Science

nitpicker

(7,153 posts) Thu Jan 3, 2019, 04:47 AM Jan 2019

China lands Chang'e 4 probe on 'dark' side of moon

https://www.dw.com/en/china-lands-change-4-probe-on-dark-side-of-moon/a-46938770

China lands Chang'e 4 probe on 'dark' side of moon

Date 03.01.2019
Author Richard Connor

China's Chang'e 4 probe touched down on the far side of the moon, state broadcaster CCTV reported on Thursday morning. The successful "soft landing" marks a groundbreaking development in space exploration, being the first time that a spacecraft has landed on the side of the moon that faces away from Earth.

Chang'e 4, which is named after the Chinese goddess of the moon, entered its planned orbit to allow the landing on Sunday. It landed in the Von Karman crater, which is in the lunar South Pole's Aitken Basin, at about 0226 UTC.

Change'e-4 is set to release a rover to map contours and surface features around the crater and look at the geology.

The crater is deep, and it is thought to possibly contain rocks that have never been seen before. Chinese scientists hope the probe might return with soil samples, bringing back clues about how the moon formed.
(snip)

6 replies

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Highlight:

China lands Chang'e 4 probe on 'dark' side of moon (Original Post) nitpicker Jan 2019 OP

So glad to see this great article, and the info. the other side gets sunlight, too! Judi Lynn Jan 2019 #1

The Moon is tidally locked in its orbit PJMcK Jan 2019 #2

They put a comm satellite in orbit around the Moon a year or two ago. Igel Jan 2019 #3

Thanks! PJMcK Jan 2019 #5

In orbit around the Lagrange point beyond the moon, so always in line of sight with the Earth muriel_volestrangler Jan 2019 #6

Every year I start out talking about the Moon's phases by standing in front of my projector. Igel Jan 2019 #4

Judi Lynn

(160,638 posts)

1. So glad to see this great article, and the info. the other side gets sunlight, too!

Reply to nitpicker (Original post)

Thu Jan 3, 2019, 06:28 AM

Jan 2019

I honestly thought it WAS the dark side of the moon, and always dark.

It's tremendous learning how wrong that is.

Thank you, nitpicker. Looking forward to everything they want to tell us about new moon info.

PJMcK

(22,055 posts)

2. The Moon is tidally locked in its orbit

Reply to Judi Lynn (Reply #1)

Thu Jan 3, 2019, 09:48 AM

Jan 2019

That’s why we only see one face of it. In time, it’s gravitational influence on Earth will slow Earth’s rotation. Additionally, the Moon’s orbit is expanding little by little so that sometime in the distant future, solar eclipses will not have the same solar coverage since the perceived size of the Moon will have decreased.

One question that isn’t clear to me: How does the lander communicate with Earth since it doesn’t have line-of- sight for transmission?

Good job by the Chinese!

Igel

(35,362 posts)

3. They put a comm satellite in orbit around the Moon a year or two ago.

Reply to PJMcK (Reply #2)

Thu Jan 3, 2019, 01:10 PM

Jan 2019

Chang'e 4 shoots its signal to the comm satellite, which relays it to Earth.

I have to assume there's still a silent period when the comm satellite goes behind the Moon. I have to assume that it uploads as much data as possible when there's line of sight between the probe and the satellite, and then it squirts that data to Earth when there's line of sight there. There'd only be immediate communication for fairly short periods immediately before and after blackout.

PJMcK

(22,055 posts)

5. Thanks!

Reply to Igel (Reply #3)

Thu Jan 3, 2019, 01:47 PM

Jan 2019

I assumed it was something like that.

I'm feeling lazy and could have looked it up myself so I'm grateful for your response, Igel.

Is it too late to wish you a happy new year?

muriel_volestrangler

(101,385 posts)

6. In orbit around the Lagrange point beyond the moon, so always in line of sight with the Earth

Reply to Igel (Reply #3)

Thu Jan 3, 2019, 07:58 PM

Jan 2019

Targeting the far side of the moon, which due to tidal locking never faces the earth, required the prior launch of a relay satellite to the second Earth-moon Lagrange point some 65,000-85,000 kilometers beyond the moon to facilitate communications.

Named Queqiao (‘Magpie bridge’ from Chinese mythology), the satellite has been in a halo orbit around this gravitationally stable libration point since June, from which has constant line-of-sight with both the terrestrial tracking stations—situated in China, Namibia and Argentina—and the lunar far side.

https://spacenews.com/change-4-makes-historic-first-landing-on-the-far-side-of-the-moon/

A halo orbit is a periodic, three-dimensional orbit near the L1, L2 or L3 Lagrange points in the three-body problem of orbital mechanics. Although the Lagrange point is just a point in empty space, its peculiar characteristic is that it can be orbited. Halo orbits can be thought of as resulting from an interaction between the gravitational pull of the two planetary bodies and the Coriolis and centrifugal accelerations on a spacecraft. Halo orbits exist in any three-body system, e.g. the Sun–Earth–Orbiting Satellite system and the Earth–Moon–Orbiting Satellite system. Continuous "families" of both Northern and Southern halo orbits exist at each Lagrange point. Because halo orbits tend to be unstable, stationkeeping is required to keep a satellite on the orbit.
...
Robert W. Farquhar first used the name "halo" for these orbits in his 1968 Ph.D. thesis.[1] Farquhar advocated using spacecraft in a halo orbit on the far side of the Moon (Earth–Moon L2) as a communications relay station for an Apollo mission to the far side of the Moon. A spacecraft in such a halo orbit would be in continuous view of both the Earth and the far side of the Moon. In the end, no relay satellite was launched for Apollo, since all landings were on the near side of the Moon.[2]

https://en.wikipedia.org/wiki/Halo_orbit

Igel

(35,362 posts)

4. Every year I start out talking about the Moon's phases by standing in front of my projector.

Reply to Judi Lynn (Reply #1)

Thu Jan 3, 2019, 01:16 PM

Jan 2019

And putting kids in various places, then having them report what they see.

When I cast a shadow, the student in shadow sees only the dark side of me, "new moon". But that means the student on the other side only sees the side of that's brightly illuminated, "full moon." To the side they see quarter. Replacing me with a basketball gives them the necessary shapes--crescent, quarter, gibbous.

Oddly, they're seniors and having been taught this for years they never actually engaged enough with the material to think, "Oh, what does this actually look like?" They memorized words for a test. Then they forgot the words.

The only time the Moon is in complete shadow, all dark, is when it's in the Earth's shadow. Stay tuned for that--what, January 21's the lunar eclipse?

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