Launching satellites
Launching a satellite into orbit requires consideration of a number of major science ideas. These include gravity, circular motion and atmospheric drag.
Launching satellites into orbit
Dr Allan McInnes explains why satellites need large rockets to launch them into orbit and why many satellites are launched from near the equator.
Point of interest:
A satellite in an equatorial orbit provides repeated observations of the same area. An example of this is a weather satellite.
Atmospheric drag
Satellites need to be placed in orbit high above the Earth’s atmosphere so that the drag of the atmospheric gases doesn’t make the orbiting satellite slow down.
An altitude of 100 km has been adopted by the United Nations as a working definition of where the Earth’s atmosphere ends and space begins. This is called the Kármán line after the Hungarian-born aerospace pioneer Theodore von Kármán. He calculated at this height an aircraft would have to travel at the same speed as an orbiting satellite (7.85 km/s, about 24 times the speed of sound) in order to have enough aerodynamic lift to maintain its height.
Making a satellite orbit at that height, however, is impractical due to the atmospheric drag of the very thin atmosphere, so most satellites are placed into orbit well above the Kármán line at altitudes between 350 and 1,500 km.
Vertical structure of the atmosphere
The line at 100 km above the Earth’s surface is the official start of space. Theodore Kármán calculated that a plane flying at this altitude needs an air speed that matches the orbital speed for 100 km.
Download a PDF version of this diagram.
There is gravity in space
A lot of people think that there is no gravity above the Earth’s atmosphere. The truth is that gravity keeps pulling an object towards the centre of the Earth even if the object is far above the Earth’s atmosphere. The force of gravity pulling you towards Earth at an altitude of 100 km compared to that acting on you if you were on a 10 m high diving board only varies by about 20 N.
We know this from Newton’s universal law of gravitation, which takes the form
F = G mEmO/d2
where
F = force of gravity in newtons
G = the universal gravitation constant
mE = mass of the Earth
mO= mass of the object
d = distance between the object centres.
Using this formula shows that the force of gravity acting on a 70 kg person on a 10 m diving board is 688 N compared to 667 N for the person at an altitude of 100 km above the Earth’s surface.
Energy needed to reach an altitude of 100 km
The work that needs to be done on a 1 kg object to reach a height of 100 km above the Earth’s surface is calculated in the following way.
work done = gravitational force x vertical height
= (1 x 9.8) N x (100 x 1,000) m
= 980,000 joules
If we account for the fact that gravity is decreasing very slightly as distance from the Earth increases, the corrected value is 967,000 joules.
To supply this amount of energy per kilogram of load, you would need a very powerful and well designed rocket. Since the rocket and fuel also have mass, there needs to be additional fuel to lift the mass of the fuel and rocket into space.
But there is still a huge problem. Even though this 1 kg mass has reached space, it would still fall back to Earth because there is still a very strong pull of gravity attracting it towards the centre of the Earth.
Keeping satellites in orbit
Dr Allan McInnes tells us about forces acting on satellites once they are in orbit, how to keep a satellite in orbit and the purpose of a graveyard orbit.
Points of interest:
The US Federal Communications Commission requires all geostationary orbits to commit to moving to a graveyard orbit.
About one-third of satellites succeed in moving to one.
Speed of orbit
To balance the strong gravitational pull, the 1 kg mass must be given additional energy to place it in orbit around the Earth. An object will fall back to Earth unless it has enough orbital speed.
To calculate the orbital speed needed, we combine Newton’s law of universal gravitation with a circular motion equation. The net result of this is an equation of the form
V=√[(G x ME)/R]
where
V = orbital speed
G = the universal gravitation constant
mE = mass of the Earth
R = the distance from the centre of the Earth to the object in orbit.
To keep the 1 kg mass in orbit at an altitude of 100 km, an orbital speed of 7.85 km/s is needed.
Newton’s orbital cannon
Newton reasoned that, if the cannon ball was fired with exactly the right velocity, the ball would travel completely around the Earth, always falling in the gravitational field but never reaching the Earth, which is curving away at the same rate that the projectile falls. It would be placed in orbit around the Earth.
The extra energy needed to make an object travel fast enough to stay in orbit is more than 30 times as much as the energy needed to lift it to an altitude of 100 km.
This means that, even though it takes nearly a million joules of energy to lift a 1 kg mass to an altitude of 100 km, it takes over 30 million joules of extra energy to give it enough speed to stay in orbit around the Earth.
Rockets need to be big enough to carry enough fuel to provide all of the energy needed to reach the correct altitude and speed. Rockets that carry satellites into orbit need to be incredibly large.
Ariane 5 rocket
On 14 October 2006, Ariane 5 delivered the Optus D1 satellite into geostationary orbit (160°E). This satellite is the communications satellite used by Sky TV among others.
Related content
Learn about the Rocket Lab orbital launch site at Mahia Peninsula in New Zealand and why they've chosen this site to launch rockets into orbit.
Activity ideas
In the Rocket launch challenge activity students use a rocket launch simulation. They change parameters such as mass, thrust and drag to make a rocket go as high as possible and launch a payload 400 km above the ground.
Now that you’re a rocket scientist, try your hand at building a fit-for-purpose satellite to be launched by the Electron rocket.
