How does a spacecraft change course?
In order to know where a ship is, NASA needs to know two things: how far it is from Earth and its location in space. Generally, NASA uses the downlink, or radio signal from a spacecraft to a radio telescope in the DSN, to tell where it is.
The distance between Earth and the ship is measured by sending up a radio signal from Earth with a time code on it. The spacecraft "bounces" back the signal, and people on the ground can see how long it took to travel from Earth to the ship and back. Since all radio waves travel at the speed of light, scientists can look at how long it took for the signal to make it to the ship and back and figure out the distance it traveled. The angle that the radiotelescope is pointing when it receives the signal tells the direction of the ship.
A more precise way of measuring uses two radio telescopes. When a ship is in space, it sends a signal back to Earth. Three times a day, this signal can be received by two different DSN radio telescopes at once. They can compare how far the ship is from each signal. They then get the distance to a known object in space that doesn't change its location, like a pulsar, (pulsing star), and from the three locations, (two telescopes and a pulsar) they can use a technique called triangulation to get the ship's location.
Some spacecraft, like DS1, can use asteroids and other objects in space to figure out where they are. Using a process called Optical Navigation or OpNav, pictures are taken of particular asteroids. The asteroids' location relative to the spacecraft are used to determine position, and the position is compared to where the ship should be. At that point the ship can do a course correction. OpNav needs at least three objects to compare and uses triangulation to figure out a ship's location.
What is a course correction?
What is AutoNav?
What is triangulation?
What are uplink and downlink?
What is DSN?
What are radio waves?
How does DS1 do a course correction?
When does DS1 do a course correction?
How can we tell a spacecraft's speed?
What are vectors?