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

Orbital direction definitely matters.

You’ll notice most rockets are launched as close to the equator as their country can and generally launched towards the East, this gives them a bit of a speed boost to help get into orbit but not much. Getting into orbit requires a speed of 8000 m/s but launching from the equator instead of the pole only saves you about 400 m/s.

To go from Earth orbit to a further out planet like Mars or Jupiter you need to add more energy to your spacecraft. Since your orbit is now referenced to the sun then burning the engine in the same direction of Earth’s orbit gives you a 40 km/s head start. If you tried to get to Mars by burning the opposite direction you first need to slow down 40 km/s, then speed back up by 40 km/s, ***then*** add the energy to get you out to Mars’s orbit. We don’t have a rocket that can do that so we always go the same direction as Earth