Let’s imagine we’re in a rocket ship in a circular orbit, and we want to go back to earth. You might think you need to point towards the ground and turn your engine on, but remember how we got up here–we’re in orbit because we’re going sideways very fast. The most efficient way to come down is to point backward along our orbit, to lower the height at the opposite side.

What happens if we do point straight downward? Well, we would start going downward, but because we aren’t pointing straight backward, we aren’t actually reducing our speed, only changing the direction of the orbit. It would take much more energy to come back to earth this way, and because we aren’t actually reducing our speed, it would be much more dangerous, because we would be entering the atmosphere faster than if we had pointed backwards instead.

In a worst case scenario, we would run out of fuel before re-entering the atmosphere. This is very bad, because as we fall towards the earth, we start moving faster. Remember how moving faster at the lowest point in an orbit increases the height of the highest point? If we don’t hit the atmosphere, the top of our orbit will end up even higher than it was before!