Talk:Molniya orbit

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[edit] Sirius Satellite Radio

This article claims that SSR uses satellites in Molniya orbits, defined as having a 12-hour period. However, SSR's own article states that they use 24-hour orbits, which could be Tundra orbits, but not Molniya. —The preceding unsigned comment was added by Pakaran (talk • contribs) 18:28, 13 February 2007 (UTC).

And in fact, the tracking site linked from this article confirms that they have 24 hour (or rather, 1 sidereal day) periods. [1] [2] [3] -- Pakaran 18:35, 13 February 2007 (UTC)

[edit] Van Allen belt

The article states that: "A Molniya orbit is not suitable for manned spacecraft as it repeatedly crosses the high-energy Van Allen belt." However, it would seem that any long range space mission whould have to pass through the radiation, such as the Moon mission, say. Or does the "not suitable" in this sentense mean something like "not advisable" rather then "impossible"? Maxim K 22:11, 24 October 2007 (UTC)

Manned missions would generally be flown on a trajectory to avoid the worst of the Van Allen radiation and thus to pass quickly through the thinnest part of the belt (see here, for example). Since the path of the Molniya orbit is effectively fixed by the orbital element parameters needed to achieve the Molniya configuration then I guess that it would be difficult to configure a Molniya orbit to avoid the Van Allen radiation in this way. Moreover the fact that an object in a Molniya orbit would repeatedly pass through the Van Allen belts on each orbit would make the radiation issue much more of a problem. In this case, I think that "not suitable" is probably the correct language. It's not "impossible" (the manned spacecraft *could* be constructed with large amounts of shielding I suppose) with enough money being spent. MarkPos 12:11, 25 October 2007 (UTC)

OK thank you, that's sort of what I suspected but wasn't sure. I like that website. Maxim K 08:18, 26 October 2007 (UTC)

[edit] Orbital period and dwell time

The article says "The most typical Molniya-type orbits have a period of 12 hours, making them appear over Russia twice a day, for about eight hours." If the Earth's rotation means the apogee alternates over two points on the surface, as the linked chart suggests, does that mean Russia is visited only once a day? -- Regregex 20:06, 29 October 2007 (UTC)

Yes, you are correct of course. It's obvious really... after 12 hours the satellite has made one orbit and returns to the apogee, but the Earth has only made a half-rotation and thus the apogee is now over a point on the Earth 180 degrees away in longitude. The dwell over one apogee is around eight hours, therefore three satellites in three orbital planes are required to provide service to one region, e.g. Russia, and as a "by-product" service could also be provided to the other region on the opposite side of the Earth, if required. If no-one else does it first I'll try and tidy up the article a bit to correct this point. --MarkPos_{(User Page | Talk | Contribs)} 11:28, 30 October 2007 (UTC)