How come an orange inside the ISS just floats in the air? 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, “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.”


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.

The Little Rocket that Was

Let’s revisit Sir Isaac Newton’s cannon.

In the above mentioned article we had placed a level on the cannon and shot the cannonball at 1440 feet per second parallel to the ground until it fell and bounced and rolled and came to a stop.

We didn’t want to shoot it up in the air in a big arc to see how far we could get it to go because we knew if we shot the cannonball up in the air in an arc, it would go much further. Naturally. We were simply interested in the effect of gravity on a projectile as it moves forward and we chose to level the cannon and shoot straight out.

So now let’s do just that, but because we know the cannonball can only go so fast, we’ll do it with a rocket. We’ll shoot the thing up in the air into a big arc and see where it comes down.

Roar! We launched. The burning rocket fuel accelerates our rocket faster and faster until it’s going 20 times the speed of sound. That’s Mach 20. Pretty lickedy-split I dare say. It’s going faster than any jet plane that I ever flew in went. In fact, it’s going so fast that every second it travels almost 5 miles (4.9 miles to be precise). I’d be scared out of my wits if I was riding that thing.

Okay, so this rocket just goes so fast and so far, and then you know what happens? Well, it runs out of fuel of course. It can’t burn forever. We only put so much of that rocket fuel inside of it, so once we light it and run to the side and it roars to the sky it’s going to run out of fuel after awhile.

So now here it is, way up in the sky. It went so high, so fast, that it actually went higher than the air. The sky around the rocket went dark, the stars came out and it got really quiet.

Yet it’s still going forward in that big arc even though we’ve run out of fuel, because essentially we threw something up in the air really fast.

Technically we didn’t even need rocket fuel, we could have used a slingshot if we could have achieved that speed. Alas, trial and error has shown me that unless you give it that extra boost as it goes up, it’s never going to go 4.9 miles per second. I’ve been through a lot of slingshots as a kid and never managed to shoot a projectile going that fast. Good thing, too! My neighbors would have been upset.

As our burned out rocket shell continues to move upward, it’s going to slow because of gravity. After all their’s no more rocket flames to boost it any higher. When it reaches the top, we call that the Aphelion, it levels out and slowly starts to curve down toward the Earth. Wherever it hits, somebody’s going to get really upset.

Except it’s so high and going so fast, that as it arcs downward toward the ground, accelerating faster and faster, it completely misses the planet! It just goes zooming right by planet Earth, it’s course warped by gravity, and whipped all the way around to the other side and then flung out into space again.

Marvelously, this happens again and again, much to our amazement. It doesn’t look like that rocket’s going to crash down at all! It’s in what they call an elliptical orbit.

That my friend, is another example of what keeps things in orbit. The speed one needs to achieve orbital velocity is 4.9 miles per second, or 7.9 kilometers per second. At that speed things go up, then free-fall to the earth, miss the earth, and keep going round and round.

Sometimes the orbit is highly elliptical, or if you’re very clever, you can make it almost round. The Earth’s orbit around the sun is elliptical, and the moon’s orbit around the Earth is also elliptical (that’s what a super moon is all about, when the moon is both full and closer to the Earth).

I’m sorry, Sir Isaac. Your cannon didn’t cut it this time. Had to make a rocket. If you want to see how this works, click here!