\n<\/p>\n
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\u2019s bound to slip through our sun\u2019s 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.<\/p>\n
That\u2019s 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\u2019s 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\u2014which, thanks to its alien, interstellar provenance may be the oldest comet anyone has ever seen.<\/p>\n
\u201cI didn\u2019t get any sleep for like 35 hours,\u201d says Bryce Bolin of Eureka Scientific in California, who rushed to release a preprint paper and arrange additional observations following 3I\/ATLAS\u2019s discovery. \u201cIt ruined my weekend.\u201d<\/p>\n
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Stefanie Milam of NASA\u2019s Goddard Space Flight Center is part of a group that had reserved time on JWST to observe an interstellar object\u2014if the researchers were fortunate enough for one to be discovered. But the group\u2019s luck was tested when it couldn\u2019t reach the lead of its program\u2014Martin Cordiner, also at Goddard\u2014to kick the observations into action. \u201cHe was hiking in Maine when the object was discovered, and we could not reach him\u2014he was completely off the grid,\u201d Milam says. \u201cWhen he finally got back, his phone just blew up. I said, \u2018You\u2019re never allowed to go on vacation again!\u2019\u201d<\/p>\n
So why exactly are astronomers so eager to observe this object, and what do they hope to learn?<\/p>\n
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.<\/p>\n
One team of astronomers has already begun doing just that, using the high velocity of the object with respect to our sun\u201460 kilometers (37 miles) a second\u2014to argue that it might have come from the vicinity of our galaxy\u2019s 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\u2014which is where our sun serenely orbits.<\/p>\n
A thick-disk origin might mean that 3I\/ATLAS is extremely ancient, more than eight billion years old. \u201cIt\u2019s from a star that\u2019s potentially not even there anymore,\u201d says Michele Bannister of the University of Canterbury in New Zealand, a co-author on the work.<\/p>\n
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. \u201cWe get similar answers,\u201d 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.<\/p>\n
This image shows the observation of interstellar object 3I\/ATLAS when it was discovered on July 1, 2025.<\/p>\n
ATLAS\/University of Hawaii\/NASA<\/p>\n
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 units (one fifth the Earth-sun distance) from the Red Planet.<\/p>\n
Although early observations have led astronomers to categorize 3I\/ATLAS as a comet, at the moment, it\u2019s not behaving exactly like one. The object doesn\u2019t display a large tail or enveloping coma of cast-off gas, only a hint of dust\u2014but that is expected to change soon. As it traverses the asteroid belt between Mars and Jupiter and basks in the sun\u2019s radiance, its surface should warm enough to sublimate ice, venting sufficient material to form a large coma and perhaps a prominent tail.<\/p>\n
A substantial coma would be like a curtain drawn over astronomers\u2019 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.)<\/p>\n
Initial estimates suggested 3I\/ATLAS might be up 20 kilometers (12 miles) across\u2014very big for a comet\u2014but most astronomers now think it is much smaller. \u201cIt\u2019s probably somewhere in the range of one or two kilometers,\u201d says John Noonan at Auburn University in Alabama. That would be somewhat comparable in size to our first two interstellar visitors: 1I\/\u02bbOumuamua, 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.<\/p>\n
If 3I\/ATLAS turns out to be much bigger, 10 kilometers (six miles) or more, this would pose problems for preexisting estimates of how many big interstellar objects reside in the galaxy. \u201cIt\u2019s statistically extremely unlikely we should ever see something that size,\u201d Noonan says. \u201cTheorists don\u2019t like that. But as an observer, I would love to see a really weird, big object.\u201d<\/p>\n
As well as its size, one of the key properties astronomers want to know about 3I\/ATLAS is its rotation rate\u2014something they might discern by watching the object\u2019s 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.<\/p>\n
\u201cCertain ways of kicking these objects out tend to make them spin up,\u201d 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.<\/p>\n
\u201cYou could do this when stars die,\u201d Taylor says. \u201cThey 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.\u201d<\/p>\n
The rotation period can also tell us more about the shape of 3I\/ATLAS\u2014a steady rotation suggests a fairly spherical form, whereas a fluctuating rotation speed might suggest a \u201cwonky shape,\u201d Taylor says, like that of \u2018Oumuamua, which was estimated to be cigar- or pancake-shaped.<\/p>\n
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\u2019s chemical composition.<\/p>\n
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.<\/p>\n
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.<\/p>\n
3I\/ATLAS streaks across a dense star field in this image captured by the Gemini North telescope’s Gemini Multi-Object Spectrograph (GMOS-N). The image is composed of exposures taken through three filters, shown here as red, green and blue.<\/p>\n
International Gemini Observatory\/NOIRLab\/NSF\/AURA\/K. Meech (IfA\/U. Hawaii) Image Processing: Jen Miller & Mahdi Zamani (NSF NOIRLab) (CC BY 4.0)<\/p>\n
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. \u201cYou would expect a lot of hydrogen coming from these water-rich irradiated objects, if this is really as old as [thought],\u201d Noonan says.<\/p>\n
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.<\/p>\n
\u201cWe can really home in and see what this thing looks like,\u201d she says. \u201cBorisov had a pretty boring chemistry, but it wasn\u2019t like any object in our solar system\u2014there was hardly any water at all but a lot of carbon monoxide and hydrogen cyanide. With JWST, we\u2019re hoping to see a lot of carbon dioxide [on 3I\/ATLAS], maybe even water, if it\u2019s as pristine as people are projecting.\u201d<\/p>\n
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. \u201cThe Lowell Discovery Telescope is really well suited to observations close to the horizon,\u201d says Nick Moskowitz, an astronomer at Lowell Observatory. \u201cWe will be able to track it closer in to perihelion than other facilities.\u201d<\/p>\n
An unlikely additional capability will be at Mars, where spacecraft such as NASA\u2019s 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. \u201cIt\u2019ll be pretty large and apparent in the sky,\u201d Noonan says, providing the object kicks into activity as hoped. \u201cThey\u2019ll be able to see the coma,\u201d giving us an insight into 3I\/ATLAS\u2019s activity near the sun that would otherwise be impossible to see from Earth.<\/p>\n
A big unknown about 3I\/ATLAS is whether it will actually survive its close encounter with our sun. While \u2018Oumuamua 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.<\/p>\n
The same fate could befall 3I\/ATLAS. \u201cBorisov fragmented, which is pretty usual for comets,\u201d Bannister says. All eyes will be on our latest visitor to see if the same thing happens again.<\/p>\n
An additional quirk of 3I\/ATLAS\u2019s 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 and thus much closer to Earth\u2019s orbital plane, allowing careful comparisons between the solar wind in both places.<\/p>\n
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. \u201cWe can potentially calculate the speed of the solar wind,\u201d she says, by noticing the impact of the solar wind on the purported comet\u2019s tail, if one materializes.<\/p>\n
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.<\/p>\n
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\u2019s 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.<\/p>\n
If no suitable interstellar object is found, Comet Interceptor will instead be sent to one of several intriguing comets of our solar system. \u201cIt is possible we could get an interstellar object, but we have to be really lucky,\u201d says Colin Snodgrass, an astronomer at the University of Edinburgh, who is deputy lead on the mission.<\/p>\n
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\u2014a real but weak hint of how many are out there, waiting to be found.<\/p>\n
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\u2014but they\u2019re typically just so faint that they\u2019re 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.<\/p>\n
Rubin is expected to see somewhere between six and 51 interstellar objects in its 10-year survey. Seeing such a population will tell us \u201chow unique, or varied, planetesimal formation is across different parts of the galaxy,\u201d Bannister says, referring to kilometer-scale objects thought to coalesce around newborn stars that become the feedstock for planets\u2014and, when kicked to a system\u2019s hinterlands, become a reservoir of comets.<\/p>\n
One puzzling question is why we haven\u2019t 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\u2019s skies at speeds and trajectories that clearly convey their interstellar origins.<\/p>\n
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 \u201cis telling us something, but we don\u2019t know what that is yet,\u201d Moskowitz says. \u201cI think that\u2019s going to be one of the major questions: Why are we seeing these big cometlike things coming through the solar system, but we\u2019re 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.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"
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