The return to Earth of China's Shenzhou 10 spacecraft has wrapped up an important chapter in spaceflight. China has now completed all crewed activity with Tiangong 1, the nation's first space laboratory. Tiangong 1 has had a long and busy service record since its launch in 2011, and the laboratory has now completed all of the major tasks assigned to it. Tiangong 1 is now in its twilight phase, but this does not mean that its story is complete.

Tiangong 1 is the first step in a long-term program to build a large Chinese space station. It has demonstrated critical tasks such as rendezvous and docking with Shenzhou spacecraft. Tiangong has received dockings from three missions, Shenzhous 8, 9 and 10.

The last two missions both carried astronauts to occupy the laboratory. It has shown that China can support astronauts in comfort for extended periods. There have also been experiments with manual dockings and different approach patterns for dockings. This rather small module has achieved a lot over its lifetime.

Let's consider the state of the laboratory right now. The interior looks fine on video footage. It has been largely depleted of logistics supplies for astronauts, making further occupation difficult.

No problem. China had never planned more than two crewed expeditions to Tiangong 1, and it has completed both of these successfully. The final expedition, conducted by the Shenzhou 10 crew, was actually longer than some analysts (and probably many Chinese mission experts) had expected when Tiangong was launched. Extra supplies carried with the crew permitted this longer stay, with the extended mission approved long before Shenzhou 10 lifted off.

Tiangong 1 is unfit for further habitation but it is still an operational spacecraft. It seems probable that mission controllers are checking the state of the cabin atmosphere for evidence of deterioration or infestation by microorganisms, a difficult long-term problem on space stations.

They will also be monitoring the performance of the spacecraft itself, which has now been in orbit for a long time. Tiangong 1 also contains two Earth observation cameras, which are probably operational and operating.

The first major action to be performed by Tiangong in its twilight phase will be an orbit change. Mission controllers will soon raise its orbit slightly. Over the next few months, it is possible that mission controllers will become more adventurous in controlling a spacecraft that is no longer so critical to maintain.

They could change its orbit or orientation to explore thermal stresses on the vehicle. Other tests of the module under extreme conditions, with no risk to a crew, could also be attempted.

Ultimately, the Tiangong 1 mission must come to an end. China has probably drawn careful plans for this. Mission controllers are probably saving fuel for a controlled re-entry at some point in the future.

The module will probably be de-orbited harmlessly over the Pacific Ocean. It seems unlikely that much debris will reach the water but it could be photographed as it is destroyed by atmospheric friction.

How long will Tiangong 1 remain in orbit? There's probably no urgent need to terminate its mission. While it remains aloft, the cameras can still be used and engineering studies can be performed. This is icing on the cake for the mission, providing more data for little extra investment.

The spacecraft has shown itself to be highly robust, suggesting that a technical malfunction in the near future is unlikely. The factors that will ultimately decide the fate of Tiangong 1 are atmospheric drag and the module's fuel reserves.

China will want to keep a good margin of error in both the fuel and the perceived drag levels. The latter is more difficult than the former. The state of the atmosphere at the border with space is notoriously fickle, as solar activity causes it to rise and fall. Also, spacecraft are not simple, atmospherically stable objects.

It is difficult to calculate how they behave when they encounter the thicker layers of the upper atmosphere, and drag levels can change with orientation and turbulence. This is one reason why predicting the fall zones of satellites is so difficult. Tiangong 1 is also a hollow object with a large surface area and two big drag-inducing solar panels. This makes it even more susceptible to the trickery of the atmosphere.

So we can expect China to gain some more use out of Tiangong 1, but not too much. Chinese statements have suggested that Tiangong's lifetime will probably not go further than another three months.

After Tiangong 1 re-enters, its legacy will live on. Data gained from its long performance is probably still being studied. This will help to build its successor, a module known as Tiangong 2. This spacecraft is expected to be larger and more sophisticated than Tiangong 1, and will probably launch in 2015 or 2016.

After that, we can expect work to begin on China's large space station. This station will probably be launched in the early 2020s, and will give China an even stronger foothold in space.

Dr Morris Jones is an Australian space analyst who has reported on China's space program for SpaceDaily.com since 1999. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.