The Shenzhou 8 spacecraft will fly with no crew on board, and will be sent to rendezvous and dock with China's Tiangong 1 space laboratory, expected to be launched at some time before Shenzhou 8 flies. The descent module of Shenzhou 8 will return to Earth at the end of the flight, carrying the experiment package.

The duration of the mission is noteworthy. This will be the longest flight ever for a Shenzhou spacecraft in its full capacity. It also gives clues to how China will send astronauts to the Tiangong 1 laboratory, and other space laboratories to follow.

The primary purpose of the Shenzhou 8 mission is to test rendezvous and docking procedures in space. It's also a dress rehearsal for the Shenzhou 9 mission, which is expected to carry astronauts to the Tiangong 1 laboratory in 2012. Thus, we can expect the overall mission plans to match each other fairly closely.

China has openly discussed its plans for sending astronauts to its first space laboratory for years, but has released few specific details on the missions.

One question that has preoccupied analysts is the expected duration of an expedition to Tiangong. The space laboratory is fairly small, and offers little room for astronauts to inhabit. Questions have also been raised about the logistics of such a mission.

The Tiangong laboratory and the Shenzhou spacecraft that will dock with it have fairly limited room for supplies. Issues of food, water and oxygen supplies would almost certainly rule out a long stay aboard the laboratory.

This writer, along with other analysts, has previously suggested that a crewed flight to Tiangong of roughly two weeks was likely. Some analysts suggested around three weeks. A three-week mission for Shenzhou 8 supports all these estimates, but still doesn't tell us exactly how long the Shenzhou 9 mission will last.

If Shenzhou 8 is meant to be a shakedown cruise for a crewed expedition, it will try to be thorough in its testing. This means that we can expect this to cover all of the normal requirements for a crewed mission, plus a bit more. So Shenzhou 8 is probably going a bit further with its endurance, to prove that it's spaceworthy for a long time.

This mission is probably designed to last a few days more than will be required for a crewed flight, just for good measure.

If this is the case, then astronauts will probably stay aboard Tiangong 1 for roughly two weeks. A stay of around two and a half weeks is possible.

At this stage, it is unclear how long Shenzhou 8 will fly alone at the start of its mission, while it approaches Tiangong 1. It's possible that they will copy current Russian strategies, and cruise for about two days to the laboratory.

However, China is also introducing a more powerful version of the Long March rocket to launch this mission, and may opt for a more direct trajectory. If crew consumables are scarce, it makes more sense to get to the laboratory as quickly as possible.

Then again, Shenzhou 8 is doing this for the first time, and it's also a modified vehicle. There will probably be a solo "shakedown cruise" in orbit before the spacecraft is cleared to move on to the docking phase of its mission.

On a crewed flight, the time spent in transit will influence the time available on the laboratory. But we can't be sure if Shenzhou 8's cruise will match that of Shenzhou 9.

So, we can probably say for sure that crewed expeditions to Tiangong 1 will last no longer than three weeks. Exactly how much time will be spent on board the laboratory is unclear. But the mission plan suggests that China is determined to get as much crew time on the laboratory as it can, given the resources available.

Dr Morris Jones is an Australian space analyst and writer. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.

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