Sending the back-up spacecraft on its own mission makes sense. The spacecraft was already built, and the integrity of its design was confirmed by Chang'e 1's successful mission.

By changing a few instruments, the mission can perform tasks that weren't carried out by the first orbiter. China has added a high-resolution camera to Chang'e 2, and has also added a small surface impactor. The mission will also fly to the Moon with a faster trajectory.

The existence of the back-up Chang'e spacecraft was publically disclosed shortly before Chang'e 1 was launched, and the likelihood of its launch as a follow-up mission was also apparent. But this is not the end of China's plans for robot lunar exploration.

China has long advertised its plans for landing two types of spacecraft on the surface of the Moon. One is expected to deploy a small rover to drive across the Moon, while another will return lunar rock samples to Earth. Both types of mission are expected to use the same type of common landing stage, with different payloads for different missions.

Chinese media statements have previously suggested that there would be one of each type of lander, with the rover mission scheduled for around 2013 and the sample- return mission for around 2017. This seemed like a highly plausible mission timeline.

Without a lot of overt discussion in the general Chinese media, there seem to be reasons to believe that China is actually intending both missions to be carried out by twin spacecraft. It seems that there could be two rover missions and two sample-return missions on the manifest.

A June 2010 edition of the Global Times, China's internationally-focused state newspaper, carried a generally mundane overview of the Chang'e lunar program, but included an interesting fact table at its end. This table outlines the Chang'e 1 mission as the first phase of China's lunar exploration, dubbed "Lunar orbit". It then goes on to outline phase 2, called "Moon landing".

Three missions are included in this phase: Chang'e 2, Chang'e 3 and Chang'e 4. A short blurb mentions this phase including landers and rovers. The third phase, dubbed "Returning", is focused on retrieving samples, and includes the missions Chang'e 5 and Chang'e 6.

Whoa! Six missions on the manifest, when just a few years ago, we had expected only three!

Let's dissect the plans. Chang'e 2 is, strictly speaking, an orbiter, but the inclusion of a lunar impactor will certainly count as a lunar landing, albeit a hard one.

The Chinese have previously stated that this mission was intended to provide support data for future landers, and the deorbit and descent phase, coupled with tracking the fall, will certainly help. We could then say that Chang'e 3 and 4 are true landers, each carrying a rover. Similarly, Chang'e 5 and 6 are probably both sample return missions.

The low-key approach to publishing this information could be deliberate. China was very coy about disclosing the existence of a back-up spacecraft for Chang'e 1, and only revealed this shortly before its launch.

The mission planning for the landers is probably similar. China is probably preparing two identical spacecraft for each mission phase.

If the first mission fails, the backup can be sent in its place. If the first mission succeeds, the back-up spacecraft can be re-tooled with a few different instruments, and sent to fly an additional mission. Calling a second spacecraft a "backup" or a "follow-up" is a subtle distinction, and the definition can only be settled after the first one is launched. China can thus hedge its bets against failure while avoiding any overt signals that it lacks confidence in the mission.

Let's assume that China's first soft lunar landing is successful, and the mission leaves rover tracks in the regolith. What would be done with the second rover-bearing lander? China may elect to make no changes at all. Simply landing in a different area could produce variety enough in the science data, especially when the Moon offers so much geological diversity.

There's a chance that some instruments could be changed, or new ones added. This could depend on how well certain experiments perform on the first mission. Some instruments may work perfectly, but still be deemed unworthy of a second flight.

The sample-return missions, Chang'e 5 and 6, are likely to offer less potential for variation. These missions are likely to be heavier and more complex to operate. The potential for extra payload space that doesn't support the main task could be minimal.

We can still probably assume that Chang'e 3 will fly in 2013 or soon afterwards. The Global Times article mentions phase 3 of the plan (featuring Chang'e 5 and 6) as "starting from 2017".

This gives a nice time interval for reviewing the performance of Chang'e 3, and considering any modifications that may need to be made to its twin, Chang'e 4. Even if all goes well, there may be a decision to space the launches out, allowing a regular pace of flights for personnel behind the landings.

Dr Morris Jones is an Australian space analyst and writer. He has covered the Chinese space program for more than a decade. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.

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