So one of our readers asked us this question the other day: When rockets launch they must have tonnes of momentum from the earth spin and orbit. Is this used to their advantage when plotting a course?

ANSWER

Short answer: Yes. By launching near the equator rather than the poles, you can get up to a 1,000 mph boost just from the earth’s rotation. Additionally, you should launch to the east to get this benefit.

Long answer: In low earth orbit you need to be going 17,000 mph – it doesn’t matter which direction. But at the equator the earth is rotating 1,000 mph counterclockwise, so if you launch east into a counterclockwise orbit you only need to gain 16,000 mph. If you launch to the west into a clockwise orbit you need to gain 18,000 mph.

By launching west, you’re basically starting out with a 1,000 mph deficit that you have to overcome, like running the wrong way on a moving floor.

In the US, this is why we launch missions from Florida – because it’s pretty far south, and if you’re launching to the east you’re over ocean and not populated areas.

On the other hand, Israel, doesn’t have this advantage. They have to launch their Shavit rockets (Hebrew for ‘comet’) to the west so that they pass over the Mediterranean, rather than potentially hostile neighbor states. This puts their satellites on “retrograde orbits,” meaning they orbit the earth opposite the direction of pretty much every other man-made satellite. While that may seem cool it comes at the expense of more fuel, limiting them to payloads 30% smaller than if they were launched eastward.

ISS

To add to this, when it was first proposed, the ISS was originally sold as a station that could be built up as a sort of staging station for Moon or Mars missions, among other things. However, in order to make it international (and get something like 20% of the funding from the Russians) we had to place the ISS in an inclined orbit that made it useless for this task.

It needed to have the inclination, because Russian rockets launched from Russia, and they lacked the power to reach an equatorial orbit after starting out at such a high latitude. That deficit being the energy lost from not being at the equator to start with, and from having to correct their inclination once in orbit. So the ISS is where it is right now precisely because the Earth gives you momentum, but the Russians couldn’t take advantage of that.

The above answers the earth’s rotation benefit. Adding to that concerning your earth’s orbit benefit question, the answer is yes also. When we launch a mission to someplace like another planet, the satellite also has the initial prograde orbit velocity the earth has while orbiting the sun. We accelerate the satellite from it’s earth’s orbit in a direction close to the same prograde direction as the earth’s orbit around the sun, requiring less fuel for it to reach escape velocity from the earth, and also causing the satellite to start increasing it’s orbital distance from the sun. This is the usual initial method used for missions to the outer planets (Mars and beyond). For missions to Venus or Mercury which are in orbits closer to the sun, we would make the acceleration retrograde the the earth’s orbit around the sun. We need to slow down the satellite’s orbital speed around the sun so it will “fall” into a lower orbit around the sun to intercept the Venus or Mercury orbits. Various combinations of the two methods can be used for missions that need to leave an orbit around the earth and escape earth’s gravity to go to other celestial destinations.

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Last Update: June 16, 2016

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