Navigation is all about knowing where you are, and knowing where you want to be. On earth this is usually pretty straightforward – we have detailed maps of every corner of our fair planet, and four handy directions to tell us how to get from A to B – north, south, east and west. ‘North’ is towards the Earth’s magnetic north pole, and ‘south’ is in the opposite direction, towards the Earth’s south pole.
These north-south directions aren’t going to be mean much when you’re in space, away from Earth. That said, astronauts rarely have to stop by and ask the Martians for directions home. Spacecraft and satellites are constantly exchanging signals with Earth. This back and forth signalling allows their position and velocity to be calculated very accurately – it’s like an interplanetary GPS. Astronauts also have very accurate star maps of much of the Milky Way, meaning that if they lose contact with Earth they can locate themselves using their relative positioning to known stars.
And instead of using north, south, east and west to describe the direction a spaceship is heading, astronauts who are in orbit cane use different terms: prograde, retrograde, normal, antinormal, nadir and zenith. This diagram helps to explain:
The red arrows in the picture above point along the direction of orbit, called prograde (orbit direction) and retrograde (the opposite). The blue arrows are perpendicular and called normal and antinormal, and the green arrows point towards the centre of the planet. Towards the planet is called nadir, and away from the planet is zenith.
However, if you’re a bit further out (and by a bit further out we mean several thousand light years further out) and well away from orbiting a planet, then things start to get fuzzier as you can’t use these rules. At the outer edges of the Milky Way, and beyond, there is a lot more uncertainty about the exact location of stars, making navigation difficult.
The solution for deep space navigation comes in the form of pulsars – astronomical lighthouses for travellers. A pulsar is the collapsed remnants of an exploded star that emits a powerful beam of radiation while spinning at an incredibly high speed. This spinning means pulsars effectively flash just like a deep space lighthouse. And because individual flash frequencies and locations of many pulsars are well known, even in deep space, they are perfect reference points for inter-galactic navigation.
So if you’re lost in space without a star map don’t panic, just follow a pulsar and it’ll guide you home…
On second thoughts, don’t get lost in space.
Answered by Nick Waszkowycz.
Image credit: Dr. Case on flickr