I noticed in the video of the recent Iridium-6/GRACE-FO launch that the fairing separation happened at an altitude of about 143 kilometers. This is significantly higher than any altitude I can remember. I first started watching this last year and noted the altitude for several launches here.
@awksp's excellent answer explains the relative constancy of the altitude as likely originating from a standardized spec. The answer is thorough and worth reading, but here is a snippet from there:
...after the 3-sigma high theoretical free molecular heating for a flat plate normal to the free stream drops below 1135 W/m$^2$ (360 Btu/hr ft$^2$) based on the 1962 United States Standard Atmosphere.
After seeing the 143 km, I went back and checked several more recent SpaceX launches and compiled more numbers that can be compared with the table in the previous question. It seems Iridium-6/GRACE-FO stands out with a uniquely and significantly higher altitude for fairing separation than any other recent launch as well, including two other Iridium launches.
Question: Why did the fairing separation here happen at such a high altitude compared to so many other SpaceX launches? Solar activity-induced atmospheric heating? Abundance of caution for this payload combo? Anomaly? Just to see if we're watching? Something else?
MISSION speed altitude post 2nd stage km/hr km ignition (sec) ------- ----- ---- -------------- Iridium-6/GRACE-FO 8754 143 28 Bangabandhu Sat- 1 8978 111 53 TESS 7055 107 24 Iridium-15 8052 110 42 HISPASAT 30W-6 8979 111 48 PAZ Mission 6509 119 20 CRS-14 8158 115? 29 GovSat-1 9222 111 52 FH Test Roadster 10196 115 28 Iridium-4 7578 111 30 CRS-13 5982 106? 32
Screenshot Click for full size.