Science fiction and fantasy stories often take readers to strange new worlds. But how do they get there? The mechanics of that can vary from none at all in the case of high fantasy to highly technical in hard sf. Writers have no shortage of interesting ideas, some of them relatively realistic and others wildly improbable. Let's look at a few.

Before we figured out that the atmosphere ends a hundred miles or so overhead, the most logical idea for getting to the Moon and planets was simply to fly there. The Icarus legend was one of the early stories to use that method. Poor Icarus melted his wings when he got too close to the Sun, but later characters made successful journeys in chariots pulled by swans, in balloons, and even, during a short window of time when our knowledge of aviation exceeded our knowledge of interplanetary space, in aeroplanes.

We did eventually learn that air grew too thin to support flight at about 50-60 miles up. We needed something that didn't rely on air...like a big cannon! Sure, we could blast a projectile to the Moon, and if we put enough padding on the floor and made the gun barrel long enough, people could survive the acceleration.

Well, actually, no. Jules Verne told a great story, but the science was off by a few orders of magnitude. In reality, to actually make it to the Moon his passengers would have been a layer of red paste on the projectile's floor, held firmly in place by the top of the projectile, which would be indistinguishable from the bottom after slamming into—and through—the atmosphere at over 25,000 miles per hour (the minimum speed needed to escape Earth's gravity).

Verne knew about rockets—indeed, the Chinese knew about rockets long before Verne—but nobody seriously considered using them for space flight until the early 20th century. Why? Well, because rocket fuel won't burn in space, of course, and rockets need something to push against.

That was of course nonsense, but people who should have known better (I'm looking at you, New York Times) dismissed the idea out of hand because of that misconception. The Times finally changed their tune 49 years after dissing Robert Goddard, when Apollo 11 made it to the Moon and back under rocket power.

The truth is that rockets carry their own fuel and oxidizer, and they don't need anything to push against. In fact, when they do have something to push against, it can damage the rocket. The backwash of flame, pressure buildup, heat, and flying debris can do a lot of damage in the first few seconds of a launch.

Rockets work by throwing stuff out the back. For every action, there's an opposite and equal reaction (Newton's third law of motion), so tossing something backward moves the tosser forward. If you toss it really hard, the forward force increases by the same amount. So we talk about rocket efficiency in terms of specific impulse, which is a fancy way of saying how hard the engine throws things.

The easiest stuff to throw behind a rocket is the spent fuel, so that's what most rockets do. Different fuels give them different specific impulses, with liquid hydrogen and liquid oxygen being near the top of the list. Those have to be chilled to very low temperatures, though, to stay liquid, which is why you see ice falling off the sides of big rockets when they launch. That's moisture in the air that froze to the fuel tank while it was waiting for launch.

The trouble with rockets is that they have to lift their fuel as well as their payload, and that means they need more fuel to lift the fuel, and more fuel to lift the fuel to lift the fuel, and that quickly snowballs to the point where the rocket is 363 feet high and carries six million pounds of fuel to lift a 100,000-pound Apollo mission to the Moon.

We need something better.

The 1950s and '60s were not only the heyday of rocket development, but the heyday of nuclear bomb development. Not surprisingly, people (Freeman Dyson, among others) figured out how to use nuclear bombs to propel rockets.

No, seriously! It was called Project Orion, and the idea was to put the space capsule on top of a really massive metal plate, then set off a small nuclear bomb underneath the plate. The explosion would toss the plate up into the air, and with it the space capsule, and if the plate was sufficiently massive the acceleration wouldn't squish the passengers into strawberry jam.

They even tested this in 1957 during Operation Plumbbob, in which a 2,000-pound steel plate was put atop a test shaft and blasted upward at about 150,000 miles per hour. Needless to say, it vaporized in the atmosphere, but it did prove the concept. We just needed a bigger metal plate, which would go slower. That was a welcome realization: a heavy payload actually helped make the thing work.

What keeps the Orion vehicle from falling back down? When the acceleration from the first bomb peters out, you set off another one directly below the pusher plate. That's right: you keep chucking nuclear bombs out the back. Blam, blam, blam, all the way to orbit. Which led to my favorite line in Larry Niven's and Jerry Pournelle's Footfall: "God was knocking, and he wanted in bad."

Okay, we still need something better.

Nuclear power certainly provides more bang for the buck, but we need something that doesn't go bang quite so energetically. Nuclear fission requires a certain minimum mass to become self sustaining, whereas fusion can occur in any size reaction chamber so long as the temperature and pressure are high enough. And fusion provides far more energy per pound of fuel, too. In theory, then, fusion rockets are the way to go.

There's just one little problem. We're still trying to figure out how to make a fusion reaction repeatable with the kind of regularity you'd need (several hundred tiny little blasts per second) for space travel. Heck, we're still trying to figure out how to do it at all. The problem isn't getting the fuel (hydrogen) to explode; it's recovering enough energy from the explosion to power the equipment to get the next fuel pellet to explode. In theory it should work, but we're nowhere near achieving break-even yet. (There's an old and uncomfortably accurate adage: "Fusion power is the energy source of the future, and always will be.")

Maybe brute force isn't everything. Specific impulse is the name of the game in rocketry, and few types of rocket have a higher specific impulse than the ion drive. Ion drives achieve their high efficiency by accelerating small amounts of mass to really high velocities. Kinetic energy is equal to 1/2 mass times velocity squared, so that Vee-squared term starts to build up quickly when the numbers grow large.

Ion drives use electrically charged plates or grids or magnetic fields to accelerate individual atoms to extremely high velocities. The amount of thrust is very small, though, often compared to the weight of a piece of paper. However, when that piece of paper keeps pushing day after day, endlessly, the tiny force adds up to a large velocity change in the spacecraft. We have successfully used ion propulsion on many solar system exploration missions, probably the most well known of which is the Dawn spacecraft that went to Vesta and entered orbit there, then broke orbit and went to Ceres, where it continues to orbit today. It used less than 1000 pounds of propellant (xenon gas) for its entire mission—after reaching orbit. It required a conventional rocket to launch Dawn into space, but once there it used its ion thruster to move into the asteroid belt.

To give you an idea of how powerful the engine is, it can accelerate the spacecraft from 0 to 60 miles per hour in...four days.

Ion drives are great for robotic missions, but we need something with a little more punch to move people around. What else can we try? Well, if we're making a wish list, throwing stuff out the back seems kind of wasteful. How about something that just pushes on the payload but doesn't require throwing anything away?

We call such a pipe dream a "reactionless thruster." Several attempts have been made, most famously the "Dean Drive" of the 1950s. The Dean Drive purportedly could reduce its weight on a scale without any visible means of thrust, but subsequent investigation showed that the effect was most likely an interaction between the machine's vibration (which was substantial) and the scale's springs.

More recently we have the EmDrive, which uses a metal cone bombarded on the inside with microwaves and supposedly produces a small but measurable thrust. Various groups, including a NASA team, have investigated this device and have published conflicting reports, but none have come up with a reliable, repeatable device, much less a good theory as to why it should work. I'm going to go out on a limb here (a very wide, hefty limb) and say that it's bunk. I would love it to be possible, but Newton's third law is pretty clear on the matter and I doubt we're going to beat it.

There is a free lunch, sort of, if you don't mind low acceleration. I'm talking even lower than ion drives, but the energy source is free: Sunlight. Light does push on things; it just pushes very, er, lightly. So to propel a spacecraft using sunlight, you would need a very light craft and a very large sail. Even then, the thrust would be measurable in Kleenexes, or maybe even milli-Kleenexes. And that thrust gets weaker with the square of your distance from the Sun, so it's really only good for the inner solar system.

Of course you could shoot a big laser at your light sail. People are seriously considering this. It would probably work, too, out to the point where targeting the sail becomes not just problematic, but downright difficult. Try keeping a Frisbee aloft with a garden hose spray nozzle and you'll get an idea of how hard "difficult" can be.

Plus the sails would have to be incredibly thin, which means incredibly fragile. And they would need to be folded up for launch and successfully unfurled in space, because they can't lift off from Earth's surface.

The biggest problem with space flight is the first hundred miles. We live at the bottom of a gravity well, and climbing up out of it is the hardest part. As Heinlein said, "Once you're in orbit, you're halfway to anywhere." So how about antigravity?

If we could neutralize gravity, we could just float into space. Start at the equator and the Earth's rotation would fling you outward. H. G. Wells came up with the idea of antigravity back in 1901, and we've been wishing for it to actually be possible ever since. Problem is, Einstein pretty much proved that gravity is a warpage of space around a massive object, not an actual force that can be blocked by anything else. Again, I'm placing my bet with the physicist and guessing that antigravity will prove to be impossible.

Most, if not all, of the above methods are really good only for travel within the solar system. Even the nearest stars are so much farther away (thousands of times as far) that we need something entirely different if we want to get there in any kind of reasonable time. Various schemes have been proposed, but there's one possibility that really intrigues me. If gravity is simply the warpage of space, what happens if we warp space intentionally? Specifically, what happens if we warp space in front of our vehicle in such a way that it acts like a big gravity source, and we fall into it? If the warp generator moves with the ship, we keep falling forward into the warp, accelerating faster and faster and faster and faster without throwing anything out the back at all.

It gets better. Nothing in space can exceed the speed of light, but space itself can be warped at any speed we want. The warp drive could theoretically move a spacecraft right past the speed of light and keep on going.

Wait a minute. Gene Rodenberry was right? Warp drive is real?

Maybe. We call it the Alcubierre Drive, because physicist Miguel Alcubierre did the math to prove that it might actually be possible. He postulated that space could be squeezed in front of a spaceship and stretched behind it, pushing a ship along. This isn't exactly gravity, but it's similar.

Is the Alcubierre Drive any more likely than the Dean or the Em-Drive? I'm going to go out on a limb here and say, "Sure it is." A person has to believe in something, right?
 

Jerry Oltion has been a science nut since he was old enough to spell "curious." He has written science fiction almost as long, and has done astronomy somewhat less. He writes a regular column on amateur telescope making for Sky & Telescope magazine, and spends many, many nights a year out under the stars.