Slashdot Mirror
NASA To 'Lasso' a Comet To Hitchhike Across the Solar System
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.
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.
I love Jesus, except for his foreign policy.
"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.)
poon tagging across the cosmos. radical dude.
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.
"A door is what a dog is perpetually on the wrong side of" - Ogden Nash
When a journalist asked which towel size would be used, NASA was not available for comment.
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.
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.
I RTFA and this is the graphic I was referring to. http://www.nasa.gov/sites/defa...
I love Jesus, except for his foreign policy.
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.
I love Jesus, except for his foreign policy.
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.
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?
Quattuor res in hoc mundo sanctae sunt: libri, liberi, libertas et liberalitas.
Seeing that the past tether attempts https://en.wikipedia.org/wiki/... haven't been so successful I think this type of mission is a bit premature.
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.
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.
"So long and thanks for all the fish."
Yeah, they wouldn't want the line to wrap around the comet and kill it....
Cheap storage VM.
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!"?
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
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.
Democracy is a sheep and two wolves deciding what to have for lunch. Freedom is a well armed sheep contesting the issue
*furiously makes addition to bucket list*
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 probe would need a battery with a magically high energy density and quick charging capability.
Maybe they should use a big spring, like a Slinky. For 5 gs, they would need a spring with a stretch of 2040 km and a spring constant K of 100 GN/m (a mighty stiff spring). And the bad news is that it slams the probe into the body at 10 km/s.
Didn't Wile E. Coyote do this in a cartoon? I don't recall that it ended well.
Left MS Windows for Linux Mint and never looked back!
Vote for Bernie in 2016!