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How would a spacecraft traveling near light speed avoid a (relatively dense group of) asteroids? Or suppose such spacecraft is designed, how would the physics work for steering it inside such a "cloud" of asteroids?

If it simply by designing the spacecraft to have huge acceleration, wouldn't that mean lots of constraints on the material the spacecraft can be made of?

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Hi user. Welcome to Physics.SE. This question seems highly imaginary. This part: "Suppose a spacecraft is designed" - looks rather science-fictiony to me... I strongly argue that, "If that would happen, Then all other constraints would also have to be solved..." Please keep in mind that we're humans. Think of it this way - When we try to launch space vehicles, we'll think of every necessary step to do that. If we were to launch a "near $c$ speed" vehicle, before that - we'll have to solve the question: "What about celestial object impacts?"... –  Waffle's Crazy Peanut Jan 12 '13 at 14:08
    
While this question could be phrased better, in my opinion, it's OK :) –  Manishearth Jan 12 '13 at 14:34
    
@Manishearth: Hi Manish. Well, What if he doesn't rephrase? :-) –  Waffle's Crazy Peanut Jan 12 '13 at 14:46
    
@CrazyBuddy: I said "While this question could be phrased better, in my opinion, it's OK". Which means that the q is OK in its current form, though it can be improved. –  Manishearth Jan 12 '13 at 14:54
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5 Answers

up vote 2 down vote accepted

The speed of protons in the LHC is at 99.9999991% of the speed of light.

You can see what happens when a proton hits a proton travelling in the opposite direction at those speeds.

higgs candidate

A candidate event in the search for the Higgs boson, showing two electrons and two muons (Image: CMS/CERN)

Now the spacecraft in question presumably is traveling at a similar fraction of the speed of light with respect with the asteroid cluster. 8 TeV (the energy of the proton in the above interaction) is still enormous energy at contact. What will happen is that the individual protons of the spacecraft will react as elementary particles creating an enormous number of secondaries similar to the image above. Immediate destruction of spacecraft.

Avoidance can only happen by detecting the presence of the cluster some fraction of a light year away from it and plot course accordingly.

IMO the main danger to such a fast traveling spacecraft will come from the low density ions in space, which will produce such interactions inimical to life. A strong magnetic field might throw away ions, but there are also neutral particles in the cosmic dust which cannot be stopped easily. A very strong and very massive shielding system will be necessary.

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Yes, you'd need large acceleration if you want it to avoid things in very small time, t. YOu could also have explosives to blow up asteroids. You wouldn't want to collide with the asteroids since if the asteroid is much more massive than you you would either blow up , or if you had elastic armor you'd just bounce away.

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"or if you had elastic armor you'd just bounce away" Uhm, no. Even speeds of mere km/s make a mockery of armor: everything is a liquid in collisions at those speeds. If you ever have a chance to watch a talk by someone who does high speed collision physics you should go. It's worth it. –  dmckee Jan 12 '13 at 4:24
    
even freshmen?? –  raindrop Jan 12 '13 at 4:26
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As far as I know, 20 or 25 km/s is the maximum velocity achieved by practical spacecrafts. If such a Sci-fi craft exists (as in Wall-E - where the craft cruises through the debris), we have a lot of choices. Physics is not necessarily needed. Sci-Fi is very good.

Spacecrafts (I mean, those used in space-travel) of the past and present are mostly guided by a bunch of astro guys on ground. If they were able to design the crafts and guide them through the asteroid belt (like the Voyager twins) by using some transmitter-receiver mechanism as in RADAR or SONAR or a Computer-aided Guidance system, then we could think of the future easily.

First, we'll be in an age of space-travel, where we'll have the farthest seeing telescopes (best ever ones than Hubble or James), and we'll have all exact locations and events (that may happen during travel, like future impacts on our space ship, etc.). So, guiding the craft wouldn't be so difficult like making some Brownian motions like the molecules do, or even land in several asteroids resting some time, like that.

If it simply by designing the spacecraft to have huge acceleration, wouldn't that mean lots of constraints on the material the spacecraft can be made of?

Not only the material because, the material is required only to provide resistance to the impact-able asteroids or when you're planning to dash through all the objects on your path. If the craft is planned to make sudden turns on its way, it should also have opposite thrusters with more or less the same power. If I'm allowed to talk still, I'll go into Sci-Fi.

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Its not possible under our current technologies. Even a small rock can blow out the entire ship because its mass would be very high in that relativistic frame.

As the question is fictional in nature, I can provide one solution from sci-fi world: Deflector Shields. Force field based deflector shields aren't completely fictional. There have been attempts to create real ones. So, if you're going to make a spaceship which could travel near $c$, try inventing perfect deflector shields first.

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e=mc2 tells us that, as an object gains velocity it becomes more "massive" (not larger but carries more mass) when approaching near light speeds, you could hit a planet, and it should be like driving through a mm thick wall of water

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This is not true. A massive object is not generally less brittle than a low-mass object. Consider the simple example of a tiny, low-mass rubber ball and a massive brick. Throw both of them at a concrete wall, which will survive? –  Wouter May 18 '13 at 23:04
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protected by Qmechanic May 18 '13 at 17:01

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