Statements in the Chinese media have suggested the commander of the Shenzhou 5 mission, expected to lift off around October 10, will be selected shortly before the flight. But it is unclear if there will be one yuhangyuan (astronaut) or two on board this historic launch.

From a technical perspective, a one-person debut mission would be the most sensible. It provides more room for freedom of movement, in case problems develop on board the spacecraft. It also allows power and life-support systems to be stretched further. Additional safety gear that may not be carried on subsequent Shenzhou missions could also be included.

Engineers would also suggest that the mission should not only have the smallest crew, but the shortest duration. Again, this would be a precaution to minimise the exposure to risk. A mission lasting roughly 24 hours, as carried out by the first Shenzhou test mission in 1999, is possible.

A one-person flight, lasting roughly one day, would be the safest and most conservative mission profile that could be used for Shenzhou 5. But China could still keep its options open, even after the spacecraft is launched.

It is possible that China is preparing two timelines for Shenzhou 5, which could explain the vague statements regarding its mission duration. The mission could be nominally set for a one-day flight, which would allow Shenzhou 5 to complete all of its primary mission goals. Simply getting a yuhanguan into orbit and bringing him safely back to Earth is all that matters for this flight.

But Shenzhou 5 and its commander are both likely to be functioning well after 24 hours in orbit. Telemetry on the state of the vehicle will be monitored closely through the flight, giving ground controllers a reliable view of its status. If there have been no serious problems with the mission after a day, it's possible that mission control will order an extended mission.

Based on orbital mechanics and previous test missions, the next most logical mission length is roughly seven days. This would give the yuhangyuan on board enough time to conduct a few experiments, which would probably be limited to photography, and monitoring his own physical condition. Chinese officials have suggested that the experiment package on board Shenzhou 5 will be much smaller than previous uncrewed flights, which were crammed with scientific payloads.

A week-long mission would also allow more media coverage of the mission, essential to the propaganda gains that have always accompanied human spaceflight.

Of course, a one-day flight by Shenzhou 5 will not necessarily indicate that anything went wrong on the mission. It will probably suggest that China is being cautious and incremental in its flight program, and has simply elected to bring the spacecraft down as soon as possible.

China is unlikely to say how long Shenzhou 5 will remain in space when it is launched, but this would not necessarily indicate an open flight plan. China has never been specific about announcing mission durations, even when the lengths were obviously fixed in advance.

The pros and cons of crew size and mission duration are interesting to consider, but spaceflight has always been influenced by economic, managerial and political factors. These can be as significant as engineering considerations. We don't know exactly what China will select for its mission profile, and even after it returns to Earth, we probably won't know the full story behind the design of the flight. But the most important aspects of the Shenzhou 5 mission will be revealed to us in just a few weeks.

Dr Morris Jones is an Australian university lecturer. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.

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