how do satellites stay at the same elevations and speed in orbit?
In: Engineering
Here’s the funny thing, the speed that it’s traveling is what keeps it at that altitude. If it were traveling faster it would get further from Earth, slower it would get closer, slow enough and it would burn up in a fire ball for us all to enjoy.
They keep going because there’s very little to slow them down. That’s kind of why that movie with Sandra Bullock, which I enjoyed, was really kind of BULL, because if things were suddenly accelerated they would not stay in the same orbit they would move away from earth.
They don’t. Any atmosphere encountered will create friction and slow the satellite down. Even the moon is changing elevation by moving 3.78 cm away from the earth each year. Only Lagrangian points are stable for a longer time. They are points near two large bodies in orbit where a smaller object will maintain its position relative to the large orbiting bodies.Wikipedia
An object in motion tends to stay in motion unless acted upon by an outside force. The only force acting on a satellite is gravity, but they were initially launched at a speed where, in the time it takes them to fall down to the earth, they’d pass it. In other words, they’re falling and failing to hit the ground.
In the absence of anything else, they’ll keep orbiting forever. In practice, satellites occasionally need to activate their thrust to gain additional speed as the trace gasses slow them down, otherwise they’ll slow down until their trajectory dives towards the Earth.
They stay at the same speed because it’s a vacuum. There’s nothing to slow them down.
They stay at the same altitude because they are in a circular orbit. That is to say, they are moving so fast that the ground curves away from them at exactly the same speed that gravity causes them to fall.
Basically we throw them high enough that they get to a point that there is no aerodynamic drag (there can still be a tiny amount in certain orbits, but it’s a known quantity now that is planned for).
When it’s high enough, we get it going fast enough sideways that as it’s falling back towards earth, it “misses” and keeps falling.
Of course launches don’t go straight up and then straight sideways. The efficient path is called a gravity turn, in which the rocket starts vertical and slowly tilts towards horizontal until it’s basically burning parallel to the earth’s surface.
Think of throwing a ball. The throw has an arch to it. No think about throwing that ball so hard that by the time it’s at it’s at the top of it’s arch and begins falling back down, the earth has moved out from under it, and it just keeps falling forever, with earth continuing to move out from under it.
For the most part there is no atmosphere to slow down a satellite, it is purely under the influence of gravity so the orbit will remain the same for many years as long as it doesn’t get too low, but that doesn’t mean they always stay at the same elevation and speed.
Geostationary satellites always stay a certain distance away from Earth and travel in a circular orbit once every 24 hours, but many imaging satellites will have a very elliptical orbit which brings them very close to Earth at high speeds for part of it and then launches them far away at low speed until they slow down and fall back in for another pass. This gives a nice mix of close approaches to get detailed images, and not spending too much time in the thin atmosphere causing them to slow down and burn up in the atmosphere
Satellites in a [Molniya orbit](https://en.wikipedia.org/wiki/Molniya_orbit) will change their elevation and speed constantly throughout their orbit to keep them moving in that very elliptical shape.
Mainly because there’s no air in space to slow them down [1].
Think about swinging a ball at the end of a string, around and around. The ball “wants” to go flying away in a straight line, but goes in a circle because you keep diverting its course by pulling on the string.
Gravity does the same kind of thing. It pulls satellites (and the moon) in circles around the Earth. And the Earth around the Sun.
You need the right starting conditions (position, speed and direction) to get a circular orbit though. It’s also possible to get an elliptical (oval-shaped) orbit, where you fall gaining speed toward a low point (perigee), then climb losing speed toward a high point (apogee). Most satellite orbits are circular, except for maybe a few science missions.
[1] Not strictly true, there’s a tiny bit of air at typical satellite altitudes, but it takes years / decades / centuries to add up enough to make a satellite fall out of the sky.
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