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Orbits for Tiangong
by Morris Jones
Sydney, Australia (SPX) Aug 27, 2011

There's a possible strategy that the Chinese could employ between visits by Shenzhou spacecraft. It's clear that Tiangong 1 will be a lonely laboratory for much of its life, with no visiting spacecraft. During this time, mission controllers could boost the laboratory into a higher "storage" orbit that will prolong its overall mission.

The impending launch of China's first space laboratory will end years of waiting for this important mission. It will also hopefully begin to answer some of the many questions that remain unanswered about China's latest spacecraft.

While China will probably remain coy about some details of the mission, one issue will be settled fairly soon after launch. What sort of an orbit will Tiangong 1 enter?

Tiangong 1 is expected to remain in orbit for roughly two years. This is far longer than even the longest extended missions of Chinese Shenzhou spacecraft, which will eventually be used to carry astronauts to Tiangong.

Fighting orbital decay will be an issue for this mission, and there's one easy way to do that: Start off in a fairly high orbit. This will allow Tiangong to gradually reduce its orbit over an extended period. Tiangong also has its own propulsion system, which will probably be fired periodically to raise the space laboratory's orbit.

Thus, it seems reasonable to expect that Tiangong will enter orbit slightly higher than previous missions in China's human spaceflight program.

So far, all launches of China's Shenzhou spacecraft have been made to fairly low orbits. The orbit of Shenzhou 5 is typical, at an altitude ranging from 200 to 343 kilometres. This low orbit makes recovery of the mission easy. Less thrust is required to come back home. It's also an orbit within the capabilities of the Long March 2F rocket that launches the Shenzhou spacecraft. But there's a tradeoff.

Even with the occasional firing of thrusters to raise their orbits, the discarded orbital modules of Shenzhou spacecraft struggle to stay aloft for more than a few months. Tiangong is a much larger spacecraft, and it will encounter more atmospheric drag than a Shenzhou orbital module. Staying alive at a low altitude will be tricky.

Even so, at its lowest operational altitude, we can't expect Tiangong 1 to fly too much higher than previous Shenzhou missions. Perhaps it will raise its orbit by a few tens of kilometres. The perigee, or lowest point of the orbit, should be higher, even if the apogee (highest point) is about the same.

Any higher would challenge the capabilities of the rocket that launches the laboratory. Tiangong is to be launched aboard a Long March 2F/G rocket, which will make its maiden flight alongside that of the laboratory itself.

This rocket is basically a modified version of the Long March 2F rocket, which has launched every previous Shenzhou spacecraft. Crew escape systems have been removed, and the payload fairing has been changed to accommodate the laboratory.

The laboratory is heavier than a Shenzhou spacecraft, but not much heavier. As compensation, taking out the escape rocket and stabilisers for ejection has probably saved a lot of weight on the rocket itself. This should make a modestly higher orbit possible. As a final nudge, Tiangong's own thrusters could be fired to raise it high soon after launch.

A very high orbit could also possibly create problems for the Shenzhou spacecraft that are expected to not only rendezvous with it in orbit, but return at the end of their missions.

Getting back to Earth from a higher orbit requires more fuel, and reduces the safety margins if the propulsion system doesn't perform properly. We expect that the unmanned Shenzhou 8 mission will launch soon after Tiangong, so the Chinese can't afford to place the lab out of easy reach.

There's a possible strategy that the Chinese could employ between visits by Shenzhou spacecraft. It's clear that Tiangong 1 will be a lonely laboratory for much of its life, with no visiting spacecraft. During this time, mission controllers could boost the laboratory into a higher "storage" orbit that will prolong its overall mission.

Tiangong can either be left to naturally lower its orbit over a long period afterwards, or have its orbit quickly lowered with more thruster burns before another Shenzhou spacecraft is launched to it. Similar tactics were used with early Soviet space stations.

It's also possible that Shenzhou spacecraft launched to Tiangong will also help to maintain its orbit. After docking, they could fire their own engines to boost the orbit of the two docked vehicles. Orbital reboost from docked spacecraft is used to keep the massive International Space Station in orbit, and it would be even more effective with a small laboratory.

The planned altitude is a mystery, but we can probably guess the orbital inclination. All previous Shenzhou missions have flown at an orbital inclination (angle) of roughly 42 degrees. This type of orbit is mostly dictated by the position of the launch site at Jiuquan, which will be used for Tiangong as well as Shenzhou. There's no reason to expect this to change for Tiangong at launch, or at any other stage in its mission.

Even if the Chinese don't supply us with the details, the orbit of Tiangong will quickly be worked out by space tracking systems around the world, including amateurs. Watching for changes will be interesting, and will give clues to how Tiangong's marathon mission is progressing.

Dr Morris Jones is an Australian space analyst and writer. Email Replace NOSPAM with @ to send email. Dr Jones will accept media inquiries.


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