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.