Why don’t these “thousands” of satellites fall back to Earth, if the likes of the ISS has to use booster rockets to stay up there?

The atmosphere doesn’t just “end” at a certain height. It just gets thinner and thinner.

In low Earth orbit, where the ISS is orbiting, there is still some atmosphere. It’s very, very thin, but it’s there. At the top of Mount Everest, just 5 and a half miles above sea level, you need oxygen to survive a long duration. At the International Space Station, 250 miles above sea level, you need a space suit.

Atmosphere, however, is still there – but very thin. Over time the ISS needs a boost to stay in orbit because it slows down due to atmospheric drag.

Most objects orbiting the Earth will succumb to falling eventually, but the higher they are, the longer they’ll stay. Some, way out in geostationary orbit 26,199 miles high, atmosphere is virtually non-existent, so those satellites will probably stay up there a few million or billion years.

We know gravity keeps the Moon in orbit. Why is there no gravity on the ISS?

This is an image of Newton’s Canon. Given a high enough elevation, shooting a canon-ball at increasing velocity will cause it to travel further and further until it just goes into orbit, falling around the earth.

That is what’s happening inside the ISS. It’s called free-fall – the same as Newton’s canon-ball.

The ISS is falling toward the earth. The only reason it doesn’t crash to the ground as it falls is because it’s also moving horizontally so fast that as it falls, the earth curves away and they just keep missing, again and again, around and around. That’s called an orbit.

This is why we don’t say the astronauts on the ISS are weightless. We say they are in microgravity.

As an example, let’s say you get into an elevator at the top of the tallest building in the world, and the elevator drops in free fall. Fun! You would fall at the same speed as the elevator and to an observer you’d appear to be weightless. In fact, if you held an orange, it would appear to float out of your hand.

That would end tragically, however, when the elevator hit the ground. Not fun anymore!

That tragic end doesn’t happen to the ISS because the ground just curves away under them and they keep missing Earth! Inside, however, the oranges float around and the astronauts float because the ISS and everything inside it is falling at exactly the same rate.

Therefore there is gravity up there – almost the same gravity that we experience on the ground. If the spaceship could stop moving forward and just hover somehow (it can’t) then everybody would be able to walk around just like we do here on Earth. It’s only because the ISS is in free-fall around the earth that the people and things inside appear to float.

If an intelligent species preceded us on Earth and wanted to leave a record of themselves in a time capsule, then one of the Lagrange points would be a good place for it. Is anyone looking for one there?

Great question! You asked specifically if one of the Lagrange points could be a good place to hide a time capsule from the past. The answer is “no” because, you see, an orbit in a Lagrange point is inherently unstable and would eventually either fall to Earth or fall to the Sun.

According to space.com, “L1, L2 and L3 are all unstable points with precarious equilibrium. If a spacecraft at L3 drifted toward or away from Earth, it would fall irreversibly toward the sun or Earth, “like a barely balanced cart atop a steep hill,” according to astronomer Neil DeGrasse Tyson. Spacecraft must make slight adjustments to maintain their orbits.”

Lagrange_points

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.