Re: Eccentricity

From: Bill Frost (
Date: Fri Jun 09 2006 - 00:22:39 EDT

  • Next message: Marcus Massey: "RE: Eccentricity"

    Hi Tom,
    To give you a quick reply, satellites are launched with a specific mission. Supposing you have a requirement for reconaissance or communications relay and you want the service to be equally available to the northern and southern hemispheres. Eg Iridium or the GPS constellation or TRMM. Then you launch a near circular orbit, like HST (Hubble Space Telescope), where apogee and perigee are within 5 metres of each other.
    However, some missions require the service to be largely based over one hemisphere, such as the Soviet or CIS communications satellites and these were placed in a Molniya orbit, where the perigee would be very close to earth and located in the southern hemisphere. As a consequence of Kepler's second law, the satellite passes this point at very high velocity, therefore minimising the time wasted on us ungrateful antipodeans, then shoots off to an apogee of around 35,000 km or so. One of the Molniyas has an apogee of 79 km (79 x 1,550 km), so it has to be almost glowing red hot by then!
    There's a good article on Wikipedia ( Then visit Heavens Above and 'Select a Satellite'. Type in %molniya% and it will bring back a complete list. Select a few of these as HA does a good job with their graphical displays of the orbit. 
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