When explorers started going around the Earth, they used the stars to find their way around. While the oceans and plains stretched out indefinitely, the stars were dependable guides. But when we travel into deep space, what will we use to find our way?
What will be the lighthouse that our future starships use to navigate?
One of the biggest challenges in space travel is the fact that, after people have traveled, they presumably want to come back again. Both Russia and the United States briefly entertained visions of winning the space race by dropping astronauts off on the Moon and having them bide their time there until their respective homelands could find a way to get them back. Similarly, some people have floated plans to colonize the Moon and Mars by shipping people one way. At least, though, the Moon and Mars are often in visual range of Earth. If people need to get back they can at least point the way. There's no such luck for eventual travelers in deep space. When the universe is spread out in three dimensions around us, and each part looks just about like every other part, how will future starships find their way?
The X Factors
The Milky Way galaxy is one hundred thousand light years across and a thousand light years thick. It has two hundred billion stars, some within its plane, and some around it. Outside the Milky Way are another, roughly, 8500 galaxies observable from Earth, but in a single image from the Hubble Ultra Deep Field, astronomers think that there are another 10,000. This adds up to hundreds of billions of galaxies in the Universe, meaning that it's hard to find consistent observable landmarks. In space, no one can remember where they parked.
Even if a space ship managed to leave a trail of breadcrumb galaxies on its way to wherever it was going, it's not necessarily going to get home again. Breadcrumbs don't move. Galaxies do. And so does home. Space agencies are used to moving targets, but as starships venture out farther, they have to navigate based on more and more moving points. One unexpected shift in gravity affecting a galaxy that was once a marker, and no one gets home again.
And while some people pin their hopes on wormholes, they'd have to be pretty specific. Put a spaceship down even slightly off course and the entire lay of the universe looks different. The parallax effect, the fact that objects in the foreground will shift compared to their background when looked at from a different position — similar to the way near objects jump back and forth when you look at them through first one eye and then the other — would make billions of stars shift in relations to billions of other stars.
Building on the Current Network
Since we already have a lot of stuff whizzing around space, there are existing navigational and control systems. Currently there is an International Deep Space Network, with three massive antennas placed on three different places on Earth, each roughly one hundred and twenty degrees from each other, checking position on various space craft. The antennas are in the Mojave desert, in the United States, just outside of Madrid in Spain, and outside of Canberra, Australia.
There are European, Indian, and Chinese Deep Space Networks as well, and they all take advantage of one of the few easy things about space: it's easy to make signals omnidirectional. Three stations on Earth are all that you need — get thirty thousand kilometers away from Earth, and you're always in view of an antenna. Place an antenna in space, and let it send out radio signals in all directions, and you've got a beacon that shines everywhere.
Of course, as explorers get farther and farther out they'd need a longer and longer chain of beacons sending out signals that can lead them home. And assuming that each of these beacons is dependent on signals from the last to keep from straying off course, then if there's even one break in the chain, the entire system could go down. If one antenna on Earth went down, we might lose one third of the starships out there.
Even if everything works perfectly, there's an error of four kilometers for every astronomical unit traveled from Earth. An astronomical unit is the distance from the Earth to the sun — a tiny unit in the grand scheme of things. Although four kilometers is even tinier, a chain of mistakes could add up. This has caused some people to look for more natural landmarks, that will continue under their own power.
Pulsars as Natural Signals
Pulsars are stars that have collapsed in on themselves in a specific way, which could turn out to be very handy.
The Earth's electromagnetic field is oriented from pole to pole. The Earth also spins around an axis that goes from pole to pole. Although the two are not exactly lined up, the rotation of the Earth doesn't involve a dramatic rotation of the Earth's electromagnetic field. It's like spinning a cylindrical bar magnet around its central axis. As it spun, any magnets around it wouldn't feel a huge change in the magnetic pull on them.
Pulsars, however, are stars whose magnetic poles and axes of rotation don't match up at all. They are more like a cylindrical bar magnet being twirled like a baton. Any other magnets around a pulsar would feel a strong variation in its pull as it was spun around. This causes pulsars to emit strong, regular beams of radiation.
Some pulsars measure their rotation in milliseconds, and with the accuracy of atomic clocks. If a pulsar's pulse sweeps past Earth at a specific time, and then sweeps past a space craft at another, it's possible to determine where the two are in relation to each other. This is a plan that has been proposed as a back-up system for space craft going to Mars, but there are a lot of pulsars out there, and their exact periods could help interstellar travelers figure out their position compared to other known natural objects. It's true that eventually even the best pulsars will wind down, but they're far more shock-proof than the average space craft carrying a human made beacon.
Interstellar travel will never be easy, but getting a good map would be a start.