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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

#61

Post by 0nce »

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…

Lagrange points are found near any couple of large bodies. They are points where a smaller object will maintain its position relative to the large orbiting bodies.

The Chinese relay satellite is orbiting the L2 point of the Earth-Moon system. That is beyond the Moon on a line which goes from Earth to Moon. Hence, this L2 point orbits the Earth like the Moon does, and the whole system orbits the Sun as well.

Now if the spacecraft would be exactly at that point, the Moon would be blocking communications between the probe and Earth antennas. So instead, it follows a halo orbit which is quite stable around that point, but goes over the Moon horizon so that there is always a direct line of sight between Earth and the spacecraft.

Not a stupid question at all !

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

#63

Post by rukuu001 »

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,…

Beautiful story, thanks for sharing :)

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

#65

Post by fspeech »

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 balance is between Moon and Earth gravity. However it is more subtle than that at L2 as both gravitational forces are in the same direction so they don't apparently cancel. The way to see it is to realize that L2 is relatively stationary to the Moon but not the Earth. As the L2 point is farther away than the Moon but has the same angular velocity as the Moon, Earth gravity alone is not able to keep any object at the L2 in synchrony with the Moon. One may want to recall that Earth gravity is just right for the Moon in its orbit so any satellite further away will have a longer orbital period than the Moon. The Moon gravity therefore supplies the additional force needed to keep an object at L2 orbiting the Earth in synchrony with the Moon. This balance (Earth gravity + Moon gravity = centrifugal force) determines the location of L2.

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

#66

Post by BattyMilk »

0nce wrote: post #61
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…

Lagrange points are found near any couple of large bodies. They are points where a smaller object will maintain its position relative to the large orbiting bodies. The Chinese relay satellite is orbiting the L2 point of the Earth-Moon system. That is beyond the Moon on a line which goes from Earth to Moon. Hence, this L2 point orbits the Earth like the Moon does, and the whole system orbits the Sun as well. Now if th…

Thank you for taking the time to explain. Very helpful :)

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

#67

Post by GraemeL »

tikumo wrote: post #62

The dark side of the moon, perhaps the perfect place for a big telescope?

The far side of the moon isn't dark. It has the same day/night cycle that causes the phases of the moon visible from Earth.

On the other hand, it is shielded from radio emissions from Earth and might be a good place for a radio-telescope.

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

#68

Post by BattyMilk »

fspeech wrote: post #65
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 balance is between Moon and Earth gravity. However it is more subtle than that at L2 as both gravitational forces are in the same direction so they don't apparently cancel. The way to see it is to realize that L2 is relatively stationary to the Moon but not the Earth. As the L2 point is farther away than the Moon but has the same angular velocity as the Moon, Earth gravity alone is not able to keep any object at…

Thank you for the good explanation!! Cheers!

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

#70

Post by wallace_f »

auslander wrote: post #30
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…

> No one has caught anything with a rope or a cable, as far as I know Soo good to have an expert here, thanks!

Poe's law. But at least you afforded the parent commenter the charity to feel good about their smug comment while still making your point.
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