Movie spaceships have a particular look. I’m sure you’re familiar with it. Somewhat patterned after NASA’s near-Earth spacecraft, they’re ungainly, bloated things, and always non-aerodynamic even if they happen to also occasionally enter in the atmosphere (where they nearly always inexplicably float with no visible means of countering gravity). People think that being fat and adorned with protuberances is realistic, because space is empty. So even as TIE Fighters roar noisily through vacuum, they can have gigantic panels without worrying about them flapping in the breeze.



Sci-Fi at its sci-finest Ships that will eventually venture outside our solar system will have a much different look. The reason is because space isn’t empty, especially inside a galaxy. The Milky Way’s Ships that will eventually venture outside our solar system will have a much different look. The reason is because space isn’t empty, especially inside a galaxy. The Milky Way’s Interstellar Medium is everywhere.



Right now, our sun is traveling through a 60 light year bubble of gas at a speed of about 25 km/second; we can detect this as an interstellar wind of helium hitting the earth in the winter, which is when Earth is in “front” of the sun in its direction of travel through the cloud. Compared to the Milky-Way-interstellar-medium’s average density of around 1 molecule per cubic centimeter, it’s quite thin. The cloud has a density is 0.1 molecules per cm 3 , because it itself is inside a void about 300 light years in diameter that is even thinner.





This is about as good as you’re ever going to get for interstellar travel. Just not good enough. Even at these low densities you don’t want to run into this medium at even modest fractions of the speeds necessary for "science fiction" interstellar travel.



Say for instance, that you’re in a ship clipping along at a tenth the speed of light. This means getting to the nearest star, Proxima Centuri, in about 42.5 years (assuming instant acceleration, which doesn’t happen, but never mind ). At that speed, each centimeter your ship's bow plows through 1.799e+12, or 1,799,000,000,000 centimeters of space, per minute. Even multiplying this by 0.1 molecules per cm3, leaves you with a rather substantial 1.799e+15 CPM (Counts Per Minute) of what is effectively alpha-particle radiation at double its normal energies (around 10 MeV each), for every square meter of your forward hull.



Now alpha radiation isn't terribly dangerous. It can be stopped by paper. But the energy it dumps into materials causes embrittlement over time. Worse, the problem dramatically increases the faster you go. When you get to about one third the speed of light, not only are you hitting triple the number of particles per minute, the impact between the bow of the ship and the interstellar medium is so energetic, that atomic reactions start to occur, both fusion and a process called nuclear spallation. And no matter how much you thicken it, lighting the skin of your ship on thermonuclear fire isn’t good, even if it cuts the jaunt to the nearest star from 42.5 years to only 14.



Before I explain the solution to this, let’s explain why the other potential solutions don’t work.

The applied phlebotiunum of jumps in hyperspace, subspace, slipspace, or what have you, isn’t a solution. Leaving our universe is science fantasy. I’ll explain why in some other article.



Using a strong electromagnetic field to move the interstellar medium out of the way simply doesn’t work at the speeds we’re talking about. First, to move the gas, you need to ionize it, which is nearly impossible at length. Second, even if you could, the kind of electromagnetic field that would be required to move particle out of the way before they impacted the ship, would be in the Tesla energies more associated with magnetars. If your ship is traveling at a third the speed of light, then every atom a ten thousand kilometers away, would have one tenth of a second to be moved perpendicular far enough to completely avoid your ship.



Plating the hull with fusion-resistant iron-58 (along with neutron absorbing materials like boron, cadmium, indium, or silver) can work, if you make it thick enough. Even then, near light space weathering will be a serious issue. About 1% of the interstellar medium isn’t gas, but other things like dust. Micrometeorites hitting any flat surface at a decent fraction of the light speed will detonate with substantial force, doing significant damage. And given the enormous amount of space you have to travel through to get anywhere, this is going to be a routine occurrence for any big "cruiser" class spaceship.

So what would work? Call them “needleships”, because that's what they will have to look like. Dozens, if not thousands, of kilometers long, they will taper to a point.

A Needleship. Very long. Its nose starts thinner than a hair, and widens as needed.

This solves the problem of space impacts because even at relativistic velocities, near misses at an oblique angle will result in a purely electromagnetic interactions with the sides of the ship, knocking pesky atoms, micrometeorites, and potentially even larger things, to the side as it passes by.The only thing that could ever bother a needleship is hitting a full sized asteroid dead on. Yet given how rare space rocks are expected to be in the interstellar medium, this shouldn't be much of a risk, even given the enormous volume of space that all ships will have to travel through to get from one star to another.This isn't quite all the danger near light-speed travel presents. The most powerful theoretical propellant we know is antimatter. If sufficient quantities of this could be made, you could have a ship that sustained a 1 Gravity acceleration continuously. (This is the closest we could ever get to the floating spaceships so common in SciFi.) That would allow them to take advantage of time dilatation. A 4.2 light year trip would only feel like a month of travel, and if you kept it up, you could travel major distances without realizing how much time was going by. The drawback is that unless you were very careful, you'd burn up.When a ship is in a reference frame that is nearly C out of phase with the standard universe, the Greisen-Zatsenpin-Kuzmin (GZK) limit starts to have an effect. The Cosmic Microwave Background is normally invisible due to its redshift from the universe's expansion, but as a ship approaches the speed of light, it blueshifts back into visibility. Go fast enough, and passengers will start to see the leftover light from the big bang's fireball as a visible glow in the direction of their travel. Go faster still, and this ubiquitous light blueshifts into UV, X-Ray, and eventually it turns into Gamma radiation. At a certain point, your ship starts to experience photodisintegration as the gamma rays from that (and starlight) penetrate through the hull. This is bad for the paint.Now admittedly, the speed you have to achieve to actually get in trouble with the GZK limit is, pardon the pun, astronomical. But it's important to note that it's stillthan the speed of light. So even if mean old Dr. Einstein and his relativity hadn't ruined all our SciFi fun by saying that FTL, travel (absent a preferred reference frame) is equivalent to going backwards in time, wecouldn't even close to the "C" without being bathed in a shower of particles and photons that would effectively become like a stream of unavoidable deadly cosmic rays.So the faster your ship goes, the more it is going to need to look like a needle.As you design ships that get very close to the speed of light out of phase with the galaxy, you will have to make these ever longer and thinner, due to relativistic length compression.You might also note that these ships don't look particularly comfortable for human beings to inhabit.That's okay.Astrophysicists are already leaving dangerous space travel to machines specifically designed to operate in hostile environments. There is increasing evidence that even freefall environments aren't good for our long term health. Luckily, we can use other means of transferring our squishy, decidedly not radiation-proof bodies, between the stars.I've outlined one means of doing so in my novelette Original Blues , which (for the Memorial Day Weekend), I'm currently giving away as a celebration that my next novel, Party of Assassins , is complete. Pick it up right now from Amazon. I don't know if it's Hugo Award worthy, but at least for the moment, it's free.