Using the SGP4 propagator, I calculated the minimum distance between ISS and Tiangong-2 for this year.
I obtained 12.32 km for
2017-09-27 17:29:36.376 UTC (which is the day
270.7289). I used the TLE
17270.72310185 for the ISS and the TLE
17270.58030751 for the Tiangong-2.
That distance seems very small. Is there any minimum safe distance to keep?
EDIT: I uploaded the graphs that I use to analyze the close encounter geometry: http://cristianopi.altervista.org/_Incontri.html (Sat1 = ISS, Sat2 = TG2).