With the successful launch of the Falcon Heavy rocket to a (presumptive) Mars orbit, I found my thoughts going to the feasibility of space travel. Specifically, why we are massively limited in our ability to reach the stars. With our current technology, Mars may be about the limit of our reach. Why? Physics.
Let me preemptively caveat all of this by saying that I'm neither an engineer or physicist. Thus, when I create models, I take shortcuts in cases where I either don't know the answer or where the math proves too daunting. For example, calculating the amount of energy required to leave earth orbit depends on everything from air temperature to the location of the launch site (it takes less energy to launch from the equator, due to the spin of the earth. That's one of the reasons Cape Canaveral is located in Florida, I'd reckon). To simplify, I calculate based on acceleration from relative zero in the absence of gravity wells - this should consistently overestimate the speed/acceleration of the spacecraft in question. So when I show that something is pretty impossible, it's actually really impossible.
The Gas Problem
For all their ingenuity, SpaceX's launch technology is fairly primitive. It's what we've been using since the 1930s. Sure, there have been improvements in the aerodynamics, launch protocols, material sciences, etc., but every single spacecraft launched to date has essentially rode a barely-controlled, violent explosion into space. This is really inefficient (explosions are, after all, definitively the release of massive amounts of stored energy) and, at least equally importantly, mass intensive.
Let's use an example to illustrate how much gas it takes to go anywhere. Take the Falcon Heavy (for which I am drawing information here), the most advanced rocket on the planet. Imagine you want to go to the nearest star, Alpha Centauri. It's a short 4.22 light years. A puddle jump, in the scope of things. To make the trip, all we have to do is get to the speed of light and wait a few years (more than 4.22 years though; remember that we have to speed up and slow down to slower-than-light speed, but let's ignore that for the moment. I'm also ignoring time dilation, mainly because that's pushing the limits of my maths ability).
Imagine you wanted to make the trip in the Falcon Heavy. Let's also pretend you could survive the force of the thrust for as long as was required and that you could shrug off interstellar radiation and a hundred other things. How much fuel do you have to take with you?
I'll attach the math below, but here's the short, rough calculation: each booster rocket can produce 4.79 meters per second of acceleration. The Falcon Heavy has three, so it can accelerate 14.37 meters per second as long as the boosters are going (this is in the absence of gravity, a favorable situation for the hypothetical traveler)*.
How long to get to the speed of light? Easy math - divide light speed (300 million meters per second) by 14.37 and we find it'll take 241.6 days at full burn to get us up to light speed. That's a lot, but how much gas do we need in our spaceship to start with to burn for 241 days?
Per the internet sources on such things, the Falcon's engines effectively burn their entire fuel supply (a reported 1,187,100 kilograms) in about 162 seconds, so let's ballpark their consumption at 7,328 kg/sec. To burn the craft up to the speed of light, we'd need... let's see... 152,985,380,928 kilos of rocket fuel. That's 337 billion pounds, or about half the weight of earth's human population. I'm confident we don't have even one percent of this amount of rocket fuel on our planet.
Actually, I lied. This is only half the gas you need. Midway through the trip, the spacecraft must flip and begin burning its engines in the opposite direction to decelerate. Otherwise you're going to shoot right by your destination too fast to even see it. So you need 700 billion pounds of fuel.
My favorite of Newton's laws: Wherever you go, you leave something behind.
Those of you expecting me to have a solution to this problem in the next paragraph are in for a rough ride. In the world we live in, the Falcon's full primary burn is sufficient to accelerate the ship to a measly 0.00141% of the speed of light. That sucks. It's also why it takes us seven-ish months to get to Mars. Even within our solar system, the distance is so great that we have to wait until the orbital relationship between Mars and Earth is favorable for a short trip (this is called the Hohmann transfer window and, for Earth/Mars, is every 26 months). If it's this hard to get to the nearest planet, getting out of the solar system is impossible for a manned mission.
More efficient engines that require less mass. People have been fucking around with ion engines for a while. These are more efficient, but the force they generate is a tiny, tiny fraction of a percent of the force of the big, powerful (and I use that term relatively) rocket boosters. In other words, they're too slow. Renewable energy sources (via solar power) is another idea, but the sun becomes just another bright star in the sky when you get out there far enough. Same problem with heat energy from solar radiation.
Gravity boosts, where a spacecraft flies close enough to a planet to gain momentum from gravity, are another possibility. But let's put it in perspective: Voyager One used gravity assists from both Jupiter and Saturn and it still has just barely left the solar system some forty years later. They're only so powerful.
Point to point instantaneous travel via Einstein-Rosenthal bridge or other not-fully-understood phenomenon. Advantages: fast. Disadvantage: currently indistinguishable from magic. Possibly-to-probably theoretically impossible.
One of the really wonky solutions I've heard is a propulsion method called nuclear pulse propulsion. We detonate a nuclear device (hopefully in space!) and the spacecraft gains momentum on the force of the shockwave. Apparently the nuclear shockwave can be pretty powerful, even from a modest nuclear blast. Fun fact: the fastest man-made object ever recorded was a allegedly manhole cover ejected by a nuclear blast, which reached a speed of 150,000+ mph. Obviously, the force of the blast would turn the passengers into liquid goo, but then it becomes a materials question instead of a fuel issue, and I've always been a fan of flipping to a different set of problems when you're faced with an impossible situation.
Then there's the slow-and-steady idea: a colony ship capable of sustaining life indefinitely. But anyone who's ever been on a long car trip knows that there's no way a group of people locked up together in a 500-foot spacecraft for 200 years would not kill each other.
So, interstellar travel is about as impossible as walking between different casinos in Las Vegas during August. And none of these solutions solve any of the secondary issues I've mentioned in passing. Radiation tends to screw us up pretty badly. There's a lot of it outside earth's ionosphere and we're not very good at blocking it. And even if we had a perfect rocket engine that wouldn't melt after 240 days of spitting a fireball several football fields long, we'd still have to survive it - imagine enduring multiple Gs for years on end with no break.
I suspect there are other issues, but these are the ones that are the real limiters that I can think of off the top of my head. In order to get anywhere, we have to solve every single one of them. And that's why we're stuck on this rock, at least for the foreseeable future.
And now it seems like things are a little screwed up with even our modest reach for Mars. Maybe the Tesla roadster that Musk stuck in the rocket will come up with something.
*The Falcon has a second booster, but it's a little bitch engine compared to the first stage, so I ignored it. That's what you do for little bitch engines.
Noah's Inner Monologue
Scribblings of a man who can barely operate an idiotproof website.