Pundits immediately suspected that political factors had intervened to change the mission. The new launch time would place the mission between the end of the Beijing Paralympics and China"s national day on October 1. With memories of the Oympic Games still fresh in the minds of the world, the space mission could hog the limelight, and strengthen China"s international profile.

The muddled pattern of announcing the change added credibility to this claim of last-minute interference by the politburo. Chinese media sources announced a change, then withdrew the claim, then re-established the change with a more narrowly defined range of dates. Evidently, people close to the program were caught by surprise.

China made vague claims about the "window" for launch being more favourable during this period. This was perplexing. It was not as if Shenzhou 7 had to rendezvous with the Moon, a planet or anything else. Technical reasons for the change seemed scarce, unless the "window" was related to propaganda.

Now, China Daily, a state-run newspaper, has released a story claiming that the position of the sun will be different, which means that the spacewalking astronaut will leave the spacecraft in sunlight, and not return in "total darkness".

Is this claim credible? It"s not entirely dismissible. The sun"s overall pattern of illumination of the Earth does change with time, as the passage of the seasons demonstrates. Sunrise and sunset will change for a spacecraft in a fixed orbital plane, just as they do for someone on a fixed location on Earth.

There will be a slight amount of change over the difference of a few weeks, but not a lot. Is this really enough to influence the course of the spacewalk?

We have no information on the projected time of launch or orbital parameters for Shenzhou 7. So it"s difficult to independently check how, or if, this will influence the sunlight on the flight. But China may want to get more sunlight when the spacecraft is near or over China, where tracking facilities are more common. This would arguably be the best time and place to do the spacewalk.

China is a large country, but Shenzhou won"t stay over it for long. The spacecraft will complete an entire orbit of the Earth in roughly one and a half hours. The spacecraft will be in sunlight for roughly half that time, and in darkness for the rest. Assuming he leaves his spacecraft in daylight, the first Chinese spacewalker will need to move quickly if he wants to get back inside before night falls.

Some Chinese reports have claimed that the spacewalk should take less than an hour. It may indeed be a very short walk if China wants solar illumination at all times.

Possible gaps in communications with the spacecraft during the overall mission could be an issue. But China has beefed up its tracking network with more ships, and has also launched a data relay satellite into geosynchronous orbit. Communications with Shenzhou 7 should be quite good, wherever it is and whatever its orbital path. In any case, China can simply move its ships where they are most needed.

Shezhou also depends on solar panels for its power. The next mission will pose challenges in this regard. Shenzhou 7 will not carry the small set of solar panels normally found on the Orbital Module, as these would interfere with the spacewalk. Larger panels on the Instrument Module will provide power. There will be less power generated on this mission, and more crew on board. Perhaps a higher level of sunlight is needed to compensate for power shortages.

The issue of safety for the spacewalker would seem to be the most pressing of all these issues, but does China really need to change the launch time by this much to go for a spacewalk in the sun? They could simply adjust the launch time by hours instead of weeks, to account for the 24 hour day-night cycle. They could alter the trajectory of launch. They could simply plan their spacewalk to take advantage of the conditions they have.

Regardless of the technical excuses that could be advanced, one factor is common to all the theories. Every issue raised in this article, and by the Chinese, would have been obvious long before now! Technical planning would have identified these questions and developed a mission plan that satisfied them.

But it"s also possible that some mission planners have suddenly turned cautious, and asked for better conditions for the spacewalker. Maybe.

The official explanation provided for the last-minute change cannot be automatically dismissed, but it must also be placed under scrutiny.

Dr Morris Jones is an Australian spaceflight analyst and the author of "The Adventure of Mars". Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email. Dr Jones will take media inquiries on the Shenzhou 7 mission.

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