climb over 90 miles above the ground where the air particles are rarefied. It is not
enough to climb up to that altitude; it is also necessary to be traveling five miles
per second in order to defy the Earth’s gravity and not fall back down. At an altitude of 93 miles, gravity is reduced by 90% with respect to its value on ground,
and at 248 miles where the ISS is orbiting, by 95%. For this reason, the speed
needed for orbital insertion varies with altitude. If you want to reach an orbit close
to the Earth, called low Earth orbit (LEO), which lies between 93 and 620 miles,
you need to reach a speed of 16,700 mph. A satellite or nuclear warhead travelling
on a ballistic missile – basically a space rocket missing the last stage – takes 45
minutes to make half a tour of the Earth.
If the orbital insertion speed is greater than 16,700 mph, then we leave the
Earth, and if we are lucky, we find ourselves on an elliptical orbit along which we
approach our planet and then slowly move away from it in a perennial back and
forth.
Firing powerful engines at a speed of 25,000 mph, we enter an orbit towards the
sun and, if we have done the correct calculations, after a three-day trip we reach the
Moon. If we want to escape the deadly attraction of our star as we move towards it,
we need to have an engine four times more powerful, which at the moment only
exists in the movies. So, in space around Earth as well as in the solar system, we do
not travel on spaceships with the engine always running. Instead, we move like
stones on predefined orbits such as cosmic freeways, thanks to a mathematical
system conceived by Sir Isaac Newton in the late seventeenth century.
Now, let’s forget about the deep space for a moment and focus on our planet to
better understand how satellites work. In LEO there are satellites taking pictures
of any place on Earth; in the politically correct jargon they are called Earth observation satellites. They have onboard telescopic cameras that point to the Earth’s
surface for spying or monitoring activities on specific sites on the ground, but if
they turn towards the stars and planets, they take astronomical images and do scientific research.
You cannot take a picture of everything from space, since there are limits to
photo resolution beyond which images are blurred because of the wavelike nature
of light. Then there is the problem of the very short time of overflight. At an altitude of 93 miles, a satellite flies over a territory of 95 miles in about a minute. In
that time, it can take pictures of a few sites, not of the entire area. To overcome
this, some satellites are launched into higher orbits such as the geostationary one,
called GEO, at 22369 miles above the Earth’s surface. Up there, the gravitational
attraction is 42 times lower than on the ground and a satellite travels at a speed of
1.9 miles per second, taking 24 hours to make a complete tour of the planet. In
practice, it takes the same time as the Earth’s rotation. This means that a geostationary satellite has the same angular speed as the planet, so it always seems fixed
from a ground site, even if it actually moves along the orbit.
4 Fundamentals of Astronautics
enough to climb up to that altitude; it is also necessary to be traveling five miles
per second in order to defy the Earth’s gravity and not fall back down. At an altitude of 93 miles, gravity is reduced by 90% with respect to its value on ground,
and at 248 miles where the ISS is orbiting, by 95%. For this reason, the speed
needed for orbital insertion varies with altitude. If you want to reach an orbit close
to the Earth, called low Earth orbit (LEO), which lies between 93 and 620 miles,
you need to reach a speed of 16,700 mph. A satellite or nuclear warhead travelling
on a ballistic missile – basically a space rocket missing the last stage – takes 45
minutes to make half a tour of the Earth.
If the orbital insertion speed is greater than 16,700 mph, then we leave the
Earth, and if we are lucky, we find ourselves on an elliptical orbit along which we
approach our planet and then slowly move away from it in a perennial back and
forth.
Firing powerful engines at a speed of 25,000 mph, we enter an orbit towards the
sun and, if we have done the correct calculations, after a three-day trip we reach the
Moon. If we want to escape the deadly attraction of our star as we move towards it,
we need to have an engine four times more powerful, which at the moment only
exists in the movies. So, in space around Earth as well as in the solar system, we do
not travel on spaceships with the engine always running. Instead, we move like
stones on predefined orbits such as cosmic freeways, thanks to a mathematical
system conceived by Sir Isaac Newton in the late seventeenth century.
Now, let’s forget about the deep space for a moment and focus on our planet to
better understand how satellites work. In LEO there are satellites taking pictures
of any place on Earth; in the politically correct jargon they are called Earth observation satellites. They have onboard telescopic cameras that point to the Earth’s
surface for spying or monitoring activities on specific sites on the ground, but if
they turn towards the stars and planets, they take astronomical images and do scientific research.
You cannot take a picture of everything from space, since there are limits to
photo resolution beyond which images are blurred because of the wavelike nature
of light. Then there is the problem of the very short time of overflight. At an altitude of 93 miles, a satellite flies over a territory of 95 miles in about a minute. In
that time, it can take pictures of a few sites, not of the entire area. To overcome
this, some satellites are launched into higher orbits such as the geostationary one,
called GEO, at 22369 miles above the Earth’s surface. Up there, the gravitational
attraction is 42 times lower than on the ground and a satellite travels at a speed of
1.9 miles per second, taking 24 hours to make a complete tour of the planet. In
practice, it takes the same time as the Earth’s rotation. This means that a geostationary satellite has the same angular speed as the planet, so it always seems fixed
from a ground site, even if it actually moves along the orbit.
4 Fundamentals of Astronautics