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| Public domain image via Pixabay. |
It's a question a child, or for that matter a curious adult, might ask. And what about the International Space Station, or ISS, something the size of a football pitch? How can that stay aloft? The fact is that the Moon, the ISS and any other object in orbit are actually falling down. However, just like someone chasing a rainbow to get to the proverbial crock of gold, the Moon never reaches the ground.
Launching Projectiles
Trajectory of a projectile fired horizontally. © Eugene Brennan
Imagine you throw a ball or stone horizontally out a window, or a cannon on top of a tall building fires a cannonball. Once the ball leaves the muzzle of the gun, it doesn’t travel any faster horizontally. In fact, it slows down due to air resistance, known as drag. However, we’ll neglect this and imagine the cannonball is travelling through a vacuum. As it’s moving outwards horizontally, it’s also falling because of gravity. The combined horizontal and vertical motions cause the cannonball to follow a curved path. In fact, the trajectory is a shape called a parabola. The higher the initial speed of the cannonball, the further it’ll travel outwards before it hits the ground.
What if the ground isn’t flat, but curved?
If the ground is flat, the projectile will eventually hit the ground. But imagine if it’s fired at a much higher velocity, so it goes over the horizon. This time, it’ll take longer to hit the ground because the ground drops away below it due to the curvature of the Earth. As the initial velocity is made higher and higher, the cannonball keeps travelling further and further horizontally but continues to fall downwards. However, the ground keeps dropping below it because of the continuing curvature of the Earth. So it’s moving away from the Earth’s surface because of the curvature, but at the same time falling and getting closer to the Earth. The two motions cancel each other out, and eventually the projectile will circle the Earth until it gets back to its starting point. It will then do this indefinitely without further assistance from an engine or other means of propulsion. We say that it has reached orbital velocity.
At this point, the spiralling trajectory becomes a near circle. For a low Earth orbit of 200–2,000 km altitude, orbital velocity needs to be 7.7–6.9 km/s (27,772–24,840 km/h or 17,224–15,435 mph). This is why such huge rockets are needed to launch satellites into space. The engines have to accelerate satellites to about 10 times the speed of an AK-47 assault rifle bullet. Orbital velocity also decreases with distance, so the Moon orbits the Earth at a mere 3,683 km/h, or about four and a half times the cruising speed of a jetliner.
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Image courtesy Brian Brondel, CC by SA 3.0 via Wikipedia |
Escape Velocity
If an object's velocity is increased sufficiently, it can break free of Earth's gravity. The escape velocity at ground level on the Earth's surface is 11.19 km/s (6.95 miles/s), or 25,031 mph. Spacecraft travelling to the Moon or to other planets must reach this speed so they can travel on an outward trajectory.
Air resistance, known as drag, does slow down satellites somewhat because space isn't a perfect vacuum and some gas molecules exist at orbital altitudes. This causes satellites to spiral closer to Earth, and they sometimes need to be boosted back to their original orbits. According to New Scientist, 7.5 tonnes of fuel are used each year to maintain the ISS at its orbital altitude. In theory, a satellite could operate close to ground level, but once the thrust from a rocket engine is turned off, it would soon slow down and spiral downwards towards Earth due to drag. Spy satellites are sometimes used in low Earth orbit, but have a limited lifespan due to this drag.