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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 both the
Moon, the ISS and any other object in orbit are actually falling.
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 cannon ball. 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 motion 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 off 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
because of the curvature, but at the same time falling and getting
closer. The two motions cancel each other out and eventually the
projectile will circle the Earth until it gets to its starting point. It
will then do this indefinitely without further assistance from an
engine or other means to propel it. We say that it has reached orbital
velocity. Also the spiralling trajectory becomes a near circle. For low
Earth orbit of from 200-2000 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. Engines have
to accelerate satellites to 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 3683 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 outwards 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 the 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.