Currently. Intercepting it and returning it from wherever it is 100 years from now could theoretically be easier than intercepting it from when it passed by in the present.
Or if not perhaps in 1,000 years retrieving it and studying it will be trivial; if it's still that interesting by then.
Is there even any such stable orbit? From which minor burns could easily set final, post-slingshot trajectory?
The downside is you can’t just park yourself around that planet waiting for the opportunity to do such a slingshot. Hyperbolic trajectories are inherently single-pass maneuvers, unlike bounded elliptical orbits. You could conceivably park in a high-energy, loosely-bound elliptical orbit, but to do this parking you still have to lose some of your energy relative to the planet in order to achieve orbital capture, and do a burn to regain that energy later. This is probably counterproductive if your ultimate goal is a solar escape trajectory.
That’s not even the most serious difficulty, though. Intercepting an extrasolar object (aside from a brief flyby, that is) requires not just achieving any solar escape trajectory, but specifically one in the same plane as the target. And even once that is achieved, an actual hyperbolic rendezvous is an extremely difficult maneuver, not to mention time-consuming: with the delta-V budget available to chemical rockets, I’d guess (without having done any calculations) that you’re looking at many decades or even centuries to complete the rendezvous.
Due to the constraints of physics and chemical rockets, spacecraft trajectories are hyper-optimized based on knowledge of the exact mission plan. I don’t think a general-purpose vehicle stationed in the outer solar system would be suitable here.
Stuff like NERVA or Project Orion stayed proposals not because they're impossible, but because there's nowhere we really need to get that fast to justify the risks/expense/fallout.
"We need to get to this interstellar starship" would change the cost/benefit ratio of developing the tech needed.
Which is to say - I'm all for it! The sooner we make those breakthroughs, the better.
In summary, you're pretty much right. If we could launch in 2017, we could have done it, but I don't think we have any spares of Rosetta or similar intercept missions sitting around.
Now - whenever people accuse me of never finishing anything I shall respond that I'm actually achieving TRL2 across a broad front.
Don't underestimate the gap between a science/jobs program like NASA vs what happens where there is an acute national interest. A derelict alien spacecraft would present an objective that no world power could ignore. Nation states taking enormous risks and spending hundreds of billions would make our current space program look like hobbyist work. In that setting, I would hesitate to say what is impossible.
It may be going fast, but partly that's because it sped up a lot through the inner solar system, as it climbs out of the Sun's gravity well it will slow down a great deal. And the solar system is big. It won't be light-years away in a few decades, of course, it's not going nearly that fast, it'll take nearly two centuries to get 1000 AU from the Sun. So we have all of that time to go out and do a flyby mission. We could put one together today that would reach the asteroid within the next 30-40 years if we really wanted to (at a modest cost of a few billion dollars). If we went wild and spent tens of billions on a mission we could do a rendezvous mission in 20 years. Meanwhile, launch systems are getting better and cheaper so almost certainly we'll get to a point where a realistic 10-20 year cruise timeframe mission is not just feasible but affordable, and then we'll go take a look at it.
That's a lot of confidence. This is the first interstellar object passing through imaged. However, we believe several of these objects pass through the solar system per year. Why haven't they been detected until now? We didn't have sensitive telescopes doing whole sky surveys like Pan-STARRS until recently. As more projects like LSST come online, we will undoubtedly discover more interstellar objects. And we will learn a lot more about them from Earth-based observations. Perhaps we will get to the point with telescope sensitivity where we can detect them years away so they intercept is simpler. But I do not share your confidence that we will send our first probe to explore interstellar objects to Oumuamua just because it was the first one discovered.
It doesn't help us much, since we'll be going against gravity too. ∆v required to reach the object won't change significantly over time, and with present technology, it's the strongest factor which determines what we can or can't do in reasonable time right now.
If our craft can accelerate fast enough to intercet it, it’s position is still irrelevant since we know it’s trajectory.
The trouble is computing these trajectories is pretty tricky and if you get even one manouver wrong you crater or burn up in the atmosphere.
Fastest object we've ever made was Juno, which went 25miles/sec (5 years to Jupiter)... this is traveling at ~30 miles/sec, and it's already getting pretty far from us: https://en.wikipedia.org/wiki/%CA%BBOumuamua#/media/File:Oum...
"Practical" in the sense of yes, the physics work, for the ones we know of. And assuming there was reason to quiet the Green crowd from freaking out about us launching nuclear bombs into orbit.