15 Things You Didn't Know About Outer Space

[Payä Tìrol]

I think Pandorapedia or something mentions that it uses two propulsion methods: An orbital laser Array at Earth against some sort of sail to propel it outbound, and a proton/antiproton torch to slow it at Pandora. After cargo is swapped, the two valkyrie shuttles are left behind on Pandora to process Polyphemus for fuel, and the reverse is used to return to Earth.
The ISV is pretty clearly carrying fuel with it, and I'm also pretty sure it was mentioned somewhere that, yes, a collision with a speck of dust at .7c would likely cause the destruction of the ship, but such collisions would be infrequent.

[Payä Tìrol]

Yeah, it's kind of ridiculous, the amount of KE an object moving at relativistic speeds gets to be pretty ridiculous, and it's sort of abused in other science-fiction universes that take liberties with physics. In Halo, for example, the station based MAC guns are supposed to fire a 3000-ton slug at 0.4c...

[kewnya txamew'itan]

such collisions would be infrequent.

Over the distance and time span we're talking about (to reach Pandora), whilst they'd still be infrequent, the probability of there being at least 1 collision (assuming no anti-collision systems).

Given that there are a few (call it 7.5) hydrogen atoms per m^3 of space (roughly), that equates to a density (taking length contraction into acount) of about 15 atoms/m^3, assuming the dish of the venture star is the widest point (which is 330m) then for every metre the venture star travels it will hit about 1.3 million hydrogen atoms which is ~16MJ (to put that in perspective, that's roughly equivalent to burning 1/8 gallon of petrol) of KE a second of ship time, that's certainly not too much, but that's still going to add up to some pretty significant damage over the entire journey (it would be possible to work out how much, but I'm not sure when I should be applying time dilation and when I shouldn't).

For, this level of KE damage, pushing ablative ice or using large amounts of ablative armour should both be viable solutions.

[Payä Tìrol]

Oh, I'm not talking about collisions with free hydrogen atoms. The density of those are like 0.1-1/cm³, and collisions, as you mentioned, would be frequent enough that you'd have to design some sort of magnetic or ablative shield to protect the ship against it. The collisions would also be mostly spread out over the entire forward cross section of the ship.

I'm talking about an actual collision with something the size of a grain of sand, maybe <1-20mg. Say you have a 10mg speck of dust.

m=m₀ (1-(v/c)²)^1/2
m=1.40e-5 kg
KE=mc²-m₀c²
KE=((1.4e-5)kg(c²))-((1e-5)kg(c²))
KE=1.26e12J - 8.99e11J
KE=3.6e11

That's roughly 0.7% of the output of the first fission weapons, concentrated in whatever unlucky mm² part of the ship that happens to get hit.

[Ean Hufwetulyu]

Oh, I'm not talking about collisions with free hydrogen atoms. The density of those are like 0.1-1/cm³, and collisions, as you mentioned, would be frequent enough that you'd have to design some sort of magnetic or ablative shield to protect the ship against it. The collisions would also be mostly spread out over the entire forward cross section of the ship.

I'm talking about an actual collision with something the size of a grain of sand, maybe <1-20mg. Say you have a 10mg speck of dust.

KE = (mc² / (1-(v/c)²) - mc²)
=(8.99e11J/0.51)-(8.99e11J)
=8.63x11J
That's roughly 1.5% of the output of the first fission weapons, concentrated in whatever unlucky mm² part of the ship that happens to get hit.

Catch me up rutxe

[Payä Tìrol]

Oops, I did that wrong. It's actually about half that value...

Basically, that's the Kinetic Energy of a 10mg object moving at 0.7c (7/10th the speed of light), or ~210 000 km/s.
As you start dealing with the physics of objects moving at a significant fraction of the speed of light, classical mechanics (such as KE=1/2mv²) starts being very wrong.
For example, if you tried 1/2mv² here, you'd get 2.2e11J, which is a ~40% error.

[Nì'awtua Eyktan]

Yeah, it's kind of ridiculous, the amount of KE an object moving at relativistic speeds gets to be pretty ridiculous, and it's sort of abused in other science-fiction universes that take liberties with physics. In Halo, for example, the station based MAC guns are supposed to fire a 3000-ton slug at 0.4c...

Here's a conversation in Mass Effect 2 about the problem with such weapons.

Mass Effect 2 - Newton's Law in Space (HQ)

[kewnya txamew'itan]

Oh, I'm not talking about collisions with free hydrogen atoms.

True, but the density I used is almost exactly the mean density of space and easier to estimate than including relatively large dust particles, my solution will spread all the damage out evenly across the ship's cross section (and journey time) as opposed to it all potentially being concentrated in one enormous impact.

[Étienne]

That was a sick read.

[Payä Tìrol]

Yeah, but collisions with the interstellar medium are likely to be a planned consequence of interstellar travel, a collision with something larger is likely to ruin your whole day

[Ean Hufwetulyu]

mllte

[kewnya txamew'itan]

Yeah, but collisions with the interstellar medium are likely to be a planned consequence of interstellar travel, a collision with something larger is likely to ruin your whole day

Yeah, but only because such impacts concentrate a large amount of KE in the same point.  Given that the estimates I used are estimates for the mean density of any section of instellar space, the total amount of KE dumped onto the ship will be broadly similar between one that suffers a dust impact and one that doesn't, the reason the second will survive and the second one quite probably will not, is to do with the KEm^-2 dump which will have a much higher peak value and quite probably destroy (or at the very least render inoperable) large portions of the ship without adequate ablative material (be it ice or armour).

[Payä Tìrol]

Yeah. I think we're basically agreeing, just with different words...
I also cringed in Halo 2 when Cortana told Lord Hood that the Covenant Fleet was holding "just out of range" of the defense cannons. :/

[Ean Hufwetulyu]

when do think we have first contact?

[kewnya txamew'itan]

Yeah. I think we're basically agreeing, just with different words...
I also cringed in Halo 2 when Cortana told Lord Hood that the Covenant Fleet was holding "just out of range" of the defense cannons. :/

Yeah, I think we are too. Of course, whilst there is no maximum range for the defence cannons physically, there would be a range above which the targeters would not be able to resolve the target or else the cannons might not have the precision of aim to aim at the target with a reasonable probability of a hit.