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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.
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"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.)
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.
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.
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.
Even with 3.6GPa ultimate tensile strength (2.75x10^6 N.m/kg specific strength) of carbon nanotube ropes, 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) / 2x10^3kg = 1x10^8 N.m/kg
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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