Earlier today a Proton-M rocket took off, carrying a Sirius FM6 satellite radio payload destined for geosynchronous orbit. Nothing super exciting, Briz-M upper stage for transfer burns and then the payload itself has some small engines on it for final geosync insertion.

The weird bit is the orbital inclination. Check this out: Sirius FM6 orbital track

It launches on a 51º inclination, then does a plane change to 0º once it hits geosynchronous altitude! Plane changes aren't cheap; I would have expected it to be much more efficient to just launch as close to 0º as possible and make whatever minor corrections are needed to get into geosync. At first I thought it might be because it was dropping off other satellites enroute, but the mission timeline doesn't list anything like that.

So, why would they use such a high-inclination orbit?

  • 3
    $\begingroup$ Note that the upper stage only brings inclination down to 23 deg. -- the rest is taken out by the satellite itself. $\endgroup$ – user29 Oct 25 '13 at 18:56
  • $\begingroup$ Plane changes are a lot cheaper at high altitudes than they are in LKO, since you're moving more slowly and don't have as much momentum to kill off. $\endgroup$ – Sean May 15 '19 at 22:21

Launches from Baikonur Cosmodrome are tilted to a higher inclination than the site's azimuth to the equator (roughly 46° North depending on the individual launchpad used) to avoid the early launch ground track going over the territory of China in case of an aborted or failed launch, so any booster rockets don't fall on the Chinese territory and can be retrieved easier, and to simply avoid violating the China's airspace or requiring permission for an overflight even with successful launches.

To prevent any of such complications, the inclination is elevated to 51.65° and the ground track the launcher makes in the ascent phase doesn't go over that territory at all.

   enter image description here

   Rockets coming from the Baikonur Cosmodrome, Kazakhstan are sent into orbits inclined 51.65 degrees to avoid overflying China.

That is also one of the reasons for the International Space Station's (ISS) orbital inclination at 51.65 degrees, since Baikonur Cosmodrome is a site for most of the ISS related launches, at least in recent years. Quoting one of Astro Bob's blog posts:

The U.S. worked with Russia to pick the best orbital tilt. Since Russia launches Progress and Soyuz from Baikonur (latitude 46 degrees N), a high inclination orbit made economic sense. It also lets astronauts study more of the Earth’s surface compared to an orbit closer to the equator. 75 percent of the planet and 95 percent of its inhabited lands are open to view.

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