On ISRO's C37 launch, I claimed that ISRO plans to study the last stage C37 for '10 orbits or so
I was watching the live launch and caught the post-launch interviews in passing. I was very happy for ISRO doing the study on the C-37 last stage.
Now please check out the video below (from 58 mins to 1:03 mins) (I had linked the video on my previous post as well):https://www.youtube.com/watch?v=yPtFNJ2SSUw
ISRO's director K. Sivan (I think) clearly mentions that ISRO will study C37 last stage for 10 orbits
(and leaves the stage passivation and degenerate orbit implied).
ISRO has been doing stage passivation since PSLV-C12/C-14 (2009) and is a standard practice on all PSLV launches since then. What ISRO does for stage passivation? It basically empties out the propellant tank (remember they are hypergolic propellants and will explode in extremes of space) so that any uncontrolled explosion does not create a debris field. Also removes any stored energy like draining the batteries.
ISRO has been doing extensive prediction analysis of space objects going back to at least 2 decades (IMHO). In fact by 2009/10 ISRO has one of the best space object reentry estimation. For example., ISRO predicted that the final stage of GSLV-F04 launched in 2007 will re-enter in 2010/2011 and it precisely did.*
Given this it can be assumed that ISRO has the tools of placing the last stage in a 'degenerate' orbit where the last stage can re-enter (and thus de-orbit) in a precise time.
Let us take ISRO's GSLV's GTO. ISRO places INSATs in GTO at 170 km x 35945 km orbit., the perigee 170km is low to be influenced by earth's atmosphere. Yes., there is a drag on the last stage by earth's atmosphere and the object ceases to be a space junk and burns up in earth's atmosphere within 2-3 years (or sooner).
In theory, space is a vacuum., but where does space begin and earth's atmosphere end? The karman line (100 km) is considered as the boundary between earth's atmosphere and space. However that is an arbitrary line., earth's atmosphere very gently merges into space (and that is the best way to describe it in english - from my limited intellect).
In fact - Karman line is so arbitrary that., at 120 km earth's atmosphere has a very noticeable effect. That is any object at 120km orbit loses velocity very fast and de-orbits (or re-enters) rapidly.
Earth's atmosphere is also affected by solar heat/radiation, pull of moon and sun etc and hence is not a perfect sphere. In fact one can safely assume that upto 500 km., earth's atmosphere has affects on orbiting bodies.
Hence pure theoretical equations will tell us that an object placed at say 170km orbit will be in space for several years or decades., but in reality - the object will re-enter within 2-3 years. In fact anything placed say around 500/550 km will have significant orbit decay - in fact without any external means to maintain orbit., the orbit decay happens in 15 years (at 500 km)**. It can happen even sooner or later., it depends upon the type of the body. A satellite with its solar array deployed will have larger decay rate compared to a streamlined body. Hence if you place a nano-sat with solar array deployed., you can safely assume that in 7-8 years it will re-enter earth's orbit.
Or if you launch sats., ensure that the last stage remains within 500/550 km and it will de-orbit. Putting it in a degenerate orbit, that is even by lowering the perigee to say 400 km hastens the decay by a significant factor.
So if you are a responsible space launcher like ISRO - what will you do? Basically passivate the stage and park it in a degenerate orbit. GSLV stages will burn up in <2 years and PSLV stages will burn up <10 years. You are basically helping Earth clean up the space junk easily by a little effort from your end using the already extra propellants.
*Anybody who gets surprised 'now' that ISRO does stage passivation is definitely uninformed
** All numbers rounded off. Eschewing physics and equations and formulas.