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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.
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".
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I wonder how they can work out the delta V budget for the entire trip though.
"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.
So basically, it is a gravitational slingshot, but more complex due to the use of a line. I am really curious to what the amount of energy does to the asteroids and comets' own trajectory, and if it literally fires back at us. If you take speed out of an asteroid, a somewhat circular orbit around the sun could become an elliptical one the interferes with Earth's orbit.
Nae king! Nae laird! Nae yurrupiean pressedent! We willna be fooled again!
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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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.
When a journalist asked which towel size would be used, NASA was not available for comment.
Careful where you sling that thing. You'll put (THE ENTIRE PLANET'S) eye out.
this Hitchhiker could be the guide to the galaxy
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.
This is possibly the best Slashdot headline this year...
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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 actually attaching to the object.
Uhm no. Look at the definition of delta-V again and come back to us.
Delta-V is not measured in km/s.
Yes, it is.
It should be km/s/s or something similar.
That's acceleration, not velocity.
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?
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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 same headline on ESPN and Slashdot.
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that the fuel is the cheapest part of space flight? How are we going to colonize the universe if even our tiny probes need to save fuel?
See you Space Cowboy
How are they going to deal with the massive electric current induced in the tether?
Pequod; with apologies to Herman Melville.
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 story, people in their spaceships end up flying out in more-or-less random directions, and hoping to find something interesting in the process.
I love Mondays. On a Monday, anything is possible.
So by tethering ourselves to this comet and "harvesting" some energy we have effectively changed (slightly) it's orbit.... maybe a good thing, maybe a very bad thing.... I smell a Saturday afternoon movie in the making.
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.
You don't exist. Go away.
Anyone else concerned that the previously known orbit of the comet will be modified by this interaction and that there are possible consequences the next time it is expected to make a close pass?
Here's why this is the stupidest plan ever. To land on a giant space rock going 100 miles per second or whatever, you have to come up along side it at 100 miles per second then land on it. That means the spacecraft had the ability to reach 100 miles per second anyway and didn't need the giant rock. There is also no wind resistance or drag in space. So if you can get the spacecraft to that speed anyway, why do you need the rock?
Grab your towels everyone!
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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.
Seems to me that you don't want a lasso but rather a very long tightly coiled spring.
Boy, that was embarrassing for you!
I just hope they remember their towel. I mean, it is the most important thing to have when hitchhiking in the galaxy.