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.
The answer to the question is that it’s a long way away, it moves all the time (sometimes it’s a very long way away on the other side of the sun), it’s expensive, it’s very dangerous, it’s a dead planet, there’s nothing humans can do there that our less expensive rovers can’t do, we can’t breathe the air, we can’t grow anything there, there’s no legitimate scientific reason to go and last but not least nobody would fund it, especially Congress.
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.
Because of Joe’s ball, the Earth may fall into the sun and the moon may fall from the sky.
Meet Joe. Joe has a ball, and his ball is a great problem for the whole world.
Because of Joe’s ball, the Earth may fall into the sun and the moon may fall from the sky. That’s a big problem for a small ball in Joe’s hand.
The problem is that Joe drops his ball and then tosses it to the side (see illustration at right).
You see, when Joe drops his ball it accelerates at a spectacular rate of 32.2 feet per second per second.
Joe’s ball, as does any falling object, doesn’t just drop. It drops faster and faster, whether he just drops it, or tosses it to the side! If he tosses it to the side, it will hit the ground at the same time as if he just dropped it. The forward movement of the ball doesn’t slow down the downward acceleration of the ball toward the ground. (We are assuming here, that there’s no wind or air to slow it down, okay? Just leave that out for now.)
Now, Let’s say Joe has a bullet in his hand and drops his bullet. Like the ball, the bullet’s gonna fall to the ground at the same rate the ball did.
Let’s next give Joe a gun, and have him shoot the bullet. The bullet is going forward, just like the ball that Joe tossed. But the forward movement of the bullet from the gun barrel doesn’t slow the downward acceleration of the bullet as it falls to the ground. It just falls to the ground a ways off because the bullet was going fast.
Finally, Sir Isaac Newton shoots a cannon. And his cannon is very powerful. The cannonball goes so fast, so far, that as it falls to the ground, the ground curves away underneath it, and it goes around the world in an orbit.
But why? Both the ball and the bullet fall to the ground at 32.2 feet per second per second. They accelerate toward the ground, but Isaac Newton’s cannonball doesn’t. Satellites stay in orbit, the moon stays in orbit, the Earth stays in orbit, and they don’t accelerate toward the ground. How come?
The Earth is falling into the sun and the moon is falling from the sky.
Once upon a time, Sir Isaac Newton had a cannon. Noisy thing that. Orbitologists call the figurative device “Newton’s cannon.”
I managed to borrow one of these cannons from the National Museum of Orbitology and Conjecturism, and am standing next to a large pile of cannonballs (see image).
Depending on all sorts of factors, like how much gunpowder, how big the cannonball, etcetera, I’ve discovered the cannonball leaves the cannon around 1440 feet per second. Alas, after leaving the cannon, the speeding cannonball gets affected by wind, gravity and distance it has to travel.
The first factor is the wind resistance. At 1440 feet per second on a calm day, the spherical ball of lead immediately encounters 1440 feet per second wind resistance in the opposite direction of flight. The cannonball’s gonna slow down.
The second factor is gravity. No matter how fast that thing travels through the air, it’s still going to fall toward the ground for the same reason we plant our feet here. Gravity.
A third factor, I suppose, is if we’re shooting at a target or just an open space to see how far the ball will travel. Since I want to see how far the ball will go, I’m in an open field.
A fourth factor is trajectory. Am I shooting level to the ground, or in a big arc? Obviously it’s going to travel farther if I angle the barrel of the cannon up and shoot the ball in a big arc. So what to do? Let’s just put a level on the barrel and shoot it parallel to the ground to see how far the projectile is going to travel before wind resistance and gravity pull that hurdling ball of metal down to the unyielding ground below and then it bounces and rolls until it comes to a halt.
Now that we’ve fired the cannonball, let’s take a tape measure and see how far it went before it hit the ground. Surprising distance it seems for such a big heavy object. I wonder if we could make it go any farther?
This is the example of Newton’s Cannon. Given a condition of no atmosphere, and enough speed, that ball would travel around the curve of the earth, hitting further and further away from us. Speed it up even more and it will never hit the ground (see illustration).
That’s an orbit.
Umm….wait. Something’s seriously wrong here. My brain puts up red flags. I’m a layman, not a real scientist, so what do I really know about physics and all that stuff. I’m a buff isn’t it enough? Seems to me that a falling object should accelerate, not just fall at a constant speed. Seems to me the rate of acceleration of a falling object is (Googling it now) 9.8 m/s/s. “Free-falling objects are in a state of acceleration. Specifically, they are accelerating at a rate of 9.8 m/s/s. This is to say that the velocity of a free-falling object is changing by 9.8 m/s every second.” –physicsclassroom.com
So the cannonball should not just curve around with the curve of the earth, but accelerate downward as it falls, thus never achieving orbit and always hitting the ground. Therefore nothing can orbit anything and the moon can’t stay in the sky and the Earth is going to fall into the sun. The International Space Station is doomed tomorrow and all the GPS satellites are going to fall down. Forget about Dish Network, DirectTV, SeriusXM Radio, weather satellites and Google Earth. It’s all coming down.
Well, obviously wrong, but why wrong? This is a question this layman has pondered over many an hour sitting in pondering places at various pondering moments in this pondering life. I suppose I’m going to have to just ponder up another Google search. I will ask this question another way posted here, and after you read that one you can read here and then here!
Satellites don’t have or need engines to keep them in orbit. Once launched at their elevation and velocity, there’s very little atmosphere to slow them down. They just go.
Imagine firing a bullet parallel to the ground. After the initial launch of the bullet from the explosion in the gun barrel, the bullet needs no engine to keep it going. It will slow down due to air resistance and hit the ground a few hundred or thousand feet away.
Now imagine that there was no air on Earth, like on the moon. When you fire the bullet parallel to the ground there will be no air resistance to slow it down. Nonetheless, it will still fall to the ground because even as the bullet goes forward gravity pulls it down.
But the Earth is also round, not flat. So if your bullet went fast enough the Earth would curve down as the bullet moved forward and the bullet would never hit the ground. It would be in orbit.
Once launched at orbital speed in the near vacuum of space a satellite just keeps going and going. Never slowing down because there’s practically no air resistance up there. But there’s still gravity up there pulling it down.
You are correct that it needs to maintain its orbital velocity. But it does keep initial velocity because of lack of air resistance to slow it down. As gravity tugs it down the curvature of the Earth falls away and the satellite keeps going round and round, in orbit, without any engines.
Even way up there, however, it’s not a complete vacuum. There’s hardly any air. It’s a near vacuum. But over time because there is some air, albeit almost none, it does slow down. That’s why satellites occasionally fall and burn up on their way down.