
China lands Chang'e-4 on far side of Moon - docbrown
http://www.planetary.org/blogs/jason-davis/change4-success.html
======
arduanika
"...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.

~~~
tanilama
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, with only one rabbit being her
companion.

Sounds familiar? Yep I believe if you are a Naruto fan, you can make a lot of
connections.

~~~
scotty79
> with only one rabbit being her companion.

So that's why one of their rovers was called "Jade Rabbit"

~~~
freeflight
The "moon rabbit" [0] is a rather popular interpretation of the Moons surface
in Asian folklore.

Japanese/Korean people believe the rabbit is pounding the ingredients for rice
cake.

[0]
[https://en.wikipedia.org/wiki/Moon_rabbit](https://en.wikipedia.org/wiki/Moon_rabbit)

------
jackconnor
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.

~~~
stcredzero
_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 Lagrange points has been
known about for a long time. If you want to cite something more recent and
innovative pertaining to Lagrange points, astronomers, rocket scientists, and
engineers have been working on fuel saving chaotic trajectories through the
Lagrange points.

Arguably, the fact that they are the first to implement the L2 halo does win
the Chinese space agency some innovation points.

~~~
FigmentEngine
actually things can and have been caught. Space tethers have been succesful
[https://en.m.wikipedia.org/wiki/Space_tether_missions#TSS-1R...](https://en.m.wikipedia.org/wiki/Space_tether_missions#TSS-1R_mission)

~~~
vidarh
Yes, but not in _this instance_.

------
A2017U1
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 Earth's EM
interference.

Also out left field is the 3kg "self sustaining" biosphere of silkworms and
plants with a camera inside. Interested to see how long it survives.

~~~
sowbug
I'd interpret the stated duration as a criterion for whatever budget they came
up with. Building something that'll last 90 days in a harsh environment is a
lot less expensive than one that needs to last at least a year. And assuming
the folks monitoring the rover aren't working for free, the budget needs to
include their cost of employment for a fixed amount of time.

~~~
trothamel
I'll note that the mars exploration rovers Spirit and Opportunity were meant
to last 90 sols (Martian days). Spirit lasted 2,210, and (assuming it doesn't
reply) Opportunity lasted 5,111.

When you have everyone working on the mission trying to make sure their part
isn't the one that causes the mission to end before the 90 days are up, you
get a long-lived rover.

~~~
dingaling
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 have flown
years ago and we'd be on JWST Gen 3 by now, doing actual science.

~~~
avar
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 time they can either just say spend an
extra 10 kg of payload to make the metal thicker, and be assured that it's
"definitely 90, or way more" and move on to the next task.

Or, they can consider them lasting beyond 180 a failure. Then they'll need
tens of thousands or hundreds of thousands of hours of design and testing to
ensure that various types of Martian terrain wear them out at just the right
rate, and no more or no less.

Easier to just say "this isn't worth micro-optimizing", eat the fixed cost of
the extra payload, and move on. There's going to be tens of thousands of
components on the rover that are like that, e.g. try finding a camera sturdy
enough to operate on Mars but just fragile enough to reliably fall apart in
180 days.

In aggregate that's why these rovers are exceeding their life span. If it's
truly important that missions are extended for whatever reason that can just
be decided as a matter of policy. Stop talking to the rover after 90 days, or
hand it over to hobbyists.

~~~
nkozyra
> 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.

------
vermontdevil
Awesome. Too bad US freezes China out in working with NASA on space
exploration. I think we all should partner up despite our political
differences.

~~~
Retra
Do you know why NASA doesn't partner up with China? Because it sounds like you
think those political differences are insignificant.

~~~
Rebelgecko
It's currently illegal for NASA to work with China. There's been a line in the
NASA appropriations bills for the last 5 years or so saying none of the money
they're receiving can be used bilaterally or in cooperation with China

~~~
cuspycode
How about working with Taiwan? I think they too have a space program, although
I have no idea how big it is.

~~~
avmich
"The 'One China' policy is not a card on the bargaining table," said Paul
Haenle, a former China director at the U.S. National Security Council. "It is
the table itself."

------
skilled
Can someone explain how are these types of devices safe from hackers? I mean,
it talks back to the space agency, right? Just curious is all. Haven't seen
anything related to this question.

~~~
Rebelgecko
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 programs don't
bother. With something like the New Horizons, why waste precious CPU cycles
with encryption/decryption when it takes such a tremendous effort to
communicate with it. The Deep Space Network costs something upwards of
$1,000/hour, and requires huge dishes and lots of land.

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

~~~
ohitsdom
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](https://eyes.nasa.gov/dsn/dsn.html)

~~~
stallmanite
Thanks for this link, never heard of this and it’s been fun for my son and I.

------
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?

~~~
0nce
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 !

~~~
Symmetry
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.

~~~
hanoz
So how can this craft orbit a point that is not a gravity well but a gravity
hill, so to speak?

~~~
Symmetry
It's a gravity saddle! From Earth's perspective it's a hill in the r direction
but a well in the phi/theta directions. To orbit at a location always visible
from Earth it needs to move in the phi/theta directions but moving the the r
direction doesn't help, so it doesn't.

~~~
Latteland
And you clearly know this but i had to refresh my own knowledge of spherical
coordinates -
[https://en.wikipedia.org/wiki/Spherical_coordinate_system](https://en.wikipedia.org/wiki/Spherical_coordinate_system)

------
peter303
Israel and India may be sending landers to the Moon too. India has sent
orbiters to the Moon and Mars.
[https://en.m.wikipedia.org/wiki/Mars_Orbiter_Mission_2](https://en.m.wikipedia.org/wiki/Mars_Orbiter_Mission_2)
[https://en.m.wikipedia.org/wiki/Chandrayaan-2](https://en.m.wikipedia.org/wiki/Chandrayaan-2)

An Israeli group got pretty far with an entry in the lunar lander XPrize. But
the prize reached its deadline dates and expired. I hear they may be shopping
for a deep space launch vehicle.

~~~
m000z0rz
I think the Israeli group is SpaceIL, currently scheduled to launch (alongside
a satellite) on a Falcon 9 in February. [https://spaceflightnow.com/launch-
schedule/](https://spaceflightnow.com/launch-schedule/)

------
DoreenMichele
China releases photos from first-ever mission to land on the dark side of the
moon

[https://www.businessinsider.com/photos-dark-side-moon-
china-...](https://www.businessinsider.com/photos-dark-side-moon-china-
change-4-landing-2019-1)

~~~
betterworldb
Is there a way around the paywall for business insider?

~~~
DoreenMichele
The photos were originally published here:
[http://www.cnsa.gov.cn/n6758823/n6758838/c6805034/content.ht...](http://www.cnsa.gov.cn/n6758823/n6758838/c6805034/content.html)

It's the original source. I chose to post the BI article because the original
source is in Chinese.

~~~
paraditedc
I don't blame you, but next time you go to a foreign website, you can look out
for icons that resemble national flags or EN.

There is usually an English version of the website available for important
government websites (as with this case, the button is on the top right
corner).

~~~
DoreenMichele
Thanks. I'll keep that in mind. I have lousy eyesight. It's easy for me to
miss details like that.

------
m3nu
Their logo has similarities to a certain sci-fi TV show..

[https://www.space.com/22743-china-national-space-
administrat...](https://www.space.com/22743-china-national-space-
administration.html)

~~~
rsynnott
I mean,
[https://en.wikipedia.org/wiki/Air_Force_Space_Command#/media...](https://en.wikipedia.org/wiki/Air_Force_Space_Command#/media/File:Air_Force_Space_Command_Logo.svg)

~~~
99052882514569
I mean,
[https://en.wikipedia.org/wiki/Roscosmos#/media/File:Roscosmo...](https://en.wikipedia.org/wiki/Roscosmos#/media/File:Roscosmos_logo_en.svg)

------
ams6110
FTA: "In addition to its value as a scientific exploration target, the Moon’s
quiet, airless far side makes it one of the best places in the inner solar
system for science applications like radio astronomy."

It what way is any part of the moon not "quiet and airless" ?

~~~
godelski
The sun is really loud. [0]

But in all seriousness, it produces a lot of radio signals. So it is
completely accurate to call it loud. The moon doesn't have a magnetic core so
it doesn't produce its own noise like Earth does.

[0] [https://youtu.be/Rvvsw21PgIk?t=17](https://youtu.be/Rvvsw21PgIk?t=17)

~~~
black-tea
The sun shines on the "dark side" just as much as it shines on the near side.

~~~
En_gr_Student
Think about what you just said, in context of why one side is light and one is
dark. Where does the light come from?

~~~
SmellyGeekBoy
The "dark side" of the moon refers to the side that is always pointed away
from Earth. It has day/night cycles just like the rest of the moon does.

Also try not to take this condescending tone with people. Especially when
you're wrong.

------
sizzzzlerz
Does China have an equivalent to NASA's Deep Space Network tracking stations?
By deploying large antennae in the US, Spain, and Australia, the US was able
to maintain 24/7 contact with the space missions. Granted, this isn't a manned
mission but I'd still think that they'd want to have continuous comms to their
lander.

~~~
ptero
There is little value in the continuous comms to the unmanned missions. You
generally want good tracking during maneuvers (which is where things go
wrong), but outside of those motion is highly predictable.

And the comms to the unmanned missions are mostly for forensics (outside of
active science or other active payload functions) -- if a problem arises that
the automation on board cannot solve it is highly unlikely that a difference
between real-time and an hour later warning would matter.

~~~
Rooster61
I'd agree with you for anything outside the Earth's SoI, but the Moon is
somewhat unique in that we DO have the ability to try to correct things in
near-real time.

It's one thing to have to wait 16 minutes to grab telemetry and issue commands
to something on Mars, but having only a few seconds lag seems like something
mission designers/operators would take full advantage of. The idea of actively
controlling a rover rather than having it perform pre-programmed routines
seems highly desirable.

~~~
ptero
> The idea of actively controlling a rover rather than having it perform pre-
> programmed routines seems highly desirable.

Rover, probably (which I considered payload functions). For delivery vehicle
during passive parts of the trajectory the value is doubtful.

Plus, the standard way of getting a ground station outside a country's land
footprint is to put one on a ship. That's what Soviet Union did and I suspect
this is what China is doing now.

~~~
Rooster61
> Plus, the standard way of getting a ground station outside a country's land
> footprint is to put one on a ship.

Never heard of a ship based ground station, and that's saying something as I'm
in the industry. How does that work? I'd imagine the antenna would need to be
on a massive gimbal, yeah?

~~~
genera1
From the top of my head there is Soviet
[https://en.wikipedia.org/wiki/Kosmonavt_Yuriy_Gagarin](https://en.wikipedia.org/wiki/Kosmonavt_Yuriy_Gagarin)

In principle this isn't that different from ships designed to track ICBMs, and
I've seen designs both with spinning parabolic radar antennas in radomes and
regular antennas on gimbals. In fact many of those tracking ships were used in
space programmes, but I'm not certain, whether just for one way telemetry or
two-way comms

------
dandare
How much lighter is the lander compared to Apollo moon lander? I mean, how far
are the Chinese from sending people to the moon? The Russians landed a
Lunokhod too but were very far from sending people to the moon.

~~~
eps
Russians were on a verge of manned mission, but the program stalled with
Korolev's death and then scrapped altogether for non-technical reasons,
primarily because of the regime change.

~~~
computerex
No they weren't. What's your source? Their solution (N1) had inherent flaws
that would have taken a long time to work out.

~~~
eps
Documentary on the subject that interviewed people from the project. What's
your source for the "would have taken a long time" bit?

Edit - ok, read up on it and N1 was not indeed flightworthy. There is however
little information on the nature of technical issues and how long they
would've taken to resolve. The program collapsed first and foremost because of
the politics and personnel/finance issues.

~~~
computerex
The N1 used closed loop cycles, which was something new at the time and was a
lot harder to get right. Also the n1 had some 30 engines that all had to
operate perfectly so the inherent risk of failure was higher. The rocket was
never even man rated.

~~~
avmich
USA rocketry school tradition of counting engines by chambers makes R-7 a
rocket which uses 32 engines at liftoff 4 main chambers for each of 4 RD-107
and 1 RD-108 (20 chambers) and 12 steering chambers (each with about 3 tons of
thrust, about as much as a chamber of British Gamma-8 engine) - more than N-1.
Yet R-7 is a pretty reliable rocket overall. Also Falcon Heavy has 27 engines,
which not too many complain about.

No, engines didn't need to operate perfectly. N-1 has less gross liftoff mass
(GLOM) than Saturn-5, yet N-1 had _higher_ liftoff trust. That's partially
because it was supposed to complete the mission with up to 4 engines out
during flight.

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

~~~
GraemeL
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.

~~~
aflag
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.

~~~
KineticLensman
The amount of reflected Earthlight would depend on your location on the Moon's
nearside and the phase of the Earth.

The corollary of one side of the Moon always facing the Earth is that the
Earth hangs in about the same place in the sky for any given point on the
Moon's surface. Over a period of (earth) days the Earth's position would
slightly but visibly oscillate due to libration [0]. This might slightly vary
the incoming Earthlight (analogous to the Moon's illumination at the horizon
vs. at the Zenith).

A much bigger effect, however, would be due to the phase of the Earth. As the
Moon orbits it will sometimes be between the Earth and the Sun, sometimes
further away. On the sun-side, the Earth would be more 'full' and thus there
would be more light to reflect. This would significantly vary the incoming
light.

[0]
[https://en.wikipedia.org/wiki/Libration](https://en.wikipedia.org/wiki/Libration)

~~~
philwelch
A "full Earth" would coincide with the sun fully illuminating the far side of
the Moon, and a "new Earth" would coincide with the sun fully illuminating the
near side of the Moon, so wouldn't you only get maximum darkness anywhere on
the Moon during a lunar eclipse?

However, counterintuitively, I think you'd be better off with a full Earth and
no direct sunlight than a new Earth and full direct sunlight, making the near
side a slightly better place for an optical telescope. The near side of the
moon even has the opportunity to experience eclipses.

------
billfruit
It is kind of surprising that the Chinese choose mystical names for their
space probes, rather than names of revolutionary origin (I guess Long March,
is an exception).

~~~
philwelch
After the Cultural Revolution, the PRC seemingly decided that turning their
backs on centuries of culture might not have been the best move. And by
culture I mean the subset of "Chinese" culture they deem compatible with their
planned future society; cultural and religious "fringe" groups are still
brutally repressed.

------
dbcooper
Hopefully they find some carbon in the crust. Only traces have been found so
far on the moon.

~~~
computerex
Sorry for my ignorance, but what's the significance of carbon in the crust?

~~~
thecanman
Most important building block of life

------
datumy
amazing communication system.

------
maloneyg
Damn..it is an impressive accomplishment by China.

------
golfer
From the NYT article [0] on the topic: "We Chinese people have done something
that the Americans have not dared try.” -- someone associated with the Chinese
space agency

Why so insecure? Not a good look.

[0] [https://www.nytimes.com/2019/01/02/world/asia/china-
change-4...](https://www.nytimes.com/2019/01/02/world/asia/china-
change-4-moon.html)

~~~
XorNot
I'm pretty happy if China wants to throw shade at the US over space program
accomplishments. I'd prefer a measuring contest over space accomplishments
then building more earth-bound weapons systems.

~~~
joejerryronnie
And how long before the space program accomplishments start looking just a
little like space-based weapons systems. I’m sure Chinese military leadership
can extrapolate as well as the Pentagon can.

~~~
pugworthy
Military interest in the moon goes back quite a ways. See
[https://en.wikipedia.org/wiki/Lunex_Project](https://en.wikipedia.org/wiki/Lunex_Project)
for example, and
[https://en.wikipedia.org/wiki/Project_Horizon](https://en.wikipedia.org/wiki/Project_Horizon)

------
metabagel
But did they find a Nazi moon base?

[http://ironsky.net/](http://ironsky.net/)

------
dbrgn
A great day for listening to Pink Floyd!

------
soheil
Nice

------
erikb
And until now nobody is complaining how this moon landing was fake because Jim
Bridenstine used a 3D simulation instead of a photo. I'm impressed.

------
dopylitty
The headline should be "China claims to successfully land..." Given the
government and business tendency in China to make claims that later turn out
to be fabricated nothing they announce should be taken as true without
independent verification.

~~~
Symmetry
How clever of them to have faked a rover failure with Chang'e 3 then, so that
we'd believe them now if they report that the rover works. But seriously, we
haven't seen any indications of the CNSA faking anything. China isn't a
person, it's a whole big country. The fact that Bernie Madoff defrauded a huge
number of people doesn't mean that you can't trust NASA.

~~~
Symmetry
I don't want to sing the praises of CNSA's openness here. They have a history
of keeping things secret until they work out, like the Soviets did. And
probably there are failures we'll never hear about. But if they say they
landed a probe on the far side of the Moon then there isn't reason to doubt
them.

