China's Tiangong 1 space laboratory is alive and well in orbit. For most spacecraft, that's a nominal state. But for this mission, it's strange. Tiangong 1 was launched in September 2011 with an advertised lifetime of two years. It played host to three dockings by Shenzhou spacecraft - Shenzhous 8, 9 and 10.

The last two carried crews of three astronauts to live on board the laboratory, which is really a small space station. When the last crew departed Tiangong in June this year, Tiangong had essentially completed all of its main tasks.

We expected China to de-orbit Tiangong sometime close to the end of September 2013, based on public statements made in the Chinese media last year. This analyst watched as Tiangong remained stubbornly aloft for a month after the originally advertised deadline, but didn't find anything too odd in this. An extended mission was possible if fuel reserves and functionality were good. Keeping Tiangong in space for another month would allow more data to be returned from the mission. However, the spacecraft has now been in orbit for much longer than a simple last-minute extension of the de-orbit date. What is China doing with Tiangong?

The simplest explanation we can imagine is that China planned this all along. The originally published timeline for ending the mission could have been a conservative estimate, based on expectations of a higher level of fuel consumption. Outsiders don't have a view of Tiangong's actual fuel reserves, but we can guess that the tanks aren't close to empty. Otherwise China would have steered Tiangong to a controlled re-entry weeks ago.

Tiangong's altitude also seems healthy, based on open-source data from satellite tracking sites. It's not exactly grazing the lower atmosphere. Based on this, we could easily expect Tiangong 1 to remain in a stable orbit for weeks, depending on solar activity and drag. The fact that the Sun seems to be fairly docile right now suggests that the upper atmosphere will also be placid in the near future, which would also help to preserve Tiangong's orbit.

This is clearly a carefully planned extended mission. But what are the objectives? We can rule out some of the wilder theories. There will be no more Shenzhou spacecraft launched to Tiangong 1. Crew supplies have been mostly depleted. An uncrewed docking by a Shenzhou spacecraft would be a largely pointless exercise. In any case, China does not seem to have a spacecraft and launch vehicle ready to carry out such a mission, and will launch no astronauts at all in 2014.

Could China be planning to use Tiangong as a target for another mission, such as a close fly-by from another satellite? Again, this seems unlikely. Last year, China aroused a lot of interest with a trio of experimental satellites launched atop the same rocket. One has reportedly conducted fly-bys of other satellites, prompting fears of some sort of military experiments. This analyst will not rule out some of these theories, but does not believe that Tiangong itself will be approached by another Chinese spacecraft. Fuel reserves and orbital mechanics for the aforementioned orbital snooper make this unlikely. Let's not even consider the possibility that Tiangong will be targeted for an anti-satellite weapon test.

So Tiangong will probably fly solo for the rest of its mission. This does not prevent it from carrying out tasks.

Tiangong 1 has two large Earth Observation cameras at its rear. It is possible that China has extended the mission to make better use of these cameras. China is also testing the performance of the laboratory on an extended mission, to see if its parts can go the distance. Much of the gear on board this laboratory is destined for future use with another Tiangong laboratory and the large space station China expects to build around the end of the decade. More testing now means less uncertainty later.

This analyst expects that Tiangong will now be allowed to slowly decay in its orbit until it reaches a semi-sustainable altitude. At that point, China will probably begin preparations for a controlled de-orbit burn from its engines. We don't know exactly where and when China intends to draw the line in terms of its termination altitude, or when it will reach this point.

If this is the case, we cannot be sure of exactly when Tiangong will re-enter, and the Chinese themselves probably have an open mind on the subject. But Tiangong seems destined to remain in orbit for longer than most analysts probably suspected.

Dr Morris Jones is an Australian space analyst who has written for spacedaily.com since 1999. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email. Dr Jones will answer media inquiries.