NASA To 'Lasso' a Comet To Hitchhike Across the Solar System 99
evilviper writes: Traveling around space can require a lot of fuel, to help fight the fuel bill NASA has a spacecraft concept that would hitch a free ride on one of the many comets and asteroids traveling across our solar system. 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.
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.
They should know better (Score:2)
Unfortunately, it seems the NASA scientists made one fatal mistake... They didn't bother to read the opinions of Anonymous Cowards on the internet, who just last year throughly explained why the idea "makes no sense", and flies in the face of "basic orbital mechanics". [soylentnews.org]
Re:They should know better (Score:4, Informative)
The idea is that you can cross the orbit of a comet - a flyby - with a lot less fuel than it would take to match its orbit and keep station.
Think of last month's New Horizons mission. Imagine harpooning a long tether into Pluto at the closest point of the flyby, then using it to change NH's trajectory enough to put it into orbit with less fuel. This wouldn't work because of the huge delta-V involved, but at the lesser speed of a comet flyby, perhaps.
Re:They should know better (Score:5, Funny)
So that makes sense, except I'm not sure about the ~10 km/s delta-V. Isn't that almost literally the same thing as getting in your car, shooting a lasso out of a gun in the path of a gigantic mountain passing by ten times faster than a bullet, and shouting "Hey... watch this!"?
Re:They should know better (Score:5, Funny)
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Maybe that's why the article explains how it is actually proposed to work: the satellite harpoons the comet while letting the tether continue to give out line. Tension is slowly applied, letting the satellite accelerate to the comets velocity. The requires a "ridiculously" long tether.
So it does make sense... like catching a train with a fishing line and then reeling yourself in over the few thousand kilometers.
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I am going to suggest you try that with braided line and not mono-filament. You're going to actually want to tighten up your drag too, assuming you're spin casting. This is most certainly not a good idea for those who fly cast for the fish I am usually after.
For a stupid ass animal that is a fish, they're sure tough to please sometimes. I expect a train to be a bit easier to hook but you're going to have an 'interesting' time reeling it in.
Re:They should know better (Score:4, Insightful)
Given how Philae was unable to harpoon itself onto the surface of Churyumov-Gerasimenko, I have extremely little confidence in JPL's plans to use a harpoon to perform massive deltaV changes sucessfully.
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Well, uh... those harpoons didn't actually fire.
I mean, yeah, they failed to anchor the probe, but that was because they didn't even activate to begin with.
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The cometary surface was at least 10 times harder than expected.
If you want a harpoon to work you'll probably need an explosive tip and umbrella barbs to hook your white whale^W^Wcomet and I suspect that you're going to need unfeasibly long line with amazingly high breaking strain simply to handle the delta-V
It's a nice idea but we probably don't (yet) have the technology to pull it off - and when we do, the space elevator problem will be solved, which makes launch masses far less of an issue.
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I am going to suggest you try that with braided line and not mono-filament. You're going to actually want to tighten up your drag too, assuming you're spin casting.
I would think the tether would be rather specific. To generate power from it, Piezoelectricity is the only thing that comes to mind.
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No, just a reel of cable connected to a generator/motor. Shoot harpoon, reel drives generator making electricity through regenerative braking. Once the probe comes to a halt, it reverses the generator (i.e. operates it as a motor) and winches itself down to the surface.
The scales and power levels involved are pretty sketchy though - slowing a 1000 kg probe at 5 g's from 10 km/s to zero will deliver an average power of 50 GJ / 200 s ~ 245 MW. That's a lot of power to try to store or dissipate. That 1000 kg p
Re:They should know better (Score:5, Informative)
It really depends on what the difference in velocity is. The AC's are correct that if you are in the same orbit then you don't get anything. But I think what is being suggested is you would be in different but intersecting orbits. Think of Spiderman shooting a web onto a moving bus or train.
Something that is questionable is you would still need a way to kill your angular momentum otherwise as you wind yourself down to land your angular velocity will increase.
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I RTFA and this is the graphic I was referring to. http://www.nasa.gov/sites/defa... [nasa.gov]
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Something that is questionable is you would still need a way to kill your angular momentum otherwise as you wind yourself down to land your angular velocity will increase.
Two obvious methods:
1. When you fly by and shoot the harpoon, do so as close as possible to the target.
2. Shoot more than one harpoon. This not only gives you redundancy, but you can control your rotation by fiddling with the tension on the different cables.
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You and the other poster are correct. You would need 3 harpoons far enough apart to be able to kill the rotation. But you would have to do it very quick because if you are 100 km away and your harpoons are only 1 km apart that triangle is pretty steep.
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They can call it the teatherball landing.
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Something that is questionable is you would still need a way to kill your angular momentum otherwise as you wind yourself down to land your angular velocity will increase.
If the spacecraft trajectory intersects that of the comet, then the angular momentum is zero.
This will not be the case exactly, because then the spacecraft would crash into the comet, but the distance doesn't have to be very large either. The resulting angular momentum would be relatively easy to eliminate using a conventional thruster when the tether is at maximum length. The fuel usage would be a very small fraction of what would have been required to rendez-vous without the tether.
If not in a hurry, then
unfortunately pretty unrealistic... (Score:5, Informative)
"carbon nanotube tether anywhere between 62 and 620 miles long attached to a diamond-tipped harpoon"
Considering that the few attempts at space "tethers" have (I think) ended in failure and their cable lengths were much shorter, I think it is highly unlikely that they'll be able to make this work. They've also got to be able to "shoot" the harpoon at at spot on an asteroid that is neither too hard so that it'll bounce off of shatter the harpoon or so soft that the harpoon cannot have purchase. Of course, this is only worthwhile if the spacecraft and asteroid are traveling at a high velocity relative to each other. That way the spacecraft will either get a big savings in energy because it can use the tether to slow it down relative to the asteroid (and potentially generate a ton of energy through resistive braking!) or it could use it to "swing" around and dramatically change its direction of travel (like a gravity assist but with much more latitude). So the harpoon would be hitting the asteroid at kilometers per second and would need to "stick". (A mechanism to cut the cable or release the harpoon might be necessary if this system is to be used more than once).
As long as we are using carbon nanotubes and diamonds perhaps we should use a large lasso instead with micro thrusters positioning it for optimal placement. In any case IF this system could work then, yes, the spacecraft could go swinging through the solar systems using asteroids (small bodies with no atmosphere and little gravity) like Tarzan uses vines hanging from trees. I fear that the engineering difficulties are so great and the risk (you probably only get one "shot" during a flyby) will make this impractical.
Side note: - This idea is related to my, ahem, own idea of using a spacecraft that lands on an rotating asteroid, and then, using a tether, slinging off pieces of the asteroid into space. This could allow a very modestly sized spacecraft to divert the trajectory of an asteroid because it would be harnessing the energy of the asteroids rotation and converting it into kinetic energy. By landing (gently, no harpoon necessary) onto the asteroid's equator and extending a tether beyond the asteroids "geosync" orbit it could keep it permanently taut using a counterweight. Then, just like a space elevator, it would ferry material up to and beyond the geosync point, generating energy (to power the elevator) beyond that point using resistive braking before it flings the material into deep space. Properly timed releases could impart a directional thrust to the entire system. (If the asteroid is rotating fast enough the system is small enough that carbon nanotubes wouldn't be needed.)
Re:unfortunately pretty unrealistic... (Score:4, Insightful)
Considering that the few attempts at space "tethers" have (I think) ended in failure and their cable lengths were much shorter, I think it is highly unlikely that they'll be able to make this work.
It has already worked. It got NASA another day of good PR and reminded Congress and the public that they're doing something with the money they're given. Mission accomplished!
Calculations (Score:5, Interesting)
So you harpoon a comet with a velocity relative to you of 10 km/s. You have 1000km of tether. That means that if you didn't apply any braking, you'd have just 100s before your tether ran out. With little bit of physics, I find that the probe would need to accelerate at 50m/s^2 (over 5g) to match velocity with the comet before running out of tether (which takes 200s because now it is accelerating). So the tether and the harpoon need to withstand a tension of 5 times the earth-weight of the probe, without breaking or pulling the harpoon out of the comet. Now assume the probe is 1000kg. (For simplicity I'll ignore the mass of the tether and the rotation of the tether reel, although that would probably be a deal breaker too.) Force = 1000kg x 50m/s^2 = 50000N. Distance acted over = 1000km = 10^6 m, so work = 5x10^4 x 10^6 = 5 x 10^10 J. It is 200 s before the tether runs out, so power = 5x10^10/200 = 2.5x10^8W = 250 MW. That power has to be stored and/or dissipated, and you have at best 1000kg with which to do it.
It all gets very much easier if your relative velocity with the comet is much lower, but then you're not gaining much, and intercepting a comet with only few km/s relative velocity is very hard in itself.
It is a pretty idea, but I can't see it working with anything vaguely like current technology.
Does anyone care to poke holes in my reasoning?
Re:Calculations (Score:5, Informative)
No, you're right.
Further assuming that 10% of the spacecraft weight is the tether, we have a 0.1kg/km tether capable of holding 5000kg. Nothing like that exists.
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Even with 3.6GPa ultimate tensile strength (2.75x10^6 N.m/kg specific strength) of carbon nanotube ropes [aip.org], it won't work.
Assuming a 2 tonne craft (as specified in the article), assuming 100% loading:
Required specific strength, assuming a rope with a mass of 2 tonnes, giving a total mass of 4 tonnes: (10^6m x 2x10^5N) / 2x10
Re:Calculations (Score:4, Informative)
They're talking about the theoretical strength of SWNTs, which is upwards of 120GPa. But the highest ever measured SWNT strength, last I read, was around 60Ga - and that's the properties of individual tubes (ropes don't even approach it).
Whenever you're reading something and it mentions needing a "carbon nanotube tether", toss whatever you're reading in the "sci-fi" category. Not even the hard sci-fi category. And all for what - a ~6 year Pluto transit time? Lame.
Don't they have anything better to research?
Heck, even I can think of a more plausible approach than that - one that doesn't require unobtanium at least. Forget the "diamond anchor", land a microsat on it (approaching comet, not a retreating one). Yeah, that takes a lot of delta-V, but if it's just a microsatellite, then that's not a lot of mass. Then, forget about the "carbon nanotube tether"; use a space fountain between the large craft and the lander. Space fountains (such as paired coilguns, for example) are plausible with today's technology, requiring no unobtainium.
But the whole concept of delta-V from a comet is just not a worthwhile avenue to pursue either way. Way too much difficulty and mechanisms for failure for way too little reward.
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Even if you managed the unobtanium cable, there's another fun problem. By the calculations above that's 250 MW for 200 seconds, which is about 10^7 kcal if all converted to thermal energy in braking. So even given some 95% efficient energy storage flywheel, you just dumped 5*10^5 kcal into a 1000 kg probe in three minutes (or 2.5 GJ if you prefer). That will end quickly, and not well (though maybe a 10 ton probe without cryogenic fuel could manage it - better upgrade that cable to uberite).
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I just wonder if this will slow the comet down or alter its course. I doubt it has enough mass to do so, but...
Also, I did a quick run in my head and your math looks okay. Physics was not my major, however.
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The holes are in the assumptions of 10km/s delta-V, 1000kg craft and 1000km of tether. Make it a 100kg craft using 100km of tether and a much smaller delta-V leads to a much different result. A 1km/s delta-V is nothing to sneeze at. Additionally, half of that energy is transferred into the velocity and angular momentum of the captured object.
It's certainly a better idea than mine: wrapping the probe in a loose wad of double sided sticky tape and aiming directly at the comet.
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1000km came from the article, which discussed a tether of 62 to 620 miles length.
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I think you are right. This is why they use the magic of CARBON NANOTUBES!
Until we can produce those more than a few centimeters in length are are not quite so flamable... this tether will be just as feasible as carbon nanotube space elevators.
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No oxygen in space to make it flammable.
Also less distance so slightly more feasible then a carbon nanotube space elevators.
snowcrash! (Score:2)
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I was thinking Ender's Game, Bean's super strong cable he used to orbit a "asteroid". As I recall that didn't work out so well for him, he got pretty badly smacked and he wasn't even moving that fast.
This would however get YT's seal of approval.
Gravitational slingshot (Score:2)
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Carbon nanotube tether eh? (Score:3)
I was reading the blurb, half-thinking that this is cool that this is happening" - you know, after the resounding success of the Rosetta mission, anything to do with a comet seems possible. And then I read "carbon nanotube tether" and I stopped reading, as this is yet another NASA dream that'll never happen, like the space elevator.
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" after the resounding success of the Rosetta mission"
The lander completely failed to attach. The harpoon and ice screws both failed completely.
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I assumed that was the joke.
Seriously long tether needed (Score:1)
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You could use a nice flat "strap" instead of a round "rope". Far less difficulty transferring rotational forces.
Re:Seriously long tether needed (Score:5, Informative)
It'd need either a pretty long tether - at a minimum 500km assuming a 10G acceleration and 10km/s initial relative velocity, or 5000km for a more sedate 1G acceleration.
Unmanned systems can be designed to tolerate far more than 10G. I once worked on a project to build and program a device that would be inserted into the nose of an artillery shell. We designed it to withstand 15,000 G. Many mechanical wristwatches can withstand over 1000G.
Here is a list of Orders of Magnitudes of Acceleration [wikipedia.org].
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True but I'd worry more about the high force causing the harpoon to pull out.
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You're talking about something that is compact and has a mass of between a few grams and a few tens of kilograms.
The article is about a 2 tonne spacecraft attached to a tether, which is anchored by a harpoon, and the article talks about braking forces of less than 5G.
A 500km long carbon nanotube rope (3600MPa Ultimate Tensile Strength [wikipedia.org]) strained to 75% breaking strength (2700MPa) and handling a force of 200 kilonewtons (2 tonnes times 10G) would need to have a cross-sectional area of about 70mm^2, and would
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I can't even begin to imagine how one is going to have a hypervelocity anchor attach to a comet rather than just blowing it up, and how any anchor attached to a comet would withstand 5 Gs. Comets aren't exactly the most structurally sound of objects...
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Land the tether on the comet first (possibly a spinning comet so that at one point the tether is slower then the comet.).
Then any space ship wanting to hitch a ride only needs to grab the tether and ride it out.
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You need to go and visit your local ice-climbing goods store. The tool you're looking for is called a "deadman [google.co.uk]" (or the mini-version, the "deadboy"). Nobody makes them commercially bigger than about 25cm on edge, because that can, even in loose powder snow, provide the couple of tonnes force needed for fall arrest.
A derived system was actually deployed on Philae as one of the redundant attachment systems. That
Comet to hitchhike across the Solar system (Score:5, Funny)
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Why? Delta-V is not acceleration. If you want to change your velocity by 10km/s, you can do it by accelerating at 10m/s/s for 1000 s, or by 10km/s/s for 1 s, or by 10000km/s/s for 1ms.
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V = Velocity
Delta = Change
Delta-V = Change in Velocity
Aka, from velocity X to velocity Y, the delta-V would be Y - X
Obviously the units of "Y - X" are going to be the same as the units for Y and X.
Thus delta-V has the same units as velocity.
Aka km/s or similar.
There should be a prize (Score:2)
This is possibly the best Slashdot headline this year...
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I do my best. This one really isn't my favorite, but the other admittedly wasn't on slashdot:
http://pipedot.org/2V6P [pipedot.org]
ICBMs are how fast? (Score:2)
We've got a pretty bad record so far at attempting to intercept objects moving at delta Vs approaching orbital velocities (which 10km/s is definitely that). That's using a guided projectile to attempt the intercept, and counting intercept as close enough to damage via explosive charge. Based on that I can't see us firing a tethered harpoon containing no guidance or propulsion engines, and having any expectation at all of hitting something moving that fast. And that's not even considering the question of
Past mission history (Score:3)
Not often you see.... (Score:2)
How to deal with massive electric current (Score:2)
Project Ship Name Idea (Score:2)
A different way to use a comet (Score:2)
I think that NASA's idea is interesting...very challeging, as other have noted, but worth it if it could be done.
I have been toying with the idea for an SF story using comets. Spaceships would wait for a comet to come by, then embed themselves into the tail of the comet, and use some kind of ramjet propulsion to accelerate out of the inner solar system. Obviously comet tails are not dense at all (a less dense vacuum than what can be made on earth) but the ion tail should be manipulatable.
Anyway, in the st
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Well, more like the attitude. I dig it.
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Almost all known asteroids and comets are vastly bigger than what you are envisioning. For example, here's the comet Rosetta [iflscience.com] is exploring.
do they know basic physics? (Score:2)
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Wouldn't it be nice if NASA would stop coming up with these hair brain ideals to get around the solar system and just blew the dust off the old nuclear rocket programs from the '60s. With some new research and with the new technology we have today these rockets would be more practical than "lasso'ing a fucking comet." A nuclear rocket could be a probe to pluto in a few months instead of a few years. Hell it could get anywhere in the solar system in a few months.
Someone should write a guide... (Score:2)
Grab your towels everyone!
Did NASA just re-invent this? (Score:2)
Because I have a NASA document somewhere which I believe is from the 70s or 80s, which talks about hitching rides in this way. Except, in that case, they were going to use a net rather than a tether.
Use a spring (Score:1)
Seems to me that you don't want a lasso but rather a very long tightly coiled spring.