ROSAT - The physics of satellite earth re-entry.

[metalslug 31]

Link

I have a question for those in the know;
This satellite was placed in orbit in 1990 and served until 1999, and now 12 years on it is re-entering earth.
It does not have a propulsion system so... why now ?

I am assuming there was no active mechanism to control it's orbit altitude during it's 9 years of service.
Likewise there was no mechanism during it's 1999 shutdown to propel it to a rapid earth re-entry orbit.

I am risking a guess that the satellite may have intentionally been placed in a very gradual earth re-entry orbit on the day it was launched, closing the distance to earth by a few miles each year, in a path calculated for re-entry sometime after it's service life.
Is this how it works ? ..or is ROSAT's re-entry a fluke?

[wolfriverjoe 1,340]

I'm no expert, but here goes:

There really isn't a "boundary" between atmsophere and "space".

The atmosphere just keeps getting thinner and thinner.

Low earth orbit is still in a very, very small amount of atmosphere. There is a very tiny amount of drag from that. Stuff like the Space Station has to use a small amount of thrust every once in a while to keep the orbit from decaying.
They didn't put a propulsion system on RORSAT because it's planned life was well short of the predicted life of the orbit.

So your guess that it was in a "very gradual earth re-entry orbit" is sort of correct. Not so much a path calculated for re-entry, more the knowledge that the orbit would decay.

And I'm pretty sure I got at least a little of this wrong, someone can (and will) correct it.

Edit to add: NASA Clicky

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[Squeak 17]

orbital decay

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