pseudo-fall. In addition, the Moon orbits the Earth and is attracted by its gravity,
just like the ISS. It makes a round trip around our planet in 27 days at a speed of
620 miles per hour, in a sort of constant fall towards the Earth – an event that luckily for all of us never happens.
Sir Isaac Newton had the brilliant intuition that the gravitational physics
between the Earth and the Moon was the same as that of an apple falling to the
ground from a tree, and so he translated the laws of gravity into equations.
Spaceships and satellites switch on their engines only when they need to change
position or height, i.e. to accelerate or brake. This means that spaceflight mostly
happens with lights and engines switched off on rigid highway lanes, following a
precalculated path thanks to the mathematical laws discovered by Newton.
Now, imagine that you are on board the ISS, floating inside the pressurized
module and checking the orbital data on the control panel: altitude at 248 miles
and speed at 16,700 mph. You feel safe because this means the station is flying
nicely and not falling. You receive a phone call from Houston telling you that you
have an imperative commitment on Earth and you must return urgently. If you
were the guest star of a Hollywood science fiction movie, you would enter the
Soyuz capsule attached to the Russian Zvezda module, turn on the retro-rockets by
detaching from the station, point the nose of the spacecraft down – it would be
difficult to find it because the Soyuz is a sphere, but let’s pretend that you can – and
you would accelerate to descend immediately into Earth’s atmosphere.
If you did this in real life and not in a movie you would risk speeding up, missing the angle of return, bouncing on the Earth’s atmosphere and never returning
down. If you were really in the Soyuz’s command seat, you’d have to slow down
by firing your engines in the opposite direction of flight. That way, the ship would
gently go down, deviating its orbit downward until it met the upper layers of the
atmosphere, generating enough friction to return to Earth like an incandescent
meteorite.
This is the way re-entry from space really works, and it is in fact a dangerous
manoeuvre. One of the main risks for astronauts is friction with the atmosphere.
The high kinetic energy of the spaceship, traveling at the amazing speed of five
miles per second, must be dissipated as heat. The space shuttle slowed down from
16,700 mph to zero in thirty minutes to land safely on Earth. During this time, the
friction with the atmosphere created an ionized plasma at 1,800 degrees Fahrenheit,
which surrounded the vehicle, making it a sort of burning meteorite in freefall.
The re-entry manoeuvre was critical. Unfortunately, in 2003 it was fatal for the
Shuttle Columbia, which disintegrated over Texas due to a crack in the heat shield.
Air drag helps slow down the descent speed from space if we want to re-enter
the planet. It also prevents us from orbiting close to the Earth’s surface. If there
was no atmosphere, we could enter into orbit even a few miles above the ground –
a manoeuvre that can be done on the Moon where there is no air. But on Earth as
well as any planet with an atmosphere, if you want to go into space you need to
Fundamentals of Astronautics 3
just like the ISS. It makes a round trip around our planet in 27 days at a speed of
620 miles per hour, in a sort of constant fall towards the Earth – an event that luckily for all of us never happens.
Sir Isaac Newton had the brilliant intuition that the gravitational physics
between the Earth and the Moon was the same as that of an apple falling to the
ground from a tree, and so he translated the laws of gravity into equations.
Spaceships and satellites switch on their engines only when they need to change
position or height, i.e. to accelerate or brake. This means that spaceflight mostly
happens with lights and engines switched off on rigid highway lanes, following a
precalculated path thanks to the mathematical laws discovered by Newton.
Now, imagine that you are on board the ISS, floating inside the pressurized
module and checking the orbital data on the control panel: altitude at 248 miles
and speed at 16,700 mph. You feel safe because this means the station is flying
nicely and not falling. You receive a phone call from Houston telling you that you
have an imperative commitment on Earth and you must return urgently. If you
were the guest star of a Hollywood science fiction movie, you would enter the
Soyuz capsule attached to the Russian Zvezda module, turn on the retro-rockets by
detaching from the station, point the nose of the spacecraft down – it would be
difficult to find it because the Soyuz is a sphere, but let’s pretend that you can – and
you would accelerate to descend immediately into Earth’s atmosphere.
If you did this in real life and not in a movie you would risk speeding up, missing the angle of return, bouncing on the Earth’s atmosphere and never returning
down. If you were really in the Soyuz’s command seat, you’d have to slow down
by firing your engines in the opposite direction of flight. That way, the ship would
gently go down, deviating its orbit downward until it met the upper layers of the
atmosphere, generating enough friction to return to Earth like an incandescent
meteorite.
This is the way re-entry from space really works, and it is in fact a dangerous
manoeuvre. One of the main risks for astronauts is friction with the atmosphere.
The high kinetic energy of the spaceship, traveling at the amazing speed of five
miles per second, must be dissipated as heat. The space shuttle slowed down from
16,700 mph to zero in thirty minutes to land safely on Earth. During this time, the
friction with the atmosphere created an ionized plasma at 1,800 degrees Fahrenheit,
which surrounded the vehicle, making it a sort of burning meteorite in freefall.
The re-entry manoeuvre was critical. Unfortunately, in 2003 it was fatal for the
Shuttle Columbia, which disintegrated over Texas due to a crack in the heat shield.
Air drag helps slow down the descent speed from space if we want to re-enter
the planet. It also prevents us from orbiting close to the Earth’s surface. If there
was no atmosphere, we could enter into orbit even a few miles above the ground –
a manoeuvre that can be done on the Moon where there is no air. But on Earth as
well as any planet with an atmosphere, if you want to go into space you need to
Fundamentals of Astronautics 3