Catching an asteroid with harpoon tether?

[laukejas]

Hello all,

The most crazy idea popped into my mind as I was reading on various proposed NASA methods to send probes to asteroids and comets with very different orbits than Earth, making these kind of journeys extremely dV expensive, if probe were to match it's orbit with the target asteroid, instead of fly-by.

The idea is as follows. Probe flies by the target asteroid/comet within several kilometers, and as it does, it fires a harpoon with a flexible tether attached. Probe carries some significant length of the tether, maybe several hundred kilometers (should be quite possible with nowadays technology, lightweight nano-tubes, and similar). As soon as harpoon hits and digs a bit into the asteroid, probe engages brakes which stop the rate of tether cable release, therefore, negating the relative velocity between asteroid and the probe. When tether release is stopped, probe "reels in" the tether, pulling itself towards the asteroid at slow speed, and after it reaches it, it has just a small relative velocity to kill. Tether can then be released or used as an anchor for landing.

Problems I see with this idea are as follows:
1) Since initial relative velocity may be several km/s per second by the time of fly-by, the harpoon must withstand the impact, not breaking the tether, and securing itself firmly in asteroid/comet;
2) Should the harpoon fail to lock, or if it were to miss the target rock altogether, the mission is screwed for good;
3) Braking and reeling in should be done quickly and with considerable G forces, because every second means extra kilometers of tether to carry.
4) Due to the nature of this "elastic collision", the orbit of asteroid/comet would be changed to some extent - but then again, who cares?


What are your thoughts? How feasible is this idea with current technology? Are there more problems than I mentioned? I'm sure I'm not the first to think of this, but I couldn't find anything on google.

Your comments, please! :tiphat:

P.S. Made some math. If probe were to fly by with 3km/s relative velocity, and break at 100m/s (around 10 G's, something that current tech can still endure), it would take 30 seconds to do the braking, and the braking distance is 45km (of tether), also quite reasonable.

 

[Artlav]

That sounds very familiar.
I've drawn this some years before...

But can't quite remember what the idea was.

I think plan A was actually to hook an asteroid, then slow down magnetically.

A cable reeling out simply won't work - there is nothing that can spin fast enough, and at these speeds "solid matter" ceases to be usefully solid.
Even if there was - you're talking about a cable that can survive a dynamic load of needing to spin up the reel with it's own mass on it to multiple orbital velocities - that alone is one heck of a lot of force. Any reel would be just torn out of it's mount way before it can get up to speed.

So, you have a straight run of a strong cable, with the probe threaded on it.
One end of the cable is a hook, that catches the asteroid.
The cable stops suddenly, and the probe finds itself moving relative to it.
Now all that is left is to apply brakes.
Somehow...

Plan B is just to use enough padding for the payload to survive a lithobraking.
Anything can survive a drop with enough padding, as customers of Russian Post prove every day.

 

[laukejas]

Alright. But the forces depend on the size of the reel, right? At first, I thought of an internal reel. That limits its size. How about an external reel, which can be made much larger?

This is crude illustration of what I have in mind. Reel is surround the probe, fixed on several beams which have a electro-magnetic or some other kind of holder, which allows to spin up or slow down the reel without spinning the probe itself.

Also, there is a tether mount outside the reel (it doesn't interfere with the reel, but it's hard to illustrate in 2d painting, just imagine it's on the other plane, going around the reel), which guides the tether so it leaves the spacecraft on the line of CoG, so it doesn't spin the probe itself while braking.

As to spinning up the reel fast enough, I have different idea: harpoon, being lightweight, could be shot from spacecraft at some reasonable speed which is tolerable by reel spinning (something like several hundred meters per second), and then accelerated with micro rockets towards the asteroid, gradually spinning up the reel. Rockets would detach before impact, impact speed could be significantly slower, and the reel would engage brakes after the harpoon locks itself in the asteroid.

In this case, all depends on the weight of the harpoon and the thrust needed to accelerate it to relative velocity after passing the target asteroid.

Another thing popped in mind - if fly-by distance is greater, it would give reel much more time to spin up, reducing the forces.

 

[laukejas]

I have a better idea. What if reel would spin up BEFORE shooting the harpoon, leaving some considerable amount of "loose" tether, then harpoon shoots, using up that tether, and reel still accelerates, so by the time harpoon digs into asteroid, reel is spinning at the needed velocity to tighten the loose tether?

 

[jedidia]

so by the time harpoon digs into asteroid, reel is spinning at the needed velocity to tighten the loose tether?

If you have the power to spin up the reel to such speeds, you might as well have the power to accelerate the probe...

 

[Artlav]

I'll be more concerned about the reel.
The rim of it should be spinning at the closing velocity - 3 km/s in your case.

The wheels on supersonic cars are precisely engineered things, that spin only at 300-400 m/s.
The tips of the blades of a jet engine are spinning at only 200-300 m/s, and need superalloys to stay intact.
Some propellers have been sped up till the tips are supersonic.

I'm not readily aware of any macroscopic thing spinning fast enough that the rim is at 3-4 km/s.

If that was possible, then the most common rocket engine would be lead pellets accelerated to 3-4 km/s escape velocity by spinning wheels at the bottom on the pellet tank:

Mythbusters tried that (as a machine gun, not a rocket), and it was a bust.

 

[robertinventor]

Actually, there's an old 1980s paper describing the idea of harpooning an asteroid - wonder if that's what you are thinking about?

The idea is for a gravity assist slingshot - but perhaps it could be used for capture by the asteroid as well.

Tethers and asteroids for artificial gravity assist in the solar system

You could have just - an elastic cable to absorb the shock, and - fire the cable well before you get to the asteroid, pass it to one side, and the cable does a slingshot around the asteroid - ends up with the cable wrapping around the asteroid - perhaps many times if it is a small asteroid - and as that happens you reel in the cable, to slow down just like firing retro rocket, you'd have a fair bit of time to do it before your spaceship finally hits the asteroid because all the cable has been wrapped around it.

e.g. maybe 300 meter diameter asteroid, about 1 km circumference, then your 100 km cable is 100 turns around the asteroid, and - suppose say, 3 km per second approach, pretty fast, just as a for example, then that's 3.5 minutes for the first orbit at 100 km - would be well over an hour to wrap entire cable around the asteroid - so you would have plenty of time to use the cable to slow down. Could reel it in for more deceleration, and let it out for less - and t finally release it so you avoid hitting the asteroid and end up in close orbit or similar enough trajectory to use rocket motors for the rest of it.

Seems feasible to me .