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Now that the ESA has landed a probe on a comet, namely Rosetta's Philae Probe, could we possibly land a probe on Halley's comet? Fuel seems to be a limiting factor for interstellar expeditions - could using Halley's comet as a free taxi, shutting down all equipment once landed to save fuel and energy, aid in such expeditions?

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    $\begingroup$ Also, you might be interested in SpaceEx.SE $\endgroup$ – Patrick M Aug 12 '15 at 4:11
  • $\begingroup$ And also, Buzz Aldrin's Mars cycler idea, en.m.wikipedia.org/wiki/Mars_cycler $\endgroup$ – user81619 Aug 12 '15 at 7:07
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    $\begingroup$ why is landing relevant to get free energy? $\endgroup$ – Skaperen Aug 12 '15 at 9:27
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    $\begingroup$ A free taxi to where? It keeps coming back. $\endgroup$ – Jodrell Aug 13 '15 at 13:42
  • $\begingroup$ I suppose you could catch a lift past the orbit of Neptune but then what. $\endgroup$ – Jodrell Aug 13 '15 at 13:48
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There seems to be a fundamental misunderstanding as to how movement in space works. In space there is no air friction, that is, once you are moving toward your destination, you don't need a continuous source of power to keep going. Landing on a comet doesn't buy you anything, since in order to land you must first match the comet's orbit, at which point the comet could disappear and you could still shut down your engines and follow the exact same path it would have taken you.

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    $\begingroup$ I already knew that but when you put it that way it does make this question seem quite silly $\endgroup$ – SemperAmbroscus Aug 12 '15 at 2:45
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    $\begingroup$ Hey, they used a harpoon on 67P/Churyumov–Gerasimenko! It didn't work, but the idea has Kerbal-merit. If you put a low-mass harpooning probe in Halley's orbital trajectory, you could achieve a significant transfer of momentum. It's not like we don't have time to plan for a mission or don't know exactly when and where to be. In short, you don't catch up to the comet, you let it catch up to you. $\endgroup$ – Patrick M Aug 12 '15 at 4:00
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    $\begingroup$ @PatrickM: The kind of velocity you would want to get from a comet would be in the km/s range. The harpoon would need to be made out of some very sturdy stuff. Imagine trying to harpoon a fighter jet as it flew past you. $\endgroup$ – Dietrich Epp Aug 12 '15 at 7:26
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    $\begingroup$ The path would be the exact same, if you ignore the tiny gravitational pull from the comet. If you want to take advantage of the change in path caused by the gravitational pull, you are better off using something heavier than a comet. (Jupiter is a popular choice due to being a lot heavier than a comet.) $\endgroup$ – kasperd Aug 12 '15 at 8:04
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    $\begingroup$ You don't have to match the comet's orbit to land on it. It's just the preferred method if you prefer to maintain your vehicles general 3D shape. $\endgroup$ – ericksonla Aug 12 '15 at 20:06
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As @ChrisWhite has already stated, catching up with Halley's comet offers no appreciable benefit since you'd already need to be traveling at the same speed in a frictionless environment.

Where you could get a big boost is if your probe was positioned in the comet's path, either coming across it at an oblique angle or simply stationary, relative to the comet.

Assuming unbreakable meta-materials are available to you, you could then use a harpoon to grab onto it, or simply allow it to hit your probe, transferring a very sizeable amount of kinetic energy to your device.

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    $\begingroup$ I love the "allow it to hit your probe" approach. We're going to have to build tougher probes, or, maybe, rubber ones. :-) $\endgroup$ – userLTK Aug 12 '15 at 10:47
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    $\begingroup$ Just select "No crash damage" in the cheat toolbar. Oh, wait...that only works in Kerbal, not real life. $\endgroup$ – gleedadswell Aug 13 '15 at 1:42
  • $\begingroup$ I cannot find it ATM, but I think in the 80's there where some studies about how this - momentum exchange with a comet - could be done. $\endgroup$ – Daniel Jour Aug 13 '15 at 9:06
  • $\begingroup$ Presumably the transfer of energy would alter the comet's orbit to a greater or lesser degree? $\endgroup$ – Robin Elvin Aug 13 '15 at 9:42
  • $\begingroup$ @RobinElvin assuming the comet has more mass then your probe, the change in orbital velocity to the comet would be less kinda like bouncing a 1/2 oz ping pong ball off a moving 15 pound bowling ball. $\endgroup$ – Keith Reynolds Aug 14 '15 at 8:31
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The sense in which a comet could be a "free taxi" is that it is a big source of volatile compounds. A probe that lands on a comet could, in principle, use these to refuel. A comet may be easier to use for this purpose than a large icy moon because of its low gravity.

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  • $\begingroup$ Either you could bring your own oxidizer which is heavy relative to the fuel and thus offers very little benefit over bring both oxidizer and fuel, or you can use photo voltaic energy for electrolysis while the comet is relatively close to the sun. The asteroid/dwarf plant Ceres might also be of interest to mine for hydrogen and oxygen while still being close enough to use solar cells for electricity. $\endgroup$ – Keith Reynolds Aug 14 '15 at 9:48
  • $\begingroup$ Yes, when I said "refuel" I meant "get new fuel and oxidiser". I was picturing electrolysing the water in the comet. There would probably be other volatiles from which you could extract both some fuel and some oxidizer, though not necessarily hydrogen and oxygen. $\endgroup$ – gleedadswell Aug 18 '15 at 3:25
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Conservation of energy means that it would take you just as much energy to catch up with the comet and make a soft landing (in the ideal case where you ignore the extra fuel costs for landing) as you would need to launch the spacecraft in a similar orbit as the comet. What you could do is let the comet give a bounce to the spacecraft, but I don't think this is technically feasible, you'll end up smashing the spacecraft to pieces.

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    $\begingroup$ conets don't stop to pick up riders $\endgroup$ – Skaperen Aug 12 '15 at 9:25
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As earlier answers have stated this method doesn't work to save fuel directly, however it might still be useful to hitch a ride on a comet using it for a gravitational assist that the probe would not survive without more shielding than is practical. Placing the body of the comet between the Sun and the probe would presumably shield it from much of the solar radiation and heating that would be experienced.

Therefore whilst not directly saving fuel this might allow the use of more direct flight paths so, were the comet not used, the probe would require the use of additional fuel to reach the same end point safely in the same duration.

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  • $\begingroup$ The maximum gravitational assist you can get from a body is escape velocity / sqrt(2). The escape velocity of a comet is so low it really isn't worth it. Better try Jupiter. space.stackexchange.com/q/6582/6117 $\endgroup$ – Level River St Aug 12 '15 at 14:18
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    $\begingroup$ Sorry it appears I wasn't clear. I'm not proposing using the comet itself for a slingshot but using the comet to protect the probe whilst it passed round something hazardous that could damage it. Both comet and probe being slingshot, and then the probe hopping off afterwards. $\endgroup$ – Carcophan Aug 12 '15 at 15:10
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    $\begingroup$ Oh i see. Unfortunately a probe launched from Earth cannot realistically get much slingshot from the sun, since it's already orbiting the sun, and its priority is to escape that orbit. The best way is to set a course for Jupiter, such that the craft runs out of kinetic energy (velocity relative to the sun) just as it reaches Jupiter's orbit, and time it so it's just as Jupiter is approaching. Then it swings around Jupiter ending up at nearly twice the velocity that Jupiter orbits at. It works because Jupiter orbits further out than earth so Jupiter is able to impart orbital energy to the probe $\endgroup$ – Level River St Aug 12 '15 at 15:25
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Fuel, or better energy indeed is the limiting factor. You can steal energy using a gravity assist, but comets simply don't have enough gravity. Jupiter is much more massive, and a gravity assist around Jupiter would in fact make sense for an interstellar probe.

Still, it's only a single boost as you fly by Jupiter, and you have to spend the fuel to reach Jupiter's orbit.

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  • $\begingroup$ Yes, but that single gravity assist from Jupiter can get you pretty much anywhere, including out of the solar system. You can get to Jupiter from Earth with relatively little delta-v by using one or more gravity assists from Venus and Earth (that's how the Galileo probe got there). Mars is somewhat less useful for gravity assists because of its lower mass. $\endgroup$ – gleedadswell Aug 13 '15 at 23:09

protected by Qmechanic Aug 12 '15 at 8:04

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