Cecil said something that confused me. He said that you need an increased velocity to have a higher orbit.
I may be confused, but I thought as you increased the orbital radius it would require a DECREASE in velocity of the orbiting object to remain in orbit.
This is due to the balancing of the centripedal force vs. the gravitational force. Centripedal is a MV^2 / R function. Gravitational force (weight) is a M1 M2 * G / (R^2) function. Basically the higher weights closer to the earth must be balanced with an INCREASING velocity. Not a decreasing one.
If an object is in orbit and you increased it’s velocity, I would think it would hit an “escape velocity” and no longer be in orbit.
If you decreased the velocity but kept the same altitude, then it would crash to the earth.
If you increased the distance to the earth, and then decreased the velocity, then you could have a higher orbit.
Credit where it’s due: I’m the guy who confused you, not Cecil. And I can see how you’d be confused: If you have two circular orbits, one further away than the other, then the one further away would, indeed, be slower. But, if you’re in a circular orbit and speed up, you’re not in a circular orbit anymore: You’re in an elliptical orbit. Or, if you speed up enough, to escape speed or beyond, you’re in a parabolic or hyperbolic orbit which will never return to Earth. But we don’t need to speed up quite that much.
As a concrete example, let’s consider the Shuttle, in low Earth orbit (LEO), launching a satellite into geosynchronous orbit (GEO), which is much higher. The satellite starts off going at LEO speed, along with everything else on board the Shuttle. Then, a booster on the satellite increases its orbital speed. The satellite is now in an elliptical orbit, with perigee (closest approach) at LEO height, and apogee (furthest approach) at GEO height. This is called a transfer orbit, since it’s used to transfer from one orbit to another. As it’s climbing up this orbit, it’s getting slower, since its kinetic energy is being converted to potential energy (equivalently, you can say it’s slowing down because gravity is pulling it back). By the time it gets up to GEO height, at its apogee, it’s actually going slower than GEO speed, so it can’t stay up that high without help. If nothing were changed, it would then fall back down from GEO height, back to LEO height, and the orbit would repeat. But that’s no what we want, so when it hits GEO height, we fire the thrusters again, to speed up the satellite again, and now it’s in a circular GEO orbit. So we fired the thrusters twice, and each time, we sped up the satellite, but it slowed down even more, in between, from gravity.
Perhaps a better way to think about it is energy. An orbit with a larger average radius has more energy than a lower one. You have to add energy to your rocket to get it higher, right? You’d think that makes you go faster, but in reality a higher orbit has a slower orbital velocity, for the reasons Chronos pointed out.
To drop into a lower orbit, you have to lower your energy, but paradoxically that speeds you up, because a lower orbit means a faster orbital velocity.
OK, so maybe thinking about it in terms of energy doesn’t help.
Yes, that makes more sense now. I can see what I was missing. You have to go faster in order to increase your radius (and potential energy), but once there, you can go slower.
What is your current speed sitting in front of your computer? The surface of the earth at the equator is rotating at about 1,000 MPH. The earth is orbiting the sun at some speed, the sun is moving as part of the Milky Way, and the Milky Way is moving away from the center of the universe. Speed depends on a point of reference. My question is, what is the point of reference for an object in orbit. Is it the surface of the earth, the center of mass of the earth, etc.
Oh, I see. The frame of reference for an orbiting body is the non-rotating frame where the center of mass of the system is at rest. That whole system may in turn be orbiting around yet another body, such as the Earth and its various satellites orbiting the Sun, but you don’t worry about that.
Just a quick question…the column explained how just throwing a baseball “down” towards the Earth would just cause it to oscillate back into a higher orbit. But surely there’s a speed the baseball could reach to overcome this? I mean, if the baseball was somehow thrown from orbit towards the Earth at a speed of 90% of C, it would hit the Earth’s atmosphere before it had a chance to swing back into a higher orbit…wouldn’t it? Or not?
Something like that. Vector sum the velocities. If the added velocity is large enough, it swamps out the starting velocity and the resultant is into the Earth’s surface. But that’s a very large velocity.
Ok. I’m just a lowly accountant, and not necessarily the most adept at understanding orbital dynamics. So, will somebody please explain, in plain English, this whole “oscillating” thing.
How is it possible to throw something towards a large body in space and not have gravity take over and pull it ever closer? I can understand that the velocity would produce a more eliptical orbit, but in the vacuum of space (and with gravity) it should still continue downwards towards the Earth.
How can a baseball possibly increase (get higher in) its orbit when it’s going towards the Earth. Since when does gravity push away an object?
Let’s see if I can answer two questions at once. If you’re in the shuttle, in a nice circular orbit 150 miles above the surface, and you throw a baseball as hard as you can straight towards the earth (from your perspective), what would happen? If the atmosphere starts 50 miles up, then the baseball needs to go 100 miles away from the shuttle to hit it. But the shuttle orbits the Earth in a little more than an hour, so in about 20 minutes, it’s made a quarter turn. Let’s say you throw the baseball 100 mph. After 20 minutes, it will not have reached the atmosphere, but will have “missed” the Earth, now going sideways to it instead of straight at it. Since it’s now lower than the shuttle and going too fast for that orbit, its speed will carry it out to a point farther away from the shuttle. It will be in an elliptical orbit.
From the shuttle’s point of view, that elliptical orbit looks like the ball is alternately going lower, then higher, then lower again. I think in this case, the orbit period would then be slightly different, so it would drift farther away too, either forward or behind the shuttle. I think it would be behind, but I’m no rocket scientist.
Gravity does not cause things to move towards the Earth, it causes them to accelerate towards the Earth. If I’m standing here on the surface, and I throw a baseball straight up, it’ll keep going up for a while before turning around and coming back down. When the ball is moving up, it’s slowing down, and then when it’s moving down, it’s speeding up.
It’s similar for a baseball thrown from orbit. It starts off going down, and while it’s doing so, it’s getting faster and faster. But due to the orbit, as CurtC explained, at some point, it’s not going down any more, but sideways. At this point, it’s just as if it were thrown sideways with that speed. And that speed is great enough that it’s able to move away from the Earth, again, for a while.
I have a question of my own to add to this. Do objects in higher orbits travel slower because the effect of gravity is not as strong so it does not need to travel as fast? I am sure it is a simple question but I want to know if it is true before I spout it off to make myself seem smarter.
