So you want to write a story that takes place in a plausible “sci-fi” stetting. One where the advanced technologies and exotic locales are part of setting, but not in the foreground of the story itself. On the other hand, you might only mention something in a paragraph or two, or use a location to provide texture to a scene, but not have the scene dependent on the location.
You might be tempted to “just wing it” and use what you vaguely remember hearing second-hand, in passing, years ago. However, our memories are not perfect, and that can lead to something that bears less resemblance to reality than a Saturday morning cartoon.
Space is big.
Read that first sentence again, it really rather understates the case. However, size does not require complexity. While space is big, it can be summed up under several different, simple, zones.
Regardless of zone, though, questions of temperature sometimes come up. You may have heard that space is cold. Well, this is oversimplifying the situation; space itself has no temperature, and is made of the same type of vacuum insulation that is found in a thermos (only better). If there's nothing nearby, then heat can just (slowly) leak away so it can take a very long time for something to freeze depending on a number of factors. Also, you may have something hot in the area, which could heat up your satellite or spacecraft as though it were sitting next to a radiator. On top of all of that, energy sources and running equipment on a spacecraft generate a lot of heat, and may cook the spacecraft unless it has (fragile) radiator fins or some other fantastic means of cooling.
You're on one. Earth is a good example of this type of location. As is Venus (crushing pressure and heat), Jupiter (even more air pressure) and others. Mars also falls in this category, but only just barely; its air is so thin that if you want to fly around there, you'll need an aircraft specially designed for ultra-thin air, normal aircraft would just crash. In short, though, if it's got wings, it can fly on one of these places.
You can see one almost every day. If it weren't for some circumstantial details, the Moon would count as a planet itself. These places come in two varieties, ones big enough to be round, and ones that are complicated lumps. For the ones that are big enough to be round (and if you've got some way of dealing with the lack of air) you can walk, run, or otherwise move around much like on Earth. For the smaller ones, move carefully; a careless jump could send you zooming away, never to return.
In short, though, the key factor here is that wings, or anything except rockets, don't work. If the space shuttle were to miraculously be taken to a landing approach to the Moon's surface, the shuttle would crash.
An area around a planet where a spacecraft or other object will stay if it's freefalling with the right speed and direction. Another word that's almost as accurate is the word “coasting” (which may read better for objects in deep space). A craft that's taken battle damage and lost power while in orbit, will generally stay in orbit. A short description of orbit is freefalling sideways around a larger object.
The lowest of the low. For Planets with Atmosphere, this is where the vacuum of space is flawed, and crafts and objects are less than perfectly stable here. For Planets without Atmosphere, this is the area so close to it that hills and valleys on the surface can distort the orbit (generally downward) over time. Many satellites and most space stations and such are placed here, as it is the easiest place to reach from the ground. For Earth, this zone starts at an altitude of about 200 km (110 N. Miles or 125 S. Miles) and goes up to several thousand. Time for one full orbit here is short; 90 minutes for one orbit is one typical example.
Note that this area is the only place where a “decaying orbit” makes any sort of sense, and the time for the orbit to decay depends on altitude. At the altitude of the ISS (~300 km) the decay time is measured in years, and gets longer with greater altitude. Orbits with decay times under a month are hardly proper orbits at all (this is really where “decaying orbit” applies, higher orbits ignore the decay outside of planning for the occasional boost back up). Also note that orbits are generally stable. If a craft is in low orbit where any decay is measured in months, and it gets shot up in a battle… it's still in a low orbit where any decay is measured in months. If a spacecraft needs to be constantly thrusting to “stay in orbit” it's not in a normal orbit and is instead referred to as being in a “forced orbit.” Or even just hovering in extreme cases. For satellites and spacecraft very long decay times are generally ignored, but are calculated for larger objects; Mars' moon Phobos is in a decaying orbit that will have it crash into Mars in about 50 million years.
Above low orbit—starting at 2,000 km or 1,243 S. miles above Earth—but below synchronous orbit, satellites and craft here achieve long term stability, but the added distance makes working with the ground take a fair bit more capability. Around Earth, this zone has usability limited as the Van Allen Radiation Belts fill most of the area (with a usable gap between the inner and outer belts). The Van Allen Radiation Belts (and similar belts around Jupiter and other places) are where incoming deadly space radiation (mostly from the Sun) gets trapped, and going outside of that means dealing with the steady trickle of incoming radiation. Passing through the belts unshielded is like getting a 20th century chest X-ray, while staying in them is ill-advised. Once outside the belts, satellites and spacecraft generally need more shielding if they're there for the long term.
Time for one full orbit is longer than any Low Orbit, but shorter than one local day.
Earth's GPS satellites are located roughly in the middle of Medium Orbit.
Geosynchronous orbit refers specifically to Earth, but around any planet the same principle applies for each one's own synchronous orbit. This is the orbit whose time for one orbit matches the length of the day of the planet below. Note that these are merely synchronized. Changing either (the planet's day or the satellite's orbit) leaves the other alone, that just means that the times for each no longer match. Weather satellites and satellites for TV are placed here.
For Earth, this is at an altitude of about 42,000 km (26,000 S. miles). For Mars, this is at an altitude of about 36,000 km (22,500 S. miles). Venus rotates so slowly that any orbit synchronous with its surface would intersect the sun.…
Above synchronous orbit. In this area, the time of an orbit is longer than a local day. Venus and the Moon lack this entirely, at least in a pure sense. You might see orbits that take longer than 24 hours referred to as “high” orbit even if that's still shorter than a local day.
This overlaps with high orbits (or “medium” orbits for planets without true high orbits), and whether you refer to something being in orbit or deep space depends on context. Deep space covers both interplanetary space, and interstellar space; there's not really much difference. Note that this may also cover the space between Earth and the Moon; most diagrams never show how far apart the two are (when everything else is to scale, at least).
There are a number of technological items common in Sci Fi. Some of them are based on current research, while others are purely props with no known grounding in reality. The focus here is on items grounded in reality, although how other items interact with reality will also be covered.
While these have existed on Earth for several decades now, these are still, in some ways, technology still in the early stage. Windscale and Chernobyl were roughly first generation designs. Three Mile Island and Fukushima were second generation (with partial upgrades). Various groups have done third and fourth generation designs. Earth has enough fuel to run second generation nuclear reactors for about 40 years, which will actually only burn a few percent of the fuel. Higher generation designs can burn more of the fuel (making it last longer), or switch to more abundant fuels entirely.
Waste can be treated simply as “the more dangerous it is, the more quickly it breaks down.” Also, higher generation reactors can burn “waste” as fuel and the resulting waste from that occurs in small amounts and breaks down quickly (in weeks to months).
The “clean” twin of nuclear reactors. Contrary to most hype, a fusion reactor will not provide infinite, cheap energy. A one GW fusion reactor will provide exactly as much power as a one GW coal plant, and will generally be of a similar size. I. e., a large facility. Fusion's promise is instead in abundant fuel (and reliable, instantaneous shutdown). Also, energy from early designs will be more expensive than from any other source.
As of the early 21st century, there are three reactor designs seriously considered as eventual power sources. How much of this is just hype for each one is left as an exercise for prospective authors and readers, and it may be worth noting that the optimism around fusion is nearly identical to the optimism that surrounded nuclear fission before nuclear reactors went into general use.
The first contender is currently expected (barring schedule slips) to have a demonstration reactor running by the 2030's. It'll produce power, but at prices too high to compete on the market. Commercial reactors could follow, but each would have a multi-decade construction time, and all will be very large (with the first being complete as early as the 2050's).
The first generation of this type would use deuterium (a stable isotope of hydrogen, and extracted from seawater) and process lithium into tritium (an unstable isotope of hydrogen) to burn the two together. The ash produced would qualify as nuclear waste for about 10–20 years as it gradually breaks down. Also, the reactor chamber itself will become radioactive in use and qualify as nuclear waste for a few decades after it's too old to run anymore.
Later generations of this design could burn pure deuterium, or eventually Helium-3. Also, in a realistic Helium-3 reactor (unless there are surprising advances), there will be lots of deuterium and other secondary reactions, and it will still make the reactor chamber radioactive.
The other two serious contenders (seemingly less likely than the design above) look to burn plain hydrogen and boron with effectively no secondary reactions. If either of these work out, they also claim to scale down to sizes small enough to put on carriers or submarines. They'd still have a similar power output as current shipboard reactors, though.