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Perigee
Aug 25, 2011 15:38:27 GMT -4
Post by Glom on Aug 25, 2011 15:38:27 GMT -4
The number I think you're looking for is called the "vacuum perigee". This is what the perigee of the entry orbit would be if the earth didn't have an atmosphere. I think it's typically around 80 nautical miles, but that's from memory. It's low enough that the spacecraft is captured by the atmosphere and slowed from its near-escape velocity to guarantee capture, then it typically rises again and enters a second and final time. The purpose of this is to spread out the heat loading over time. To nitpick, wouldn't "vacuum perigee" be the perigee if Earth was a point mass? Or is "vacuum perigee" specifically for such a specific situation?
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Perigee
Aug 27, 2011 0:10:12 GMT -4
Post by ka9q on Aug 27, 2011 0:10:12 GMT -4
I think "vacuum perigee" refers to the earth as it actually is, just minus the atmosphere.
That's actually harder than it sounds. The earth isn't perfectly spherical; to a first order (literally) it's squashed at the poles and bulges out at the equator. The difference is actually substantial; the earth's equatorial radius is 6378.145 km while its polar radius is only 6356.8 km. That's a difference of 21.345 km, more than twice the height of Mt. Everest.
So when you're computing perigees, it matters whether you're talking about an "average" spherical earth or the oblate earth as it actually is, and whether you're talking about that point on the orbit that comes closest to the earth's center or closest to its surface.
The devil is in the details...
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