So You Want To/Write a Hard Science Fiction Story With Space Travel: Difference between revisions

All orbits are shaped like ''conic sections.'' If your space ship is moving slower than the "escape velocity" -- or more precisely, if its total kinetic energy and gravitational potential energy is less than the gravitational potential energy it would have at an infinite altitude -- its orbit will be shaped like an ellipse. If it's moving ''precisely at'' the escape velocity, its orbit will be shaped like a parabola. If it's moving faster than the escape velocity, its orbit will be shaped like a hyperbola. This is assuming, however, that your space ship spends all its time from this moment forward in free-fall, without firing its engines.

The formula for how long it takes to make one complete orbit was first deduced by Johannes Kepler when studying the motions of the planets around the sun. Sir Isaac Newton expanded on this formula so that it applied when orbiting ''any'' object. The formula is:

The sad fact is, though, that no other planet we've detected thus far is even remotely habitable by human standards. The bigger ones are Jupiter-like balls of gas, while the smaller ones are almost universally airless. The few worlds we've found that ''do'' have both an atmosphere and a solid surface have been blanketed in gases that no human can breathe, at pressures anywhere from [[Useful Notes/Mars|near-vacuum]] to [[Useful Notes/Venus|90 times Earth's sea level]]. While it's theoretically ''possible'' that a planet out there might harbor life as we know it, it would have to fit a long, narrow list of parameters, and even then, the kind of life that might have actually evolved there will most likely be very different from the multicellular-eukaryote-rich biome inhabiting Mother Terra.

How far away from the star the Goldilocks zone is depends on the star's energy output. A very dim red dwarf star, like Wolf 359, would require a planet to be only about 1.5 million kilometers away from it to receive as much energy as Earth does from our sun -- that's only 0.01 A.U., 1% of the Earth-sun distance. A bright and powerful star like Sirius A, on the other hand, would require a planet to be 5 A.U. away from it to receive as much energy as the Earth does from the sun.

Interestingly, both of those distances have potentially disastrous consequences. If a planet is only 0.01 A.U. away from its star, the star's tidal influence is going to be enormous. The strength of tidal forces varies directly with the larger object's (i.e. the star's) mass, but inversely with distance ''cubed''. The tidal forces on a planet only 0.01 A.U. from a star 1/10 the mass of the sun are, therefore, going to be 0.1 / 0.01³ = '''100,000''' times as strong as the tidal forces the Earth experiences from the sun. This all but guarantees that the planet will be locked in synchronous rotation with its star -- that is, its rotational period must match its orbital period, so the same side is always facing the star. One side of such a planet would be in perpetual daylight, while the other would be in perpetual night. The climate on such a world would be much different than the climate on Earth.

Worse, many red dwarf stars -- Wolf 359 included -- are ''flare stars'', which emit semi-regular bursts of X-rays every bit as powerful as those emitted from a flare taking place on the sun. At 0.01 A.U., that much ionizing radiation can easily disassemble the organic molecules necessary for life. And X-rays can scatter (e.g. bend around corners), which is why the dental hygienist always leaves the room and closes the door when (s)he takes an X-ray picture of your teeth. Regular flare outbursts so close by probably means that any life would have to be buried underground.

So what do you do when you ''need'' your characters to be able to move between the stars faster-than-light, or [[Transporters and Teleporters|teleport]], or have a [[Tractor Beam]], or do any of the other myriad things that our current best guesses at the law of nature say are impossible? You set the technology up in such a way as to '''limit the damage''' to your story and your setting. Maybe your [[Deflector Shields]] are magnetic, and can only affect charged particles and ferromagnetic metals -- and your spaceship needs to open up holes in its shields to shoot iron slugs or particle beams at an enemy. Maybe the high speeds needed to traverse interplanetary distances in days or hours are imparted not by your space freighter's own engines, but by planetside pushers that will only push it onto a predictable course, thereby eliminating the threat of rogue spaceship commanders turning their vehicles into WMDs. Maybe your transporters only let you beam between one transporter pad and another (unlike the transporters in a certain [[Star Trek|softer SF franchise]]). Maybe the violations of the Laws of Thermodynamics needed to make [[Stealth in Space]] work are curtailed in some way that prevents you from getting useful energy out of any warm object (which, like some types of [[Reactionless Drive]], would have driven your Space Oil companies out of business).

[[FTL Travel]] is one of the bigger thorns in the side of the Hard SF genre. Special Relativity makes it absolutely clear: it is physically impossible to accelerate an object with any kind of mass so that it's moving faster than the speed of light. Even accelerating an object ''to'' the speed of light would require an infinite amount of energy. However, we've also pretty much established that there are no other technological species on any planet in the Solar system other than Earth. If we want to have space adventures involving high-tech aliens, we'll have to travel to other star systems, and the distances involved are so enormous that it would take years to get from one star to another if you were limited to sub-light speeds. Science Fiction writers have had to compromise, and allow ''some'' means of travelling faster-than-light which didn't turn their universe into something totally unrecognizable to a modern reader. Therefore, the ability to move faster-than-light has received more attention in SF than any other fantastic concept as to ways to Limit The Damage of having it around.