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
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.
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