How do direction work in space because north,east,west and south are bonded to earth? How does a spacecraft guide itself in the unending space?
In: Physics
The best part of this question is the number of folks is the aerospace and astronautical field that are willing to chime in.
Thanks to all of you, I learnt more than I expected to. Much appreciated.
As an add-on to D1Foley’s comment, check out Quill18’s “Kerbal Space Program for Complete Beginners” series on youtube. He covers this stuff and does a preeeeeetty good job of it.
Play [Kerbal Space Program](https://www.kerbalspaceprogram.com/). Here is a helpful xkcd to help understand why it will help.
[https://xkcd.com/1356/](https://xkcd.com/1356/)
Also why you won’t be ready for that NASA position.
[https://xkcd.com/1244/](https://xkcd.com/1244/)
Astronautical engineer here.
Spacecraft are equipped with a subsystem called Attitude Determination and Control System.
This subsystem can contain various tools including Star Trackers, Horizon Sensors, and Sun Sensors for navigation.
There are lots of stars in space, and a lot of them are so far away that they appear fixed, i.e. they do not seem to move.
A star tracker is basically a camera that scans the space for star patterns. Then it compares the image with the database to estimate its orientation.
Sun sensors find the Sun (obviously) and are generally used for solar panel pointing etc. Horizon sensors use infrared to find orientation based on the planet’s horizon line.
This is the navigation part. For control, there are reaction wheels, magnetorquers, reaction control thrusters, and more. RWs spin to generate a moment in the desired axis, so there are mostly 3 of them. Magnetorquers use magnetic field of the planet to change orientation. RTCs are small thrusters that are placed on large spacecraft to perform small correction/orientation maneuvers.
Earth based directions (North/South/East/West/Up/Down) don’t work, so we create a new “frame of reference”.
A frame of reference is a way of looking at and measuring things. Walking around your neighborhood, you use N/S/E/W, but if you were walking on a huge cruise ship sailing through the ocean, you would use Fore/Aft/Port/Starboard, no matter which direction the boat was pointed. We would say we are moving towards the port side, even if the boat is moving west, so Pot is actually south. We would say we’re walking towards the Port side at 1.6 km/hour (1 miles/hour), even if the boat is moving forward through the ocean at 32 km/hour (20 mile/hour).
In the same way, we can create different frames of reference for outer space. One frame of reference when you are orbiting close to earth, another when you are far from earth and orbiting the Sun, another when getting close to the moon / Mars, etc…
A great and fun way to experience this is to play Kerbal Space Program.
I actually work in the space industry, so I feel qualified to answer this. As other commenters have alluded to, there are two parts to this question: reference frame and navigation.
In science and engineering, when describing motion you need a base coordinate frame. To start, you need a fixed reference point and direction to base the coordinate frame on. The typical reference is the vernal equinox, which is an imaginary line pointing towards a distant star called Vega. For our purposes, the position of Vega is fixed, so it makes a good reference. From there we can build our axes, but this will depend on the physics involved.
For a low-earth orbit spacecraft we use the Earth-Centered Inertial frame (ECI), which has an origin at the center of the earth, x axis pointed towards vernal equinox, z-axis pointed through the north pole, and y axis perpendicular to both x and z.
A base reference frame should be “inertial,” or non-rotating and non-accelerating, in order to make the physics work out. For an interplanetary spacecraft, the ECI frame is NOT inertial, because it is fixed on the earth which is accelerating around the sun. In this case we define a different frame: sun-centered. In this case the origin is at the center of the sun, X-axis pointed towards vernal equinox, z axis perpendicular to the ecliptic (plane that Earth’s orbit makes around the sun), and y axis perpendicular to X and Z.
Now, for navigation: we use devices called Inertial Measuring Units, or IMUs, to constantly measure acceleration and rotation. Think of them as fancy accelerometers and gyroscopes like you have in your phone. If we know where we start, and we keep track of all the accelerations, we can figure out where we end up. The previously described reference frames give us the language to describe this (in terms of X, Y, and Z coordinates). We can improve knowledge of our position with dead reckoning, where we CHECK our distance and speed with radar measurements. If we send a signal to a spacecraft and it takes 20 minutes for that signal to get back to us, then by knowing the speed of light we can say exactly how far it has travelled, which makes the estimate we got from the IMU more accurate.
EDIT: I think forget what I said about Vega. The X axis is defined by the mean vernal equinox, which is when the southern and Northern hemispheres receive the same amount of light (around March 21st). At this point, you can draw a straight line from the sun though the center of the earth and that line will intersect Earth’s equator. Because of this, it is by definition perpendicular to the north pole.
Aerospace engineer here!
The short answer is basically however you want it to!
The long answer is something called frames of reference.
A frame of reference, or reference frame, is how you determine your position and orientation relative to another object. On Earth we tend to use down as the direction earth is pulling us, up as the opposite and then north/south/east/west for planar (side to side, forward-back) directions. In space however, there is no absolute frame of reference.
You could be x miles from the earth and y miles from something else. (This also effects velocity but we won’t go into that unless someone asks).
So which reference frame do you use? Whichever one works best. Some times the math is easier if you use earth as a reference frame, sometimes it’s easier if you use the sun.
Spacecraft are able to determine their position and orientation through a combination of on board sensors (like star sensors) and off board trackers (like radar). Beyond that, it is typical to describe their position and velocity as an orbit. These orbits can be described using a few variables that indicate the size, orientation, and direction of the orbit. These are called “Keplerian Elements.”
So, for example if you wanted to convey information about a satellite above the Earth, you wouldn’t say “It’s 500Km above the ground, moving 7km/s in the Northwest direction” but you could say, “The satellite’s orbit has a semimajor axis of 6800km, with an eccentricity of .01, inclination of 23 degrees…”
Of course, there are other ways of keeping track of and describing these, but that’s one of the most basic ways.
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