[EspadaV9]

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.

[kqgnkqgn]

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.

[themgt]

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.

[andrekandre]

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...

[signa11]

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

[kristianp]

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.

[lazide]

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.

[mcswell]

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?

[simne]

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.

[lazide]

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.

[motoboi]

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.

[pandemic_region]

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

[danparsonson]

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.

[grues-dinner]

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.

[actionfromafar]

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.

[thegrim33]

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.

[actionfromafar]

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.

[TheOtherHobbes]

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.

[mcswell]

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.

[bmiekre]

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