Light is Law

299,792,458 meters per second through vacuum. Round up to 300,000 km/s. Nothing goes faster. (Except, perhaps, monarchy)

Information and radiation travel at light speed. Getting anything made of matter even close to that speed takes a whole lot of energy and is subject to all manner of weird effects like time dilation.

Space is Big

Interstellar travel is not a casual affair. Interplanetary travel isn't either, but interstellar travel is always a big deal.

Interstellar travel takes a long goddamn time. Long enough that cryo, digital storage, or life extension will be more or less required.

Interplanetary travel can still take days, weeks, months.

Rerouting a ship in transit is not a lightly-taken endeavor.

News travels at light speed. Long-distance communication will have lag.

Don't expect big interstellar empires to form. Communication lag and travel time will put hard limits on that quickly. Expect to see a lot of small alliances and confederacies, getting smaller as you go up from habitat to planet to system scale.

If you use space habitats a lot a single solar system can contain trillions of people and more places to visit than you can actively comprehend.

It's the Great Filter, Charlie Brown!

They died off from nuclear war / catastrophic climate change / plague / etc.

They are trapped by high gravity, icy crusts, hyperdense atmospheres, etc.

They are intelligent, but lack the ability to make tools.

They aren't intelligent, and their tools don't have proper analogs to normal technology.

They're not going to progress past simple tools for a few hundred thousand years.

Every Gram Counts

It's Not Easy Getting Into Space

Space elevators - A hugeass cable stung up to a space station in geosynchronous orbit.

Skyhooks - A rotating cable that dangles down into the atmosphere and swings a ship up and around to launch them into orbit.

Mass drivers - A fucking huge railgun. Great for launching raw materials off of whatever airless rock you're mining them out of, and if it's long enough you can just make a launch loop.

O'Neill Cylinders are the Shit

Stellar Laser Highways are the Shit

Spaceships are not Boats

Missing the solar sail, alas.

You Will Not Explode and or Immediately Freeze in a Vacuum

In Knowing the Rules, You May Now Break Them

Bonus: 200 Posts!

Listening to the recentepisode of gg no re , I thought it might be useful to provide a helping hand for people who are interested in running a sci-fi game but don't have the time or inclination to do a research deep-dive.I will be linking to the videos of Isaac Arthur throughout this post. If you want more in-depth exploration of any of the topics here (and many more besides), absolutely listen to him.In short:The apparent discrepancy between the size and age of the universe and the total lack of detectable alien life (for an interstellar civilization would get HUGE in a relatively short amount of time) is called the Fermi Paradox . The reasons offered to explain why this might be are many, ranging from the plain (the physical conditions for life are rare, intelligent life is rare) to the more elaborate (the aliens are all just really, really good at hiding from us) and among all of these proposals are and a category of them are called Great Filters: factors that prevent life from reaching an interplanetary or interstellar civilization.There are a whole lot of them. The most plain are just that the physical conditions for life as we know it are rare or that intelligent life is rare. Sensible, but less gameable. Other options include but are not limited to:Skerples also offers the following:(Check out his full post on the matter) , but a solid summary is that the most human aliens are still nothing like us, and the most Earthlike worlds are nothing like Earth. Anything that resembles humans or Earth was probably designed as such (metahumans and space habitats can come into play here.)Mass needs energy to move it. Fuel and propellant have mass. The more mass you have the more fuel and propellant you need.Space aboard a ship is at a premium. You've got to balance everything for your specific trip, because you are trying to hit one moving object with another moving object launched from a third moving object all while trying to keep some fragile bags of mostly water and meat from dying.Gravity is fighting you the entire way up and rockets are inefficient. So there are a lot of better ways to get up there. Including but not limited to!Imagine a cylinder 20 miles long and 5 miles across. You can make it bigger if you have better materials. Rotate it so that the interior walls have 1 G of gravity, while the caps and central axis will have none. String some together in a chain or cluster, embed them into an asteroid, surround it with all the support systems you might need.Congratulations, you have just made where most of the population in a space-faring society will live and the easiest way to do a classical space opera. Now you can hop between radically different environments and cultures in a couple hours, in a spaceship that's more like the family RV.The important thing to remember is that space habitats like these are better for human habitation than planets: you don't need to fight a gravity well to leave, you can tailor the interior environment much more easily, you can churn them out by the dozens or hundreds from single asteroids.Imagine a laser . Now imagine that it is so fucking huge that it can push a spaceship. Build an entire network of them. Now you have a means of accelerating (and slowing) your ships that doesn't require them to spend fuel, thus allowing them to carry more stuff and more people.It's still slower than light, but it's damn effective. Can be used as communication hubs too.James Cameron'swas not a good movie. But it has one thing going for it: TheThis is a proper spaceship.Just look at this beaut, this sheila, this absolute unit.Look at those radiators! See that little bit by the debris shield, with the two rotating arms? That tiny thing there? That's the crew capsule.This thing has a maximum cargo capacity of 350 tons, a max speed of 0.7 C, and can get you to Alpha Centauri in six and three quarters years flat.What I'm getting at here is that this is what a hard sci-fi ship will end up looking like: a tiny little submarine attached to a whole hell of a lot of propulsion.You can survive in open space for as long as you can hold your breath (get the air out of your lungs first, lack of pressure will make it expand and that's no good at all). Still likely to get a nasty dose of radiation but having no air is the big danger.Arguments over the relative hardness / softness of any given science fiction media are both stupid and dumb. You're making a story or running a game, and that's the most important thing. Do what works for you, do what makes sense - I like aliens, so I tend to be looser on them than I am with, say, FTL communication. Tweak what you want, gloss over what bores you. The opposite is also true - if there's something that really gets you interested, embrace it and see where it can take you.I was going to have post 200 be announcing my Patreon, but that's taking its time and I want to keep making things. So this is post 200 (and the first of the post-G+ era).Holy shit. 200 posts in just a scrape (month and a week) under two years. I can't say much else besides thank you. I certainly wouldn't have kept it up this long without all of you out there reading and commenting and providing feedback.There's so much yet to do, and I have barely begun.