Traveling around space can be hard and require a lot of fuel, which is part of the reason NASA has a spacecraft concept that would hitch a free ride on one of the many comets and asteroids speeding around our solar system at 22,000 miles per hour (on the slow end). Comet Hitchhiker, developed at NASA's Jet Propulsion Laboratory, would feature a reusable tether system to replace the need for propellant for entering orbit and landing on objects.
The spacecraft would first cast an extendable tether toward the object and attach itself using a harpoon attached to the tether. Next, it would reel out the tether while applying a brake that harvests energy while the spacecraft accelerates. This allows Comet Hitchhiker to accelerate and slowly match the speed of its ride, and keeping that slight tension on the line harvests energy that is stored on-board for later use, reeling itself down to the surface of the comet or asteroid. A comet hitchhiker spacecraft can obtain up to ~10 km/s of delta-V by using a carbon nanotube (CNT) tether, reaching the current orbital distance of Pluto (32.6 AU) in just 5.6 years.
Unfortunately rocket scientists apparently don't read SN, or they'd know from discussions last year that it simply won't work. It seems that the idea defies "basic orbital mechanics" and "makes no sense".
Yeah, my first thought is that there is too much delta-v for a grappling hook and cable to be practical. It'd be worse than trying to accelerate your skateboard by hooking on to race cars during a race. Hooking on to passing cars happened in Snow Crash, but that after all is fiction, no matter how scientific.
All depends on the material science that goes into the harpoon and how long of a tether they can use. A skateboard could safely tether to a race car if it had a mile long tether. With diamond nanothreads exceeding 500x the strength of steel, and being much lighter, I don't see why this is impractical.
With diamond nanothreads exceeding 500x the strength of steel, and being much lighter, I don't see why this is impractical.
Or better yet, power the craft with magical fairies and unicorn farts!
No joke! Alone neither of those would work be enough, but the fairies are able to increase the power AND efficiency to levels we can not even measure.
Yeah, my first thought is that there is too much delta-v for a grappling hook and cable to be practical.
Well, what matters are the magnitude of the forces involved (acceleration), the tensile strength of the cable, and the mass of the whole assembly (compared to a more conventional thruster). As well secondary issues, like how reliable the system is. The idea in the article is that you fly by the comet, fire an anchor into it, and the cable unwinds as the spacecraft flies by the comet. This powers the spacecraft via a generator, and simultaneously slows the craft (relative to the comet).
For the concept to be useful, I imagine that the cable would be unwinding for the entire usable life of the spacecraft. Which means it needs to be unimaginably long, low mass, and have high tensile strength. The article suggests carbon nanotubes, which of course make all sorts of cool things possible except that, well, nobody has ever actually managed to build a cable out of the stuff.
For reference, 10 km/s delta-v is appox. the same as the delta-v budget of Dawn (after separation from its launch vehicle), which carried 425kg of Xenon propellant.
There was a game for Sega Genesis called Skitchin' that you had to get around town by hanging onto cars. It was actually a pretty fun game.
My armchair analysis seems to think that the deta-v might be too great for a tether. Just some napkin calculations suggest that even if your tether is 100km long, then the craft would be subjected to 50Gs of acceleration.
the craft would be subjected to 50Gs of acceleration
No worries there: the cable would simply snap :)
But apart from that, they claim it could be up to 620 miles long, that's 1000km in the rest of the world. Which would reduce the acceleration to 5G. Still quite a bit but feasible. As for the rest of the ingredients, I'm not so sure those are all that feasible. But one can dream. And research. And that might even result in something valuable.
This is possible: https://en.wikipedia.org/wiki/Fulton_surface-to-air_recovery_system [wikipedia.org]
But trying to attach a cable to a comet that's traveling many times faster... You'd need a lot of cable that can be released at high speeds and won't tangle...
And you definitely have to be very very accurate in timing and positioning if you're hoping to get much of a boost. Otherwise if you're having to adjust and match, you've probably spent almost as much energy by then.