satellite re-entr

Dear Chris and Heather,

First of all I want to say that I love the show. I'm glad that is has returned, the audio format works quite well. I was a bit put down when I noticed it was audio only, but I quickly turned around when I heard how good the show is.

In episode 18 you discussed the satellites coming down to earth and seemed a bit confused as to why these come down in an uncontrolled fashion. The reason I decided to mail you is in the hope of explaining why this is.

With all the satellites we launch it gets pretty crowded in space, especially in some of the more useful/ commonly used orbits. In order to avoid having dead satellites in places where an active satellite could be, there are some guidelines. These guidelines dictate that you either have to move your satellite to a graveyard orbit or let it come down and burn-up in the atmosphere at end of life.

The graveyard orbit is located a few hundred kilometers above the altitude for geostationary satellites, about 36000 km altitude, GEO satellites typically go there at end of life. They will in theory remain at this orbit indefinitely.

For other satellites, typically in LEO (low earth orbit) which are between 200 and 2000 km altitude, they bring them down and let them burn up. The guidelines state that the satellite has to burn up within 15 years after it's end of life, so it is not necessary to bring it down immediately in a controlled fashion.

Bringing it down immediately cost a lot more fuel then putting it in a decaying orbit and letting the drag of the atmosphere bring it down the rest of the way. The difference in fuel consumption can correspond to several months of operational lifetime, and thus to several months of making money with your satellite.

That is if a satellite operator abides by the guidelines. Sometimes operators make a mistake, or just don't care, and are unable/unwilling to dispose of their satellite. The problem with the guidelines is that there is no way, not even between parties that signed it, to enforce these guidelines in any way. Let alone punish those who break it.

I hope this clarifies the reasons behind the uncontrolled de-orbits a bit. There is only a very small number of satellites that are brought down in a controlled fashion. Most controlled re-entries don't burn up and are recovered intact, such as manned spacecraft or satellites containing analog storage devices.

Satellites that get a controlled burn-up in earth atmosphere are either very large, such as Mir(2001) and ISS(targeted for 2016 or later) or have to be brought down prematurely, such as the Compton Gamma Ray Observatory(2000).

Thanks for the weekly science updates and I hope the show will have a long and healthy second life.

Sincerely,

Duncan
Bsc student Aerospace Engineering, Technical University of Delft