B 3I/ATLAS, Interstellar Comet

[Astronuc]

3I/ATLAS, also known as C/2025 N1 (ATLAS) and formerly designated as A11pl3Z, is an iinterstellar comet. It was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station at Río Hurtado, Chile on 1 July 2025.

Note: it was mentioned (as A11pl3Z) by DaveE in a new member's introductory thread.

Post in thread 'Brian Cox lead me here!'

Aug 8, 2025

missile = comet, I guess?
https://earthsky.org/space/new-interstellar-object-candidate-heading-toward-the-sun-a11pl3z/

• DaveE

https://earthsky.org/space/new-interstellar-object-candidate-heading-toward-the-sun-a11pl3z/

One source mentions the comet traveling at 68 km/sec, or 244.8 km/hr! The Wikipedia article indicates speed at perihelion of 68.3 km/sec.
https://en.wikipedia.org/wiki/3I/ATLAS

the newly discovered object was entering the inner Solar System at a speed of 61 km/s (140,000 mph; 220,000 km/h) relative to the Sun, located 3.50 AU (524 million km; 325 million mi) from Earth and 4.51 AU from the Sun, and was moving in the sky along the border of the constellations Serpens Cauda and Sagittarius, near the galactic plane.

https://en.wikipedia.org/wiki/3I/ATLAS#Discovery

It will pass between the orbits of earth and Mars, with the Comet's trajectory projection on the ecliptic intersection the orbit of Mars twice.

https://science.nasa.gov/solar-system/comets/3i-atlas/

Comet 3I/ATLAS poses no threat to Earth and will remain far away. The closest it will approach our planet is about 1.8 astronomical units (about 170 million miles, or 270 million kilometers). 3I/ATLAS will reach its closest point to the Sun around Oct. 30, 2025, at a distance of about 1.4 au (130 million miles, or 210 million kilometers) — just inside the orbit of Mars.

 

[OmCheeto]

I also ran across the discussion in the new persons thread.
It got me interested in how long it would take the comet to travel interstellar distances at its approximate speed. Here are 4.

3I-Atlas (v∞ = 58 km/sec)	Proxima Centauri (nearest star)	Ross 154 (nearby in the direction of Sag A*)	average distance of nearest 100 stars	Sagittarius A* (black hole at the center of our galaxy)
distance (ly)	4.25	9.7	17	27,000
time to travel (years)	22,000	50,000	88,000	140,000,000

{2025.08.14 20:08 PDT: edited to fix speed booger, per Filip Larsen
Odd that I made the mistake as I had noticed the v∞ speed in the wiki article.
Chalk this up to 'Senility Now!' }

 

[Filip Larsen]

It got me interested in how long it would take the comet to travel interstellar distances at its approximate speed.

It looks like you used the perihelio speed at 68 km/s and not the hyperbolic excess speed (i.e. the speed far from the Sun) estimated at 58 km/s, so the time to travel should probably be around 17% larger.

 

[OmCheeto]

It looks like you used the perihelio speed at 68 km/s and not the hyperbolic excess speed (i.e. the speed far from the Sun) estimated at 58 km/s, so the time to travel should probably be around 17% larger.

Fixed!
I think I may have originally used 58 km/s in my calculations, but I kept adding distances, and by the time I'd finished, I went back and double checked Astro's post to see if I was repeating something he said, which I think is where I saw 68, and decided that I'd made a typographical error, and in error, I unfixed it.
In any event, thank you.

 

[Filip Larsen]

Fixed!

And points for rounding the numbers to significant figures!

I assume the idea for the table is to get a grasp for what a speed of that magnitude means for travel time and thus on purpose ignores where 3I/ATLAS is actually heading. But just in case anyone now wonders if the comet will reach the center of our galaxy in around 140 million years, then the answer is no since 3I/ATLAS, just like the solar system, is in an orbit around the galactic center. However I have so far not read (or had time to try calculate it myself) to what extend this encounter between the two has significantly changed the galactic orbit of the comet, but from the small deflection angle I would suspect it has not.

 

[OmCheeto]

And points for rounding the numbers to significant figures!

Thanks!

Spoiler: Grampa's long winded excuse...... Ok to ignore.

The original format, while 'significant figures' ugly, was done on purpose, as along with my brain, my eyes are going bad, and the commas looked like decimal points, and made it quite confusing to look at. Obviously that was before I had added Sag A*.

After I fixed the numerical error, I decided that 'Sag A*'s travel time negated the whole 'old people like me might get confused' argument. And then some weird sense of 'transcription error' paranoia was overcome and I followed my hero's lead and dropped everything down to 1 or 2 decimal points.
I guess you could call it 'conversational significant digits', which is obviously not the same as that of the scientific sense.

I assume the idea for the table is to get a grasp for what a speed of that magnitude means for travel time and thus on purpose ignores where 3I/ATLAS is actually heading. But just in case anyone now wonders if the comet will reach the center of our galaxy in around 140 million years, then the answer is no since 3I/ATLAS, just like the solar system, is in an orbit around the galactic center. However I have so far not read (or had time to try calculate it myself) to what extend this encounter between the two has significantly changed the galactic orbit of the comet, but from the small deflection angle I would suspect it has not.

The table was inspired by two things:
1. Brian Cox's most hilarious comment in human history. I was curious how long ago, some "sentient molecular cloud", had propelled a "giant space turd" at us.
2. The comet is coming at us from the direction of the galactic center.

Given that I had no clue what stars are in the galactic center direction, I started with our nearest neighbor. After quite a bit of googling, I discovered Ross 154.
I then went and found an old list of our 100 nearest neighbors and was quite surprised how close they were.

In the end, I decided that 3i/Atlas could have come from near the galactic center.
But this prompted a couple of new questions:
1. How many stars did 3i/Atlas significantly interact with if this were true?
2. If it were 'A LOT', then it seems like it would have run out of gas by now, indicating this was a nearby star supplied comet. Which begs the question; "Will we one day be able to determine the approximate distance these interstellar comets travel from an analysis of their outgassing?"

I had a similar question about how far Oumuamua had traveled. But as often is the case, didn't know how to phrase the question without sounding like an idiot.

 

[Filip Larsen]

The comet is coming at us from the direction of the galactic center.

This is only relative to us in the solar system. Relative to our galaxy the comet is in an orbit similar to that of the solar system where it (in lack of other close encounters) has been and will get a bit closer to the galactic center, but not significantly so. The orbit can be seen at https://en.wikipedia.org/wiki/3I/ATLAS#Origin_and_age, but as mentioned one should take the multiple "round-trips" depicted to only be an indication of the orbit before or after the current passage, and not as an accurate prediction of there the comet has been or will be for billions of years. It could be interesting to know with what precision (in time and space) the actual in- and out-bound trajectory can be calculated relative to the stars in the local arm of the galaxy. Considering the great distance between stars I suspect it might be possible to give the orbit a sort of statistical measure under the assumption that close star passages for one specific interstellar comet really is rare.

Edit: a larger image of the same orbital view from the team behind is included below. I was not able to find the exact orbital elements used, if such data even makes sense for galactic orbits.
https://ras.ac.uk/news-and-press/re...-interstellar-object-may-be-oldest-comet-ever
https://ras.ac.uk/sites/default/files/2025-07/Fig3-map_faceZoom-unlabelled.png