First, its speed does not depend on its altitude. Two planes can fly at the same
altitude at different speeds. On the contrary, in space, there is a precise relationship between the distance from Earth and speed. Satellites, spaceships or space
stations that move in the same orbit – i.e. at the same altitude from the Earth’s
surface – always have the same speed. This varies with altitude. The closer to
Earth a satellite is orbiting, the higher its speed is and the shorter its travel time
around the planet. The relationship between orbit altitude and speed in space is a
precise mathematical rule – thus, those fast and reckless spaceship manoeuvres
can only be seen in movies.
Second, an airplane flying in the atmosphere travels through the air, which is a
compressible fluid, to sustain the wings allowing it to manoeuvre. By moving
ailerons
1
on the wings or the tail rudder, an airplane shifts the flow of air around it
and thus changes direction, in the same way that a boat manoeuvres in the water
with its oars or rudder.
There is no air in space, so a satellite must always use rocket engines to manoeuvre. This has important implications in a space project because, depending on the
type of mission, it will be necessary to precisely calculate all the necessary fuel,
including a contingency reserve, and fill the tanks before take-off. To date, it is not
yet possible to refuel in space or return to Earth to fill up and then take off again,
as airplanes do. Moreover, one of the most widely used propellant for spaceships
is hydrazine, a nitrogen chemical mixture that is highly toxic and harmful to
humans and the environment, so refuelling can only be done on Earth in closed
and protected areas.
Third, a flying airplane always encounters air drag, which continuously
slows it and tends to bring it down. To avoid this catastrophic event, its engines
must always be running, providing the necessary thrust not to crash. This does
not happen in space. Satellites go into orbit thanks to a rocket, named launcher,
that carries them inside its fairing; the powerful engines of the launcher defy
the Earth’s gravity, bringing the payload to the limits of the atmosphere over
90 miles high.
At that point, just like a slingshot, the launcher releases the satellite. The satellite then begins to turn around the Earth with the same speed – about 5 miles per
second – provided by the rocket itself. In practice, the spacecraft remains in constant balance between the Earth’s gravity and the centrifugal force, following a
curved trajectory parallel to the Earth’s surface. It is attracted by the planet’s gravity, but if the rocket has provided it with the right speed, the spacecraft remains in
orbit and never falls back.
It may sound weird, but it works. An astronaut inside the ISS feels weightless –
not because there’s no gravity, but because he or she is in a state of perpetual
1
Ailerons are the hinged surfaces in the trailing edge of an airplane wing, used to control lateral
balance.
2 Fundamentals of Astronautics
altitude at different speeds. On the contrary, in space, there is a precise relationship between the distance from Earth and speed. Satellites, spaceships or space
stations that move in the same orbit – i.e. at the same altitude from the Earth’s
surface – always have the same speed. This varies with altitude. The closer to
Earth a satellite is orbiting, the higher its speed is and the shorter its travel time
around the planet. The relationship between orbit altitude and speed in space is a
precise mathematical rule – thus, those fast and reckless spaceship manoeuvres
can only be seen in movies.
Second, an airplane flying in the atmosphere travels through the air, which is a
compressible fluid, to sustain the wings allowing it to manoeuvre. By moving
ailerons
1
on the wings or the tail rudder, an airplane shifts the flow of air around it
and thus changes direction, in the same way that a boat manoeuvres in the water
with its oars or rudder.
There is no air in space, so a satellite must always use rocket engines to manoeuvre. This has important implications in a space project because, depending on the
type of mission, it will be necessary to precisely calculate all the necessary fuel,
including a contingency reserve, and fill the tanks before take-off. To date, it is not
yet possible to refuel in space or return to Earth to fill up and then take off again,
as airplanes do. Moreover, one of the most widely used propellant for spaceships
is hydrazine, a nitrogen chemical mixture that is highly toxic and harmful to
humans and the environment, so refuelling can only be done on Earth in closed
and protected areas.
Third, a flying airplane always encounters air drag, which continuously
slows it and tends to bring it down. To avoid this catastrophic event, its engines
must always be running, providing the necessary thrust not to crash. This does
not happen in space. Satellites go into orbit thanks to a rocket, named launcher,
that carries them inside its fairing; the powerful engines of the launcher defy
the Earth’s gravity, bringing the payload to the limits of the atmosphere over
90 miles high.
At that point, just like a slingshot, the launcher releases the satellite. The satellite then begins to turn around the Earth with the same speed – about 5 miles per
second – provided by the rocket itself. In practice, the spacecraft remains in constant balance between the Earth’s gravity and the centrifugal force, following a
curved trajectory parallel to the Earth’s surface. It is attracted by the planet’s gravity, but if the rocket has provided it with the right speed, the spacecraft remains in
orbit and never falls back.
It may sound weird, but it works. An astronaut inside the ISS feels weightless –
not because there’s no gravity, but because he or she is in a state of perpetual
1
Ailerons are the hinged surfaces in the trailing edge of an airplane wing, used to control lateral
balance.
2 Fundamentals of Astronautics