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"So you can just slow down and make a sharp left in space?" - Mystery Science Theater 3000
Isaac Newton turned the world of physics upside down when he observed his first law of motion:
A body in motion will tend to stay in motion at a constant velocity unless acted upon by an outside force.
This was earth-shattering stuff when he introduced the notion. Several hundred years ago.
The average writer for science fiction TV shows, however, is a little behind the times, because he doesn't quite get it.
Basically, goes the misconception, if your engine breaks down in space, you are boned. The ship will very quickly slow to a complete stop. Your helm officer will say "We're adrift." This means something between, "We're moving at a random (low) speed and direction," and "We're not moving at all, practically speaking."
Writers do this because if the engine in your car breaks down, you come to a stop, and if the engine in your boat breaks down, you drift at a random low speed at the whims of the currents. Based on these experiences, it feels intuitive to write that spaceships will act this way.
However, the reason these things happen is friction. On a straight road, with good tires, you can coast quite a long way on even a slight downgrade. In space, where there is no friction with the road or the air to contend with, you can coast forever, or until you hit something, which, given how big space is, is awfully unlikely. Which means that if your engine stops working in space at top speed, it's not going to give you any trouble getting to your destination. In fact, space vehicles spend most of their time outside the atmosphere with their engines off in this fashion. Stopping when you get there, of course, is another matter.
Note that there is some friction in interstellar space, due to hydrogen particles. However, these are dispersed enough that they'd really only matter over huge distances - huge even by astronomical scales.
For that matter, the entire notion of "We only have enough fuel to get so far," is a little suspect: if you've got enough fuel to reach top speed, you've got enough fuel to go anywhere; once you reach top speed, you can just shut the engine off and coast. Of course, it would become a problem if you don't have enough fuel to stop at the end - or if, for whatever reason, you have to turn somewhere, or if your engine fuel doubles as power generator fuel, which would cause a black-out in your ship (which, if it comes up in a Space Friction plot, generally means the crew has a few hours to restore power before running out of air). None of this should be a problem if you know your situation before you set out and can plan accordingly (being boned in the event that you lose your fuel supply en route is possible, but then the problem is that you're either going to miss your destination... or you're going to hit it).
Heck, when it comes right down to it, there's also not really any such thing as "top speed", other than the speed of light; earth-bound vehicles have a top speed only because there's a speed at which the engine can no longer out-perform friction, and there's a point at which the environment will break the vehicle. Space ships won't shake apart from going too fast (though they might be obliterated by interstellar dust or irradiated by blue-shifted cosmic background radiation).
Exotic propulsion systems of the sort needed to exceed the speed of light are exempt from the normal laws of physics and can reasonably be presumed to expend energy even at a constant speed, just holding the laws of relativity in check, but it's curious that the effects of such drives always cause space to behave exactly like an ocean.
A form of Hollywood Science.
Examples:
- Star Trek, unsurprisingly. In fact, in an instance where someone takes advantage of inertia (Star Trek The Next Generation: "The Battle") by letting a derelict coast alongside them, everyone else is amazed by the notion - and, indeed, it turns out that his decision was prompted by his being Not Himself. In the other known instance ("Booby Trap"), it takes Picard being inspired by ancient records to come up with the idea of escaping an engine-detecting minefield by pulsing the engines once and then coasting.
- In Star Trek IV: The Voyage Home, a ship whose power is zapped by a probe is actually seen to coast to a stop.
- Halfway justified, in that the warp engines are what generate the Warp Field, which in turn allow for faster-than-light travel. If the engines go down, so would the warp field, thereby forcing the ship back to sub-light speed (which is very slow by comparison)
- Firefly, despite the evidence that all space travel is sub-luminal, has Serenity stranded in open space when its engine breaks down, and frequently features the notion of having "just enough" or "not enough" fuel to get somewhere. Because it's All There In The Manual, ancillary sources explain that Firefly's Artificial Gravity is also used as inertia cancellation and can move quickly using a relatively weak engine. When this system shuts down the ship comes to a dead stop as its inertia returns.
- The "Serenity stranded" scenario is partially Justified when the column of flame shoots out the front of the ship, providing thrust that would oppose the direction of motion. Whether there the thrust would be sufficient is up for debate. The "just enough" or "not enough" fuel thing is also partially justified: presumably there would be some direction changes involved if the solar system is as crowded as they imply. Direction changes would likely require fuel. Again, how crowded the system is and how much fuel the directional changes would require is up for debate, but at least partially justifiable.
- Further justified in the 'Out of Gas' example due to the fact that Wash was using "creative navigating" to avoid Alliance patrols. I.e. they weren't actually pointing at their ultimate destination when the engine failed.
- Pretty much every other space-based Science Fiction series does this at some point as well. Space 1999, Farscape, the old Battlestar Galactica, and many others have all presented the victims of engine failure as effectively motionless.
- One episode of Space 1999 even showed one of the Eagle ships rocking in space. Gah!
- Exception: Babylon 5 and the new Battlestar Galactica portray spaceships moving realistically according to Newtonian physics, with Babylon 5 even showing damaged vessels with no engines gliding helplessly out of range of help. Ships with gravity-based technology can move in a more Star Trek or Star Wars manner; watch the White stars dart around the comparatively lumbering Earthforce Omega Destroyers which, having no gravity-based technology, maneuver far, far more like spacecraft we have today.
- Averted by Cowboy Bebop - in one episode, the Bebop is out of fuel, but the characters are unconcerned and are just killing time while the ship coasts to its destination. Despite this, Cowboy Bebop is not entirely a realistic series using newtonian flight physics. There are plenty of occasions where the ships behave in newtonian-correct ways (maneuvering thrusters, braking with forward-firing engines...), but plenty more when they don't (the dogfights in space, for instance, follow atmospheric flight patterns).
- Averted like most other Did Not Do The Research tropes in Planetes. Most of the early story was about collecting debris that were dangerous precisely because items in space never slow down or stop.
- Both averted and used in Futurama, in the episode "Godfellas" - when Bender is launched out of a torpedo tube, he keeps going. He's realistically able to slow himself a bit by throwing away pieces of treasure he had stored in his compartment, but runs out of objects to throw before he's stopped himself completely. However, the ship cannot catch up to him because it was moving at "top speed" when they launched him, so he was moving even faster.
- Given the way the Planet Express Ship moves (or more accurately doesn't move, rather moving the entire universe around it), this may be somewhat justified.
- Given that the first tenet of relativity is that there's no difference between an object moving relative to the universe and the universe moving relative to the object (at least, when no acceleration is occurring, which there isn't in this scene), this doesn't justify anything.
- Is a major driver in the plot of Poul Anderson's novel Tau Zero, where the Bussard-ramjet ship Leonora Christie suffers a failure of part of its engine. You see, Bussard-ramjet ships can accelerate near the speed of light using magnetic fields to fuse interstellar hydrogen to drive the ship; the reaction is self-sustaining once started. Problem is, as the ship accelerates relative to the rest of the universe, so does the oncoming hydrogen in the ship's path, which would result in the instant destruction of the ship if the fields fail. The ship has a special decelerator module on its engine for slowing down safely. Guess which portion of the ship's engine fails... resulting in the ship's crew having to Wait It Out until the universe decays enough to allow the engine to be shut down and repaired. They then have the problem of finding a new home, as humanity is now long dead due to the eons that have passed outside the ship. Unlike most that use this trope, this is based on *real* science.
- In the Xbox 360 game Project Sylpheed, your ship steers as if there's air resistance in space, with this becoming more pronounced in atmosphere. In a related note of bizarreness, cutting the engines and coasting works even in atmosphere, despite the fact that the ship should fall out of the sky if it's not done in an effectively zero-G environment.
- In the video game Star Control II, variously used, subverted, justified, lampshaded, and parodied. Hyperspace and Quasispace have friction, resulting in a continuous need for fuel, and the ship slows to a stop when fuel runs out. However, space travel in solar systems and in battles obeys Newtonian physics, and fuel expenditure only occurs when using the engines to alter course. Yet the Arilou Lalee'lay, hinted to have perpetrated the various alien abductions and flying saucer appearances on Earth, fly saucer-shaped ships that stop and start instantly with no inertia, as often seen in depictions of flying saucers.
- The FreeSpace video game series. Not that it's fun trying to chase after a ship that suddenly became disabled when it was on its afterburner. Besides, you've got to worry about that invisible barrier 150km away from your starting position that causes you to "collide with yourself" and blow up.
- Averted very well in the Galactic Civilisations PC game series, although most gamers probably wouldn't notice. Star systems really are bazillions of light years apart - trying to cross the galaxy using even the Humans' galaxy-changing Hyperdrive would take years. Evolving the "engines" branch of the Tech tree involves finding more effective and powerful ways to fold normal space and move in Warp space, both of which literally reduce the distance between two points. In theory, this would mean that ships could be justified in "running out of fuel": they're still travelling at near-light speed, but can't operate their Hyper Warp Mk III drives, meaning that the voyage will take far too long to complete.
- Transformers The Movie (the animated one) had a variation of this; Astrotrain, in space shuttle mode, pleads to his passengers to "jettison some weight, or we'll never make it to Cybertron". Parodied here
 .
- Stargate Atlantis averted this. When the city's hyperdrive broke down, they didn't keep moving faster than light, but they DID keep drifting at a fairly high speed. Not fast enough to get them where they were going for a few hundred years, but fast enough for them to go through The Asteroid Thicket and have several close calls.
- Freelancer averts this one to some degree; there is no friction when you kill your engines, but somehow the friction reappears as soon as you start them.
- Averted to a small degree in Tachyon: The Fringe. While it abuses the trope in normal flight, there is a button you can hold to continue moving in your current direction at the current speed. You can even spin around and fire backwards. A real pedant could use this system to fly the ship in a (pseudo) real fashion!
- The Escape Velocity series averts this, as once you're moving, you can stop holding down the arrow key and the ship will continue on its current direction unless you attempt to stop it (you can even change the direction the ship is facing without changing your inertia!). This has led to what some players call the "Monty Python Maneuver", where you run away from enemy ships, then turn the direction the ship is facing (you'll still keep moving in the original direction!) and fire at the ships. Due to the fact the ships are coming towards you, they'll go right into your weapons while you remain out of theirs. Strangely, this excellent strategy has yet to be followed on most television series...
- As mentioned above Babylon 5 is an exception here. The B5 fighter pilots love this maneuver.
- Shown in GuavaMoment's Lets Play X-Com: Apocalypse as a sign of Tynam's growing insanity and the excessive levels of Did Not Do The Research present in X-COM: Interceptor.
- Averted in the Honor Harrington novels, as plenty of attention is paid to the difference between acceleration and motion - to the point that moving too fast can leave you blazing past the enemy's fleet without time to take a shot at him.
- Averted in the Frontier games by Frontier Developments, along with other space tropes. Ships that run out of fuel will continue to drift forever through a solar system.
- Averted in Colony Wars 2, where one mission has you tow a frigate out of harms way when its main engine failed; the problem is that it drifts to an asteroid and if you pull too much on the nose sideways, it'll create a torque and the sides may collide with the asteroid. Also, your own spacecraft drifts when your engine is disengaged; you'll rarely, if at all, have any time sitting put without any motion (although, you drift in a lateral motion, and like the asteroids there, you never stupidly rotate in place like most Hollywood asteroids do).
- In Spaceballs, a spaceship brakes... and even emits a kind of shower of sparks, like a car would with burnt rubber.
- ...but it is Spaceballs after all.
- In the MMORPG Eve Online, this is taken to rather ridiculous extremes for an otherwise acceptably scientific game. Not only does space have friction in EVE, but avid fans have actually done the math and determined that space in the EVE universe has the consistency of WD-40. When paired with the fact that a ship traveling on traditional propulsion methods actually has a top speed and an acceleration curve, it strains Suspension Of Disbelief.
- Averted in Red Dwarf - Starbug's engine is disabled, and they're in trouble because they're headed right at a planet and traveling entirely too fast for comfort.
- Justified in the Lensman stories: the key to interstellar flight is a device that cancels a spaceship's inertial mass, so top speed is determined by the point at which a spaceship's thrust is exactly counterbalanced by the friction of the interstellar medium. Since a spaceship doesn't have inertia, turning off the engines causes it to come to an instant stop.
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