Re: Final trajectory (Re: 2021-035B decay estimates)

From: Marco Langbroek via Seesat-l <seesat-l_at_satobs.org>
Date: Sun, 9 May 2021 22:17:07 +0200
Op 9-5-2021 om 11:04 schreef Marco Langbroek via Seesat-l:
> 
> I have updated my blog post here:
> 
> https://sattrackcam.blogspot.com/2021/05/reentry-predictions-for-chinese-cz-5b.html


I have, earlier today, further updated this post with a new bit of analysis.

I fiddled with the area-to-mass ratio in GMAT untill I got a splashdown near
2:24 UT close to the position in the Maldives reported by China.

The resulting trajectory (map is in the updated blog post) has it pass near the
location over the Arabian peninsula reported by CSpOC in their TIP by 2:14 UT,
the time for that TIP.

The modelled altitude at that time at the CSpOC location, then comes out at ~100 km.

CSpOC says that their TIP times and positions are usually based on when the
object crosses the 10 km altitude limit in their reentry model.

The CSpOC TIP has a very narrow uncertainty margin of +- 1 minute. Several
analysts (Jonathan and me for example) have long suspected that such TIP's with
the +-1 minute error margin are in fact not output of a reentry model, but based
on actual satellite observations of the reentry fireball, e.g. by the SBIRS system.

100 km altitude is comparable with the altitude where meteoric fireballs start
to become visible.

So the 3265 km and 10 minute discrepancy between what China reports and what the
CSpOC TIP says, might be because China refers to sea impact, while the CSpOC
time and position might be the start of the reentry fireball at high altitude as
observed by SBIRS (and not the 10 km mark in a reentry model).

Of course, with GMAT models, it is always a simplification (the proverbal
"spherical cow in a vacuum" etc.), as it does not take mass loss over the
reentry trajectory (with changing area-to-mass ratio as a result) due to
ablation and fragmentation into account, so the ~100 km is a ballpark figure
(but for this early phase of the reentry, probably close to what it was, if the
Chinese reported location near the Maldives is right).


- Marco


-----
Dr Marco Langbroek  -  SatTrackCam Leiden, the Netherlands.
e-mail: sattrackcam_at_langbroek.org

launchtower:    http://launchtower.langbroek.org
Station (b)log: http://sattrackcam.blogspot.com
Twitter:        _at_Marco_Langbroek
-----

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Received on Sun May 09 2021 - 15:18:26 UTC

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