Where are the satellites?

I’ve been really high in an aeroplane and very fast, but still only slightly higher and faster than many thousands of people every day who travel on airliners that typically fly around 30,000 ft at around 900 kmh. At 46,000 ft the sky directly above is dark and the curvature of the earth is clearly evident, you can even make out the atmospheric band hugging the horizon reminding you of how thin the height of the atmosphere we can live in is.

In fact at 46,000 feet the world is hostile to life, it’s cold, there is not enough air to breath and the pressure is extraordinary low – much higher and you need a pressure suit to stop your blood boiling. I was watching a video of a SR-71 and the pilot was commenting that they often flew at around 80,000 ft, still way lower than the Hubble Space Telescope at 569 km in altitude but still very very high. At 80,000 ft there’s hardly any atmosphere above but it’s still not space, though almost just as hostile to human life. 100km can be described at the point where the atmosphere stops, or becomes so negligible that for most purposes it’s not far off the vacuum of space. That is also known as the Kármán line. The Sputnik orbited at about 215km and the ISS is at about 350km in altitude and even at that height there’s enough residual atmosphere to create a slight drag requiring the ISS to adjust its height periodically.

NASA

Above the height of the ISS is all of the satellites we have come to rely one in everyday life, like GPS, satellite TV and the weather satellites that help us predict if it will be nice enough to go to the beach tomorrow.

So how do you get something into orbit?

Newton figured out that if you fired a canon ball fast enough it would get to the point where the Earth was falling away below the canon ball at the same rate the ball was falling, essentially putting the canon ball in a state where it would never hit the ground. If you add atmosphere to the model then you have to make sure that the canon ball is above the atmosphere or the drag would slow it down so that the Earth would not be falling away at the same rate the canon ball was falling so the ball would come back to Earth.

Every height has an optimal speed to keep the object in orbit so the ISS has to travel at 28,000 kmh to stay at around 350 km altitude. To stay in a stable orbit higher you don’t have to go as fast, so a GPS satellite in a Medium Earth Orbit travels at around 14,000 kmh at 20,350 km in altitude. For the geostationary communications satellite at 35,786 km they need to be travelling at just over 11,000 kmh. Our own Moon travels at 3,700 kmh and its at a distance from Earth of just under 300,000 km (1 light second).

GPS satellite from http://www.gps.gov

Orbits

There are loads of different types of orbits but essentially they can be characterised into three groups elliptical, parabolic and hyperbolic. A circular orbit is essentially an elliptical with both focal points in the same place. A parabolic orbit occurs when Newton’s canon ball (mentioned above) is at escape velocity. A hyperbolic orbit occurs when the velocity is faster than the escape velocity of the thing being orbited. These are the orbits when stuff does not come back, so best not to fire your satellite too fast – unless you want it to go somewhere else of course. The elliptical orbits are what all of the satellites do because they are going slower than escape velocity. The orbits are divided into three groups based on height. Low Earth Orbit (LEO), Medium Earth Orbit (MEO) and High Earth Orbit (HEO).

LEO is below 2000 kms in altitude and includes the ISS, Hubble and things like the Iridium satellites. MEO is where the likes of GPS hang out and it extends from 2000km to about 35,000km and the HEO heights are where all of the communications satellites in geostationary orbits. There’s obviously a load more satellites than I’m mentioning here but that gives you the idea. So the next time you are on an airliner heading off overseas or somewhere, then think for a moment that you’re up at the edge of the atmosphere and there’s really not that much between you and the ISS, there’s certainly less than what’s between you and the ground but don’t worry to much, because you’re not going anywhere near fast enough to pop into orbit.