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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

#91

Post by nkozyra »

avar wrote: post #72
dingaling wrote: post #50

Earlier quoted context omitted.

The flip side to over-engineering is ever-ballooning budgets, fewer missions and more chance of random events curtailing the few missions that are flown. If the requirements state 90 days, design for 90 days - even if 180 days is achievable 'for free'. Otherwise there is no feedback loop to inform future requirements and 90 becomes an implicit 2000.b If JWST was designed for two years and a hard shutdown it would hav…

You're making the false assumption that designing something that last at least 90 days, but ends up lasting 2000 days is more expensive than something intended to last at least 90, and at most 180. Planned obsolescence of hardware operating on another planet can be hard, much harder than over-engineering it. Consider the wheels on the rover as a simple example (Curiosity has had structural issues with those). Next ti…

> You're making the false assumption that designing something that last at least 90 days, but ends up lasting 2000 days is more expensive than something intended to last at least 90, and at most 180.

I think this demonstrates the risk op was getting at. Designed for 90 and lasting more is not an implicit assurance it will last up to 180. It changes expectations and that always Cascades.

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

#93

Post by _void »

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,…

Smite fans will know all this lore as well :) Chang'e main here.

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

#94

Post by ohitsdom »

skilled wrote: post #76
Rebelgecko wrote: post #37

Earlier quoted context omitted.

A lot of satellites and probes "back in the day" more or less depended on security through obscurity. You would need to know what frequency (or frequencies) to use, and how to encode the commands. Your ground station also needs a bigass antenna, and a good idea of where to point it. Now most if not all satellites use encryption and/or authentication for command and control, but I wouldn't be surprised if some space p…

Awesome, great to know. Didn't know about the DSN so going to read upon that.

For others that might not have seen it, there's a cool live dashboard of the DSN so you can see which spacecraft are currently transmitting: https://eyes.nasa.gov/dsn/dsn.html

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

#96

Post by Symmetry »

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…

I just wanted to add that L2 (and L1 and L3) are unstable in that if a satellite is a bit further than the exact point it will tend to get ever further with time and a satellite that's closer will tend to get even closer. But the closer you are to the exact L2 point the smaller these tendencies are and the amount of fuel needed to remain on station is minimal, a satellite will just eventually run out some day and fall out.

The L4 and L5 Lagrange points are the stable ones. Something that finds its way there is going to stick around indefinitely. That's why many planets have asteroids in the L4 and L5 they make with the Sun. These are called Trojan asteroids and the names of the Jupiter-Sun Trojans are taken from the Trojan war.

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

#97

Post by martimarkov »

xupybd wrote: post #87
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.

They have to be careful then. Apollo now means more about the moon landings than the old traditions to me. I don’t know if that’s something the Chinese would like to have happen to their traditions.

Or Battlestar Galactica. XD

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

#98

Post by aflag »

GraemeL wrote: post #67
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.

Surely it's darker than the earth facing side. I'm no rocket scientist, but I reckon at least some of the light the sun sends our way gets reflected. From experience, a moonless night sure feels darker than when the full moon is up.

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

#99

Post by GraemeL »

aflag wrote: post #98
GraemeL wrote: post #67

Earlier quoted context omitted.

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.

Surely it's darker than the earth facing side. I'm no rocket scientist, but I reckon at least some of the light the sun sends our way gets reflected. From experience, a moonless night sure feels darker than when the full moon is up.

You're probably correct, but I also have know idea how much of a difference it would make.

One other advantage would also be the lack of atmospheric scattering, but they have made huge strides with adaptive optics so it might not be worth the extra cost of getting all the components to the Moon and assembling them.

You would also be limited by how close you could point the telescope to the Sun when it's in the sky. I think Hubble isn't allowed to point closer than 50 degrees for fear of damaging its optics/sensors.

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

#100

Post by rsynnott »

xupybd wrote: post #87
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.

They have to be careful then. Apollo now means more about the moon landings than the old traditions to me. I don’t know if that’s something the Chinese would like to have happen to their traditions.

Unless you're an ancient Greek, that isn't particularly surprising; it was not part of current traditions when used as the name of a spacecraft.
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