Going back to fundamental basics, I felt I would explain a bit about orbital mechanics. We all know that if you toss a ball weakly to someone else it doesn’t go far. If you throw it harder, it goes a little further. The faster you throw it, the further it goes before falling. Rockets are thrown so hard that they reach what’s called orbital velocity. At these speeds, if Earth was flat, the rockets would fall to the ground, but since Earth is round, the rocket keeps missing the earth. This goes for all celestial objects, but orbital velocity is relative to mass. For example, something orbiting earth requires about 17,500 mph or 5 miles/sec to stay up. To orbit the moon, you need about 3,000 mph, or .8 miles/sec. For Mars, it’s 11,000 mph or 3 miles/sec.

The reason rockets go straight up at first is because the atmosphere is thick and doesn’t like it when things go too fast. So the rockets are launched straight up for a little while until the air thins a bit, and progressively lean over to gain speed. Once they get above about 60 or 65 miles in altitude, they are technically in space, but the atmosphere doesn’t know that. You’re above the majority (close to 98-99%) of the air particles up there, and the few that reside there smash into the rocket and over time will bring everything back to earth. The traces of atmosphere can reach out to about 400-500 miles high. This is why some spacecraft (like the International Space Station) under that height have to reboost their orbit every so often.

When you’re in a stable low earth orbit, gravity is still about 88% of what it is on Earth (If you doubt my numbers; http://ksnn.larc.nasa.gov/webtext.cfm?unit=float). You don’t feel it because you’re in freefall. You do however, experience migrogravity, which is caused by the mass of the spacecraft you’re surrounded by, or near to. You don’t necessarily feel this effect, but anything with mass has gravity.

To change orbital altitude, as I mentioned earlier, you have to either speed up or slow down. As you orbit and change speed, if you want to make it circular, you have to fire the thrusters or engines again about 45 minutes later (it will never be a perfect circle due to gravity variations across the surface of any body). If you’re in a given orbit and release or launch something from your spacecraft (like a tool bag, for example) the energy imparted on the object changes its orbit so it doesn’t fly along side the craft. It will stay roughly the same shaped orbit as the craft–if you discount the drag of the atmosphere– but the shape of the orbit of the secondary object is just turned a bit and could come back later in the orbit and run into the spacecraft, unless you release the object directly in front of or behind the craft. If you have a baseball and you want to send it down to earth (discounting the effects of drag in orbit again), you’ll want to throw or launch it directly behind you relative to the direction of travel. To raise the apogee (the high point of orbit) you must throw the ball faster.

This is all there is to it for physics near planets and moons. It gets more complicated when traveling between objects, because while it may be a shorter distance to go in a straight line, it’s also less efficient. Escape velocity is when that ball you threw into orbit actually goes faster than earths gravity can pull it down.

I think I’ll end with this, because I would rather not get to the point of explaining in detail why we can’t do something like launching nuclear waste into the sun, but to touch briefly on it, if we were aiming a rocket full of radioactive material at the sun, we would need to slow down by about 55,000 or 60,000 mph. We orbit the sun at 70,000 mph. It takes a lot of energy to launch anything towards the sun.

-Jeph

This article was updated on 03/04/2010