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".
I doubt the harpoon concept will work.They should have a backup plan to use a big net, like a purse seine. [noaa.gov].
These things tend to be big rubble piles, and a harpoon might not obtain nay purchase.
You've hit on one of the central problems with this approach. The reason for catching a comet with this harpoon "propulsion" system is to get more delta-v with the same mass than some other propulsion system. But if the harpoon misses, or doesn't stick, or sticks but then breaks loose during acceleration, you've missed your absolute one and only transfer window and the probe is a multimillion dollar monument to Things That Look Good on Paper.
I don't think I like the net idea. With that kind of mass budget I'd just get a solar sail or ion engine (depending on destination, timeline and electric power budget).