Background

Tianzhou 2 was launched 2021 May 29 at 12:55 UTC. Launch had been delayed from May 19 because of a pressure leak in either the core stage or one of the boosters of the CZ 7 launch vehicle.

Originally, the Yuan Wang 5 tracking ship was on station near the Philippines in order to monitor Tianzhou separating from its launch vehicle but the delay meant that Yuan Wang 5 had to move west in preparation for its next task, the upcoming Fengyun 4B launch. Yuan Wang 6 was sent to the Philippines station as a replacement.

In the event, Yuan Wang 5's move proved fortuitous.


Tianzhou Docking with Tianhe

At the time of the original launch attempt on May 19, Chinese-language internet discussion groups were talking of rendezvous and docking occurring about 6.5 hours after launch following a fairly short chase. Tianzhou would compute and control its own trajectory. After the May 19/20 cancellations, Tianhe's orbit was lowered from 373 km to 353 km in order to set up the ground track for May 29. It was then raised again to 373 km in order to allow Tianzhou to use the originally-planned route up to Tianhe. There is a detailed list of Tianhe's orbital manoeuvres on the zarya.info page for launches in 2021.

Internet discussions still talked of a short duration flight but there seemed to be less certainty than on the earlier occasion about the length of time it would take.

Tianzhou 2 docked with the rear port of Tianhe at 21:01 UTC, about nine hours after launch. Orbit data from Space-Track pointed to rendezvous maybe one circuit earlier. At the time, the pair were above the southern Pacific Ocean, away from any land-based tracking stations but within line of sight of China's Tianlian data relay satellites in GEO.


Yuan Wang 5

Yuan Wang 5 had already reached the Fengyun 4B monitoring station just south of the equator near 180° east longitude. It was sitting below the Tianhe/Tianzhou ground track a few minutes after after the docking. At 21:23 UTC, the pair rose above Yuan Wang 5's radio horizon for a ten minute pass that put them directly over the ship at 21:28 UTC. It gave controllers a chance of direct communication between the Yuan Wang 5 control room and the spacecraft that could have proved vital if there had been some problem with docking.

The encounter had not been in the original flight plan for May 19 as there was no ship available mid Pacific. Mission controllers seem simply to have taken advantage of an opportunity created by the logistics of the Yuan Wang 5 tracking mission.