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For many years now, miners have been harvesting rare metals and other exotic materials from the asteroid belt and using Jupiter to sling-shot 10 tonne canisters of materials at the Moon, where they are collected before being refined and transferred to Earth.

At the Asteroid belt, the miners use a dedicated ship with a rail gun to insert the canisters into Jupiter's sling-shot gravity tunnel (not an actual tunnel, it's just a trajectory).

Out of seemingly nowhere (as they do), an alien mothership has appeared and is orbiting Earth, showering our homeworld in crippling EMP bursts.

Our only hope lies in our intrepid asteroid miners who hatch a plan to shoot one of the larger asteroids at the mothership.

Given that the miners are generally only used to handling smaller projectiles, would they reasonably be able to manoeuvre an asteroid into Jupiter's sling-shot tunnel and knock the alien scum out of our skies and save the day? And how long would the journey take (roughly)?

Assume:
* Hollywood levels of believability
* The aliens are too stupid to notice a Manhattan sized lump of rock heading right towards them

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  • $\begingroup$ Based on @JohnDallman's answer the mothership has to be in the same orbit, roughly eighteen months later. So assume the alien mothership needs two years to recharge its drive-systems before moving anywhere. Now it's a sitting alien duck. Asteroid whooshes in, and KER-Bang!! (Hollywood sound effects) No more mothership. Roll end credits. $\endgroup$ – a4android Nov 11 '16 at 12:32
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    $\begingroup$ Is it bad form here to note that this sounds a teeny bit contrived? "Firing small asteroids with multi-year trajectories... why, it's like a cannon...if only there was something really slow and dumb to shoot at!"... "Sir! Incoming distress call from Earth!" :-D But I'm sure there's more to this, right? $\endgroup$ – SusanW Nov 11 '16 at 13:13
  • $\begingroup$ @SusanW - Yes, of course it's contrived - this is Hollywood level Sci-Fi we're talking about here.... $\endgroup$ – user10945 Nov 11 '16 at 13:22
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    $\begingroup$ Miners, not minors. And yes, that's more fun to say than in text. $\endgroup$ – Feyre Nov 11 '16 at 14:05
  • $\begingroup$ @Feyre - Darned fat fingers, thank you. $\endgroup$ – user10945 Nov 11 '16 at 14:10
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It's certainly doable with Hollywood level science. Realistically, you can't. Here are the two problems and their potential solutions -

  1. Moving the asteroid - Your proposed asteroid is immense, it'll be practically impossible to just get it out of its own trajectory. Further, the rail gun is more or less useless because it's designed specifically for firing much much much smaller loads and that it'll require the asteroid to have very specific composition. You could somehow push/pull the asteroid using your own spaceships but that'll require the asteroid to be much smaller than Manhattan (the size mentioned in the question). But you can potentially hand wave it by making your own mining mothership humongous and/or with some future space travel tech .

  2. Aiming the asteroid - Whatever you use to move the asteroid must stay with it for most of its journey to be able to do course corrections. It doesn't have to sacrifice itself though, it can veer off from a safe distance.

If you want to reduce the hand waving, here are some ideas -

  1. Reduce the size of the asteroid and turn it into an explosive bomb instead. The miners might already be mining uranium or other materials that they can turn into a potent bomb macgyver style. It can be destructive enough to disable the alien ship without being unrealistically heavy to move.

  2. Make the load even small enough to not require your one mothership - With a smaller load you can make multiple bombs and can send one mining ship (assuming you have one big mining mothership and many mining ships that go out to mine) per bomb. Now you can even allow the alien ship to move a little and make aiming more challenging for the miners. More bombs, limited mining ships for navigation, moving target, more drama/suspense.

  3. If you must use the railgun - Just turn your regular mining loads into nuclear bombs and space equivalent of tracer rounds. Use tracers to get the aim right and bombard the alien ship with a few thousand nuclear bombs. Here more drama/suspense can be added by letting the alien ship trying to defend itself forcing the miners to get the targeting right. The fun part is how aliens won't understand just what is hitting them, to them it'd look like an especially destructive meteor shower coming from space!

Or a combination of all 3 at different stages of the battle!!

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  • $\begingroup$ Something something, "won't they spot it coming?" something something. $\endgroup$ – Draco18s no longer trusts SE Apr 21 '17 at 16:00
  • $\begingroup$ They can't see the mining ops hidden on the other side of the Jupiter. The core idea is that the mining ops is going on far away on the other side of the Jupiter that aliens aren't aware of and the miners stay hidden while using Jupiter's gravity as the slingshot. Whether the aliens are able to spot and evade something coming from as near as Jupiter is up to the writer, I personally would let them spot a big asteroid and then let the miners improvise by small size nukes machine gun style bombardment that's difficult to evade etc. $\endgroup$ – Achilles Apr 21 '17 at 16:13
  • $\begingroup$ "Sir, there appears to be a large rock flying towards the ship at high speed. Coming from a range of 329,115,316 miles and moving at a velocity of 10,000 miles/sec. If we don't move we'll be blown to smithereens in...calculating...a little over nine hours!" (Tip: Space is Big. Really big. Asker said that the alien ship is in Earth orbit and the rocks are out by Jupiter. That's a really long way away) $\endgroup$ – Draco18s no longer trusts SE Apr 21 '17 at 16:15
  • $\begingroup$ :) "The rocks are too small to be of any concern"....then 9 hours later "The rocks appear to be causing nuclear explosions upon impact! Force field is down to 7%! Too close to evade... Abandon ship" $\endgroup$ – Achilles Apr 21 '17 at 16:19
  • $\begingroup$ I'd let that slide in a holywood movie, maybe, but no where else. Its always important that the aliens not be aware that nuclear weapons are a Thing. Sorry, they've mastered interstellar travel and arrived "out of nowhere." If they can move around space like that they could dodge every rock we hurl at them, nuke-mined or not, and expend no effort doing so. Hell, they could let the nuke impact and dodge the radiation. $\endgroup$ – Draco18s no longer trusts SE Apr 21 '17 at 16:22
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Now, I may be prejudiced, but the fact that you're asking this question at all indicates to me that you're trying for higher than Hollywood levels of believability.

The first problem is the journey time. A trajectory out from the asteroid belt to Jupiter will probably take around six months, and from Jupiter back to Earth about a year. That seems to be too long for tactical purposes, and predicting the mothership's position precisely enough to score a hit isn't very plausible.

The other problem is what they use to accelerate the asteroid, which is presumably rather larger than the ten-ton packages of ore that they're shipping. Trying to use a railgun to accelerate a large asteroid is going to be like trying to fire a beach ball through a .22 rifle. It just won't work. You need to use some other way. Use everyone's ships all strapped onto the asteroid, and when it's on course, have them all unstrap and coast to a halt while the asteroid whooshes on ahead towards Jupiter. Because Hollywood.

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  • $\begingroup$ I think "coast to a halt" is where you lost the upvote. Even Hollywood tends to know better than that these days. $\endgroup$ – user Nov 11 '16 at 12:16
  • $\begingroup$ Well... Back in the late 70, NASA launched Voyager 2, which somehow managed to find it's way to perform fly-byes of Jupiter, Saturn, Uranus and Neptune without having to expend a huge amount of fuel doing so. Given advances in computational power, shooting at the Moon isn't completely implausible. It's also possible that these canisters might have a crude auto-pilot allowing for minor course corrections. $\endgroup$ – user10945 Nov 11 '16 at 13:26
  • $\begingroup$ @Pete Maybe it helps if you give an indication of the power of the rail gun and its angular accuracy. Honestly, a couple of weeks long journey wouldn't be that unrealistic, if Earth isn't on the opposite side of the solar system. $\endgroup$ – Feyre Nov 11 '16 at 14:11
  • $\begingroup$ @Feyre - Yep, but managed to do it without having to need huge fuel tanks. The implication being that the initial trajectory was amazingly accurate considering how far it had to go and how many sling-shots it had to perform. $\endgroup$ – user10945 Nov 11 '16 at 14:14
  • $\begingroup$ The Voyagers and Pioneers had onboard navigation sensors, and small engines for course corrections, which happened between slingshots. Your canisters can have that, but doing it for a much more massive asteroid is hard. I was assuming that the mothership had a size measured in single-figure miles, which would make hitting it without guidance at this range simply impractical. $\endgroup$ – John Dallman Nov 11 '16 at 14:24

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