Highly relativistic spaceflight would be fatal for passengers and instruments

In short, because the Milky Way disk sets a preferred rest frame. Other matter in the galaxy all rotates with approximately the same speed around the galactic center. Here are some numbers to give you intuition.

Tangential speed of the galactic disk, obtained by averaging over nearby stars: ~ 250 km/s

Speed of sun (and other nearby stars, in random directions) with respect to the galactic disk: ~ 15 km/s

Speed of Earth with respect to the Sun: 30 km/s

Speed of satellites in low-Earth orbit: 7.8 km/s

Speed of light: 300,000 km/s.

If you want more, this article discusses these speeds in the context of estimating the dark matter velocity distribution.

http://pa.brown.edu/articles/Lewin_Smith_DM_Review.pdf

It's fairly accessible as far as physics articles go.

Even if you get outside the Milky Way, the cosmic microwave background sets a preferred rest frame. I can't find the exact speed of the Milky Way in that frame, but the speed of the Earth is ~360 km/s. (This means the Milky Way's speed is somewhere between 100 and 600 km/s.)

The take-home message is that the although the laws of physics have no preferred rest frame, the actual matter out there in the universe most certainly does.

Because the hydrogen atoms we care about are bound in galactic orbit, and that means they have a relatively low maximum relative velocity to us. If they weren't, they would all fly out into the intergalactic void in geologically short timescales, star formation would cease, and we'd have no such radiation problems after all.

'a few rouge hydrogen atoms'

It's rogue. Sorry, pet hate, can't help myself. That and people typing 'tounge' when they mean tongue.

Alastair Reynolds describes ice shields in a lot of detail.

This also reminded me of the "Archangel" class ships described in Endymion by Dan Simmons. FTL travel on these ships instantly kills all passengers but then conveniently resurrects them once they reach the destination.

Nitpick: The Songs of a Distant Earth is actually by Arthur C. Clarke.

No, it is not.

Beall's List of Predatory, Open-Access Publishers includes Scientific Research Publishing on the list, saying "Do not do business with the above publishers, including submitting article manuscripts, serving on editorial boards, buying advertising, etc. There are numerous traditional, legitimate journals that will publish your quality work for free, including many legitimate, open-access publishers."

The NASA orbital debris programme has a few pictures of impacts on returned space craft. Obviously these craft are travelling very much slower than 0.9 c.

(http://orbitaldebris.jsc.nasa.gov/protect/impacts.html)

(http://orbitaldebris.jsc.nasa.gov/photogallery/photogallery....)

And some of the debris is quite large - (http://orbitaldebris.jsc.nasa.gov/photogallery/gallarypage/s...) (Is that tape inches?? So that lump is about 1.5 to 2 inches?)

There are existing projects researching near earth impacts from high speed objects (http://ares.jsc.nasa.gov/ares/hvit/problem.cfm) so maybe there's some trickle through to research between very fast craft and small objects.

You are making a fundamental error by assuming classical kinetics. At .9c, your first interactions would be electronic (ionization), then nuclear and radiative. The complete and initial ionization of all the atoms of 10mg of, say carbon, would be a lot less than 360 GJ. Then you'd have different interactions depending on your hull material, magnetic shielding/Bremsstrahlung losses, etc...

With some complicated magnetic fields, you could probably do something smart to take advantage the different charge/mass ratio of an ionized atom and positron and minimize interaction with a steady/confined positronic cloud. You might be able to annihilate enough electrons of the dust particle to ionize it enough to subsequently deflect it from the ship. Then you'd still have to worry about gamma ray radiation and back pressure and maybe some other interactions.

I don't really know about all of this for sure, just a thought.

That's the energy of about 20,000 rounds of 5.56mm NATO. The real problem is that it would be concentrated on such a tiny part of the spaceships hull.

So you need a very lage ablation shield

Because nobody knows how to make a Bussard Ramjet actually work. From the abstract: "Stopping or diverting this flux, either with material or electromagnetic shields, is a daunting problem."

And assuming that you can divert the protons, ironically enough, you'd have a serious problem cooling them down sufficiently to achieve fusion (because if you don't, the plasma will not be sufficiently dense to cause a high probability of fusion events, despite the high temperature); and slowing their passage through the ship so they stick around long enough to fuse would create significant drag. And after you have solved the problems of collecting, matching velocity with, and cooling/compressing the plasma, a Bussard Ramjet is only worthwhile if you have an extremely efficient system for capturing all of the waste heat from those processes and injecting it back into the exhaust. Otherwise, at any speed above about .12c, you lose more energy to drag while collecting protons than you gain back from fusing them.

Now, there's no particular physical reason why all of those problems couldn't be solved. But if you've already got some way of accelerating to >.5c, and you just need to worry about radiation shielding, building a whole Bussard Ramjet rather than just trying to push protons out of the way with minimal effort or absorb them in some very massive physical shield, is not the way to go.

Because, if I am reading the article correctly, some hydrogen (which has become radiation) will go through you anyway. This kills you. Somehow gathering up all of the radiation and trying to use it to fuel a ramjet is the same thing as trying to make some sort of shield, which the article says would be very difficult.

The article points out some reasons why it might be difficult (not impossible!) to construct the electromagnetic field required for such a ramjet, but yes this point seems to be completely lost on the author. A Bussard ramjet specifically exploits the presence of interstellar hydrogen as a valuable energy resource.

Thermonuclear pulse propulsion (a la Project Orion) can do better than that- up to 0.1c. And that requires no unproven technology; just a lot of money and a lot of politics (and a lot of fission fuel to buy with that money; I dunno if it's more than you could actually get on the market or not, but it would definitely cost A Lot).

I agree that both of the statements you gave are not productive. "Interstellar travel isn't possible" rejects all theories past and future without considering their individual merits, which is awfully arrogant. "We have enough problems down here on Earth" is short-sighted, in terms of a geological timescale. In a billion years, our most pressing problem on Earth will be that the expanding Sun will kill all terrestrial life. Hopefully some forward-thinking individuals will have worked out space travel by then.

I only read the abstract, but I don't think the article is nearly as prohibitive as your statements. It says that diverting the radioactive hydrogen is a "daunting problem" i.e. hard but not impossible. Even if Near-C space travel _is_ invariably fatal, that's not so broad as "interstellar travel isn't possible". There's still wormholes and hyperspace and whatever exotic ideas we might come up with in the future.

> Lastly, we don't really have a good physical understanding of matter at near C speed limits. With mass increasing as we approach C, I don't know we can assume the same physical properties of any material in classical models.

Yes, we do, and yes, we can. Matter travelling at near c relative to us behaves exactly the same as any other matter. That's what frame invariance means. From our perspective, yes, the ship would seem to have more mass, which is a direct result of the equivalence of mass and energy and the fact that it has a crap-tonne of kinetic energy. But from the ship's own perspective, its mass does not change at all- rather, the entire surrounding universe seems to become more massive.

>It would take like 34 days at 10g acceleration to get to C.

So why not take 340 days at 1g?

There are many problems with high relativistic travel. This is not one of them.

You need to learn more about relativity.

First, people on the ship will not feel as if they "have more mass". Their physical properties will be exactly identical, because they are at rest in their frame of reference. Flying through a galaxy at 0.999c is identical to "sitting still" while a galaxy flies by at 0.999c.

Second, you're ignoring relativistic effects in your calculation of the amount of time it takes to "reach c". You can't just divide light speed by the acceleration and convert to days. You need to include the effects of time and space dilation, in which case you'll find you have to pick a target less than c because no matter how long you accelerate you only approach c.

Not necessarily. There are some interesting potential radiation issues with "warp drives" as well. For example, at the boundary of a warp bubble of the generally-Alcubierre-like variety, space is shearing / expanding such that virtual particle pairs are incapable of recombining, just like at an event horizon. That could potentially create a great deal of radiation draining energy from the warp field. Additionally, what happens to particles that are intercepted by the travelling bubble? They don't just disappear!

Wouldn't a jump drive work slightly better in this case?

Meh. We just need radiation shields.