Why It's Never Hammer Time in Space

Have you ever been watching a sci-fi show, and heard the captain of the ship yell “All Stop!” at the helmsman? Have you ever wondered what that actually means?

Orion CEV

I once had a long debate with a fellow astrophysics student over the question of what ‘all stop’ really means for a spaceship.

See, for a real life spacecraft (lets say the Shuttle or Apollo missions) the vessel itself never stops, from the moment it leaves the planet to the moment it lands again. It is constantly moving: either orbiting earth, or the moon, or moving between the two. But it is always moving at speeds of at least 17,000 mph (all the way up to 24,000 for a lunar mission). You can only imagine that for interstellar missions, the same principle must be true, probably to an even greater extent. 

Well I decided to try and imagine the physics and do some simple calculations.

We need to remember that at any given time, planets are moving around stars, stars are moving around galaxies, galaxies are moving around clusters and clusters around super-clusters. So if you stepped back and looked at the universe from an impossibly far distance, it would look like one massive, heaving blob of ‘stuff’.

Within a Solar System or Galaxy?

Imagine that a spaceship (the red dot on the diagram) is approaching Earth. Let’s simplify things by just considering three aspects of the space-crafts ‘speed’ – Effective trajectory, Galactic orbit and Solar orbit.

Our craft is moving towards Earth (red arrow). But the Earth is cruising around the sun at a steady 30km/s. 

Red = Spaceship, Blue = Earth, Yellow = Sun, Gray = Galactic Orbit

So if the captain yells ‘all stop’, what happens? Does the distance to Earth simply remain constant? If so, then the craft is not stopped; it too is moving at 30km/s around the Sun. If not, then the Earth will drift away from the craft at 30km/s, and the crafts distance to the sun will remain the same.

But, then too is has not truly stopped. If distance to the sun is constant, then the craft is orbiting the centre of the galaxy at the same rate as the sun (the gray arrow on the diagram). This speed is approximately 220km/s.

You may answer: all stop would depend on frame of reference. If you are aboard an interstellar craft travelling between two stars, then “all stop” would entail the helmsman eliminating all motion which could change the distance between the craft and the stars other than the stars natural motion. Fine. But you still have a problem: Gravity.

Gravity is Constant

First let’s explain a conceptually simple but mentally challenging concept: Constant gravity.

If there were only two objects in the universe; you and a pea, and we placed you at opposite ends of the universe, no matter how far you were apart, you would eventually fall towards each other. 

Our real-life universe is not that empty, especially in a galaxy.

So lets say you are moving through a galaxy between two stars. You then decide to pull an 'all-stop’.

Remember, you are likely already orbiting the galactic centre at about 220km/s, so using the f=ma formula it would require a negative acceleration of 22,000 m/s^2, requiring a force of 22 million Newtons, or 2,243 metric tons.

If you did manage to do this though, you would instantly begin moving again! Why? Because of constant gravity; some nearby body would attract the ‘stationary’ craft, and you would begin accelerating toward that body. Our galaxy alone has at least 100 billion stars, not to mention a super-massive black hole at the core. That is a lot of mass.

 If you use Einstein’s visualisation of the space-time curve caused by gravity, you can imagine that it is like trying to keep a marble perfectly still within a moving cement mixer. The galactic gravity-well is uneven and constantly moving, so keeping a ship “stationary” is an unreasonable and illogical notion. The best you can hope for is to maintain a relative position within the galaxy, at the speed of 220km/s, but even this is challenging as nearby star systems would quickly exert their tug on the vessel.

So, ‘all stop’ does not mean:

• Kill Power to the engines (as you would continue moving at an even speed)
• Reverse power and kill speed along our trajectory (as you must still be moving at pace with galactic and solar dynamics)
• Stop Dead – theoretically an illogical notion