China’s second lunar mission, Chang’e 2, will perform several tasks during its flight. Some of these are purely scientific, aimed at helping us to understand the Moon and the space around it.
One task is highly practical. China is using the spacecraft to scout landing sites for future Chinese Moon missions.
China’s first lunar orbiter, Chang’e 1, made a complete map of the Moon. While this tells us a lot, it is not very useful as a guide to safely landing there. The resolution of the spacecraft’s camera was around 120 metres, which means that hazardous objects such as boulders won’t show up.
Chang’e 2 carries a more powerful camera which, according to one recent Chinese statement, boasts a resolution of ten metres. Other statements have spoken of resolutions of seven metres, or 1.5 metres, or even one metre. That’s a lot of variation. It’s partially a result of the fact that the resolution of the camera will vary with distance.
The closer the Chang’e 2 spacecraft gets to the Moon, the more it will see. Chinese media recently reported that the orbiter will descend to roughly 15 kilometres to get the highest resolution images.
China will need the highest possible high quality images if it is to accurately map landing sites for spacecraft, and determine that they are safe. If the resolution of the images is no better than two metres, then problems could easily go unnoticed. Large boulders lying in otherwise clear terrain represent the greatest threat to a safe landing.
We have seen how dangerous it can be when the mapping is insufficient. Prior to the Apollo Moon missions of the 1960s and 1970s, NASA undertook its own campaign to map the Moon with robot orbiters.
A sequence of “Lunar Orbiter” spacecraft produced the first ever global map of the Moon. These spacecraft recorded their images on ordinary film, then developed the film on-board the spacecraft and scanned the images for radio transmission to Earth. No video cameras or CCD chips were used! Amazingly, the spacecraft returned images with a resolution as high as two metres.
The Lunar Orbiter maps were good, but not good enough. In 1969, astronauts Neil Armstrong and Buzz Aldrin were descending to the surface of the Moon on board the Lunar Module “Eagle” during the Apollo 11 mission.
As they approached their designated landing site in the Sea of Tranquility, the astronauts were shocked to find their touchdown area strewn with boulders! Armstrong quickly took control and steered the Lunar Module to a nearby, safer area. A near-tragedy had been averted.
NASA knows it must have better data for its next landings. The Lunar Reconnaissance Orbiter (LRO), currently orbiting the Moon, has a maximum resolution of around 30 centimetres, good enough to see old landers resting on the lunar surface. This spacecraft, launched in 2009, was developed to support the Constellation program, a project aimed
at returning US astronauts to the Moon around 2020. Unfortunately, a US manned lunar landing within another decade now seems impossible. But data from LRO will still probably guide a new series of robot US landers to the Moon in the near future.
China expects to stage its first robot landing on the Moon in 2012 or 2013. The Chang’e 3 spacecraft will carry a small rover to the surface, and will operate a small astronomical telescope, in addition to other instruments. It is unclear if the lander has any ability to navigate around hazardous objects during its descent, as Armstrong did.
As an educated guess, it probably cannot do this. Hazard avoidance technology like this is highly advanced, and is still somewhat new to more advanced space agencies like NASA.
The best option for steering China’s robot landers to safety is to ensure that no unexpected surprises await them. This will place heavy demands on the performance of the Chang’e 2 mission.
China has recently revealed that Chang’e 3 is targeted for the Bay of Rainbows, a lava plain on the Moon. It’s easy to see why. This area is remarkably flat and featureless in low-resolution images. It’s also large, allowing plenty of room for rough navigation in the descent. A close look should confirm its suitability.
China has probably shortlisted a number of other landing sites that could be used for other landers, or possibly for Chang’e 3 if the Bay of Rainbows is judged unsuitable.
China’s lunar exploration plans are highly ambitious. Apart from deploying a rover on the Moon, China also plans a sample-return mission. This will retrieve rock samples and place them inside a small rocket carried by the lander.
The rocket will then take off from the Moon. As it approaches Earth, a small capsule with the samples inside will separate from the rocket and land in China.
The sample-return mission is expected to fly around 2017. Most reports suggest that there will be one rover-lander and one sample-return mission this decade, but this is not totally clear. One table of missions released in the Chinese media listed two rover-landers and two sample-return missions. As usual, we won’t know for sure for a while.