7 Big Mysteries about Interstellar Object 3I/ATLAS

Scientific American

17-07-2025

Earlier this month astronomers were thrilled to discover only the third known interstellar object ever seen in our solar system. Now dubbed 3I/ATLAS, the suspected comet has just zoomed past the orbit of Jupiter, traveling so fast that it's bound to slip through our sun's gravitational grip. The high speed and hyperbolic trajectory of 3I/ATLAS means it must have come from another star and was cast adrift in the Milky Way by some unknown process before it eventually, by chance, briefly swooped by our sun. It will reach about the orbit of Mars before it boomerangs back toward interstellar space, never to be seen again, at the end of this year.
That's why astronomers have been racing to study 3I/ATLAS since July 1, when Larry Denneau of the University of Hawaii first spied it using a telescope in Chile that's part of the globe-spanning Asteroid Terrestrial-Impact Last Alert System (ATLAS). Soon more powerful observatories, including the James Webb Space Telescope (JWST) and Hubble Space Telescope, will scrutinize the object—which, thanks to its alien, interstellar provenance may be the oldest comet anyone has ever seen.
'I didn't get any sleep for like 35 hours,' says Bryce Bolin of Eureka Scientific in California, who rushed to release a preprint paper and arrange additional observations following 3I/ATLAS's discovery. 'It ruined my weekend.'
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Stefanie Milam of NASA's Goddard Space Flight Center is part of a group that had reserved time on JWST to observe an interstellar object—if the researchers were fortunate enough for one to be discovered. But the group's luck was tested when it couldn't reach the lead of its program—Martin Cordiner, also at Goddard—to kick the observations into action. 'He was hiking in Maine when the object was discovered, and we could not reach him—he was completely off the grid,' Milam says. 'When he finally got back, his phone just blew up. I said, 'You're never allowed to go on vacation again!''
So why exactly are astronomers so eager to observe this object, and what do they hope to learn?
Where did 3I/ATLAS come from?
The first major question to answer about 3I/ATLAS is its origin. Tracing it back to an individual star is likely impossible, given the mixing of myriad stars in their orbits around our galaxy across billions of years. But we might be able to work out roughly the region it came from.
One team of astronomers has already begun doing just that, using the high velocity of the object with respect to our sun—60 kilometers (37 miles) a second—to argue that it might have come from the vicinity of our galaxy's thick disk. This is a puffy torus of older stars moving at high velocities above and below the main flat plane of the Milky Way—which is where our sun serenely orbits.
A thick-disk origin might mean that 3I/ATLAS is extremely ancient, more than eight billion years old. 'It's from a star that's potentially not even there anymore,' says Michele Bannister of the University of Canterbury in New Zealand, a co-author on the work.
Aster Taylor of the University of Michigan performed a different age analysis based on the trajectory of 3I/ATLAS and suggests the object is 11 billion to three billion years old. 'We get similar answers,' Taylor says. Such estimates might soon be revised if subsequent observations can show just how much space weathering the object has endured during its interstellar sojourn.
How big is it?
Currently, 3I/ATLAS is inside the orbit of Jupiter and approaching the orbit of Mars, which it will cross in October, passing about 0.2 astronomical unit (one fifth the Earth-sun distance) from the Red Planet.
Although early observations have led astronomers to categorize 3I/ATLAS as a comet, at the moment, it's not behaving exactly like one. The object doesn't display a large tail or enveloping coma of cast-off gas, only a hint of dust—but that is expected to change soon. As it traverses the asteroid belt between Mars and Jupiter and basks in the sun's radiance, its surface should warm enough to sublimate ice, venting sufficient material to form a large coma and perhaps a prominent tail.
A substantial coma would be like a curtain drawn over astronomers' eyes, obscuring their view of the object and complicating efforts to gauge its dimensions. Before that happens, a team led by David Jewitt at the University of California, Los Angeles, is hoping to pin down its size with Hubble in August. (Other telescopes might be able to determine the size of 3I/ATLAS, too.)
Initial estimates suggested 3I/ATLAS might be up 20 kilometers (12 miles) across—very big for a comet—but most astronomers now think it is much smaller. 'It's probably somewhere in the range of one or two kilometers,' says John Noonan at Auburn University in Alabama. That would be somewhat comparable in size to our first two interstellar visitors: 1I/ʻOumuamua, which was discovered in 2017 and was up to about 400 meters (0.25 mile) long, and 2I/Borisov, which was found in 2019 and was about one kilometer (0.6 mile) wide.
If 3I/ATLAS turns out to be much bigger, 10 kilometers (six miles) or more, this would pose problems for preexisting estimates of many big interstellar objects reside in the galaxy. 'It's statistically extremely unlikely we should ever see something that size,' Noonan says. 'Theorists don't like that. But as an observer, I would love to see a really weird, big object.'
How fast is it spinning?
As well as its size, one of the key properties astronomers want to know about 3I/ATLAS is its rotation rate—something they might discern by watching the object's changing brightness as it spins. The spin of 3I/ATLAS could carry clues as to how the object was ejected from its home star in the first place.
'Certain ways of kicking these objects out tend to make them spin up,' Taylor says. A close pass of a gas giant planet, for instance, could easily set the object twirling while hurling it away from its home star. Conversely, a slow rotation period would suggest the object experienced a more gentle ejection.
'You could do this when stars die,' Taylor says. 'They lose a lot of mass, and so the gravitational force on objects at the outer edge of their system goes away. Those objects become unbound and just flow out into the galaxy.'
The rotation period can also tell us more about the shape of 3I/ATLAS—a steady rotation suggests a fairly spherical form, whereas a fluctuating rotation speed might suggest a 'wonky shape,' Taylor says, like that of 'Oumuamua, which was estimated to be cigar- or pancake-shaped.
What is 3I/ATLAS made of?
If 3I/ATLAS really is an ancient cometary castaway that has been drifting through the galaxy for eons, it might be full of ice that has never been heated by a star. If so, then as it gets closer, the object might suddenly erupt into activity. While that could be bad news for measuring its size, it would aid efforts to determine 3I/ATLAS's chemical composition.
JWST and Hubble would be best suited for the task of picking apart the different species of molecules that might erupt from 3I/ATLAS. Unfortunately, however, in October, when the object will be at its warmest, closest point to our star (called perihelion), Earth will be on the other side of the sun. This will make observations from our planet almost impossible.
In November, post-perihelion, Noonan will use Hubble to study 3I/ATLAS and its emissions, looking for signs of substances such as hydroxide and hydrogen that can help clarify its composition.
If the object is several billion years old, as predicted, then it might be rich in water because of the suspected formation environment around older stars. 'You would expect a lot of hydrogen coming from these water-rich irradiated objects, if this is really as old as [thought],' Noonan says.
Milam and her colleagues, meanwhile, will use JWST in August and December to observe 3I/ATLAS before and after perihelion. Thanks to its keen infrared vision, JWST is better suited for teasing out the presence of molecules such as water, carbon monoxide, carbon dioxide and ammonia.
'We can really home in and see what this thing looks like,' she says. 'Borisov had a pretty boring chemistry, but it wasn't like any object in our solar system—there was hardly any water at all but a lot of carbon monoxide and hydrogen cyanide. With JWST, we're hoping to see a lot of carbon dioxide [on 3I/ATLAS], maybe even water, if it's as pristine as people are projecting.'
Although the overall view from Earth degrades as the object approaches perihelion, some telescopes will be less visually impaired. Those operated by the Lowell Observatory in Arizona, for instance, are primed to observe 3I/ATLAS at dawn and dusk, when the sun is below the horizon. This will allow for studies even when the object will be close to our star from our planet-bound perspective. 'The Lowell Discovery Telescope is really well suited to observations close to the horizon,' says Nick Moskowitz, an astronomer at Lowell Observatory. 'We will be able to track it closer in to perihelion than other facilities.'
An unlikely additional capability will be at Mars, where spacecraft such as NASA's Mars Atmosphere and Volatile Evolution (MAVEN) orbiter may be able to see 3I/ATLAS as it passes about 30 million kilometers (19 million miles) from the planet. 'It'll be pretty large and apparent in the sky,' Noonan says, providing the object kicks into activity as hoped. 'They'll be able to see the coma,' giving us an insight into 3I/ATLAS's activity near the sun that would otherwise be impossible to see from Earth.
Will it survive?
A big unknown about 3I/ATLAS is whether it will actually survive its close encounter with our sun. While 'Oumuamua did so, Comet Borisov was not so fortunate, with the object appearing to split and break apart on its way out of our solar system.
The same fate could befall 3I/ATLAS. 'Borisov fragmented, which is pretty usual for comets,' Bannister says. All eyes will be on our latest visitor to see if the same thing happens again.
An additional quirk of 3I/ATLAS's survivability is the impact of solar wind, which may snip away any cometary tail as it is ejected. By chance, the object is entering our solar system at quite a shallow angle, much flatter than that of most comets, which means it will experience stronger solar headwinds.
Sarah Watson of the University of Reading in England and her colleagues are using this quirk to study how the solar wind traverses into the outer solar system. 'We can potentially calculate the speed of the solar wind,' she says, by noticing the impact of the solar wind on the purported comet's tail, if one materializes.
Could we reach it?
No spacecraft will be able to reach 3I/ATLAS. It is moving too fast and is too far from Earth for us to consider launching something in time.
Yet an upcoming European Space Agency (ESA) mission called Comet Interceptor, set to launch in 2029, might attempt to visit another interstellar object, if we find one within its reach. The spacecraft will be positioned past the moon's orbit away from the sun and, if a suitable target is found, will be commanded to fire its engines and try and intercept the incoming alien object.
If no suitable interstellar object is found, Comet Interceptor will instead be sent to one of several intriguing comets of our solar system. 'It is possible we could get an interstellar object, but we have to be really lucky,' says Colin Snodgrass, an astronomer at the University of Edinburgh, who is deputy lead on the mission.
How many are there?
One of our biggest outstanding questions about interstellar objects concerns their unknown abundance. The object 3I/ATLAS is our third interstellar visitor in eight years—a real but weak hint of how many are out there, waiting to be found.
Predictions estimate there are trillions upon trillions of interstellar objects drifting around our galaxy, and perhaps one in our solar system at any given time—but they're typically just so faint that they're unlikely to be found by most telescopes. This is expected to change when a new telescope called the Vera C. Rubin Observatory begins a 10-year survey of the sky later this year.
Rubin is expected to see somewhere between six and 51 interstellar objects in its 10-year survey. Seeing such a population will tell us 'how unique, or varied, planetesimal formation is across different parts of the galaxy,' Bannister says, referring to kilometer-scale objects thought to coalesce around newborn stars that become the feedstock for planets—and, when kicked to a system's hinterlands, become a reservoir of comets.
One puzzling question is why we haven't seen much smaller interstellar objects, Moskowitz says. If smaller objects are more plentiful than larger objects, as scientists expect, then we should have seen some small interstellar objects entering our atmosphere, appearing as meteors streaking across Earth's skies at speeds and trajectories that clearly convey their interstellar origins.
Detections of such objects have been claimed, but the evidence behind them has failed to convince most experts. The apparent absence of small interstellar interlopers 'is telling us something, but we don't know what that is yet,' Moskowitz says. 'I think that's going to be one of the major questions: Why are we seeing these big cometlike things coming through the solar system, but we're not seeing things that are smaller? It may have to do with the survivability of stuff out there in the galaxy, but we need more data.'