How does one "lasso" an asteroid?

[JanewayRulz!]

Besides the obvious answer... "very carefully"!

From space.com

http://www.space.com/20552-nasa-considers-plan-to-lasso-an-asteroid-for-study-video.html

Wouldn't it be "embarrassing" if instead of being trapped in the moon's orbit, it came after the Earth!

NASA Chief: "uh... didn't expect that outcome."

POTUS: "Really? Naturally, you have a backup plan on how to blow the thing up before it reaches us."

NASA Chief: "Well you see, Madame President, that was just one the points of the experiment, to study ways to mine these for minerals in space station construction, to learn how to deflect asteroids and how to blow them up before they hit Earth."

POTUS: "And?"

NASA Chief: The "how to blow them up" experiment is scheduled to begin in 3 years."

POTUS: But the asteroid is expected to make impact in 3 days!!"

NASA Chief: "Yeah... like I said, we really didn't expect that outcome or we might have started with the "exploding asteroid experiment" as soon as we captured it last year.

POTUS: 'ARGHHHHHHH1"

 

[Squiggy]

With "physics".

 

[gturner]

What I can't figure out is why they aren't cutting the mission weight by getting rid of the giant lariat and just pushing the asteroid along.

 

[Crazy Eddie]

Wouldn't it be "embarrassing" if instead of being trapped in the moon's orbit, it came after the Earth!

Probably, but then, anything small enough to be captured and dragged into lunar orbit would be too small to do any real damage to Earth. More importantly, it'll be dragged to the moon's orbit, NOT Earth's orbit, so it's more likely to careen into the path of a satellite and annoy AT&T, who probably have it coming anyway.

What I can't figure out is why they aren't cutting the mission weight by getting rid of the giant lariat and just pushing the asteroid along.

Because "just pushing the asteroid along" only SOUNDS simple and is in fact anything but.

 

[scotpens]

Sounds like a job for this guy.

 

[gturner]

What I can't figure out is why they aren't cutting the mission weight by getting rid of the giant lariat and just pushing the asteroid along.

Because "just pushing the asteroid along" only SOUNDS simple and is in fact anything but.

I was making a joke because cables and space don't get along very well. ^_^

I think pushing it is how they're going to do it, using a big expanded cup as a container, because tiny asteroids tend to have very little internal cohesion - unless they're going for a solid iron asteroid.

Pulling on an asteroid would require either a very long cable or introduce large cosine losses from using multiple thrusters that are angled so their exhaust misses the asteroid they're towing.

If all the exhaust hits the asteroid then you're just tensioning a cable and might as well dump the fuel overboard because nothing is going to accelerate. The other problem with a cable is that it's elastic and would be under tension, like a rubber band, and after engine-cutoff the spacecraft and asteroid are going to become much more intimate.

 

[Davros]

I was making a joke because cables and space don't get along very well. ^_^

example please?

 

[E-DUB]

Maybe I'm wrong here and if so someone jump in and tell me. I thought anything left in lunar orbit would eventually fall to the moon like the LEM upper stages did. Atmospheric drag is not a factor obviously but due to orbital perturbations caused by Earth. Park it at L-5.

 

[Metryq]

I was making a joke because cables and space don't get along very well. ^_^

example please?

He gave examples. There's also the problem of electric currents.

The "space tug" approach of pushing an asteroid can be problematical, too. Depending on the shape of the asteroid and the placement of the tug, the asteroid might be induced to tumble. Repositioning the tug would then be extremely difficult and may not solve the problem anyway. Should the asteroid be outfitted with a complete RCS?

 

[Crazy Eddie]

Maybe I'm wrong here and if so someone jump in and tell me. I thought anything left in lunar orbit would eventually fall to the moon like the LEM upper stages did. Atmospheric drag is not a factor obviously but due to orbital perturbations caused by Earth. Park it at L-5.

There are a couple of "frozen orbits" that are stable around the moon, generally at inclinations of 27 degrees, 50 degrees, 79 degrees and 86 degrees. An object on one of those orbits will remain more or less undisturbed for years with minimal stationkeeping.

 

[Nerys Myk]

Sounds like a job for Wonder Woman.

 

[Crazy Eddie]

I was making a joke because cables and space don't get along very well. ^_^

example please?

He gave examples.

No he didn't.

There's also the problem of electric currents.

Only within the Earth's magnetic field and in relatively low orbits. Besides, the kinds of asteroids they're looking at capturing would be about the size of a schoolbus, so a spacecraft pulling that asteroid wouldn't need a cable more than about a dozen meters long, far too short for electric current to be a problem.

Should the asteroid be outfitted with a complete RCS?

Nope. Just figure out the asteroid's center of mass and have the tug attach a cable so that it is providing thrust through that point. It then becomes possible to disconnect the cable, reposition to a different vector and then reattach the cable, again thrusting through the center of mass.

 

[R. Star]

With great difficulty.

 

[gturner]

Nope. Just figure out the asteroid's center of mass and have the tug attach a cable so that it is providing thrust through that point. It then becomes possible to disconnect the cable, reposition to a different vector and then reattach the cable, again thrusting through the center of mass.

Think through that slowly and you can see that mission planners will have a handful. How do you attach the cable to the asteroid? If you are going to try to anchor it, what kind of anchor do you use?

You can't drill into the asteroid without somehow being attached to it firmly enough to apply significant force to the drill bit, which implies that you've already driven some anchors to hold your satellite against the asteroid, which required drilling...

You probably can't glue something to the asteroid because the surface is likely covered with a fine powder, and in fact much of the outer surface might be pretty loose grains, which could throw any drilling to set an anchor out the window, too.

So you'll probably have to wrap the cable around the asteroid, but then you'll need more than one cable because you can balance a basketball in a loop of cord; it will always fall out one side or the other. If you try to loop the cord around the asteroid, there's the deployment problem of trying to get the free end of the cord to stay where you left it as the satellite makes a circle. This would be simple if you attached the free end to the asteroid, but that brings up the problem of how to attach the end of a string to an asteroid.

Using a net or bag instead of a single cable might work, but you'll need something to hold the net open so you can get the asteroid inside, and somehow you have to keep the cords that run to the net from snagging the asteroid as you try to complete the operation, and that implies arms to hold the strings to the side or some other pretty sophisticated technique.

But then you've got the problem that the asteroid is probably spinning very slowly, which is going to reel in the satellite unless the satellite orbits to match the asteroid's rotation. The only way to de-spin the asteroid is to apply a torque, but cords only work in tension, so you'll have to apply little tugs with the cord, ideally while the thrust is aimed along the desired flight vector to avoid wasting fuel. But you also need the cord to pass to the side of the asteroid's center of mass so that each pull will reduce the rotation rate, and given the relative mass of the asteroid and satellite, and that the asteroid's rotation will probably include tumbling, controlling all that will be complicated.

And when you periodically tug on a cord you're introducing dynamics where the cord's elasticity pulls the satellite back toward the asteroid after each tug, slackening the cable and tending to crash the satellite into the asteroid while getting the loose cable tangled in the satellite's antennas and solar arrays. Untangling after such an event would probably take days of mission planning to figure out how to roll, pitch, and yaw the satellite so the cable doesn't do any damage, and that would have to be done with the cable still flopping around in real time. They'd want to avoid that situation at all costs.

So cutting thrust will have to be a carefully controlled operation, more of a return to station-keeping based on video of the cable position and range to the asteroid.

In contrast, a big bucket with a lid and a thruster in back is easy peasy, and you won't lose a single grain of material, as long as the asteroid is small enough to fit in the bucket. If after capture you sprayed in some aerogel or other foam, then the asteroid/satellite assembly becomes a single solid body and dynamic control is simple.

 

[Crazy Eddie]

Nope. Just figure out the asteroid's center of mass and have the tug attach a cable so that it is providing thrust through that point. It then becomes possible to disconnect the cable, reposition to a different vector and then reattach the cable, again thrusting through the center of mass.

Think through that slowly and you can see that mission planners will have a handful.

I have, in fact I'm in the process of writing a book about that very subject. Of course, I had envisioned this as a manned mission -- which would simplify matters hugely -- but doing it with an automated mission is just a matter of better planning and specialized techniques and equipment; suffice to say, even with the manned mission, just attaching the cable to the asteroid probably takes up an entire chapter at least.

You can't drill into the asteroid without somehow being attached to it firmly enough to apply significant force to the drill bit, which implies that you've already driven some anchors to hold your satellite against the asteroid, which required drilling...

You need pitons with anchor cables to get the initial bracing force, the placement of which will have to be chosen carefully so the drill can be placed near the center of mass. Once the drill is in, it supplements the anchor cables/pitons by distributing the mass more easily. If you can figure out exactly how much force the anchors can take -- based on how deep they're buried and the structure of the asteroid near the surface -- then you have a figure for how much force you can put on when you decide to move the thing. For a carbonaceous chondrite, for example, it would be anywhere between 500 and 3000 Newtons before the anchors are in danger of breaking free. Adding more cables would allow a little bit more force, but would make it slightly harder to reposition the tug for midcourse maneuvers (although lengthening/shortening some of the cables could also accomplish that).

So you'll probably have to wrap the cable around the asteroid, but then you'll need more than one cable because you can balance a basketball in a loop of cord; it will always fall out one side or the other.

Well, the industry proposals all involve wrapping the asteroid in a large kevlar or vectran net like a giant ziplock bag, for precisely this reason. That to me seems like a good concept if you know exactly what you're going out to capture and fully understand its shape and structure, but it could be problematic if the asteroid has a funny shape or is larger than you expected. It seems to me that the "bag if and haul it" approach works best if you also bring three tons of TNT with the intention of blasting it to pieces before you start hauling it.

Using a net or bag instead of a single cable might work, but you'll need something to hold the net open so you can get the asteroid inside

Elastic ring around/within the aperture is inflated with air, expands to a larger ring.

But then you've got the problem that the asteroid is probably spinning very slowly, which is going to reel in the satellite unless the satellite orbits to match the asteroid's rotation

Which is another reason why I favor the cable/anchor method. If the asteroid is slowly tumbling, you can attach a cable on one side and use some controlled thrusts to stop its rotation relative to its intended orbit.

And when you periodically tug on a cord you're introducing dynamics where the cord's elasticity pulls the satellite back toward the asteroid after each tug, slackening the cable and tending to crash the satellite into the asteroid while getting the loose cable tangled in the satellite's antennas and solar arrays.

That's a matter of timing and throttling the thrusters in the right way in order to avoid sudden snaps of the cable from slack to taught. Gentle upthrust starts each maneuver, followed by a larger thrust once the cables are fully taught. Strictly speaking, even if we were talking about strapping a Merlin engine and 30 tons of propellant to that tug, a 500 ton asteroid isn't going to pull much faster than about 1/8th of a G and the tension on the cables won't be nearly as much as you'd expect, slackness or not. It's really the interface with the asteroid you have to worry about.

In contrast, a big bucket with a lid and a thruster in back is easy peasy

And also very heavy and very inefficient, especially if you have to have something solid on the open end of the bucket to keep the asteroid from tumbling out of it when you cut thrust. That, plus the fact that keeping the asteroid from rattling around inside the "bucket" would require a bit of engineering workup almost as difficult as attaching the tethers.

Significantly, it doesn't have to be a solid "bukcet" to encompass the asteroid. A collapsible cage that expands to the size of the asteroid and then contracts again to the smallest size it can would solve that problem and give the tug something nice and solid to attach its cables too. In that case, figuring out how to keep the cage intact becomes a different engineering problem than the otherwise GEOLOGICAL problem of finding a piece of the asteroid solid enough to stick an anchor. The nice thing about the anchor method, however, is that you still have options to redeploy to a different spot if one of the anchors breaks loose.

 

[Davros]

I was making a joke because cables and space don't get along very well. ^_^

example please?

He gave examples.

None that I can see.

 

[gturner]

Significantly, it doesn't have to be a solid "bukcet" to encompass the asteroid. A collapsible cage that expands to the size of the asteroid and then contracts again to the smallest size it can would solve that problem and give the tug something nice and solid to attach its cables too. In that case, figuring out how to keep the cage intact becomes a different engineering problem than the otherwise GEOLOGICAL problem of finding a piece of the asteroid solid enough to stick an anchor. The nice thing about the anchor method, however, is that you still have options to redeploy to a different spot if one of the anchors breaks loose.

USA just posted a story on NASA's lasso mission ( link) which has a video that's much more informative than the text.

It looks like they're using an expandable bucket for the capture phase (perhaps inflating like an air mattress, but I'm not exactly sure), and then they collapse it tightly around the asteroid. That part looked kind of like sucking air out of bag (which of course doesn't work in a vacuum). It also looked like they were about to start pulling it as you suggest, but they pushed it instead.

Pushing with the remains of an inflatable bag in between the asteroid and the satellite seems to me like perhaps the worst of both ideas - unless there's some sort of fairly rigid structure in between.

Finally the asteroid is placed in lunar orbit, astronauts fly up on SLS/Orion, dock, spacewalk to the asteroid, and then based on the theme music discover some sort of alien artifact.

Anyway, I recently red that some small chondrites are something between 30% and 50% void space internally, just a loose assemblage of boulders that happen to cling together. That might make any attempt at anchoring both easier (just stick a long pole in a hole or gap) and harder (It might just pull a couple rocks loose). One the other hand it might make processing easier because the material would already be in more manageable chunks.

 

[Crazy Eddie]

USA just posted a story on NASA's lasso mission ( link) which has a video that's much more informative than the text.

It looks like they're using an expandable bucket for the capture phase (perhaps inflating like an air mattress, but I'm not exactly sure), and then they collapse it tightly around the asteroid. That part looked kind of like sucking air out of bag (which of course doesn't work in a vacuum). It also looked like they were about to start pulling it as you suggest, but they pushed it instead.

That's concept art recycled from Planetary Resources LLC, from whom NASA seems to have borrowed the idea. Planetary Resources has spent the last year studying different methods of capturing small asteroids in mission profiles more or less identical to what NASA describes, but they don't have anything like a working model for how that would really work. The concept art is just that.

Anyway, I recently red that some small chondrites are something between 30% and 50% void space internally, just a loose assemblage of boulders that happen to cling together. That might make any attempt at anchoring both easier (just stick a long pole in a hole or gap) and harder (It might just pull a couple rocks loose). One the other hand it might make processing easier because the material would already be in more manageable chunks.

More to the point, if you have the ability to break the entire asteroid into smaller chunks, you could simply wrap a cargo net around the whole thing and arrange them into a long chain and haul that along its own center of gravity.

 

[not]

The video link clarifies they would just take samples from the captured asteroid; I thought the intent was to bring it back to earth. I'm a bit disappointed!

 

[gturner]

Yeah, a manned sample and return mission to lunar orbit? Why not just have the probe return with a coffee-can full of material and just splash-down with it? They multiplying the costs by a very large amount and pushing the schedule back to 2025.

I would think the only reason for sending a manned mission to the recovered asteroid is to do anything required to process it into useful materials, since any sampling could be done with a second probe tele-operated from Earth.