The Solar Orbit
In the case of the solar orbit it is the Sun’s gravity that is pulling on the satellite and making it go around and around the Sun. Of course, all the orbits are also solar orbits, because the spacecraft that is orbiting the Earth or even sitting on the Earth before it is launched, is already in a solar orbit – along with the Earth and everything on it, or going around it.
So when we want to place a spacecraft into a solar orbit, what we really want to do is place it in a solar orbit that is different from the one it is already in – namely, the Earth’s orbit. Usually we want to do this to make the spacecraft travel through the solar system to another planet, such as Mars or Venus, but sometimes (as in the case of Pioneer spacecraft) we just want to find out what is out there in interplanetary space.
The principles involved are just the same that are involved when satellites change from one orbit to another. We are already in one solar orbit (the Earth’s) and we simply want to change to another. If we want to change to a lower orbit, which means one nearer the Sun, we have to slow the spacecraft down from the Earth’s orbital speed. We do this by firing a rocket engine in the opposite direction to the Earth’s motion around the Sun. This slows down the spacecraft from the Earth’s speed to a lower speed, and it drops to a lower orbit, inside the Earth’s. It is still going in the same direction, remember, but closer to the Sun. If we had a powerful enough rocket to cancel out all of the Earth’s speed, our spacecraft would be stopped still and would simply fall right into the Sun.
If we want to go into an orbit that is outside the Earth’s orbit – which is what we have to do to go to Mars, for example, we fire the rocket engine in the same direction that the Earth is going. This gives it a push (the rocket fires for only a few minutes) ahead of the Earth, and it costs out of the higher (father from the Sun) orbit.
In either case, whether we want to go to a lower solar orbit or a higher one, the spacecraft has to escape from the Earth’s gravitational force. And to do this, it must reach escape speed, which is a little over 25,000 per hour. Of course, the farther out (higher) or in (lower) we want the spacecraft to go, the more we have to speed it up beyond this escape speed.
And remember, just as with Earth orbits, even though you slow a spacecraft down from Earth’s orbital speed around the Sun in order to make the spacecraft fall into a smaller solar orbit, by the time it falls to that orbit it is going faster than when it left the Earth. And even though you speed a spacecraft ahead of the Earth in order to push it into a larger solar orbit, by the time it gets there it is going more slowly than before. That is why satellites that are launched “behind” the Earth into inward path eventually overtake and pass the Earth, and satellites that are launched ahead of the Earth into outward paths eventually fall behind.
Neptune and Its Great Dark Spot