Twenty light-years
These are the nearest stars, the first places any ship from the Sun would go. Within twenty light-years there are about eighty star systems and at least fifteen brown dwarfs, bodies too small to shine as stars, drifting on their own. Each one on the map stands on a stem that drops to the plane of the Galaxy, the flat disk in which the Milky Way’s stars circle: solid if the star lies above the plane, dashed if it lies below. The stems are there because a flat map lies. Tau Ceti, Epsilon Eridani and Epsilon Indi all sit eight to eleven light-years below the plane, while Alpha Centauri and Barnard’s Star lie almost in it.
What counts as a home
How full this map looks depends on what you are willing to call a destination, and the control at the top asks exactly that. The cautious answer is a star like the Sun, or one brighter: eighteen systems. Allow any ordinary star and the red dwarfs appear, the small dim stars that make up most of the Galaxy, and the count more than quadruples. Allow anything with mass, which is how a machine that needs only rock, ice and energy would see it, and the white dwarfs and brown dwarfs join.
Which answer is right is not an astronomical question. It turns on whether people will live in habitats they build, and whether they would choose to do so around a red dwarf. The first finding works through what each answer does to the pace of settlement.
The first wave
Press Launch. Every ship leaves the Sun at the same moment, and the systems light as each is reached.
It is natural to picture the neighborhood being settled star to star, each new settlement sending ships to the next. Fuel does not require that. For a ship that cruises at a given speed, the fuel it burns depends on that speed and not on how far it goes: a star twenty light-years away costs no more to reach than Alpha Centauri. It only takes longer. So the Sun needs no stepping stones for fuel. By our estimate it could send a ship to every system on this map before the first new settlement had grown enough to build one of its own.
The order of arrival is then simply the order of distance. How long it takes depends on how fast a fusion engine can drive a ship that must also stop at the far end, and that is not known. Our middle estimate is about three percent of light speed. At that pace the first ship reaches Alpha Centauri in about a century and a half, and the last reaches the edge of this map in under seven hundred years. If engines perform as their designers hoped, halve those times. If they barely work, triple them.
What limits the reach is not fuel. It is whether a machine can fly for centuries and still work when it arrives, and nobody has tested that.
If ships cannot fly that long
To reach the edge of this map from the Sun, a ship at three percent of light speed must still work after nearly seven hundred years in flight. The choice marked Ships last any flight, under Send the ships, sets the longest flight a ship survives. Choose a limit and the map shows what follows. The Sun’s ships stop at the dashed ring. Everything beyond it waits for a settlement to grow until it can send ships of its own, which we put at about two hundred years, though it could be half that or several times longer. Ships sent by the Sun are drawn in amber and ships sent by settlements in blue-green, and each system is reached by whichever route gets a ship there first. Select a system to see its route.
Choose Anything with mass, so that every system counts, and leave the speed at three percent. If ships last 400 years, the Sun reaches 22 of the 95 systems itself, settlements reach the rest in one or two further flights, and the map fills in about 1,200 years instead of 700. If they last 300 years, the Sun reaches eight, some systems lie six flights out, reaching 94 of the 95 takes more than 2,300 years, and one stays out of reach. Near 200 years the network comes apart. At 210 years 65 systems can still be reached, the farthest of them through seventeen flights and almost ten thousand years, long enough for the stars themselves to have moved. At 200 years ships reach nine.
The answer also depends on what counts as a home. Count only stars like the Sun and the stepping stones lie farther apart: a ship that lasts 300 years then reaches six of the eighteen.
So settlement may yet move from star to star here. That is what happens if ships wear out, and whether they do is not known. If failures strike at random, no flight is impossible, only unlikely: the Sun can still reach a distant star by sending many ships and accepting that most are lost, and a route through settlements becomes the cheaper way only when those losses grow heavy. If damage builds up with time, from radiation, dust and the slow decay of materials, there is a longest flight any ship can make, and beyond it settlements are the only way onward. The control shows this second case. We think most of the damage is of that kind, but that is a judgment, not a measurement.
The distances between these systems matter in another way, whatever the ships do. Every system on this map has a neighbor nearer to it than the Sun is, typically about four light-years away. Each system’s card names its nearest neighbor among the systems shown.
Nothing here stays put
Let the stars drift and the map comes apart. Every star has its own motion, and over a hundred thousand years the neighborhood replaces most of its members. Barnard’s Star swings past in under ten thousand years. Alpha Centauri keeps closing for another twenty-eight thousand. Ross 248, a red dwarf ten light-years away today, will pass at three light-years in about thirty-six thousand years and for a while be our nearest neighbor.
A first wave sent straight from the Sun would be over long before any of this mattered; in a few centuries the stars barely move. A wave that has to work its way outward through settlements takes thousands of years, and by then it does matter. Either way it changes how to think about the place. The Sun’s neighbors are not a fixed set of destinations. They are whoever happens to be passing, and settlements planted around them will, over tens of thousands of years, be carried off in every direction. On the scale of the whole Galaxy that scattering becomes the main story.
How this map was made
Star positions, distances and motions come from the AT-HYG catalogue, which combines measurements from the Gaia and Hipparcos space telescopes with older surveys. Brown dwarfs and the faintest red dwarfs come from the SIMPLE archive of published measurements. Stars that orbit each other share one marker. Alpha Centauri moves as its center of mass moves, using the 2021 orbit solution by Akeson and colleagues.
The clock carries each star forward in a straight line at its measured velocity, which holds well over a hundred thousand years. Seventeen objects, most of them brown dwarfs, have no measured speed along our line of sight; their paths use sideways motion only, and their cards say so. Stars now outside the map enter it only if their full velocity is known, so a few faint arrivals are certainly missing. The brown dwarf census is incomplete, and surveys are still finding them.
The first wave follows each ship in a straight line, at the chosen speed, to the point where its target will be when the ship arrives. When a limit is set on how long a ship survives, a settlement can launch 205 years after its own ship arrived, the central estimate of our analysis of seed ships, and each system is given the route that reaches it soonest. The stars go on drifting while the ships fly, so a star out of reach at first can come within reach later, and the ships are followed for ten thousand years. The model leaves out the time spent speeding up and slowing down, assumes every ship and every settlement succeeds, and lets only the systems shown serve as stepping stones.