How come some things can escape gravity? After all, it’s supposed to pull everything down. Maybe it is density and not gravity.

It is escape velocity, once achieved, that allows us to escape. Here is an image to illustrate the point. Once you achieve escape velocity, gravity gets weaker as the ship goes outward, and it does not cause the ship to return.

AT_7e_Figure_02_26_L

Is it possible to mitigate the extreme gravity of a large planet with an appropriate orbiting space station?

Would it be possible to orbit at just the right altitude and speed to make Jupiter’s gravity ‘feel’ like earths?

 In any orbit, around any planet, you’re going to be in micro-gravity. You will “feel” weightless because essentially you are falling around the planet in an orbital trajectory.

Just like astronauts on the ISS float around their space station, an astronaut orbiting Jupiter would also float around his or her space craft. It matters not how far away from the surface the space craft is as long as it is orbiting above significant atmospheric drag.

In theory, by having a wheel shaped craft that spins slowly, just like in the movies, you could achieve 1G.

 

 

How Big does a Moon Have to Be?

Not very big! It would just have to be natural. Not made by man! And it wouldn’t have to be there for very long either. Just long enough for it to be in orbit.

The astronomical community doesn’t have a definition for moon other than it has to be a natural object. Thus captured asteroids can be moons. It’s even possible for a moon to have a moon!

Mars has two moons, neither of which are large enough to be round. Objects in space get round due to their own gravity. The first image above is Deimos and the second is Phobos, the Martian Moons. Deimos (top) is the smallest. It has a mean radius of 3.9 miles.

When in orbit, astronauts experience weightlessness. What is this caused by?

Astronauts are not weightless. They experience micro-gravity.

As close to the earth that they are, gravity is a huge factor. You couldn’t, for example, step outside and just float away into space.

The reason it seems to be weightlessness is that the ISS and the astronauts inside are all falling at the same speed. The forward movement causes and angular movement away from the earth and the gravity pulls downward. This balances out in a wonderful phenomena we called an orbit.

An orbit really is like shooting a cannonball so fast that as it falls to the ground, the ground curves away underneath it to the point it never hits the earth but just goes around perpetually.

For this reason, the space shuttle is falling. It’s also going forward very fast and as it falls goes around the curvature of the earth and just goes round and round, along with the people and things inside it. They all are falling at the same rate, giving the impression of weightlessness.

That’s why it’s called microgravity and not weightlessness.

 

If you fire a gun vertically from the earth’s surface, but there is no friction, would it return to earth?

Yes it would return to Earth. The escape velocity for Earth is more than 11 km per second or 33 times the speed of sound. This is about 9 or 10 times faster than a rifle bullet.

So even without air friction the bullet’s going to go up, gradually slow down, and fall.

The problem is that without friction it’s going to come down too fast and kill someone at roughly the same speed as when it left the gun barrel.

In reality if you shoot a bullet into the air, when it drops it meets air resistance and winds up stabilizing at about 30–40 miles per hour. That’s enough to hurt you, but not as fast as if there was no friction.