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Headshot of Emily Lakdawalla

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

Posted by Emily Lakdawalla

03-11-2014 12:21 CST

Topics: mission status, Chang'E program

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!

See other posts from November 2014


Or read more blog entries about: mission status, Chang'E program


Dwayne Day: 11/03/2014 04:57 CST

The plans for CE-4 remain a big question mark in my view. Assuming that it includes a rover, I would assume that China would like to gain more experience. CE-3 was an impressive mission, but there is a lot more they could have done with Yutu, and gaining extended rover experience would have a good payoff. Another question is if they plan on including a rover with CE-5. That could improve sample collection, but it would add complexity, so gaining more rover experience would be a plus. Finally, I'd add that a South Pole-Aitken Basin sample return mission was highly rated by the U.S. planetary science decadal survey. If the Chinese read and agreed with that report, they could perform a high value scientific mission before the United States conducts it. That would be a real scientific achievement to brag about.

ethanol : 11/03/2014 06:56 CST

I second the curiosity about CE-4. It was supposed to carry another rover up in 2015, but all I've heard lately is uncertainty. Does the comment that CE-5 is next mean that CE-4 has been put on hold? It is a scientifically proven fact that one moon rover is not enough. Also, it is the only mission planned to launch next year. Anyone in the know please share.

Dwayne Day: 11/04/2014 12:48 CST

The value of flying CE-4 with a rover would be both scientific and engineering experience. The ground penetrating radar on the rover was a unique instrument, and _any_ data from such an instrument is valuable. So it would be nice if China flew again and used that instrument to provide local data. For instance, provide subsurface data on a crater impact. China was not able to operate Yutu for very long, and if they flew another one, they could get a lot of experience that might be useful both for sample return and even an eventual Mars rover. JPL proposed both lunar and Mars rovers starting in the late 1960s but abandoned both while still in the study stage. It was not until 1997 that NASA eventually flew its first rover. Pathfinder/Sojourner provided NASA with a lot of valuable experience for Spirit and Opportunity. China could similarly benefit by flying another rover.

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