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Emily LakdawallaJune 16, 2011

Chang'E 2 is on its way to Sun-Earth L2

According to an article published a week ago by the Xinhua news service, Chang'E 2 departed the Moon on June 9 at 09:10 UTC. It's now headed toward a Lagrangian point in space, but not the one I thought it was headed for. Previously I've talked about it going to the Earth-Moon L2 point (on the far side of the Moon as seen from Earth) but I was wrong about that. It is actually headed on a much more ambitious journey to the Sun-Earth L2 point, which is about 1.5 million kilometers away. It will take "about 85 days" for Chang'E 2 to arrive there, which puts it at the beginning of September.

Why go to L2? It'd be a good spot for a telescope since it wouldn't be affected by Earth's nearby blazing light, and in fact the James Webb Space Telescope will eventually be placed there. (Don't fear Webb and Chang'E 2 encountering each other -- L2 is a broad region of space and it will not be a challenge for the two spacecraft to avoid each other.) But L2 is interesting for another reason -- it's a waypoint on the interplanetary superhighway, a position from which a relatively small amount of thrust can send a spacecraft to a diverse array of destinations.

Not that Chang'E 2 is likely to go to another planet. Reading the rest of the article, it reinforces my speculation about China's motivation in sending Chang'E 2 to L2: they're using the extended mission to test out their capability of handling several very big challenges that make interplanetary travel much more difficult than Earth orbit. The article comments about how operating Chang'E 2 there poses "great challenges to the country's technology in measure and control, telecommunications, data transaction and orbit design," and goes on to say:

"We are developing outer space measure and control stations in outer space and they will be capable to carry out tasks by the end of the second half next year," said an SASTIND [Science, Technology and Industry for National Defence] scientist, who declined to be named.

At that time, the satellite can be used to test the two stations' functions, the scientist said.

Challenges exist as Chang'e-2 was not designed for the additional task and it is now in extended service without extra capacities to deal with abnormal risks, Zhou [Jianliang, deputy chief engineer of the Chang'e-2 measure and control system of the Beijing Aerospace Control Center (BACC)] said.

Meanwhile, long-distance brings many problems like weakening signals and difficulties in measure and control, Zhou said. Best of luck to Chang'E 2 on its travels beyond the Earth and Moon!

Chang'e 1

Chang'e 1

Read more: mission status, Chang'E program

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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Emily Lakdwalla
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