When it flies, Shenzhou 4 will be almost identical to its predecessor. This is due to the incredible amount of tinkering and revisions that went into Shenzhou 3, where many suggested modifications were apparently examined but rejected.

The fact that Shenzhou 3 was apparently successful in its mission also validates the configuration of the subsystems that flew. After three missions, China should feel ready to start freezing the design of Shenzhou, at least for the first generation of this vehicle.

There has been speculation in the media that Shenzhou 4 may attempt to rendezvous with the orbital module of Shenzhou 3, which is still circling the Earth. If China intends to do this, it will need to launch Shenzhou 4 fairly promptly, before the orbit decays. The orbital module has its own propulsion system, but no outside observers are sure of how much impulse it can provide.

Shenzhou 2's module remained in orbit for roughly six months, but careful boosting of the Shenzhou 3 module could sustain its orbital lifetime for longer than this. Can China get its next ship up in time? It should be noted that the Chinese themselves have remained tight-lipped about any rendezvous plans, and that stories of a possible rendezvous are the product of external observations. In the meantime, it's worth examining a few more details of the last mission.

There has been much speculation as to the payloads carried on the front of Shenzhou's orbital module. Photographs, models and illustrations of every previous Shenzhou vehicle have shown a complicated arrangement of antennas, boxes and a half-circular device with spokes. Some commentators have suggested that the payload is an ELINT (electronic intelligence) package, designed to eavesdrop on foreign targets from space.

It would be somewhat unusual for such a sensitive military payload to be displayed so openly in the media, but as usual, there is no way to conclusively defeat this suggestion.

It's even possible as this author suggested in previous reports that the device was a half-and-half view of a docking apparatus and an experiment package - based on images of Shenzhou models that have been circulated.

This is clearly not the case, but my wild guess, and the ELINT suggestion, all go to show how little is really known.

Clearer illustrations suggest that this half-ring device appeared on orbital vehicles, and it seems that it holds electromagnetic feed horn antennas or some other sort of sensors that scan the Earth below at various angles. Beyond this, nobody can be sure of what Shenzhou is carrying up front.

Whatever the case, the payload on the front of the orbital module is clearly not a docking device. Currently, China does not need to install a docking apparatus to conduct basic rendezvous experiments, but eventually, Shenzhou will need one if the platform is to evolve into a space station later this decade.

No illustration or model of Shenzhou has provided any insight into what Shenzhou's docking system looks like. In fact, China has not issued any public statements on the subject.

Will it be a copy of a Russian docking system? Will it be a probe-based system, as used on most Soyuz spacecraft, or will China opt for an androgynous docking system? Will it be compatible with any spacecraft currently in use by Russia or the USA? Nothing we have learned through official channels can provide any suggestions.

China may have decided on a docking system for Shenzhou, and may have a development program for the device. But there's another possibility. China may have elected not to finalize any particular design, pending further negotiations with other parties.

In the past, China has expressed interest in joining the International Space Station, although official contacts between China and ISS partner nations has been somewhat tenuous. The prickly political situation between China and the USA has probably been the major obstacle to their entry, but situations can change.

As budgetary pressures threaten the future of ISS, and the USA seeks the cooperation of more foreign nations for international security objectives, welcoming China on board the ISS could be a realistic option for the future.

China will clearly not be invited to dock a Shenzhou with ISS until the vehicle has a proven track record of safe performance, but steady progress with Shenzhou will provide this after a few years of operations. If China is serious about joining ISS, it may simply elect to use a Russian docking system. Presumably, China would manufacture its own device after buying an original from Russia. Shenzhou could then dock with a Russian module on ISS.

Aside from knocking on the hatches of ISS, China has also expressed a wish to build its own space station. A Chinese ferry vehicle docking with a Chinese station would, in theory, have no need for anyone else's docking apparatus, but there are still reasons why China may choose an international standard.

It could save time and money by using previously developed technology. A Chinese station could receive visits from foreign spacecraft, possibly forming a rival international program. Chinese station modules could also be docked to the ISS, in addition to China's own independent station.

Even if China does develop an indigenous docking system, it could still adapt Shenzhou with other mechanisms for specific missions. Russia has flown different mechanisms on Soyuz.

Some of these questions could be answered before the end of the year, if Shenzhou 4 carries a docking apparatus. China may elect to fly a docking apparatus even if there is nothing to dock with.

The mechanical components could be tested to simply verify that they perform in space. Alternatively, a sub-satellite deployed from Shenzhou, a separately launched payload or another fully developed Shenzhou vehicle could serve as a docking target.

Prior to the launch of Shenzhou 3, there was talk of a sub-satellite to be carried on the flight. No details of its mission were revealed. Was it originally intended to be a part of rendezvous experiments that never took place, or was it simply a secondary payload on an unrelated mission?

Recent statements from China also suggest that the putative sub-satellite was not carried at all. Was it deleted from the mission at the last minute, or was this a phantom payload from the beginning? If it was carried, did it fail to separate from the spacecraft?

Morris Jones is a Sydney, Australia-based journalist. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.

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