# ISS - Tiangong 2 minimum safe distance

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).

• – uhoh
Dec 31, 2017 at 15:51
• Also, see answer to What is the “ISS's Keep Out Sphere” and what is its radius?
– uhoh
Dec 31, 2017 at 16:03
• Believe it or not, a distance of 12 km for the two stations is probably safer then 12 km for two random pieces of debris. The space stations are known to a very high precision, and thus that far of distance is pretty much guaranteed to be real. Dec 31, 2017 at 21:59
• Wow, from this distance they could even see eachother! (If it happens on the sunny side) Apr 2, 2018 at 0:42
• I've edited to bump to the active queue. Do you remember the relative orientation? Did one pass mostly above the other, were they moving nearly parallel, or was there a substantial angle between the two meaning that the relative velocity would have been of the order of a kilometer/sec and therefore almost too fast to see (if one were looking at the other)? fyi I've just asked Has an astronaut ever seen another object in LEO pass by that wasn't mission-related in any way?
– uhoh
Sep 21, 2019 at 7:30