When launching a deep-space craft, how do the rotation and orbit speeds of the Earth and direction (relative to the prior) impact the travel time and actual velocity of the space craft?

Question

To simplify it for my mind, I envision the Earth being a car, and the space craft being something like a bullet. If the car (Earth) is orbiting at 30,000 m/s, and the bullet (spacecraft) is traveling at 17,000 m/s (Voyager 1 speed), the bullet is then traveling at 47km/s, but only at 17km/s relative to the car. But, if the car’s direction changes and goes the opposite way at the same speed (as with orbit), it is actually then going 77km/s. This is confusing me 😩.

In: Physics

Answer 1

> But, if the car’s direction changes and goes the opposite way at the same speed (as with orbit), it is actually then going 77km/s.

Right. If you jump off the front of the car, then the car is hurtling toward you, so the gap between you and the car is smaller than it would be if the car were parked. If you jump off the back of the car, the car is hurtling away from you, so the gap between you and the car is larger than it would be if the car were parked. Since speed is distance divided by time and the *moving* car is your zero-point, your speed from the perspective of the car (that is, from the perspective that the car is actually perfectly still and it’s **the universe** that’s hurtling past at 30 km/s) will be faster when you jump off the rear of the car than when you jump off the front.