Changing Orbit

The orbits we discussed in the previous Unit can be explained by Newton's laws of motion as described above. Consider a spacecraft in a circular LEO (low Earth orbit).

As you recall, this orbit is possible because of the force of gravity as a result of the presence of the Earth's mass. If the force of gravity was suddenly removed at any time during the orbit, the spacecraft would continue in the direction tangent to the orbit.

The Earth is not going to suddenly disappear in order to create this effect. However, the spacecraft can use its jets to counteract the force of gravity. In this way, the end effect is the same as if the Earth has disappeared.

A spacecraft that is capable of going to another planet has to be big enough for a sizable crew (somewhere between 4 and 8 people). This means that the ship must be large enough to contain medical facilities, and enough food, water, and fuel for the round tripjourney. Most likely such a trip will result in a human landing on a planet, which means there have to be rockets for landing and taking off again plus rover equipment, a surface habitat, and scientific equipment. As you can see, any journey to another planet is a vast undertaking!

All of these requirements mean that the ship will have to be assembled in-orbit around the Earth because we do not have rockets large enough to launch a completed spacecraft capable of carrying and supporting humans to far-off reaches.

NASA has studied a number of options, and it turns out that the most practical is probably to launch all the parts of the spacecraft into Low Earth Orbit and assemble them into a complete spaceship there. Once the spacecraft is complete, the entire spacecraft can be transferred into an elliptical orbit by a separate disposable rocket

In the next unit we'll look at the kind of orbit we might use to get to another planet - whether a "cheapest" option or a "faster" option. We will also consider how to plan it so you arrive at the planet's orbt at the same place where the planet is - and not when it is on the opposite side of its orbit!

This ends Unit 5. Review, take the end-of-unit quiz, and then launch yourself into Unit 6.