Hard Space Science Fiction Notes

Popular science fiction media (Star Wars, Star Trek, etc) with artificial gravity, faster than light travel and communication gives people a distorted view of space and the distances involved. When dealing with lower technology, near future fiction, things are different.

Space is Big
It seems redundant to say that, but its a fact that everything else is based on. A common measurement used within a solar system is the AU or Astronomical Unit. An AU is the average distance between Earth and the Sun, 93 million miles or 150 million kilometers. An AU is so far that even light at 186,000 miles per second (300,000 kilometers per second) takes over 8 minutes to travel that distance. Even the Moon is an average of 238,000 miles away and light takes more than a second to travel the distance. Other measurements are based on the speed of light, light-seconds (186,000 miles or 300,000 kilometers) and others, such as light minutes, light days, etc. A light year is therefore the distance light travels in a year (5.9 trillion miles or 9.5 trillion kilometers) and is commonly used measuring the distance between stars. A parsec is 3.26 light years. The galaxy is about 100,000 light years across and about 1,000 thick. The nearest major galaxy, the Andromeda galaxy, is about 2.5 million light years away.

How Far?
The planet Mars is the next planet out from the Sun and the nearest to being habitable. But, even at its closest approach to Earth, its still about 33 million miles away (Venus is slightly closer, but completely uninhabitable). So how long would it take to get there? An object in Earth's orbit travels at about 25 times the speed of sound or about 18,000 miles an hour. That sounds pretty fast, but at that speed for example, it would take over 76 days to get to Mars. And Mars is moving a little less than 15 miles per second, it will have moved almost 100 million miles along its orbit from where it was when you left Earth. So you need to aim for where it will be when you get there; your flight path will be a long curve catching up from behind, making your flight time longer. If Mars is on the opposite side of the Sun, it can be far as 3 AU.
So you need to be travelling faster than orbital speed. One Earth gravity is equal to 10 meters per second acceleration, it is common in science fiction to rate ships in Gs of acceleration. But for real space craft, it is very difficult to accelerate at more than a few Gs for any duration. The Space Shuttle was able to climb to Earth orbit, but needed solid rocket boosters and a fuel tank larger than the shuttle itself and all that fuel was burned in less than ten minutes. Kind of like towing a 1,000 gallon fuel tank behind your car so you could drive to work. Even if you could refuel your shuttle in orbit, your maximum speed would be about 25,000 miles per hour and your trip would still be 55 days. Which brings up the next problem, since there's no atmosphere friction or significant gravity to slow you down, you need to reverse course and blast just as hard to slow down as you did to accelerate. So half your fuel is spent speeding up and half is spent slowing down at the other end. Now you can only build up to about 12,500 mph and your trip takes 110 days. And that's to the closest planets. Jupiter would be at least ten times as far and Saturn would be more than twenty times as far.

Enter Science Fiction
Most science fiction that deals with reaction drives (basically rocket engines or other engine with an exhaust) use some variety of nuclear or fusion reaction to supply power or thrust, usually at a factor of ten to a thousand times more efficient. The Expanse uses such a fusion system, called the Epstein drive. If you could accelerate at 2 G's for an hour, you would be travelling 160,000 miles per hour and your trip to Mars at closest approach would be about 9 days. But most of it is coasting through space in free fall. Then you would need to decelerate for an hour at 2 Gs, give or take some, to match speed with the planet and take up an orbit. If you could accelerate at just 1 G continuously for the first half of the trip, then flip end for end (called turnover) and slow down at 1 G for the second half, the trip would take about 30 hours. But that assumes you have enough fuel to run your engine for 30 hours non-stop.
Most advanced technology science fiction uses a combination of artificial gravity and a device called an inertial compensator to nullify or reduce the effects of acceleration. Your ship might be accelerating at 100 gravities, but you only feel 1 or 2. The Expanse does not have artificial gravity, etc, so the speed you accelerate at is felt by the crew. Three Gs is about what astronauts feel today taking off, seven or more is what fighter pilots experience while dogfighting. Warships accelerate hard during battle and the crews in The Expanse have to ride it out with acceleration couches and drugs to stay conscious.
All of the ships in the Expanse seem to have fusion reactors, but not all of them have Epstein drives. Smaller ships and shuttles carry water as reaction mass and superheat it with the fusion reactor for thrust, which is called "teakettle" mode or "flying teakettle". Some older sci fi such as Robert Heinlein's The Rolling Stones comes to mind. His Rocketship Galileo uses molten zinc as reaction mass.

Maximum Speed
Ignoring warp drive, etc, the fastest thing is light or radio waves. As mentioned, light travels at 186,000 miles per second. But it would still take three minutes to reach Mars from Earth at closest approach. Its like texting someone who is busy and can't answer right away: "Hi, are you there?" <three minute wait for message to get there and three MORE for their reply to get back> "Yes, what's up?" <six minutes> "Want to go out tonight?" <six minutes> "Sure, Chinese?" <six minutes> "No, had that last night. Pizza?" You get the idea. I ran one Traveller game where they were searching for a lost ship and they sent out a radio signal to see if they could activate the missing ship's emergency beacon. The players were shocked when I explained it would take over an hour to get a response back if the ship was orbiting the planet four AU away (4 times 8 minutes/AU = 32 minutes each way). Pluto is a minimum of 30 AU from the Sun, it took 4.5 hours for messages from the New Horizons probe to reach us when it passed Pluto.