let it be known that because the oceans slosh around on Earth as it
rotates about it’s axis (these are called ocean tides), the speed of
Earth’s rotation is gradually slowing down. Billions of years ago a day
on Earth was only about 8 hours, and now it continues to slow down. We
compensate this twice a year by adding leap-seconds as needed.
slowing of the earth’s rotation causes the moon to drift further away
from us every year by about 1.5 inches, so eventually the Moon will
escape Earth altogether and drift away.
This would be so cool. Everyone would want one, even if they couldn’t afford it. But is the concept possible?
Well, the short answer is no. It’s not possible.
To understand that “no” requires a little bit of understanding in orbitology.
You see height is not the only problem when you want to stay up in space. If you want to stay up there for sometime you’ll need to be in orbit. To get in orbit you’ve got to go really, really fast – like around 17,000 mph fast.
That’s because an orbit simply put is when an object is going so fast in one direction that as it falls it misses the planet. You’ve got to be going a lot faster than a speeding bullet.
So if you had a vehicle in your garage that could not only fly like a plane, but could continue to fly above the atmosphere and get going 17,000 mph, then you\d be bad ass.
But you can’t and you won’t, because to do all that takes a lot of fuel. That’s why we have these big ol’ rockets boosting satellites and astronauts into space. Those big ol’ rockets are filled with fuel. Once the fuel is expended then the big ol’ rocket isn’t needed anymore and it’s detached to fall back to Earth.
So sadly, the idea of having a small shuttle that you park in your garage and fly into space is just not going to happen. Ever. Sorry!
It just moves forward. The moon isn’t stationary. It has inertia and no atmosphere in space to slow it down. It just goes. As it moves forward, the gravity of Earth tugs on it and it falls, but because it’s moving forward it misses Earth as it falls and goes around, again and again. That’s called an orbit.
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.
Astronauts are not weightless. They experience microgravity.
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.
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.
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!
In two previous blog posts about Newton’s Cannon and Joe Drops the Ball I posed the question: If falling objects go faster and faster, why don’t orbiting objects fall out of orbit and crash to the ground?
The question is legitimate and also has a perfectly legitimate explanation.
The rate of a falling object is 32.2 feet per second per second, i.e. it goes faster and faster as it goes down. It accelerates on the way to the ground. So the first question is this: 1) Is an object in orbit in free fall? The answer is yes. 2) Do falling objects accelerate as they fall to the ground? The answer is yes. 3) Do orbiting objects accelerate and thus fall to the ground? The answer is no.
It’s all in the definition of accelerate. Acceleration is a change in velocity not just a change in speed. Velocity is the speed in a given direction, but because an object in orbit is always changing direction it is technically accelerating even if it’s speed isn’t changing.
The force of gravity bending the forward motion of the orbiting object changes the direction of the object. The object is accelerating even if it’s speed isn’t changing, because it is constantly changing direction.
This is alternatively explained in the article I wrote called The Little Rocket that Was.