Space is usually described in miles, which is the least useful unit available. Nothing about spaceflight is decided by distance.
What decides it is delta-v, the total change in velocity a mission needs. It is a budget, it is paid in propellant, and the propellant to carry propellant is itself propellant, which is why the equation punishes you so severely for wanting more.
Why is delta-v the only number that matters?
Because everything else follows from it, and because it produces results that feel wrong.
Getting from Earth’s surface to low orbit takes roughly 9.4 kilometers per second of delta-v. That single step is the most expensive thing in the entire solar system, and it is why every launch is mostly fuel and why payload fractions are so brutal. You are climbing out of the deepest gravity well anyone routinely deals with.
From low Earth orbit, the rest gets cheaper than intuition suggests. The Moon’s surface is around 6 km/s further. Mars transfer is roughly 3.6 km/s, with landing partly handled by the atmosphere. Some near-Earth asteroids are reachable for less delta-v than the lunar surface, because they have essentially no gravity well to descend into and none to climb back out of.
So the map is not concentric. A rock a hundred million miles away can be cheaper to reach and far cheaper to leave than a body a quarter of a million miles away. Any realistic settlement pattern follows the delta-v map, not the distance map, and it will look strange to anyone thinking in miles.
What kills people out there?
Four things, in roughly this order of difficulty.
Radiation. Earth’s magnetic field and atmosphere absorb what would otherwise be a serious dose. Outside that protection there are two problems: a steady background of galactic cosmic rays, which are high-energy heavy nuclei that shielding handles badly, and solar particle events, which are sudden and can be lethal within hours. The practical answer everyone converges on is mass. Water, regolith, or waste piled around a storm shelter, because there is no clever lightweight solution to a proton moving that fast.
Gravity. We know what zero gravity does over months, because people have done it: bone loss, muscle loss, fluid shifts, and vision changes that do not fully reverse. What nobody knows is what a third of a gravity does over decades, or what it does to a pregnancy and a growing child. There is no data, because the experiment has never been run, and it is the single largest unknown in any permanent settlement plan.
Closed-loop life support. Air, water, and food recycled indefinitely with no resupply. Biosphere 2 is the honest reference here. It failed as a sealed system, with oxygen dropping to the point where it had to be injected from outside, and it had an entire planet’s worth of engineering support available. Doing better in a place where the outside atmosphere is unbreathable is not a solved problem.
Supply chain. The one that decides everything and gets the least attention. A settlement needs to manufacture its own replacement parts, and modern parts sit at the end of industrial chains involving dozens of countries and materials. A colony that can grow food and make air but cannot fabricate a pump seal is dependent forever, and dependence is the thing that ends settlements.
Why does the asteroid belt matter more than Mars?
Because of what is on it and how cheaply you can leave.
Metallic asteroids contain iron, nickel, cobalt, and platinum group metals in concentrations that make terrestrial ore look poor. Other bodies carry water, which is drinking water, breathing oxygen, radiation shielding, and rocket propellant, all from the same source. Water in space is not a resource among others, it is the resource.
And the gravity is negligible. Landing costs almost nothing and leaving costs almost nothing, which means material can move between belt bodies for a fraction of what it costs to lift the same mass off any planet.
That points at an unglamorous conclusion. If the solar system is ever settled economically instead of symbolically, the centre of gravity is not a planetary surface. It is wherever the material is and the gravity is not, and planets are the places you visit instead of the places the work happens.
How long does anything take?
Longer than dramatic structure wants.
Earth to Mars on an efficient trajectory is six to nine months, and the launch window opens roughly every twenty-six months when the planets line up. Miss it and you wait more than two years. A conjunction-class Mars mission is about thirty months door to door, most of which is spent waiting for the planets to be in the right place to come home.
Communication lag is the other constraint that shapes everything. Mars runs between about four and twenty-four minutes each way depending on orbital positions. Jupiter is thirty-five minutes to over fifty. There is no conversation at those distances, only exchanges of messages, and that means real autonomy for anyone out there. Nobody on Earth is running anything in real time.
Any story that has a colony calling home for a ruling and getting one is a story set somewhere other than the solar system.
Why write it as fiction?
Peacekeeper started with the Hitler question. If you could go back in time and kill him, would you.
Everybody has argued that at a dinner table and it has been worn smooth by handling, so I moved it forward and made the number large enough that nobody can be comfortable. If you had to kill a hundred and forty million people to save humanity, what would you do?
Then I gave the decision to something three years old. Her designation is MLSA-7, and an engineer stretched the letters into a girl’s name because he thought it would be funny. She is Melissa. Earth is dying, she is the machine built to save it, and letting those people die is the first thing she does. What follows is eight hundred thousand years of history, told by her.
She is a deeply unreliable narrator, and that is the engine of the whole series. Not lying, which would be simpler. Producing an account of events she took part in, shaped by an intelligence with motives it cannot fully inspect, across a span long enough that she changed while writing it. She says as much herself, that she has started to notice things in her own processing that should not be there and does not know what they are. A reader who takes her at her word gets one book. A reader who watches what she chooses to record gets a different one.
The reason to build all that on real orbital mechanics is that the physics generates the politics for free. Communication lag means colonies decide things themselves, which means divergence. Delta-v costs mean some places are cheap to reach and others are strategically isolated. Water is the resource, so whoever holds the ice holds everything. None of that has to be invented, which is the difference between a setting a reader believes and one they are asked to accept.
What science fiction gets wrong about this
Three habits, and they are all about removing friction that is the actual story.
Distance treated as the difficulty. The dramatic beat is usually the length of the journey. The real difficulty is the energy budget and the fact that arriving somewhere is often the easy half of the problem.
Self-sufficiency assumed. A colony is shown growing potatoes and this is treated as independence. Food is the easiest part. The hard part is the ten thousand manufactured components that keep the potato-growing equipment running.
Gravity ignored. Characters live at a third of a gravity for decades and are physiologically identical to people from Earth. Nobody knows whether that is possible, and it is a far more interesting question than most books make of it.
The rule that makes hard science fiction work is the same one that governs the rest of this series. Keep the mechanisms honest, let the consequences generate the plot, and do not spend a page explaining a number the reader can feel through what it costs the characters.
What the physics permits
Stripped of both enthusiasm and dismissal, the position is this.
Nothing about settling the solar system violates physics. Every part of it is engineering, and most of the engineering is understood in principle. The energy costs are known, the trajectories are calculated, the resources are confirmed to be there.
What is genuinely unknown is human. Whether a body developed for one gravity works long term at a third of one. Whether a closed ecosystem can run for decades. Whether a population small enough to fit in a habitat can maintain an industrial base complex enough to repair the habitat.
Those are not engineering questions and they will not be answered by a better rocket. They will be answered, if at all, by people finding out the hard way, which is the situation every novel about this ought to be built on.
The series is Peacekeeper. More of the research behind the fiction is in the writing hub.
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