NASA Comet Hitchhiker Idea.

[Robotengineer]

http://www.nasa.gov/feature/jpl/comet-hitchhiker-would-take-tour-of-small-bodies

Seems like a bad idea IMO. How would you prevent the spacecraft from swing around on the tether and bashing into the asteroid without using at least some propellant?

[Nikolai]

How would you prevent the spacecraft from swing around on the tether and bashing into the asteroid without using at least some propellant?

The same way you keep a kite from plowing into the ground -- by changing the braking rate on the line.

EDIT: It's not perfectly analogous, and I know that. Thinking about it with kites should get you started in the right direction, though.

[PB666]

http://www.nasa.gov/feature/jpl/comet-hitchhiker-would-take-tour-of-small-bodies

Seems like a bad idea IMO. How would you prevent the spacecraft from swing around on the tether and bashing into the asteroid without using at least some propellant?

I think they are confusing differential velocity with differential rotation. There is one physical way to slow down using the target as the means, friction. If the target has an atmosphere you can aerobrake, it does not you have to slam into it. For most analysis the intercept velocity will be in the km/sec range which means that friction is striking the target like a surface to air missile, not a good idea for landing.

SO obviously this is not what they are talking about. What they are talking about is this. The problem with landing on a comet is this, the surface is always moving, there is no non-rotational reference frame in which a comet is not moving, moving surface makes landing hard.

If you tether a rotating asteroid and real in, the ship will move faster, and just like a rock climber on a cliff, will slam into the wall elsewhere as soon as tension on the tether starts to decrease.

But the rotation creates centripetal force and if the ship lets out line then the ship is propelled the opposite direction relative to the original rotation, and if friction is placed on the line itself (such as a clutch on a trany) the energy will be released as heat, if you did this over many areas on an asteroid you could slow down its rotation

E_rotation ------> hv.

This technique I have actually used in fishing. So for example if a fish is caught and you real very quickly toward the boat, you are pulling the fishes head toward you but his thrust, if he desires is the easier in your direction, which means that once he is getting close to you it can thrust away at a much higher velocity and snap the line or tangle it in the propellar. The way to stop this is to lead a heavy fish along a tangent allowing the fish to pull away from you with a drag that is about maybe 1/2 to 3/4ths the maximum force that you can apply. In addition you constantly try to keep the rod at 90' up relative to the line, this has two positive benefits, it increases the shock absorbing capacity of the rod and pulls the fish off the bottom, and action most fish will try to counter naturally by pushing down and using the bottom, shallows or obstacles to gain thrust. By reeling and letting him pull you are dissipating some of your energy against the clutch, but when he thrusts he is dissipating his energy, the fish then stays in a range of radius from the boat moving at small angles relative to the perpindicular of the line. You would think that you are losing energy because you are constantly reeling, but the fish, when it thrusts applies thrust against you at an oblique angle, because you keep is head moving obliquely toward you and hes trying to curl his head away, the result is when he thrust his head moves from obliquely toward to obliquely away before he has lost to much thrust traveling away and he relaxes bringing his head back. So only some of this energy is going against the clutch, and the rest is wasted trying to turn, creating angular drag against his body. Eventually the fish cannot supply enough oxygen to his muscles to sustain that level of heat and he will start gravitating to you at a much lower speed. They always keep one burst of speed close to the boat. Many fish, for example bill fish are often dead by the time you get them to the boat, in that last burst to get away, they frequently deplete their heart of sufficient oxygen and die, muscles cramp up immediately, like pulling a fish out of the fridge. On some catch and release fish I have actually had to flush oxygen across the gills and palpitate them gently to get them back to life. (For me the really big fish are a nuisance, I prefer pound size fish).

To do this right in space you would have to do this in a conic section, that is essentially flat on one side (not so different from fishing because you want to steer the fish away from the bow or props). You would have to repeately move in against the line without tension, thrust to tense, and then reel out or side to side basically converting rotational energy into heat.

The problem is that to slow down rotation, which i think mining an asteroid this is a good thing, you would need to constantly jerk the tethers hold fast, as we know asteroids don't take well to this, and the holdfast would loosen; the same think happens in fishing, you can tear the hook from the fishes mouth because by constant back and forth jerking the hole will widen and the counter point on the hook will have enough space to free itself (one reason why the rod is always held up and perpindicular to the line). On an asteroid no such hold will work, the only way to prevent this is to is to land an auger on the asteroid and twist it deep into the asteroid and then cement the transit to the surface with a plate with a plane that is perpindicular to the rotation vector.

On a comet like 67P the roid would probably break in 2. It would have to be tethered from both long ends. And in any case doing this on one side and not the other will constantly shift the center of rotation and destabilize the asteroid.

[Aethon]

We learned from Rosetta/Philae that our harpoon tech is seriously lacking.

[Nikolai]

We learned from Rosetta/Philae that our harpoon tech is seriously lacking.

That wasn't a problem with the harpoon; it was a problem with the propellant (nitrocellulose), discovered some nine years after launch (and a little over a year before Philae was to use it).

[Aethon]

Isn't that a part of our technology for deploying harpoons in space?

[Nikolai]

Isn't that a part of our technology for deploying harpoons in space?

Not anymore.

Taking a technique that is known to be unworkable (and that would not be used in any spacecraft designed today) and pretending that it represents our current technology on the matter is a little misleading, don't you think?

[kerbiloid]

Kerbal Attachment System contains a harpoon.

If this works in KSP, why it shouldn't in real world?

[Yukon0009]

KSP is not real world, for example KSP ions are waay more powerful than real world ions.

[KerbMav]

I cannot decide who of you two is more cute right now.

[kerbiloid]

I cannot decide who of you two is more cute right now.

https://xkcd.com/1244/

[StrandedonEarth]

Sorry for the necro, but I didn't want to start a new thread for this, and it is related.

https://www.spacedaily.com/reports/To_watch_a_comet_form_a_spacecraft_could_tag_along_for_a_journey_toward_the_sun_999.html

I think the researchers at UoChicago have spent too much time catching comets in KSP, and now have figured out how to try it for real. Catch a comet at Jupiter, as it gets slingshotted inwards? That's a three-star mission!

[insert_name]

3 hours ago, StrandedonEarth said:

Sorry for the necro, but I didn't want to start a new thread for this, and it is related.

https://www.spacedaily.com/reports/To_watch_a_comet_form_a_spacecraft_could_tag_along_for_a_journey_toward_the_sun_999.html

I think the researchers at UoChicago have spent too much time catching comets in KSP, and now have figured out how to try it for real. Catch a comet at Jupiter, as it gets slingshotted inwards? That's a three-star mission!

This actually seems reasonable, it  does not mention grabbing the comet, simply rendezvousing with it, in the paper linked at the bottom of the article they show a graph with the delta v from Jupiter, and the highest point is 0.14 km/s,  an order of magnitude below what it takes to get from a GTO to GEO, the main concern I have is more related to surviving the debris from the comet as it forms

 

[JoeSchmuckatelli]

Question about the harpoon and tether idea: won't any rotation of the target at all result in it becoming a spindle of whatever circumference matches up with the direction of rotation? 

So - the craft has to take a LOT of line just to keep from becoming a birds nest attached to the asteroid... Right? 

Although - couldn't this technique be used intentionally to wrap up the target, allowing it to reel in a thruster for slowing down the rotation prior to resource harvesting?