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Emily LakdawallaNovember 3, 2014

Chang'e 5 test vehicle flying on to Earth-Moon L2

The Chang'e 5 test sample return capsule successfully returned to Earth last week. But the sample return capsule wasn't the only spacecraft on the mission; there was also a service module, a spacecraft based upon the design of Chang'e 1 and 2. Today I learned that the Chang'e 5 test vehicle service module did not follow the sample return capsule into Earth's atmosphere. Instead, it successfully performed a divert maneuver, and is now on its way to the Earth-Moon L2 point, a gravitationally stable location beyond the Moon from which the spacecraft could be steered to any number of other destinations.

Lagrangian points

Wikimedia commons

Lagrangian points
From Wikipedia: given two massive bodies in circular orbits around their common center of mass, there are five positions in space where a third body, of comparatively negligible mass, could be placed which would then maintain its position relative to the two massive bodies. As seen in a rotating reference frame with the same period as the two co-orbiting bodies, the gravitational fields of two massive bodies combined with the centrifugal force are in balance at the Lagrangian points, allowing the third body to be stationary with respect to the first two bodies.

China has already performed this tricky piece of navigation once before, with the Chang'e 2 lunar orbiter; they sent it to the Earth-Moon L2 and then onward to fly by near-Earth asteroid Toutatis. According to China Military Online, that's not the plan for the Chang'e 5 test vehicle service module. Instead, it will stay at L2 for a little while and then enter lunar orbit, "in order to verify Chang'e 5 tasks related to flight control technology." I'm not exactly sure what that means. One thing it does mean is that China will soon have an operational orbiter at the Moon, in addition to the still-functional Chang'e 3 lander.

We do not yet know where they plan to send the Chang'e 4 or Chang'e 5 landers. (We don't even know if the spacecraft known as Chang'e 4 will even launch at all.) But if you'll allow me to indulge in a little bit of speculation, I can think of one excellent reason to have a lunar orbiter in place when you are planning future landed missions. With an orbiter, you could conceivably land something in a place you cannot see from Earth -- namely, the lunar farside. I can point you (thanks to a tip posted here) to a Chinese discussion forum where other people are speculating about the same thing -- if not for Chang'e 4 or 5, possibly even for Chang'e 6, the presumed backup to Chang'e 5. But that's pretty far in the future, and, admittedly, a long chain of speculation.

Why would a landing on, or sample return from, the farside be so cool? A large fraction of the farside is swallowed up in the South Pole-Aitken Basin, one of the biggest impact basins in the solar system. To the extent that a group of scientists ever agrees on anything, a sizeable fraction of lunar scientists would love to land in the South Pole-Aitken Basin. As one of the biggest holes in the solar system, it likely excavated lunar mantle; so there'd be the chance to see what rocks from deep inside a terrestrial planet look like (and, more importantly, what they are made of). It's also very, very far from the Imbrium impact basin, whose ejecta reached pretty much the entire nearside, possibly affecting all the Apollo samples.

Is the lunar farside in the plans for future Chinese landers? I don't know. But I really, really hope so!

Chang'e 1

Chang'e 1

Updated to add: Well, my speculative bubble has already mostly been burst. SETI Institute astronomer Michael Busch just happened to be in a place where he could ask some questions about this today. See his tweets below. I'm still holding out hope for Chang'e 6 being sent to the lunar farside!

In Irvine for the National Academies - Chinese Academy of Sciences forum on space science. A very wide range of things fall under the label

— Michael Busch (@michael_w_busch) November 3, 2014

@elakdawalla If I understood Ji Wu from the CAS Space Science Center correctly, Chang’e 5 is next with Chang’e 6 optional back-up.

— Michael Busch (@michael_w_busch) November 3, 2014

@elakdawalla Additionally: CE5 will be landing at low latitude on near-side. If CE5 works and CE6 happens, CE6 might land elsewhere.

— Michael Busch (@michael_w_busch) November 3, 2014

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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