Space flight in Science Fiction: getting off this rock

Image: the last Shuttle lift-off in May 2011. Image snarfed from the amazing APOD site.

I’ve just finished reading a book I won’t mention, for the reason that it describes something that’s impossible and inaccurate, would never be practical in the way described, and I don’t want to single out the author. I see this a lot in the slush, too: a huge space ship lands on the surface of a planet with breathable air.

Whenever I read stuff like this, it sets all sorts alarms in my head screaming Oh, no! Because you can’t do that. There is no way anything short of magic will make that realistic. Feel free to use magic, but by then I’ve lost all faith in the story as Science Fiction.

I’d like to talk a bit about why and things to consider when writing stories that involve landing on planets and transfer between planets.

Want to send something to the Moon, or Mars? Well, as long as you don’t plan on landing there, the vast majority of your fuel will be spent just getting off this rock. There is no resistance in space and once a rocket has attained speed, it won’t slow down until the engines are engaged in the opposite direction. For missions sent to orbit planets and moons, the only fuel used after lift-off from Earth will be a very small amount for course correction.

Typically, more than 90% of a rocket’s weight prior to departure will be fuel. You see, Earth has two things that make it liveable for us but are very annoying for space flight: an atmosphere and gravity.

Gravity means that you’ll need a certain speed, called escape velocity, get away from the centre of gravity, this being the planet or moon. How much depends on the size of the planet. For Earth, the escape velocity is 11.2km/s. That is more than 40,000km/h. Ouch. On the Moon, it is 2.4km/s. On Jupiter, 59.5km/s. Double ouch.

How much speed you can attain is determined by the rocket equation, which, without going into details, describes the efficiency and capacity of rockets.

One limiting factor is the specific impulse of the fuel, which determines how fast the exhaust gas leaves the rocket, and thus how much it can propel the rocket. We have more or less reached the limit of chemical fuels (which is what space flight uses today). Nuclear rockets would be faster, but understandably have drawbacks. Ion propulsion is useful once you’re in space, but not so much for getting off the surface, and there is the airy-fairy promise of antimatter. The point is that all these propulsion types have their specific impulse, and their own limits as to how effective they are.

The other element of the rocket equation is the mass of the space craft, which includes fuel. It is important to note that the rocket equation is a logarithmic function. In other words, an increase in the craft’s mass requires more than an equivalent increase in the efficiency of the rocket.

All this conspires against heavy ships. Getting very large space craft off an Earth-like planet, barring magic, can’t be done. Zip. Nada.

But couldn’t you fly a very large space ship close to the surface?

Sure, if it doesn’t have an atmosphere. Mind you, unless you’re going to crash, you’ll be going so fast you won’t be able to see anything.

Gravity declines the further away you are from the planet, but even when you have left Earth’s gravity, as long as you are in the solar system, you’re still orbiting the Sun.
All man-made objects, satellites and indeed the International Space Station, are orbiting Earth, just like the Moon. Space flight means orbiting. An object in orbit around a body needs to maintain a speed that just stops it falling into that body. This speed is higher the closer you are to the body. For this reason, the speed in orbit of Mercury is MUCH greater than that of Pluto. Or, looking at planets with many moons, you will see that the further moons move at a much smaller speed than the closer ones.

The International Space Station orbits the Earth in ninety minutes. Supposing the Earth had no atmosphere and it could orbit closer, for every metre it came closer, it would have to go faster, and it’s already going at more than 28,000km/h. Zoom indeed.

But most of these planets have this annoying thing called atmosphere, no matter how thin, and whether breathable or not. An atmosphere means air resistance, so energy needs to be spent maintaining speed. It also means wings, flaps and a rudder are useful. It means aeroplanes.

An aeroplane is not a space ship. I cannot say this often enough. A ship designed for long space flight would look, and fly, in Earth’s atmosphere just as well as the Empire State building.

So, having said all that, the most realistic treatment of door-to-door interplanetary flight includes stop-overs in orbit. Surface-to-orbit flight and space flight are such different beasts that you cannot use the same vehicle for both. Even a single surface-to-orbit vehicle is stretching the limits, because you either use the advantage of an atmosphere while you can, and need a second set of different engines, or you use rockets, and have to carry a lot more fuel (and use stages). But I’m happy to believe such a vehicle could be developed.

When you get to orbit, however, you’d need to transfer to a more comfortable, lumbering, clunky-looking behemoth that has neither the need for wings, nor similar restrictions in mass. And those lumbering things don’t land on a planet.


  1. Nice post. The single most hilarious ‘sci-fi’ film I’ve ever seen is Paul Verhoeven’s Starship Troopers. The just plain wrong stuff in that film would about fill a book. My personal favourites: 1. the big spaceships blundering around in space bumping into each other – cause hey there’s not a lot of room up there (wonder why they call it SPACE then?)

    2:the enemy bugs firing – there’s just no nice way to say this – ASS–PLASMA from their rears up into space at said blundering spaceships and;

    3. Lightly armed troops arriving on planet with no heavy artillery and no air cover at all, not even the kind of conventional weapons we have now, no fighter jets, no drones, no heavy artillery or SAW guns – and when they find the bugs they SURROUND them. In a circle. Which no trained soldiers would ever do cause you know, then they can’t fire their weapons and all, as they’d be shooting at each other. And also when they make contact, they RUN AWAY. The marine military adviser on that film must have just wanted to cry.

  2. I’m still trying to get over ‘red matter’ in Star Trek…

  3. And to think, in Star Wars (the newer crappy movies) the character’s capes flap in the breeze of space and a sound weapon makes noise in a vacuum! Oh dear.

  4. Great post – I’ve felt the same way so often. You mentioned the kids’ ebook I’ve just published – I tried to take all those factors into account. For example, shuttles are launched from the Moon by catapult. I must confess, however, I have a ship land on the Moon using thrusters. It’s established that this is new technology which can take the strain of the Moon’s gravity. Now I’m stricken by doubt!

    As for films set in space – when are they ever really science fiction? Um… the first Alien film? Maybe?

  5. You’d use thrusters to land on the Moon, because it has gravity, which you have to counter.
    Certain films are better than others. The science in Star Wars and Star Trek is pretty aweful, but other films, such as Moon, and even Avatar, do a much better job. Bending science is pretty much a given in SF, but blatantly disregarding it is neither smart nor necessary, even in the most epic of space operas.
    My kids’ SF is set on a space station, and I describe how the station has artificial gravity through the station’s rotation, and that there is no gravity in the centre (where kids of course aren’t allowed but where the go to clown around). I also describe how a ship achieves artificial gravity, and how food is produced.

  6. I just don’t think Hollywood gains anything when they sacrifice reality for special effects. People who like SF see straight through this sort of stuff.

  7. My favorite bad sf film has got to be ARMAGEDDON, which abuses science terribly. Just the video-game flight through the meteor swarm or whatever it is is worth the price of admission… a continuous burn! Where’d all that fuel come from? THE CORE was pretty rancid, too.

  8. That, and the naked-skinned “polar” beastie… and the ENTERPRISE being built on the ground… Purely idiotic.

  9. Oh, yes. Bullt on the ground. Never mind how they’d get the thing into space. They didn’t need to. It’s only a movie!

  10. In my space station, they play null-g soccer at the centre!

  11. I’ve been thinking of zero-G recreational activities (No sniggering at the back!) and wondered what the three-dimensional equivalent of a square dance might look like. Cube dancing anyone?

  12. I kind of buy the ‘building big ships on Earth’ side of Star Trek because they invoke antigravity / artificial gravity in other respects, for example the directional gravity on the starship during spaceflight. It’s physical nonsense, of course, but it is kinda consistent.

  13. Yes, I think it’s much better if you establish some sort of nonsense to make it–uhm–fly.

  14. Two words for you:

    Starflight. One.

  15. The “hardest” SF film of all time is probably 2001: A Space Odyssey. It does virtually everything right, and extensive colonization of near earth space — to include the Moon — is portrayed accurately, according to physics. From the standpoint of this writer-reader, I enjoy 2001 for its accuracy. However, I will say that I enjoy its sequel, 2010: The Year We Make Contact, much more. Despite the fact that 2010 ‘cheats’ by having sound effects in the vacuum of space.

    Film is an aural experience as much as it’s a visual experience. I think Ron Howard understood this when he did Apollo 13, which also ‘cheated’ with sound, despite being rigorous in other ways. I think audiences *need* their sound effects, in order for the movie-going experience to be more visceral. Certainly the opening scene of Star Wars doesn’t work as well if we remove the roar of the star destroyer’s engines, or the keerkow-keerkow of the laser blasters. Thus I am willing to forgive the sound effects if only because I think they are a *necessary* cheat in the motion picture and television medium.

  16. I agree with that, Brad, and I think this sort of this is forgiveable because of the medium and the film maker’s necessity to fill the soundtrack with–well–sound. This post is more about fundamental errors of understanding built in the plot. Incidentally, I watched 2001 for the first time on the plane to LA and while I enjoyed the authenticity of the setting, I thought the movie itself was as boring as hell. Oops. There, I said it.

  17. Oh dear Patty, them’s fightin’ words. I’m working on an essay at the moment on why 2001 is one of the greatest films of all time and tracking some of the remarkable cultural influence it still wields.

    Having said that, the idea of watching 2001 on a plane fills me with horror. Can’t imagine any half-decent film coming up well under those circumstances. Kubrick was trying to reach the audience on a non-verbal level, to put you into a slightly altered state. Possible on a plane? Not in a good way.

  18. I’m sure it had a great cultural influence at the time, but I didn’t watch it at that time. It’s one of those movies, like Heinlein or A.C. Clarke as writers, where you’re not actually *allowed* to say that you didn’t enjoy them without committing sacrilege.

    That said, I also watched Inception and Moon on the same flight (yeah, it’s a very long flight to LA). I found the first entertaining enough to keep watching although I didn’t feel like I understood it 100%. Moon was simply amazing. Also quite accurate, and not flashy in the special effects stakes, with long periods of silence, which is the artistic point of the movie. Just wonderful. Not boring at all.

  19. You’re allowed to say you didn’t enjoy it πŸ™‚ and I’m sure that on a plane you certainly didn’t enjoy it but I contend you haven’t seen it. I’ve seen Moon and Inception too and while I can’t really see how Inception would do well on a plane, it’s impossible to judge 2001 from seeing it on a plane. A film which utterly depends on good sound and a big screen and draws its power from being an overwhelming aesthetic experience, not just an intellectual one.

    Moon is a terrific little film with great ideas and a clever plot – but that’s the clue. It’s a little film and can be appreciated intellectually. 2001 is… a work of art.

  20. I also have to admit that I have very little patience for stuff presented to me on screen in the first place. I’d much rather grab the book πŸ˜‰

  21. No shame in admitting the obvious. 2001 is a *very* boring film, especially compared to today’s fare. I find its sequel to be far more entertaining. As for the more run-of-the-mill science and physics errors, someone in the thread mentioned Armageddon as an example of the perfect storm, between terrible story and terrible science. I agree, especially since Deep Impact was essentially a ‘correct’ version of what Armageddon could have been; and was both more enjoyable and more scientifically rigorous.

    I think what it comes down to, for me, is whether or not the movie-makers at least attempted to get it right. It’s pretty obvious (as with movies like Armageddon) when movie-makers don’t give a damn. Those are the times I get miffed, if only because it doesn’t take that much effort — and when someone like Michael Bay not only doesn’t make the effort, but seems instead to go all the way in the other direction — ludicrous implausibility — I feel like it’s contempt for the audience.

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