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China lands Chang'e-4 on far side of Moon

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Re: China lands Chang'e-4 on far side of Moon

#51

Post by isostatic »

adventured wrote: post #36
jetison_out_to_ wrote: post #33

Noticed this is the same day as NASA got the Ultima-Thule photos. Was this announced today to overshadow it, like how Uber announced their IPO basically a day after Lyft's? Want to avoid going off about conspiracy theories or who's more patriotic, just curious how the announcements actually lined up. This (wikipedia-sourced) AP wire from Oct 22, 2015 when the Pluto flyby happened said they had re-pointed the probe fo…

> Is this what a space race feels like? No. New Horizons capturing Ultima Thule is not a large enough event to warrant the Chinese feeling like they need to compete with it. Spectator wise, it's a nice event for anyone that likes astronomy. It will be ignored by most of the people on the planet, just as the Chinese landing on the far side of the moon will be. These are not seminal moments in space history.

New Horizons was the most read story on bbc yesterday morning. China’s landing is today.

It’s no Sputnik or Gagarin or Apollo, but experts at Nasa as "a first for humanity and an impressive accomplishment".

Re: China lands Chang'e-4 on far side of Moon

#52

Post by dempseye »

tanilama wrote: post #41
arduanika wrote: post #5

"...and if the dam breaks open many years too soon..." Perhaps interesting to note the origin of this name for spacecraft. The legend "Chang'e flies to the moon" (嫦娥奔月) lies behind the Mid-Autumn Festival (featuring mooncakes, as it were). Point is, the names of Chinese space missions are every bit as carefully steeped in tradition as, say, Mercury/Gemini/Apollo.

The story behind Chang'e is also interesting. Essentially, she is the wife of a legendary hero, Hou Yi(后羿), who saved the mankind from dying of drought due to the presence of nine Suns, by shooting all but one of them down. Chang'e stole the Immortal Pill, and eat it alone without her husband knowing. After she ate it, her body became to float and she flied to the moon, to live there alone forever as her punishment,…

The trisolarian plot in Cixin Liu's The Three Body Problem also seems to have borrowed from this myth.

Re: China lands Chang'e-4 on far side of Moon

#53

Post by BattyMilk »

Apologies in advance for the (hopefully not) stupid questions from a layperson...

How constant is the L2 point for the relay satellite's orbit? If I understand correctly, this is the "balance" point between the earth and sun's gravity. Does the moon's gravity affect this? If so the L2 point will shift as the moon moves towards and away from this point?

As the moon orbits the earth, presumably the relay satellite would stay put and not follow the moon in its orbit. This would mean that communication from the lander and rover to the satellite is only possible when the moon is in a particular position (between the earth and the sun), correct?

Re: China lands Chang'e-4 on far side of Moon

#54

Post by brainwipe »

A2017U1 wrote: post #4

Copying earlier comment: Wikipedia seems to say the rover mission duration is 3 months, yet the previous rover traveled for a year and transmitted a bit longer. The Chinese seemed quite coy about it failing last time (as with many things) so perhaps underpromising in this case? This all preparation for China's planned permanent radio telescope on the far side which would be a huge boon for astronomers by escaping Ear…

Regarding underpromising - there might be a little management of expectations but to paraphrase a friend in "the biz": facing away from the Earth means that the stuff you need to be reliable jumps up in complexity and with that complexity comes a foreshortening of life. She then babbled incoherently in a way I enjoyed but didn't understand.

Love the idea of a dark side telescope!

Re: China lands Chang'e-4 on far side of Moon

#55

Post by juloo »

BattyMilk wrote: post #53

Apologies in advance for the (hopefully not) stupid questions from a layperson... How constant is the L2 point for the relay satellite's orbit? If I understand correctly, this is the "balance" point between the earth and sun's gravity. Does the moon's gravity affect this? If so the L2 point will shift as the moon moves towards and away from this point? As the moon orbits the earth, presumably the relay satellite woul…

It's the L2 point in the earth-moon system, it is different from the earth-sun L2 point. And yes, the satellite follow the moon in its orbit, the point in the L2 point.

Re: China lands Chang'e-4 on far side of Moon

#56

Post by mkl »

BattyMilk wrote: post #53

Apologies in advance for the (hopefully not) stupid questions from a layperson... How constant is the L2 point for the relay satellite's orbit? If I understand correctly, this is the "balance" point between the earth and sun's gravity. Does the moon's gravity affect this? If so the L2 point will shift as the moon moves towards and away from this point? As the moon orbits the earth, presumably the relay satellite woul…

The L2 point that the satellite is orbiting is a Lagrange point [1], a place where the gravity (edit: and centrifugal forces) of the Earth and the moon balance. L2 follows the moon as the moon orbits, because the moon is what makes it exist.

[1] https://en.wikipedia.org/wiki/Lagrangian_point

Re: China lands Chang'e-4 on far side of Moon

#57

Post by andybak »

789body wrote: post #9
jackconnor wrote: post #3

The way they used the L2 point to lasso a relay satellite so they could transmit information from the rover on the far side is stunning too. This was really brilliant, China's space program has really become something.

This comment threw me for a loop - "L2" and "lasso" triggered the pattern recognition in my head for "regression regularization" and it took a moment to realize the topic was still orbital dynamics. Come to think of it, saying "Lagrangian" wouldn't have prevented my confusion - not that there's anything wrong with the terminology in OP. I'm reminded of a passage in Cryptonomicon , which I've been rereading: "Newton w…

(Can you edit the fixed width formatting? It's impossible to read without horizontal scrolling)

Re: China lands Chang'e-4 on far side of Moon

#60

Post by mkl »

turbinerneiter wrote: post #47
stcredzero wrote: post #7

Earlier quoted context omitted.

The way they used the L2 point to lasso a relay satellite So confusing to put it that way. No one has caught anything with a rope or a cable, as far as I know. Basically, they are orbiting a relay satellite around the L2 Lagrange point so that the relay has line of sight with both the landing point and Earth. Obviously, if it just sat at L2, the moon would be in between the relay and the Earth. Orbiting around Lagran…

Isn't there a solar telescope around sun-earth L1?

Yes, more than one, in fact: https://en.wikipedia.org/wiki/Lagrangian_point#Spacecraft_at...
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