting through space untethered to a star. These things form around a star and then get chucked out into space, usually through gravitational interactions of one form or another. We’ve known about rogue planets, which are planetary-mass interstellar objects, since 2000. And certainly if it’s possible for big things like planets to be flung out of solar systems, it’s even easier for tiny things like comet cores, orbiting far from the gravitational stability of their star, to get ejected.

So it only makes sense to assume that interstellar space is full of random bits of detritus, mostly in the form of icy cometary bodies that tend to lurk near solar system’s edges. But until A/2017 U1, we’d never seen one inside our solar system before. And that required a name change.

1I/’Oumuamua

In honor of this first known interstellar object in our solar system the International Astronomical Unit created a new letter designation, “I” for Interstellar. As the first, A/2017 U1 was redesignated 1I/2017 U1 before also being given the more charismatic name of ‘Oumuamua, a Hawaiian word meaning “messenger from afar arriving first”, sometimes interpreted as “scout”. 

And ‘Oumuamua proved to be weird. I just said that we suspect most interstellar objects will be of the comet variety and that’s true, but ‘Oumuamua didn’t look or act like a comet. It’s shaped like a slab, thought to be around 377 x 364 x 62 feet (115 x 111 x 19 m). We know of no comet, or asteroid for that matter, that looks like that.

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What’s more, ‘Oumuamua moved oddly, accelerating as if it was outgassing (like a comet), but not obviously showing a gas tail. This led to speculation that it was, in fact, an alien spacecraft, a claim that got a lot of attention in certain circles but has absolutely no evidence to back it up.

The latest thinking is that ‘Oumuamua’s odd acceleration was caused by outgassing, but of either nitrogen or hydrogen, gases that don’t make a bright, visible tail. This also nicely fits a recent theory that ‘Oumuamua is a chunk of nitrogen ice blasted from the surface of a Pluto-like object by an impact. That would explain why it never behaved like a typical comet—it’s not one. But it could be the first direct evidence of exo-Plutos in other solar systems.

And where was that solar system, the one that birthed and ejected ‘Oumuamua? Unknown. It came, roughly, from the direction of the star Vega, but it’s been traveling for so long that 300,000 years ago, when it might have been at the distance from us that Vega is now, Vega wasn’t there yet. About a million years ago it would have been in the vicinity of a small red star named HIP 3757.

Perhaps that’s where it came from, but backtracking it to a specific solar system could be a mug’s game. After all, its passage by the Sun dramatically bent its trajectory, sending it winging off into the darkness in the vague direction of Pegasus. Any future eyes that spot it in other systems and hope to find where it came from might be able to trace it back as far as the Sun (or perhaps not even that far) but figuring out just how the Sun’s gravity changed its path would be challenging.

2I/Borisov

Given how exciting and strange the first interstellar object proved to be, you can imagine how psyched everyone was in 2019 when a new comet designated C/2019 Q4 Borisov (after its discoverer, amateur astronomer Gennady Borisov) turned out to be 2I/Borisov, our solar system’s second known interstellar visitor.

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And, as it turned out, the first interstellar comet, because Borisov was definitely a comet. It behaved exactly like a comet should as it approached the Sun. If anything Borisov turned out to be a little boring, because it mostly looked and acted just like a comet from our own solar system, with only a few minor differences.

For one thing, it showed signs of being richer in carbon monoxide than our comets, suggesting it formed around a star that is cooler than the Sun. But that’s not surprising, most stars are smaller and cooler than the Sun. Something like 75% of the Milky Way’s stars are red dwarfs.

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Borisov never came close enough to Earth to get a great measurement of its size. It’s maybe about a mile (1.6 km) or so across, though it’s definitely not as big as it was when it first came into the solar system. Borisov is thought to have lost up to 0.4% of its mass before it reached its closest approach to the Sun purely though loss from outgassing.

That’s far more than normal comets lose. This, along with other observations of the comet’s makeup, suggests that Borisov had never encountered a star before. It was a pristine piece of a whole ‘nother solar system! Again, identifying that solar system is probably impossible, though at least one paper identified a red dwarf named Ross 573 as having been within 14,000 AU (Earth orbits the Sun at a distance of 1 AU, Pluto at about 45 AU) of Borisov 910,000 years ago. Is that Borisov’s origin point? Eeeeh, probably not. But it’s technically possible.

3I/ATLAS

This summer, on July 1, 2025, the Asteroid Terrestrial-impact Last Alert System, aka ATLAS (which is an alarming-sounding title but really means someone wanted to have the acronym be ATLAS) discovered comet C/2025 N1 ATLAS. By July 3^rd I was reporting in the Spacing Out newsletter that astronomers suspected this comet might have interstellar origins, and by the July 10^th newsletter it had the designation 3I/ATLAS.

Measurements from the Hubble Telescope put ATLAS’s size somewhere between 0.2-3.5 miles (0.32-5.6 km) across—and yes that’s a big uncertainty. ATLAS is surrounded by a very reflective envelope of gas (the coma) that makes getting a good size estimate hard, and the first size estimates were significantly larger.

Like Borisov, ATLAS is a lot like comets we see in our own solar system, though its coma has an unusually high ratio of carbon dioxide in it. This could mean the place it formed in had a higher carbon dioxide ratio than our own solar system, or it could mean something about the makeup of the comet, such as a crusty layer preventing slowing the release of other, non-carbon dioxide ices.

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And, of course, we have no idea what solar system this comet came from, but unlike ‘Oumuamua or Borisov its trajectory is hardly being affected by its passage by the Sun. It’s flying past far enough from the Sun (only just inside Mars’s orbit at closest approach) and fast enough that our solar system won’t leave much of a mark on its journey.

That path did, however, bring it near Mars on October 3^rd and on its way back out it will pass close by Jupiter on March 16^th, close enough for our Mars and Jupiter spacecraft to observe. Those observations may represent our best views of ATLAS before it disappears back out into the void. At this point the Mars orbiters have already done their thing. The government shutdown may make it take a bit for pics from NASA orbiters to be released, but it won’t affect ones from ESA orbiters.

Future Visitors

While we only know three interstellar visitors so far, we suspect that they’re here all the time. The Planetary Society reports an estimate that an average Milky Way solar system will chuck out 10 quadrillion objects into interstellar space over its lifetime. Astronomers estimate that at any given time there are 60 interstellar objects inside of Jupiter’s orbit. The tricky part is finding them before they take off again.

This is where tools like the new Vera Rubin Observatory can help. It’s estimated that Rubin will help push our list of known interstellar objects passing through our solar system into the dozens. And that could prove invaluable.

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Unlike almost any other kind of science, astronomy rarely gets to look at things up close. With the exception of planetary scientists, most astronomers need to learn about places that are unfathomable distances away by teasing every possible bit of information out of the meager amount of light we can collect with our telescopes.

Interstellar objects change that game. They’re science samples from the universe, a chance to study fragments of completely different solar systems up close, an excessively rare opportunity in astronomy. They could represent an incredible scientific boon!

Of course, we have to find them first. The hunt is on.