The science of interstellar objects is moving swiftly. Now that we have the third ‘interloper’ into our Solar System (3I/ATLAS), we can consider how many more such visitors we’re going to find with new instruments like the Vera Rubin Observatory, with its full-sky images from Cerro Pachón in Chile. As many as 10,000 interstellar objects may pass inside Neptune’s orbit in any given year, according to information from the Southwest Research Institute (SwRI).
The Gemini South Observatory, likewise at Cerro Pachón, has used its Gemini Multi-Object Spectrograph (GMOS) to produce new images of 3I/ATLAS. The image below was captured during a public outreach session organized by the National Science Foundation’s NOIRLab and the Shadow the Scientists initiative that seeks to connect citizen scientists with high-end observatories.
Image: Astronomers and students working together through a unique educational initiative have obtained a striking new image of the growing tail of interstellar Comet 3I/ATLAS. The observations reveal a prominent tail and glowing coma from this celestial visitor, while also providing new scientific measurements of its colors and composition. Credit: Gemini Observatory/NSF NOIRlab.
Immediately obvious is the growing size of the coma, the cloud of dust and gas enveloping the nucleus as 3I/ATLAS moves closer to the Sun and continues to warm. Analyzing spectroscopic data will allow scientists to understand more about the object’s chemistry. So far we’re seeing cometary dust and ice not dissimilar to comets in our own system. We won’t have this object long, as its orbit is hyperbolic, taking it inside the orbit of Mars and then off again into interstellar space. Perihelion should occur at the end of October. It’s interesting to consider, as Marshall Eubanks and colleagues do in a new paper, whether we already have spacecraft that can learn something further about this particular visitor.
Note this from the paper (citation below):
Terrestrial observations from Earth will be difficult or impossible roughly from early October through the first week of November, 2025… [T]he observational burden during this period will, to the extent that they can observe, largely fall on the Psyche and Juice spacecraft and the armada of spacecraft on and orbiting Mars. Our recommendation is that attempts should be made to acquire imagery from encounter spacecraft during the entire period of the passage of 3I through the inner solar system, and in particular from the period in October and November of 2025, when observations from Earth and the space telescopes will be limited by 3I’s passage behind the Sun from those vantage points.
As we consider future interstellar encounters, flybys begin to look possible. Such was the conclusion of an internal research study performed at SwRI, which examined costs and design possibilities for a mission that may become a proposal to NASA. SwRI was working with software that could create a large number of simulated interstellar objects, while at the same time calculating a trajectory from Earth to each. Matthew Freeman is project manager for the study. It turns out that the new visitor is itself within the study’s purview:
“The trajectory of 3I/ATLAS is within the interceptable range of the mission we designed, and the scientific observations made during such a flyby would be groundbreaking. The proposed mission would be a high-speed, head-on flyby that would collect a large amount of valuable data and could also serve as a model for future missions to other ISCs [interstellar comets].”
Image: Upper left panel: Comet 3I/ATLAS as observed soon after its discovery. Upper right panel: Halley’s comet’s solid body as viewed up close by ESA’s Giotto spacecraft. Lower panel: The path of comet 3I/Atlas relative to the planets Mercury through Saturn and the SwRI mission interceptor study trajectory if the mission were to be launched this year. The red arc in the bottom panel is the mission trajectory from Earth to interstellar comet 3I/ATLAS. Courtesy of NASA/ESA/UCLA/MPS.
So we’re beginning to undertake the study of actual objects from other stellar systems, and considering the ways that probes on fast flyby missions could reach them. 3I/ATLAS thus makes the case for further studies of flyby missions. SwRI’s Mark Tapley, an expert in orbital mechanics, is optimistic indeed:
“The very encouraging thing about the appearance of 3I/ATLAS is that it further strengthens the case that our study for an ISC mission made. We demonstrated that it doesn’t take anything harder than the technologies and launch performance like missions that NASA has already flown to encounter these interstellar comets.”
The paper on a fast flyby mission to an interstellar object is Eubanks et al., “3I/ATLAS (C/2025 N1): Direct Spacecraft Exploration of a Possible Relic of Planetary Formation at “Cosmic Noon,” available as a preprint.
The fact this thing exists and is here suggests that interstellar space is not nearly as empty as we think it is. Could that interstellar space being full of objects be the real reason why high speed (> 10% light speed) interstellar travel is impossible?
Interstellar space is largely empty. Even the inside of the solar system in the densest part of the asteroid belt is largely empty. The distance between two km-sized asteroids is millions of km. This distance is even larger outside of the asteroid belt, and even much larger in interstellar space. Such “big” objects are really not the big problem — the probability of encountering any by chance is virtually zero. You can note that whatever spacecraft exploring the solar system never approaches very close to any asteroid by random chance — either it’s a planned close approach, or it’s at best a very distant approach (millions of km at best).
What can be much more critical for near-c travels is actually tiny dust grains — of course they are much smaller, but globally much more numerous, so the probably of encountering any is much larger. And if you go a near-c speed, even a tiny dust grain can have a large energy that can produce critical damages.
In the forseeable future, not any inhabited extrasolar mission is realistically imaginable (even going to Mars is not done yet), but sending uninhabited nanospacecrafts (gramme-scale objects) is something that is being worked about at the scale of the century. We may not see the launch of such interstellar nanospacecrafts during our lifetime, but it may happen within the next few generations. The idea would be to send swarms of redundant nanospacecrafts (maybe hundreds or more nanospacecrafts), with each spacecraft having one instrument (camera etc.) — and therefore, in the whole swarm, there would be multiple times each instrument. Such tiny spacecrafts could be accelerated at a significant fraction of c (the concept imagines 20% of c (~20 years of travel) for a simple flyby of Proxima, and maybe 5% of c (~century-scale travel) if we want the swarm to enter into orbit around Proxima) by using huge lasers. Of course, we are currently far from being able to build such miniaturised spacecrafts, and to build such huge lasers — hence why I said it’s an idea that is being worked about with a realistic aim of launch at the scale of the next century.
A small complement to link the “dust grain” part and “nanospacecarft part” in my first comment: the point of having redundant spacecraft would be exactly to mitigate the risk of losing a number of nanospacecrafts (for whatever reason, it be the spacecraft having a problem, a spacecraft being destroyed by such a tiny dust grain, or whatever else) during travel. The point is that, even if part of the swarm is lost before reaching the target system (e.g. Proxima), we can expect to still have at least one (ideally several) instrument of each type in the end — and therefore being able to study the target system as expected.
Hello,
Are any teams working on technologies that would repel this dust for a spacecraft traveling at relativistic speeds? If so, which ones ?
Hi Abelard
The mass of the ISM is very constrained by Interstellar Medium studies and dynamics of the Galactic Disk. Locally there’s about 0.03 solar masses per cubic parsec – i.e. about 6E+28 kg of mass spread through 3E+49 cubic metres. On average that’s about a million atoms per cubic metre and most of them are either hydrogen or helium. To make dust grains add either carbon, oxygen, silicon or iron, which collectively form about 1% of the mass of the ISM.
For the sake of the argument, imagine it’s all oxygen and it’s combined with some hydrogen in the form of ice-grains. 1 gram ice-grains to a vehicle traveling at 0.2 c pack about a half-kiloton of TNT equivalent energy. So how many cubic metres does each ice-grain occupy by itself, if the average mass density is 2E-23 kg/m^3? 5E+19 cubic metres. In other words a cubic volume that’s 3,700 km each side.
Imagine a Laser Light-Sail is 4 metres wide. Through a cubic parsec it bores a cylinder with a volume of ~4E+17 cubic metres. Therefore, on average, it’d encounter a 1 gram ice-cube every 125 parsecs. To encounter an object 100 metres on an edge, massing a trillion times more, you’d need to travel a lot further.
Using what vehicle?
Still waiting for that information to be released. So far the study remains internal to SwRI.
I would think any assets to intercept this object need to start burning now—Juno’s status?
It may already be too late or Beresheet type lifting burns of an Mars orbiters considered for this.
We know other interstellar visitors will be coming. We need to prepare intercept missions now. We have no excuses any more.
https://youtu.be/Pke3u-HI3PM?feature=shared
As interstellar visitors start to become old hat, maybe we have something more exotic to get excited about: intergalactic visitors! According to https://aasnova.org/2021/03/23/understanding-the-origin-and-arrival-rates-of-interstellar-objects/ there should be roughly one interstellar object approaching from Milky Way halo stars per 11 years, with velocity >200 km/s.
When we go beyond the galaxy, the models for any potential extraterrestrials change. What if a halo star came from a small, ancient galaxy where every system was colonized, artifacts everywhere, but then civilization somehow died out, billions of years before it merged with the Milky Way? Then any one of these halo-derived objects – only! – might, in theory, be encrusted with alien libraries, homes, and artworks just waiting for the first probe to make it up to 200 km/s to check out an unknown visitor.
I’ll admit that’s a slender scenario, but I don’t see a way to rule it out.
When I walk around my neighborhood, I don’t see books or artwork laying on the ground, but beer cans and food wrappers. Unless ET is much tidier than humans, the first alien artifact we encounter is more likely to fall into the latter category than the former. This would still be incredibly important (alien tech/language) but not exactly the Encyclopedia Galactica…
@NS
The very background to the Strugatsky brothers’ SciFi novel: Roadside Picnic.
Junk and trash to us would be amazing to Europeans a millennium ago – paper (with or without print), plastic food wrappers, foam cups, dead batteries, aluminum cans, and even scraps of processed foods.
There is the hypothesis that life began on Earth when an ETI landing expedition roughly four billion years ago left some of their trash behind. Or they could have contaminated the planet just by their presence.
Even the famous naturalist John Muir (1838-1914) had no issue with leaving trash behind at camp sites: He assumed the detritus would just blow away with the wind into the woods and somehow disappear. Now imagine how an alien might act with their refuse on a planet not their own.
I’d like to get some confirmation on the numbers here:
Visiting interstellar comet 3I/Atlas by current tech: New Horizons model. Part 1.
https://www.linkedin.com/posts/robert-clark-94273688_dawn-moessner-gabe-rogers-stacy-weinstein-weiss-activity-7367915873266847744-81Jd
And:
Visiting interstellar comet 3I/Atlas by current tech: New Horizons model. Part 2.
https://www.linkedin.com/posts/robert-clark-94273688_visiting-interstellar-comet-3iatlas-by-current-activity-7367919978311761920-85wx
Proposes using a Falcon 9 launcher to launch a Centaur upper stage plus 2 smaller solid stages to get a 10 kg minisat to a flyby of 3I/Atlas as it crosses Jupiters orbit in March, 2026.
No rush, there are plenty more out there…
The rubin observatory is going to reveal many more surprises, I am sure.
But what can we hope to learn from these interstellar interlopers?
Granted, they may contain evidence of conditions where they were formed, or of conditions that existed long after their formation, but we still don’t know where they originated, or when, or how long they have been flying around the galaxy being exposed to who knows what conditions.
Yes, we have every reason to believe these objects are extremely common (especially when we now have the technical means of detecting them), but how their current condition or their distribution is related to their origin or to conditions in the galaxy since its formation will remain a mystery until we have collected data on an enormous number of them for a very long time. The minimum first step will be to study how their overall properties are correlated to one another. Just because some may contain more or less ammonia or methane or ice or rock than others will tell us little. Even a detailed inventory of their isotopic abundances will contribute little to our understanding on how they fit into the big picture..
No, I’m NOT suggesting we should ignore them. Any opportunity to examine interstellar objects in detail should be welcomed, and sooner or later we may stumble on some really revealing facts about them. But I suspect that most of them will look pretty much the same, and what differences they do have amongst themselves will be very difficult to fit into any theoretical framework concerning galactic or stellar or planetary evolution we can devise at this time.
I’m hoping we get lucky and find one with purely bizarre characteristics, but I doubt it will play out this way. What I suspect is that the line-up of suspects will play out to be a collection of unique misfits (like our own asteroid belt, or our catalog of planetary satellites) with little clue as to their origin or role in galactic history. Only a very long experience with these objects is likely to result in sufficient data to propose an explanatory theory, and an even longer experience will be required to test that theory.
We’ve always known these guys were out there. We’re just talking about them now because we have the tech to see them.
From the image caption:
I think this was rather poorly worded by whoever produced the SwRI piece. The PR piece was published on their website on September 3, 2025. Assuming the fast flyby probe was ready to launch on that date, it has to make what appears to be a faster than minimum energy transfer orbit to Mars and intercept the comet around November 26. That is about 3 months from launch to intercept, while the minimum energy transfer orbit takes close to 9 months, and the fast flyby probe about 6 months based on where Earth was at launch and the interception point for 3I/ATLAS. Using the Wikipedia entry it looks like the fast flyby probe would have to have been launched at the start of June 2025 to reach its interception point by the end of November 2025 in 6 months. Having said that, the SwRI paper assumed minimum energy transfer orbits to reach the various ISCs, which is what the trajector looks like in the image.
This suggests to me that the publication of the SwRI piece was 3 – 6 months after the the needed February – May launch of the flyby probe (depending of trajectory), and perhaps should have been reworded before it was published?
Astrodynamicists, please correct me if I am wrong.
This object is extremely weird…
Polarimetric Behavior: low inversion angle and extreme negative polarisation
A combination of low inversion angle and extreme negative polarization is a rare polarimetric signature in astronomical bodies, indicating a unique surface composition and microstructure. Recent observations have identified interstellar comet 3I/ATLAS as the first object known to exhibit these extreme characteristics.
Overview of polarimetric behavior Polarimetry measures how much the reflected light from an airless body is linearly polarized as a function of the solar phase angle (the angle between the Sun, the object, and the observer). A typical polarization phase curve has the following features:
Negative polarization branch: At very small phase angles (typically <20 degrees), the light is polarized parallel to the scattering plane. The polarization starts at zero, becomes negative, and reaches a minimum at a phase angle of 5-10 degree.
Inversion angle: The phase angle at which the polarization crosses zero and changes from negative to positive is called the inversion angle (alpha _0).
Positive polarization branch: At phase angles greater than the inversion angle, the polarization becomes positive and typically reaches a maximum around 90 -100 degrees
Interpretation of low inversion angle and extreme negative polarization The specific shape of the polarization curve is determined by the surface properties of the object's regolith, such as albedo, particle size, and porosity. Low inversion angle: The inversion angle is generally linked to the refractive index of the surface material. Asteroids with small inversion angles, like the F-type asteroids (e.g., 302 Clarissa, 704 Interamnia), have been linked to a high optical homogeneity in their surface microstructure.Extreme negative polarization: The depth of the negative polarization branch (Pmin) is strongly correlated with the geometric albedo of the surface. Low-albedo (dark) objects tend to have deeper negative polarization branches, while high-albedo (bright) objects have shallower ones. An "extreme" negative polarization therefore suggests an unusually dark or unique surface composition. When these two features are combined, they present a puzzle, as a low inversion angle is typically associated with a relatively shallower negative polarization minimum in objects like F-type asteroids.
3I/ATLAS: A notable case
The interstellar comet 3I/ATLAS was the first object observed with both a low inversion angle and extreme negative polarization.
Extreme values: Observations revealed a minimum polarization of -2.7% at a phase angle of 7° and an inversion angle of 17°. This polarization depth is almost twice as large as typical F-type asteroids with similar inversion angles.
Surface composition: Numerical simulations and comparisons with laboratory scattering experiments suggest that 3I/ATLAS has a surface composed of large particles made of a mixture of icy and dark material. This high level of negative polarization is consistent with a highly porous surface made of weakly-absorbing materials like Mg-rich silicates.
Interstellar origin: This unique polarimetric signature may indicate that interstellar objects, and not just those in our solar system, have a broader diversity of surface properties than previously understood.
https://arxiv.org/html/2509.05181v1
Perhaps it is a giant alien egg that will hatch when heated by the sun.
3I/ATLAS will come closest to the Sun on 29 October 2025 at 11:44 ± 00:01 UT. The comet’s perihelion or closest distance to the Sun is 1.36 AU (203 million km; 126 million mi), which lies between the orbits of Earth and Mars.
Just in time for Halloween!
Back in the late 80s and early 90s I did a lot of numerical orbital calculations on the orbits of dust in the solar system for an experiment to go on the ISS (that did not happen then).
One thing I got to do was consult the Catalogue of Cometary Orbits from the Smithsonian Astrophysical Observatory (pretty sure that is on line now). Commentaries about that catalog , which spanned from about mid 19th century to the 2000s noted that there were a small number of comet orbits with hyperbolic orbits but all those could be attributed to , mostly Jupiter, having thrown solar system comets out of the solar system.
Interesting that Edmond Halley (that Halley) thought that the orbits of the some of the 24 comets he calculated were interstellar in nature, that was 1705! (Turned out they were not.)
Comet experts puzzled over the Smithsonian catalog , long history of observations there, not having any interstellar comet orbits. Even knowing that the instrumentation is orders of magnitude now days, seems one big one , and 31/Atlas at 11 km is pretty big (largest long period comet is Comet Bernardinelli-Bernstein , at 140 km! is really big).
I don’t know has anyone checked to see if instrumentation of the 20th century could have seen Oumuamua , Borisov and 31/Atlas?
For a pretty long time now, since the 1980s, elaborate numerical modeling of the formation of the Oort cloud have shown there should be a lot of interstellar comets thrown into our Galaxy during its formation. Indeed Oort clouds have been observed in extra solar star systems, those Oort clouds may be as numerous as extra solar planets. Though one keeps in mind how large the volume of the Galaxy is.
Are there extra Galactic comets?
A small change in the Starships configuration may allow us to have these faster probes to reach these objects. If the second stage of the starship design was much smaller there would be a lot less issues with retry and reuse. With that we could use aluminium fuel or oxygen tanks (they can be sent separately) atop the second stage that would allow a lot more fuel to be sent into orbit. The tanks after emptying could be deorbited or used in space habitats.
Its this fuel in orbit that would allows us to have a lot more end velocity to reach objects faster.
https://astrobiology.com/2025/10/exomars-and-mars-express-observe-interstellar-comet-3i-atlas.html
ExoMars And Mars Express Observe Interstellar Comet 3I/ATLAS
By Keith Cowing
Press Release
ESA
October 7, 2025
Between 1 and 7 October, ESA’s ExoMars Trace Gas Orbiter (TGO) and Mars Express spacecraft turned their eyes towards interstellar comet 3I/ATLAS, as it passed close to Mars.
The two Mars orbiters had the closest view of the comet of all ESA spacecraft. During its closest approach to the Red Planet on 3 October, the interstellar interloper was 30 million km away from them.
Each spacecraft used its dedicated camera to watch the comet pass. Both cameras are designed to photograph the bright surface of Mars just a few hundred to a few thousand km below. Scientists were unsure what to expect from observations of a relatively dim target so far away.
ExoMars TGO captured the series of images shown in the GIF below with its Colour and Stereo Surface Imaging System (CaSSIS). Comet 3I/ATLAS is the slightly fuzzy white dot moving downwards near the centre of the image. This dot is the centre of the comet, comprising its icy-rocky nucleus and its surrounding coma.
ExoMars TGO images comet 3I/ATLAS — ESA
CaSSIS could not distinguish the nucleus from the coma, because 3I/ATLAS was too far away. Imaging this kilometre-wide nucleus would have been as impossible as seeing a mobile phone on the Moon from Earth.
But the coma, measuring a few thousand kilometres across, is clearly visible. The coma is created as 3I/ATLAS approaches the Sun. The Sun’s heat and radiation is bringing the comet to life, causing it to release gas and dust, which collects as this halo surrounding the nucleus.
The full size of the coma could not be measured by CaSSIS because the brightness of the dust decreases quickly with distance from the nucleus. This means that the coma fades into the noise in the image.
Typically, material from the coma is swept into a long tail, which can grow up to millions of kilometres long as the comet moves closer to the Sun. The tail is much dimmer than the coma. We can’t see the tail in the CaSSIS images, but it may become more visible in future observations as the comet continues to heat up and release more ice.
Nick Thomas, Principal Investigator of the CaSSIS camera explains, “This was a very challenging observation for the instrument. The comet is around 10 000 to 100 000 times fainter than our usual target.”
ESA’s Mars and Jupiter missions observe comet 3I/ATLAS — ESA
The work continues
3I/ATLAS has not yet revealed itself in the Mars Express images, partly because these were taken with an exposure time of just 0.5 seconds (the maximum limit for Mars Express) compared to five seconds for ExoMars TGO.
Scientists will continue to analyse the data from both orbiters, including adding together several images from Mars Express to see if they can spot the faint comet.
They also tried to measure the spectrum of light from comet 3I/ATLAS using Mars Express’s OMEGA and SPICAM spectrometers, and ExoMars TGO’s NOMAD spectrometer. At this point, it is uncertain whether the coma and tail were bright enough for a spectral characterisation.
Scientists will keep analysing the data over the next weeks and months to try to figure out more about what 3I/ATLAS is made of and how it is behaving as it approaches the Sun.
Colin Wilson, Mars Express and ExoMars project scientist at ESA says: “Though our Mars orbiters continue to make impressive contributions to Mars science, it’s always extra exciting to see them responding to unexpected situations like this one. I look forward to seeing what the data reveals following further analysis.”
A rare visitor
Originating from outside our Solar System, comet 3I/ATLAS is only the third interstellar comet ever seen, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019.
These comets are absolutely foreign. Every planet, moon, asteroid, comet and lifeform in our Solar System share a common origin. But interstellar comets are true outsiders, carrying clues about the formation of worlds far beyond our own.
Comet 3I/ATLAS was first spotted on 1 July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile. Since then, astronomers have used ground-based and space telescopes to monitor its progress and discover more about it.
Based on its trajectory, astronomers suspect that 3I/ATLAS could be the oldest comet ever observed. It may be three billion years older than the Solar System, which is itself already 4.6 billion years old.
What’s next?
Next month, we will observe the comet with our Jupiter Icy Moons Explorer (Juice). Though Juice will be further from 3I/ATLAS than our Mars orbiters were last week, it will see the comet just after its closest approach to the Sun, meaning that it will be in a more active state. We don’t expect to receive data from Juice’s observations until February 2026 – find out why in our FAQs.
Icy wanderers such as 3I/ATLAS offer a rare, tangible connection to the broader galaxy. To actually visit one would connect humankind with the Universe on a far greater scale. To this end, ESA is preparing the Comet Interceptor mission.
Comet Interceptor is due to launch in 2029 into a parking orbit, from where it will lie in wait for a suitable target – a pristine comet from the distant Oort Cloud that surrounds our Solar System, or, unlikely but highly appealing, an interstellar object like 3I/ATLAS.
Michael Kueppers, Comet Interceptor project scientist expands: “When Comet Interceptor was selected in 2019, we only knew of one interstellar object – 1I/ʻOumuamua, discovered in 2017. Since then, two more such objects have been discovered, showing large diversity in their appearance. Visiting one could provide a breakthrough in understanding their nature.”
While it remains improbable that we will discover an interstellar object that is reachable for Comet Interceptor, as a first demonstration of a rapid response mission that waits in space for its target, it will be a pathfinder for possible future missions to intercept these mysterious visitors.
Astrobiology, Astrochemistry, Interstellar