Satellites Orbiting The Earth. Tell me more!

Satellites are, to some degree, "mysterious" objects. They travel in space, which feels like an exotic place because most of us have never been there. They are so far away that we cannot see them. They usually cost millions or billions of dollars, which means none of us will ever own one personally. And so on…

Orbital mechanics can also be mysterious because there is no easy way for us to experience orbital mechanics personally. However, with a little imagination, you can understand the basic idea behind orbital mechanics very easily. It turns out that we play with orbital mechanics all the time!

Think about what happens when you throw a ball. Imagine that you are standing in a big field and throw a baseball as hard as you can -- like a pitcher. The ball might go 100 feet (30 meters) and then hit the ground. You put the ball in orbit -- It's just that a ball's orbit is very short!

Now imagine that you shot a rifle straight and level instead of throwing a ball. The bullet might travel a mile (1.6 km) before succumbing to gravity and hitting the ground.

Now imagine that you shoot a very large cannon that is able to give its shell an extremely high initial velocity. Also imagine that our world is completely covered in water to remove any worries about hills, and that the cannon is shot straight and level. Its path might look like the image to the right.

­ In this diagram you can see that the shell is going far enough to actually follow the curve of the earth for a period of time before hitting the ground.

One thing that gums these examples up is air resistance, so imagine that you took this cannon to the moon and mounted it on top of the highest mountain. The moon has no atmosphere and is completely surrounded by the vacuum of space. If you adjusted the speed of the shell just right and shot the cannon, the shell would follow the curve of the moon perfectly.

It would fall at exactly the same rate that the curve of the moon falls away from it, so it would never hit the ground. Eventually it would curve all the way around the moon and ram right into the back of the cannon! On the moon you could actually have satellites in extremely low orbits like that -- just a mile or two off the ground to avoid the mountains. And satellites could conceivably be launched from cannons.

On earth, it's not so easy because satellites have to get up above the atmosphere and into the vacuum of space to orbit for any length of time. 200 miles (320 km) up is about the minimum to avoid atmospheric interference. The Hubble space telescope orbits at an altitude of 380 miles (600 km) or so. But the principle is exactly the same. The speed of the satellite is adjusted so that it falls to earth at the same rate that the curve of the earth falls away from the satellite. The satellite is perpetually falling, but it never hits the ground!

It would fall at exactly the same rate that the curve of the moon falls away from it, so it would never hit the ground. Eventually it would curve all the way around the moon and ram right into the back of the cannon! On the moon you could actually have satellites in extremely low orbits like that -- just a mile or two off the ground to avoid the mountains. And satellites could conceivably be launched from cannons.

On earth, it's not so easy because satellites have to get up above the atmosphere and into the vacuum of space to orbit for any length of time. 200 miles (320 km) up is about the minimum to avoid atmospheric interference. The Hubble space telescope orbits at an altitude of 380 miles (600 km) or so. But the principle is exactly the same. The speed of the satellite is adjusted so that it falls to earth at the same rate that the curve of the earth falls away from the satellite. The satellite is perpetually falling, but it never hits the ground!

by "environment clean generations"

If There Were No Gravity On Earth?

Gravity is one of those things we take completely for granted. And there are two things about it that we take for granted: the fact that it is always there, and the fact that it never changes. If the Earth's gravity were ever to change significantly, it would have a huge effect on nearly everything because so many things are designed around the current state of gravity. 

Before looking at changes in gravity however, it is helpful to first understand what gravity is. Gravity is an attractive force between any two atoms. Let's say you take two golf balls and place them on a table. There will be an incredibly slight gravitational attraction between the atoms in those two golf balls. If you use two massive pieces of lead and some amazingly precise instruments, you can actually measure an infinitesimal attraction between them. It is only when you get an gigantic number of atoms together, as in the case of the planet Earth, that the force of gravitational attraction is significant.  

The reason why gravity on Earth never changes is because the mass of the Earth never changes. The only way to suddenly change the gravity on Earth would be to change the mass of the planet. A change in mass great enough to result in a change in gravity isn't going to happen anytime soon. 

Could we survive without gravity?

But let's ignore the physics and imagine that, one day, the planet's gravity turned off, and suddenly there was no force of gravity on planet Earth. This would turn out to be a pretty bad day. We depend on gravity to hold so many things down -- cars, people, furniture, pencils and papers on your desk, and so on. Everything not stuck in place would suddenly have no reason to stay down, it would start floating. But it's not just furniture and the like that would start to float. 

Two of the more important things held on the ground by gravity are the atmosphere and the water in the oceans, lakes and rivers. Without gravity, the air in the atmosphere has no reason to hang around, and it would immediately leap into space. This is the problem the moon has -- the moon doesn't have enough gravity to keep an atmosphere around it, so it's in a near vacuum. Without an atmosphere, any living thing would die immediately and anything liquid would boil away into space. 

             Without gravity, the water in oceans, rivers and lakes would disappear, leaving Earth with no water supply.

n other words, no one would last long if the planet didn't have gravity.

If gravity were to suddenly double, It would be almost as bad, because everything would be twice as heavy. There would be big problems with anything structural. Houses, bridges, skyscrapers, table legs, support columns and so on are all sized for normal gravity. Most structures would collapse fairly quickly if you doubled the load on them. Trees and plants would have problems. Power lines would have problems. The air pressure would double and that would have a big effect on the weather.

What this answer shows you is just how integral gravity is to our world. We can’t live without it, and we can't afford to have it change. It is one of the true constants in our lives!.

by "environment clean generations"

Simulate "Your" Own Gravity

Working is microgravity is not easy, and during the Gemini space program in the 1960s, there were concerns that EVAs might not be a safe way to work in space. Advances in suit design and EVA techniques ultimately made it possible, and now scientists want to make things even easier by getting the suits to simulate Earth gravity.

Simulating Earth gravity is (unfortunately) not nearly the same thing as generating artificial gravity. Instead, a gravity simulator generates resistance that is similar to the amount of resistance that you experience due to gravity. 

So for example, here on Earth, it takes effort to lift a weight, because gravity is pulling the weight down. In space, you don't have to fight against gravity, you just have to overcome the weight's inertia, which is much easier. If you were to attach an elastic strap to the weight, though, you could simulate having to apply that extra effort, even in weightlessness.

Why would anyone want to ruin the ease of weightlessness with a gravity simulator? Well, besides the health problems that can crop up from long exposure to microgravity, even the simplest of tasks take on a whole extra dimension when your body doesn't weigh anything. If you pick up an object, you'll also be pulling yourself down towards that object. If you try to twist a screw, your whole body may start to rotate. With practice and the right equipment, astronauts can get used to all this weird stuff, but NASA wants to make things much easier by designing a new spacesuit that can turn people into relatively stable platforms, like gravity does.

To make this work, the new suit will have an inertial measurement unit that can detect small angular changes. Flywheel gyroscopes will then kick in to provide simulated gravity resistance along any axis, potentially allowing an astronaut to "plant" themselves in microgravity to operate a tool. Off-orbit, the same kinds of technologies could be used for medical rehab, and Draper Laboratories hopes to have this kind of tech inside spacesuits in five to ten years.

by "environment clean generations"