Airplanes aren’t orbit either and if the engines cut off, they will fall.
To reach orbit, you need to achieve a certain speed. That’s why so fast.
Minor nitpick:
Actually, MC, it is not necessary for an object to be heading “directly away from the Earth’s centre.” So long as an object has reached escape speed, the only requirement is that it not be heading in a direction that would intersect the surface of the Earth. The escape speed is calculated using energy terms. The direction is irrelevant.
satellites, spacestations, etc. have to fire engines periodically to keep them in orbit and to keep them from falling to earth.
i accept that. how do you explain this: i cant cite a source, but i have read that there are thousands of objects, including the ashes of timothy leary, orbiting the earth. these objects have no engines, why dont they fall?
when i say objects, most of them are parts off of spacecraft, manmade objects .
They do. Some just take longer than others.
why would it take t. leary years to fall to earth. (no jokes , please).
Why wouldn’t it? It depends on a lot of things, like height, speed. etc.
dp
It is all a matter of how much drag they have. The more drag, the more quickly they will slow down and the sooner they will fall back to Earth.
Drag can be caused by either shape or height. The more cross-sectional area you have perpendicular to your velocity, the more drag (think flat surface vs a cone shape). Also, the higher the less drag there is, and the longer it will take to slow down.
At several thousand miles above the Earth, there just isn’t a lot of air left to slow you down.
Because their orbit is high enough so they’re essentially above any of the earth’s atmosphere.
The higher in altitude you go, the thinner the earths atmosphere is. Even at the height that the ISS orbits - about 400 kilometers (250 miles), there’s still a very very tiny bit of air. Over time, the drag of the atmosphere slows the spaceraft and it gradually falls into a lower and lower orbit. IIRC, the ISS loses about 6 kilometers a month. So once in a while, they need to fire rocket engines to push it to a higher orbit. When a shuttle or unmanned cargo ship has docked to the station, they’ll give it a boost with some of the extra fuel reserves they’ve got left over before they undock and return to earth.
Now if you put a satellite into a higher orbit (say, 1000 miles high), it’s above a lot more of the atmosphere. Then it would take years before it slows enough to fall back to earth.
Eric
i understand the responses and thanks again for taking time to answer. when i think about something falling, i think of it in an “on earth” kind of way. when something starts falling it goes straight down and increases speed until it reaches its max. speed ( is terminal velocity the correct term here) and it doesnt stop until it hits the ground.
There are very few things which fall straight down. Very few. You’d have to throw something straight up to make it come straight back down. There aren’t a lot of things which go straight up.
A person skydiving won’t fall straight down, as the plane they jumped out of had some horizontal velocity to it. If you drew a line in the air of the path they took, it would look like a big arc. Basically that arc ends when the line intersects the Earth.
But picture if you will the arc missing the Earth, and going around the other side. And then from that point, it does it again. And so on and so forth. Eventually, that arc, due to slowing down by air friction, might make the object hit the Earth, but it could take hundreds or thousands of orbits to do that.
You’re catching on. The thing to keep in mind is that, in terms of physics, that while it’s ok to say that something has a “speed” of 10MPH, it’s incomplete to say it has a “velocity” of 10MPH. You have to specify a direction too - it’s going 10MPH up, or 10MPH northwest, for example.
Acceleration is defined (again, in terms of physics) as a change in velocity. On Earth, we’re more familiar with acceleration where something is travelling in more or less a straight line, and getting faster and faster. An object in orbit might be going at a constant speed, but it’s direction of travel is always changing. So it is also accelerating.
The distance of the orbit from the center of the earth is related to the time for one orbit by Kepler’s law. The formula is:
d[sup]3[/sup]/t[sup]2[/sup] = a constant.
for d, the distance from the center of the earth, in meters and t in seconds the constant is about 10[sup]13[/sup]
This means that a satellite in orbit 100 miles above the surface of the earth has an orbital time of just under 1.5 hours and the orbital speed is 17400 mph.
As can be seen from the formula, higher orbits take more time. If the orbital distance from the center is doubled the time is multiplied by about 2.83.
In orbit - the Space Shuttle is always “falling” towards Earth due to Earth’s gravitational force. Depending on the height of orbit, any orbiter has an obital velocity tangential to that Earth’s gravity.
The Shuttle lands (or “deorbits”) simply by slowing down. The “simply” begins with the Shuttle “horizontally” turning so it’s engines face toward it’s direction of travel. Now it could do nothing - and each bit of friction imparted by particles would eventually slow Shuttle down enough. Shuttle instead fires its engines and quickly slows down and quickly enters Earth’s atmosphere (Shuttle again turns horizontally).
/nitpick
It’s not air friction that heats up objects moving very fast through the air, it’s air compression. Objects moving much faster than sound are moving so fast that the air cannot get out of the way, it gets plowed through and highly compressed.
As the vulume of a gas goes down, the temperature and pressure go up. Friction does add some temperature, but it is much less than the effect of atmospheric compression.
/nitpick.
This is also why some meteors explode as they enter the atmosphere. The atmospheric pressure on the leading face becomes so great that the pressure differential between the front of the object and the back squeezes it into oblivion. 