# Third interstellar body discovered in our solar system **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [July 4, 2025, 5:26am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471 "2025-07-04T05:26:13Z")\ **Posts on this page:** 16\ **Page:** 3
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [July 11, 2025, 10:58am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/41 "2025-07-11T10:58:25Z")
Maybe, maybe not. “Asteroid” and “comet” are terms made up to describe objects in this solar system: “astroids” are inert lumps of rock and metal that primarily have orbits between Mars and Jupiter, “comets” are lumps of rock, metal, and frozen gasses that come into the inner solar system on extremely elliptical orbits with an aphelion far beyond the outer planets. But there isn’t a fundamental dividing line between the two types of objects. You can have comety astroids and asteroidy comets. > **[Asteroid–comet continuum objects in the solar system](https://pmc.ncbi.nlm.nih.gov/articles/PMC5454227/)** > > In this review presented at the Royal Society meeting, ‘Cometary science after Rosetta’, I present an overview of studies of small solar system objects that exhibit properties of both asteroids and comets (with a focus on so-called active ... > **[Active asteroid](https://en.wikipedia.org/wiki/Active_asteroid)** > > Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics. That is, they show a coma, tail, or other visual evidence of mass-loss (like a comet), but their orbits remain within Jupiter's orbit (like an asteroid). These bodies were originally designated main-belt comets (MBCs) in 2006 by astronomers David Jewitt and Henry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist The firs... > **[Asteroid Ryugu Might Actually Be a Dead Comet](https://www.universetoday.com/articles/asteroid-ryugu-might-actually-be-a-dead-comet)** There are “asteroids” out past the snow line that would act like comets if brought close to the sun. And there are probably plenty of “dry” rocks tossed into the Oort cloud by the gas giants that wouldn’t be active if brought into the inner solar syestem (and we may actually get them but never see them because they don’t form comas and tails). We get dry asteroids and wet comets in this solar system because of the positions of the gas giants interacting with the small solar system objects. But in solar systems with much closer gas giants (or much hotter stars) they would be tossing out a lot more dry objects. So for this interstellar object, who knows? It is showing a faint coma and tail now. That may be the most it will or can produce, or it might become highly active. Only time can tell. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 11, 2025, 1:52pm UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/42 "2025-07-11T13:52:47Z")
Also, the things that spend all of their time relatively close to the Sun might have had a fair amount of water in them originally, but it’s all evaporated by now. Comets only retain their water for a significant amount of time because they only spend a few months per century or millennium close to the Sun, and so there hasn’t been a chance to lose all of it yet. But if this object was ejected from its origin system fairly quickly (as most things that get ejected probably are), then it might still have the moisture that our “asteroids” have lost. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [July 11, 2025, 4:28pm UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/43 "2025-07-11T16:28:15Z")
> [@dtilque](#): > > In order to be captured into solar orbit, an interstellar object would have to lose a humongous amount of velocity. Which means some other object, such as a planet, would have to take that velocity. Nitpick: it’s momentum, not velocity. If a comet loses a large amount of velocity by interacting with Jupiter, Jupiter is obviously not going to gain that same amount of velocity and go shooting off from its orbit. It will gain whatever momentum that the comet loses, but since Jupiter is so much more massive, the change in its velocity will be trivial. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 11, 2025, 4:50pm UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/44 "2025-07-11T16:50:18Z")
> [@Chronos](#): > > Also, the things that spend all of their time relatively close to the Sun might have had a fair amount of water in them originally, but it’s all evaporated by now. There are comets that spend all their time close to the Sun. Comet Encke, for example, has an aphelion of only 4.1 AU, which means it’s well inside Jupiter’s orbit. In fact, there’s 79 known comets with similar orbits, that is, those whose orbits don’t reach Jupiter’s (5.2 AU). They’re called Encke-type comets. However, they haven’t been in those orbits for very long astronomically speaking (perhaps a few thousand years). Since they have periods of around 3 years or so, their ices will completely evaporate fairly soon (astronomically speaking) and, assuming there’s anything left, they’ll become asteroids. > [@markn\_1](#): > > Nitpick: it’s momentum, not velocity. Good point. ---
**Author:** ![Fiendish\_Astronaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiendish_astronaut/32/3295_2.png) [@Fiendish\_Astronaut](https://boards.straightdope.com/u/Fiendish_Astronaut)\ **Post date:** [September 25, 2025, 7:51pm UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/45 "2025-09-25T19:51:25Z")
The comet has been estimated to weigh over 33 billion tons. Which is very large indeed. > **["Major Anomaly" As Interstellar Object 3I/Atlas Measured To Be Over 33...](https://www.iflscience.com/major-anomaly-as-interstellar-object-3iatlas-measured-to-be-over-33-billion-tons-80941)** > > Measuring the object's non-gravitational acceleration, the team believes they found something "anomalous". > A new study has attempted to pin down the properties of interstellar comet 3I/Atlas, finding it is “anomalously massive” at around 33 billion tons. This is not anomalously massive for a comet, though it is a very big one. The team say it seems very large considering the supposed sparsity of other interstellar material that has entered the solar system. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [September 25, 2025, 10:38pm UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/46 "2025-09-25T22:38:15Z")
> [@Fiendish\_Astronaut](#): > > The comet has been estimated to weigh over 33 billion tons. Which is very large indeed This seemed incredibly large to me so I looked for a reference. I knew earth would far outweigh it. What about Pluto? For comparison: “Pluto weighs approximately 1.3 x 10²² kilograms, which converts to about 13 quintillion tons (or 13,000,000,000,000,000,000 tons)” So 3I/Atlas is big, but not planet big, or even dwarf planet big. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [September 26, 2025, 1:53am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/47 "2025-09-26T01:53:30Z")
Well, there’s a lot of water in the Cylindrical Sea, so that makes it pretty heavy. ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [September 26, 2025, 4:25am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/48 "2025-09-26T04:25:00Z")
How does it compare to our moon? ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [September 26, 2025, 4:59am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/49 "2025-09-26T04:59:43Z")
I’m sure I’m being whooshed here but…. The Moon’s mass is approximately 73.5 quintillion tons, or 7.35 x 1019 tons. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [September 26, 2025, 5:05am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/50 "2025-09-26T05:05:02Z")
The cited paper says that they estimate that 3I/Atlas has a diameter of 5 km, so it’s more than 300 million times smaller than the Moon by volume. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [September 26, 2025, 5:16am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/51 "2025-09-26T05:16:04Z")
> [@Lucas\_Jackson](#): > > The Moon’s mass is approximately 73.5 quintillion tons, or 7.35 x 1019 tons. 7.35 x 1019 tons. Note that super- and subscripts don’t cut-and-paste correctly. Please fix those after pasting, especially the superscript exponents. ---
**Author:** ![Elmer\_J.Fudd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmer_j.fudd/32/1211_2.png) [@Elmer\_J.Fudd](https://boards.straightdope.com/u/Elmer_J.Fudd)\ **Post date:** [September 26, 2025, 5:23am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/52 "2025-09-26T05:23:08Z")
5 km puts it at about half the size of the dinosaur killer and with significantly less mass (33 billion tons vs 1 trillion tons), ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [September 26, 2025, 6:11am UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/53 "2025-09-26T06:11:03Z")
> [@Lucas\_Jackson](#): > > This seemed incredibly large to me so I looked for a reference Not even the biggest coment currently visible. As per the article, C/2014 UN271 is around 500 trillion tons. (Hale Bopp is estimated at around 190 trillion tons.) ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [September 26, 2025, 4:04pm UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/54 "2025-09-26T16:04:31Z")
> [@dtilque](#): > > 7.35 x 1019 tons. > > Note that super- and subscripts don’t cut-and-paste correctly. Please fix those after pasting, especially the superscript exponents Noted, thanks. ---
**Author:** ![Fiendish\_Astronaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiendish_astronaut/32/3295_2.png) [@Fiendish\_Astronaut](https://boards.straightdope.com/u/Fiendish_Astronaut)\ **Post date:** [October 10, 2025, 7:01pm UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/55 "2025-10-10T19:01:24Z")
> **[Physicists uncork a message in a bottle from another star](https://www.eurekalert.org/news-releases/1100952)** > > For millions of years, a fragment of ice and dust has drifted between the stars — a frozen message in a bottle from another world. This summer, that message reached our solar system as 3I/ATLAS, only the third known interstellar comet. Using NASA’s... The observations show that the comet is leaking water at a fast rate even when it should seem too far away from the Sun to do so. “What makes 3I/ATLAS remarkable is where this water activity occurs. The Swift observations detected OH when the comet was nearly three times farther from the Sun than Earth—well beyond the region where water ice on a comet’s surface can easily sublimate—and measured a water-loss rate of about 40 kilograms per second—roughly the output of a fire hose running at full blast. At those distances, most solar-system comets remain quiet.” ---
**Author:** ![Jasmine](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jasmine/32/2964_2.png) [@Jasmine](https://boards.straightdope.com/u/Jasmine)\ **Post date:** [October 10, 2025, 7:21pm UTC](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471/56 "2025-10-10T19:21:15Z")
I always thought of the planet killer that might someday do us in as originating from the asteroid belt or the Oort Cloud, but imagine if we end up being done in by something from another star system God knows how many light years away. Imagine how astronomically (literally) small the odds are for that to happen! Imagine how many eons that object would travel through the depths and darkness of interstellar space before its fateful rendezvous with our hapless planet! > The odds of Earth being hit by an interstellar object are considered > infinitesimal, far lower than the risk posed by objects that originate within our own solar system. Interstellar objects are extremely rare visitors to our celestial neighborhood and pass through at very high speeds, making an impact highly improbable. [Previous page](https://boards.straightdope.com/t/third-interstellar-body-discovered-in-our-solar-system/1020471.md?page=2)