Satellites, part deux










2 thoughts on “Satellites, part deux

  1. Mr. Devlin,
    i enjoyed your brief regarding the Sirius satellites orbits. i found it when trying to answer a little different question I had. i became familiar with the Sirius orbits when i downloaded a satellite tracking program to go with a star tracking program for my Android or iPad. For a moment I thought you may address my question, but unfortunately you did not.
    My question: how is this orbit maintained? You touched on it in you analysis of tne Clark’s belt – fuel. i cannot imagine these satellites carry enough fuel to make several “burns” daily, but how else are they able to change direction on a daily basis? Surely not neuclear energy, or couldnit be? this would seem to be a radical & cost prohibitive alternative for a company like Siriius, still in an enfant stage. Will an orbit of this shape maintain itself with gravitational or physical forces?

    i look forward to future reading if paths cross. maybe it’s in the stars…..

    Addison Davis
    Albany, Ga USA

    • Hi Addison, Excellent question. The Sirius satellites are in a high elliptical orbit, called a Molniya orbit. You can read more about it here. The animated picture in the posting may be a bit misleading because you can’t tell that the figure 8 motion that you see over North America is caused by a combination of the satellite’s motion along with the rotation of the earth. As the satellite comes back down from its high orbit, it gets closer to the earth and then gets slung back up into space at a very high altitude which makes it look like it’s stationary as it reaches its apogee. That’s why the figure 8 is not symmetrical, one portion is made while the satellite is close to the earth and the other portion is made when it’s back up at maximum altitude. The earth is not at the center of its orbit, instead it is at one of the foci of the elliptical path.

      There are several types of satellite orbits, for example, a low earth orbit which is just a few hundred miles up, usually circling from North to South. These satellites include some of the spy satellites and ham radio satellites. They travel at about 17,000 mph and make an orbit every 90 minutes. The GPS constellation is up around 12,000 miles above the earth and constantly moving in precise circular orbits and need minor amounts of fuel to keep themselves exactly where they are supposed to be. But needing to use fuel constantly wouldn’t work on a satellite, because it would severely limit the life of it, so they depend on the satellite’s speed and the earth’s gravity and the vacuum of space stay in the orbit with minimal need to add energy. The design life of the GPS satellites is finite, and was estimated to be around 8-10 years. Many have lasted much longer than that. All will fail eventually, and the Air Force has to keep sending up new ones to replace them. They have launched 59 GPS satellites to date of which 31 are still functional. They need to keep at least 24 functional to avoid having outages in GPS coverage. Sorry for that tangent ;-).

Leave a Reply


CommentLuv badge