Interstellar Comet 3I/Atlas: What We Know Now | Hacker News Reader

Interstellar Comet 3I/Atlas: What We Know Now

skyandtelescope.org

95 pointsbikenaga46 comments

> The Rubin team realized that their telescope had — unbeknownst to them — imaged the object starting 10 days before its discovery.

This is so cool! Vera Rubin is designed to detect lots and lots of similar things, and having a test case where they can go back and "track" 3I through data is probably a great thing for tuning their models. Can't wait to see what Rubin finds over the years.

Also, it's so cool that they found 3I pretty early, we'll now have lots and lots of data from multiple powerful observatories and probably from some remote ones as well. It's nuts that we'll probably get some images/data from probes out in the solar system (mars orbiters, maybe JUICE).

Astronomers discover 3I/ATLAS – Third interstellar object to visit Solar System (308 points, 26 days ago, 171 comments) https://news.ycombinator.com/item?id=44451329

Vera C. Rubin Observatory Observations of Interstellar Comet 3I/Atlas (3 points, 7 days ago) https://news.ycombinator.com/item?id=44638392

Feasibility of a Spacecraft Flyby with the Third Interstellar Object 3I/Atlas (3 points, 6 days ago) https://news.ycombinator.com/item?id=44649150

First Hubble telescope images of interstellar comet 3I/ATLAS (109 points, 6 days ago, 29 comments) https://news.ycombinator.com/item?id=44649653

One interesting thing I learned from this was how they determined the probable size of this comet probabilistically, rather than using direct observation - basically, based on the observations, it could either be really big (10km) or really small (0.5km), and we can basically rule out really big because we've been looking for comets for years, and during that time, to see one that is that big implies that we _should have seen_ thousands that are quite small over that time period, because the size of space objects follows a power law since they're always whacking into each other and breaking up. Since we've only seen one small interstellar object during that time rather than thousands, a large comet is so impossibly unlikely that we can conclude that it is 0.5km in size. I'm sure at some point this will be confirmed in a more conventional way, as well.

What you’re describing is Bayesian inference in action. Given how rare big interstellar comets should be, and how common small ones should be, the lack of detections makes the big-comet hypothesis incredibly unlikely. So we update our beliefs: it’s probably small. Space statistics at work

> The Rubin observations also show hints of a dusty tail, but oddly this elongation points directly toward the Sun. Typically, the push of the Sun’s radiation moves dust in comet’s tails so that it points away from the Sun.

Deceleration burn?

I guess at some point we should expect dust to launch toward the Sun as that side gets heated more, right? But I have no idea when that is supposed to happen…

While uncommon, some ordinary comets also have an "anti-tail" that points toward the sun. As a recent famous example, the very bright comet in October 2024 [0] had a prominent anti-tail. The main photo on the wikipedia page shows it clearly.

[0] https://en.wikipedia.org/wiki/C/2023_A3_(Tsuchinshan–ATLAS)

We might be the blue people

Gas released from fresh ice on the illuminated hemisphere can push millimeter-scale grains out at a few m/s, and since these particles weigh thousands of times more than typical cometary dust, solar radiation pressure at 3 au is too weak to bend their trajectories, letting them overtake the nucleus and form a sunward anti-tail. Finson–Probstein dust dynamics predicts this plume should flip to the normal anti-solar direction or fade once 3I/ATLAS moves inside 1 au later this year. Watch the position angle around September to see the theory tested.

Pity it travels too fast to decelerate enough to be captured by the Sun.

I'm inspired to re-watch Don't Look Up this afternoon.

At a closest approach of 1.8AU, you really don't have to worry about a collision with Earth.

But that film is relevant to this comet in another way -- several missions that made it into space before the administration's war on the scientific enlightenment may be able to image the comet when we can't see it (perihelion) or when they're much closer to it than we'll ever be.

Do you happen to know which missions those might be?

The linked article has a good summary.

With the number of interstellar objects being detected only going up, it would be amazing if we could get some probes to hitch a ride on them. Imagine something lasting as long as Voyager 1 but travelling 3.5x the speed as it leaves the solar system.

I would guess there all kinds of technical logistics reasons as to why this is improbable, but I agree that would be really cool.

Visiting one with a probe would surely be amazing in it's own right...but hitching a ride would mean matching velocities with them. And if you can do that...you're already in the same orbit, so the comet doesn't really help.

The comet having virtually unlimited fuel would be of great help.

No it wouldn't because once you've matched speed with it, you stop accelerating and therefore need no more fuel. The comet isn't doing anything except following gravity.

The comet probably doesn't have a great amount of fuel. Even if it's all ice, how are you going to split the water? Only if you have a fusion reactor that can do H-H fusion, or you can scrape enough dueterium out of it, can you use that. You will find no tritium as it has a short half life, and He-3 (probably) won't have been implanted by solar wind in interstellar space, so if you need that, you have to bring it or breed it somehow.

It does have a great amount of mass, so you could rendezvous, construct a mass driver or ion drive and start taking it apart and using chunks of it as extra reaction mass. That would allow you to essentially get a free reaction mass "refill". You will still need a lot (a lot) of power, and solar will be near zero.

There's disposable FPV drones that launch with 50km spools of kevlar cable. Seems like smart people could work out a way to "hitch" a probe on a comet without fully matching velocities first.

that would probably be... extremely hard?

i mean aren't we talking like km/s of speed difference? idk of any kind of material even 50km long that could absorb that kind of stretch/sheering like that...

matching velocities till you can hitch the ride. from that point onwards, you can just do…nothing (at least in that department)

They're saying if you can match velocities in the first place, you don't need to hitch a ride, because you're already travelling fast enough.

And also, good luck hitting 70+km/s with chemical rockets, even without it going in the wrong direction relative to us for that to go well.

Question: I know that our planetary probes often use planetary gravity to boost their speeds. That only works for prograde speeds, right? Because you're subtracting a miniscule amount of orbital speed from the planet and adding it to your spacecraft's speed. You couldn't whip around a planet and somehow use that to give you retrograde speeds, could you? (Presumably an airless planet, like Mercury.) Or what about using a large moon during the retrograde (relative to the planet's motion around the sun) part of its orbit?

I’m not an expert on orbital mechanics, I just want to provide some data points.

The voyager craft, which not only had very good acceleration early on (the best we could do, really), combined with exceptional gravity slingshots and a lot of time - and are by all accounts some of the fastest man made objects ever - are going 15.4 km/s and 17 km/s relative to the sun.

3I/ATLAS is going so much faster than these objects they might as well be stationary.

Even ignoring the limited amount of time we have to intercept, catching up to 3I/ATLAS would be incredibly difficult to do. Perhaps impossible with our current technology*. Like catching up to a semi-truck going full speed on a highway with a bicycle. After it’s already passed us and is a couple miles down the road.

*barring theoretical (and kind of insane and dangerous) tech like Orion drives.

Problem is another. To got additional speed from other body, you need to move very close to it and in perigee use some powerful acceleration to increase rotation speed fast, so could not use slow acceleration engines, like ion engines.

Idea of gravity-assist acceleration, mechanically is just rotation of pair tightly tied bodies (and cut tie in right moment, so one body got acceleration and other got deceleration), but as it is impractical to tie for example to Moon with rope, used gravity force.

What also interesting, gravity-assist could use not only orbital speed of large body, but also got some acceleration from rotation of large body, as for gravitation, large planet is not just one material point, but system of few smaller (sub)bodies, and closer (sub)bodies give more acceleration than others.

When you achieve speed in space, after acceleration, the speed won’t change forever unless you encounter some other force, like a celestial body gravity to change it. So if you achieve interestelar comet’s speed, you can shutdown the rocket and just travel at that speed for eternity like the comet does.

Even better: you can forget the comet, accelerate, keep accelerating until there is no more power or even a working motor while also extending a big sail to let solar wind accelerate you a little more.

At first I thought, "no silly, it has to gain matching speed first, what's the point". Then it occurred to me - if we can make something which can survive the impact, we "just" have to place it in the path of the comet and it will be swept with it.

The whole thing would be like something like shooting a bullet at a moving target, but it's an idea.

That hypothetical probe will not look anything like any other space probe before it, but more like an artillery shell. (They can survive pretty damning Gs and still run that little embedded computer, so it's not a completely insane idea, I guess.)

We would also have to detect the interstaller object plenty in advance, so the probe can be launched "comfortably" in a trajectory which will intercept at exactly where the "object" is going to be.

Some quick prompting seems to say a G force of somewhere between 16-160 billion Gs, for a CPU-equivalent object getting hit by a solid object moving at 3I's escape velocity. Compared to a "typical" artillery shell of 10-15 thousand Gs. Not sure you're manufacturing anything that could survive 6-7 orders of magnitude more Gs than an artillery shell.

Of course the G-load would lessen based on how much you sped up to match its speed beforehand, but still, I think you'd need to be pretty much sped up to near the same speed as it before you could remotely possibly survive the impact.

Wow, I clearly didn't think this through. That's brutal.

That leads to another idea - if something more substantial was placed in its path - the resulting debris and gas cloud from the impact could reveal something about the contents of the object.

Or, if it's an alien probe, it would force their hand. :-D We could see some exotic manuevering.

Or an interstellar war.

But they're probably used to it. At 61,000 m/s, 0.5mv^2 must turn every collision with a small rock into quite a big bang.

A "solid object". But it's not clear how solid a comet is (or more to the point, this or any interstellar comet). If it were a fuzzball of snow, maybe 20 km across, you could in theory decelerate through it more slowly, maybe using a very large parachute initially, then discarding that for successively smaller parachutes as you approach denser and denser parts of the comet. The EPOXI mission to the Hartley 2 comet was reportedly hit nine times by "snowflakes" coming off the comet, but not damaged (https://www.astronomy.com/space-exploration/spacecraft-sees-...).

Of course your point is probably still valid.

Could an object like 3I theoretically tell when eyes are being pointed at it?

Probably not ground-based eyes, but possibly Hubble.

i.e. "the inhabitants are scanning us"

I don't believe so, unless the object had such a powerful telescope it could image Hubble itself in high definition and literally see where it was pointing, which would require an absurdly large lens. A telescope like Hubble absorbing light is purely passive, it doesn't emit anything "back" that would tell the observed object it is being observed.

Observing passive observation systems over those distances is pretty unlikely even with an optimistic view of future technology. But perhaps not impossible.

We have all kinds of space radars though that are active.

If it had several kilometer wide imaging array, then yes. Hubble and other telescopes are totally passive optics. You need to see the distant dot and have enough resolution to identify it as a telescope and see that it is pointing at you. Seeing hubble from more than 1 au would require optics about 8km in size. This doesn't need to be one giant mirror, could be two cameras 8km apart, but synchronized optically (distance known to a small part of wavelength of light). Seeing Hubble as more than 1 pixel to tell that it's a telescope pointing at you needs something much larger.

Theoretically, any sentient being with the necessary technology to get a craft to our solar system and spot the aiming of a telescope at that distance would have even better technologies for detecting what we are doing.

It's a logistics problem: if you can build one of something, you can definitely build two.

Why build one, when you can build two at twice the price?

If you can build two, then build three. In fact, always do things in threes.

Extremely unlikely, given that there is no "active" scanning happening (no "give me a ping, Vasily"), we're just passively looking. I'd think the most likely way to find out would be listening in on our radio broadcasts (I assume someone's been talking about this object on the radio) rather than being able to directly detect the Hubble being pointed at it.

We have a LOT of energy outbound in the form of space radars and interplanetary comms. We’re lit up like a christmas tree.

Sure, but none of that was pointed at this comet specifically.

Ah, fair point - I see what you mean.

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