China could launch its second lunar probe within days. The Chang’e 2 spacecraft was originally built as a back-up to China’s first lunar orbiter, Chang’e 1, which was launched successfully in 2007. Rather than mothballing the spare spacecraft, China has sensibly planned a follow-up mission that will examine the Moon even more closely than the first.
Chang’e 2 has been fairly camera-shy for the media, and we have received only a basic disclosure of its mission. We can expect that it will look pretty much the same as Chang’e 1, a boxy orbiter with large solar panel wings. Some of the instruments have been changed, and the overall mission plan is also being tweaked.
Some changes will take place soon after launch. China is planning a more direct climb out of Earth orbit to the Moon, which will see the spacecraft reach its goal in roughly five days. This is less than half the transit time of the first mission.
Chinese officials claim that they have greater confidence in their ability to track and navigate the spacecraft, which leads to a faster trip. This is fair enough, but it will also possibly reduce the exposure of Chang’e 2 to Earth’s radiation belts. This could help the longevity of some instruments, but it’s probably not a core reason for the change.
Reaching the Moon, Chang’e 2 will be placed in an orbit “100 kilometres closer to the Moon”, according to a Xinhua report. The math is simple. Chang’e 1 was placed in an orbit of roughly 200 kilometres, so we can expect Chang’e 2 to orbit at roughly 100 kilometres above the surface. The orbital altitude has halved!
This means that Chang’e 2 is China’s first “close encounter” probe with the Moon. This is a fairly low orbit that will allow very close studies of the surface. NASA’s Lunar Reconnaissance Orbiter (LRO) is currently in a similar orbit.
It was stated that Chang’e 2 will return higher resolution images than its predecessor. It’s clear that the camera itself is a better instrument (Chang’e 1’s resolution was around 120 metres), but some of the improvements in the images will also be a result of the lower orbit. Chinese statements speak of resolutions between one metre and seven metres, depending on the distance to the Moon.
It’s a fairly large difference, and it suggests a lot of variation in the orbit. It is probable that Chang’e 2 will mostly operate in a near-circular polar orbit, like its predecessor. The spacecraft could be directed into a very low orbit during the final phases of its mission. Some reports have spoken of passes across the surface lower than 10 kilometres, a trajectory that will probably be performed during the final phases of the mission.
What will the spacecraft see? The resolution won’t be as good as LRO, which can see details on other spacecraft that have landed on the Moon. But China will be able to map the Moon with greater precision, and can use its existing global lunar map from Chang’e 1 as a reference framework for the new data.
It hasn’t been long since Chang’e 1 returned its data, but it’s almost certain that some new craters will be discovered. One crater that China may want to search for will be the impact site of Chang’e 1, which was steered to a controlled hard landing. The impact site of Japan’s Kaguya orbiter, which was also deorbited, could also be explored.
Chang’e 2 will also release an impactor to strike the Moon, a feat earlier performed by India’s Chandrayaan-1 orbiter. The Chang’e 2 impactor will serve several functions. It will give China experience in descending a spacecraft from lunar orbit and tracking it.
This will be good practice for China’s plans to softly land spacecraft on the Moon later this decade. It will also allow the surface environment to be explored more closely. The Chang’e 2 orbiter will observe the results of this impact, which is expected to generate a dust plume.
NASA may also elect to explore the crater formed by the Chang’e 2 impactor with the Lunar Reconnaissance Orbiter, assuming it’s large and bright enough to be seen by its camera. It would be ironic if NASA released such an image before China did! But it’s not entirely clear if such a crater would be visible to the cameras of either spacecraft.
Xinhua has also stated that Chang’e 2 will photograph the landing site planned for the Chang’e 3 robot lander, which is still apparently slated for 2013. This suggests that the landing site has already been tentatively chosen, or at the very least, China now has a short list of candidate sites.
Sites have possibly been earmarked for China’s other landers, which include at least one sample-return mission. The need to scout landing sites is probably a major motivation in selecting the low orbit in the first place. Seeing boulders large enough to pose a hazard to these landers will require very high-resolution imagery, and it seems likely that these landing sites will be high-priority targets when Chang’e 2 makes its lowest orbits above the Moon.
It is unclear how long the mission of Chang’e 2 will last. Keeping a spacecraft in a lower orbit requires more navigational skill, and also leads to a faster orbital decay. It’s possible that this mission will end much sooner than Chang’e 1, but this will be an acceptable compromise for the closer look.
As with Chang’e 1, the Chang’e 2 spacecraft will be launched aboard a Long March 3A rocket. This is a fairly reliable workhorse in China’s steadily growing stable of launch vehicles. It should perform well on the flight, as it did for Chang’e 1.
Soon, this long-anticipated mission will become a reality. We would like to know more about the spacecraft, but it seems that China’s public relations policies are no more relaxed than they were in the past. For the moment, we can simply think about what we know for sure, and try to make educated guesses about what we don’t.