Parts of Soyuz have also been modified as a transfer vehicle for the small Pirs module that's currently docked to the International Space Station.

But the most successful modification of Soyuz has been Progress, the uncrewed cargo vessel that has allowed cosmonauts (and astronauts from various nations) to remain aloft for extended missions.

China's Shenzhou spacecraft seems tipped to perform a similar variety of roles for China's space program, if their ambitious goals are fulfilled in the years ahead.

China has announced plans for a "space laboratory", which will be a small space station capable of supporting short visits by crews.

This is probably intended to pave the way for a larger Chinese space station, which will host Chinese astronauts (and possibly their guests) for months.

If China intends to build a sustained human presence in space, it will need to develop a vehicle capable of resupplying its astronauts in orbit. China will need its own equivalent of Progress.

The importance of logistics in human spaceflight is enormous, as recent problems aboard the International Space Station have demonstrated.

Crew rations were cut, and an expedition was almost brought home prematurely due to supply shortages. Few nations have developed their own crew launch systems, but Europe and Japan are both developing logistics transfer vehicles for the International Space Station.

Yet China seems to have said little or nothing about developing its own logistics vehicle.

Without one, China will not be able to keep astronauts aloft for more than a couple of weeks, or find that its "space laboratory" can only support one short expedition.

A large gap in China's space planning apparently exists to an outside observer, but it's doubtful that China would have ignored this.

The most logical development path for a Chinese logistics vehicle would be to modify Shenzhou into a Progress-style spacecraft. This would allow most of the sub-systems developed for Shenzhou to be re-used, bringing cost savings and a high level of inherited reliability.

It would also produce a vehicle compatible with China's existing fleet of Long March launch vehicles.

China modified its Long March 2E satellite launcher for Shenzhou with the addition of a special payload fairing and safety systems, such as an escape system.

The result was the Long March 2F vehicle, which has now launched five Shenzhou spacecraft. Perhaps we will see the introduction of the Long March 2G, to launch a cargo version of Shenzhou.

This would probably retain the payload fairing of the 2F vehicle, but delete the escape rockets and aerodynamic panels that are used for crew ejection.

What would a Shenzhou-derived cargo vessel look like? The rear propulsion module, containing solar panels, rocket engines and other systems would probably be the same.

The front of the vehicle would probably also resemble Shenzhou's orbital module: A stubby cylinder with a docking collar at its front. Radar or some sort of radio beacon for docking would also be added, to enable rendezvous with other spacecraft.

Shenzhou sports a small set of solar panels on its orbital module, but the Chinese may opt to delete these on a cargo carrier.

Presumably, power requirements would not be as great without a crew on board, or the need to provide power for a disconnected orbital module in free flight.

As with Progress, the middle portion of such a vehicle is likely to sport the greatest changes. The robust, bell-shaped descent module will disappear, as there will be no need to return a crew to Earth.

Possibly, China could elect to make the cargo version a Shenzhou a simple cylinder, from its instrument module to the docking port. This would maximize the internal volume, allowing more room for cargo.

A cargo Shenzhou would probably be used to carry oxygen, food, water and equipment to orbiting crews.

China could also use it for fuel transfer to orbiting space stations, but it could be simpler (at least on a first-generation station) to simply use the cargo vehicle's own engines to provide an orbital reboost to the space station before it is dispatched.

The vehicle would also be used as a trash dump, to gradually be filled with refuse from a space station before it is sent on a destructive re-entry.

The reluctance to discuss a cargo Shenzhou could be due to feelings that it's a long-term matter. China will probably not require such a vehicle until it has extensive experience in human spaceflight and a maturing space station program.

But a Chinese Progress could be useful in the short term. A rough road map of plans for upcoming Shenzhou missions suggests that after long-duration flight is demonstrated on Shenzhou 6 this year, China expects to demonstrate extravehicular activity (spacewalking) and then rendezvous and docking.

The latter will require another spacecraft in orbit, ready to rendezvous with Shenzhou. Analysts have regularly suggested that Shenzhou's first docking target will be the free-flying orbital module of another Shenzhou spacecraft, left in orbit by an earlier mission.

If China plans to do this, it will need to quicken the pace of its launches, as all the orbital modules launched so far have re-entered before the next Shenzhou was launched.

But China may well decide that a cast-off orbital module, which may or may not be pressurized, is a small prize for an orbital quest. Why not dock with something more substantial?

One option could be to launch two Shenzhou missions almost simultaneously, then stage a rendezvous and docking.

There could even be a crew transfer. Such a mission profile was flown on Soyuz 4 and 5, although the cosmonauts on board these early Soyuz spacecraft didn't have the benefit of a pressurized tunnel between the two vehicles.

If China prefers a more incremental approach, the docking target could be an uncrewed Shenzhou spacecraft launched prior to the docking crew. But if no crew launch or recovery is performed by the docking target, it might as well be a Progress-style vehicle.

The docking crew could use the cargo vehicle as a sort of miniature space station, supplying them with extra room and supplies for an extended-duration orbital mission.

If China eventually becomes a partner in the International Space Station, it will also probably be required to help with logistical support to the station.

A cargo carrier would thus be called into service long before China develops a large independent space station of its own.

So, a cargo version of Shenzhou would seem to have a potential role in China's space program in the near future. In any case, its usefulness in the long-term is obvious. Is China planning such a vehicle, and is a prototype being assembled behind closed doors?

Dr Morris Jones can be contacted at morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.

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