how do scientists know if something is traveling towards Earth if it’s hundreds of years away?

Question

For example, like if an asteroid was projected to pass Earth but not until 20 years from now, how do we calculate this? How is that even possible to know?

In: 25

Answer 1

Since there’s not air to slow things down in space, they kind of just keep moving the same way (an object in motion stays in motion…). The only significant force acting on objects in deep space is gravity, so if you know where something is and what speed/direction its traveling, you can predict where it will be in the future pretty accurately.