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The Chinese space craft that just landed on the moon had a tube with a biosphere in with flies and plants. What are they expecting to learn? Didn't we already experiment that on the ISS?

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Source

Chinese scientists released this image of a cotton plant germinating in its tank on the moon aboard the Chang'e 4 lander. The photograph was taken Jan. 7, 2019. Credit: Chongqing University

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    $\begingroup$ Your title is unclear on which part you're asking about. Do you mean why did they send this stuff to specifically the far side of the moon, as distinct from some other part of the moon? Or why did they send this specific stuff, rather than something else? In either case, I suspect the answer is that they wanted a mission that included living things, they wanted a mission to the far side, and the living things don't care which side of the moon they're on so they may as well kill both birds with the same stone. $\endgroup$
    – David Richerby
    Jan 17, 2019 at 11:49
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    $\begingroup$ Sorry to break it to you, but the cotton plants have died. $\endgroup$
    – JPhi1618
    Jan 17, 2019 at 16:19

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Right now, almost 100% of existing research on growth in gravity fields is basically at 0g (ISS/Mir/Skylab/whatever) or 1g.

There are a lot of questions of what happens at 1/6g or 2/3g? No good experiments to demonstrate.

Thus the Chinese lander is testing 1/6th g.

SpaceX is likely to brute force test 38% g when they get to Mars. It is entirely possible that mitigation techniques developed for 0g are not useful or needed or effective at 38% g.

The Space Studies Institute (Via Gary Hudson, of Rotary Rocket infamy (alas)) is pushing for something they call G-Lab, an orbital facility with a control facility, at 0g and then a centrifuge at some other value.

If you are interested in this topic, there is an excellent "The Space Show with David Livingston" episode where Gary Hudson discusses this issue. The Space Show does not allow transcripts so I cannot link to anything like that, you are left with only listening.

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    $\begingroup$ I suspect it would actually have been cheaper to build a dedicated ⅙g artificial-gravity mission in LEO for these experiments (basically need only tether the tubes to a counterweight and spin it up with a monopropellant thruster), than to add them to the moon mission with its much higher Δv demand. Just, “first plant in ⅙g” doesn't have quite the PR ring that “first plant on the moon” has. $\endgroup$
    – leftaroundabout
    Jan 15, 2019 at 22:31
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    $\begingroup$ Putting a small spinning plant on the ISS seems a lot easier for low G experiments too, even just two hanging flowerpots on a string, or a centrifuge $\endgroup$
    – Xen2050
    Jan 16, 2019 at 4:18
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    $\begingroup$ @leftaroundabout: A low-g centrifuge in LEO would only test growing plants in a centrifuge in LEO. Growing them on the moon tests growing them in the actual lunar environment. Of course for a full test, they should have scooped up some lunar "soil" and tried growing plants in that. $\endgroup$
    – jamesqf
    Jan 16, 2019 at 5:18
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    $\begingroup$ @jamesqf I highly doubt that plants would survive in lunar regolith, at least without a lot of pre-processing. Both on the moon and on Mars, the first plants will be grown hydroponically and more or less completely isolated from the environment, save for the gravity, and that again is pretty much perfectly emulated by a centrifuge. (You may object that a centrifuge also generates Coriolis force, but that's hardly relevant for a plant because nothing in it moves fast.) $\endgroup$
    – leftaroundabout
    Jan 16, 2019 at 8:51
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    $\begingroup$ "at 0g (ISS/Mir/Skylab/whatever) or 1g.": you should specify where the 1g experiements are. (Kidding.) $\endgroup$
    – msh210
    Jan 16, 2019 at 10:38
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They did it for propaganda or pride mostly. What sprouted quickly died because it froze. They did not have a method to protect the biosphere from the temperature swings. This is what they expected and admit.

The Chang'e-4 probe entered a "sleep mode" on Sunday as the first lunar night after the probe's landing fell. The temperature could drop as low as about minus 170 degrees centigrade.

"Life in the canister would not survive the lunar night," [experiment designer Professor] Xie [Gengxin] said.

(Xinhua)

There was no intention of seeing how well plants grow to any level of maturity. All they did was prove a seed can sprout at 1/6g. If a seed can sprout at 1g or 0g, I think it is safe to assume it would sprout at 1/6g. They might have proved a seed can sprout outside the protection of Earths magnetosphere, but the moon is sometimes in the tail of our magnetosphere. I'm not sure if it was at this time. Testing if the cumulative radiation dose over such a short mission would damage a seed could have been tested on earth. There was no real scientific value in this experiment that was not also obtainable for less effort and cost. The logical conclusion is that it was done for human emotional reasons. I'm not knocking them for that, I think it's cool.

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    $\begingroup$ Seems like there's a contradiction here; another answer says they did have a way to protect from temperature swings. Can you add a reference for the lack of protection, especially if from official Chinese sources? $\endgroup$
    – Nathan Tuggy
    Jan 17, 2019 at 3:25
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    $\begingroup$ Several articles have bits and pieces from Professor Xie Gengxin (head of "experiment"). If you search his name you can find many quotes. Here is an example: xinhuanet.com/english/2019-01/15/c_137745505.htm If you dig thru the articles found by searching him you will see some fancy wording about "special" aluminum alloy materials and such. Which is a dead tell for propaganda in China or North Korea. Unless they invented transparent aluminum, it's BS. Here is the kicker: "Life in the canister would not survive the lunar night," Xie said. Parse the words Would not vs Did not. $\endgroup$
    – Diznaster
    Jan 18, 2019 at 4:31
  • $\begingroup$ Can you show an article where they explained their heating mechanisms and biosphere climate controls? $\endgroup$
    – Diznaster
    Jan 18, 2019 at 4:32
  • $\begingroup$ "propaganda and pride mostly" and to provide the scientists and engineers involved some practical experience $\endgroup$
    – noɥʇʎԀʎzɐɹƆ
    Jan 18, 2019 at 6:07
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    $\begingroup$ I believe @NathanTuggy's comment was not a challenge, Diznaster, but a suggestion as to how you could improve the quality of your answer. $\endgroup$
    – user20636
    Jan 18, 2019 at 10:57
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According to the Planetary Society, they ran a competition$^1$ for an experiment to make use of spare capacity on the lander. This one was chosen, most likely for it's potential to cultivate interest in the mission.

$1$:

On January 8, 2016, the Chinese Lunar Exploration Program (CLEP) released an unusual "Call for Proposal" to the general public, for a small payload that may go on-board the Chang'e 4 lander and/or the planned relay satellite at Earth-Moon L2 point. The details of the call are available here (in Chinese).$^2$ The payload is limited to 3 kilograms in mass and 20 watts of power. It seems CLEP is most interested in a payload that is useful for public outreach

$2$: NB: Microsoft Word document

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