Chang'e-4 farside landing mission Options

[Thorsten Denk]

Hi all!

Cháng'é-4 is now in the final 15x100km orbit!
https://gbtimes.com/change-4-lowers-orbit-r...ide-of-the-moon
Very nice video there!

I herewith dare to predict the landing time:
02 January, 00:00 UTC (midnight) at 178.1° East.

How do I get there?
Quite simple calculations with the publicly available data:

First looking at the orbital plane:
Lunar orbit insertion (LOI) was on 12Dec at 08:39 (all times UTC)
in (- assumption! -) a plane exactly perpendicular to the earth vector, this means "face-on" and over the poles.
Furthermore: Landing site: Between 176.4° and 178.7° East. (This is from here.)
The orbital plane therefore has to rotate between LOI and landing between 271.3° and 273.6°.
Or, more precisely, the Moon under it.
Since a sidereal lunar rotation lasts 27.3217 days,
this corresponds to a time from LOI to landing of 20.590 and 20.764 days.
That falls between 01Jan 22:48 and 02Jan 02:59 (as said, UTC).
That's a period of just over 4 hours, enough for two passes.

And now, when exactly?
The insertion into the final orbit of 15x100km was early today (30Dec) at 0:55 UTC.
This injection occurs in the aposelene, this means half an orbit from periselene!
And the periselene is logically (almost) over the landing site.
The orbital period can be calculated (113.68min), so you just have to look
when a multiple-plus-zero-point-five of the orbit period falls into the the time frame calculated above.
This is the case (UTC) on 01 Jan at 23:58 (37.5rev) and on 02 Jan at 01:51 (38.5rev).

With uncertainty in the minute range and a bit of time for the final (slow) approach,
it will be midnight UTC on January 2nd.
And the backup opportunity at 177.0° East just before 2h UTC.

To all this also fits that on that day, the Moon in Beijing rises at 03:45 (Beijing time).
The landing would be at 07:58, the backup landing at 09:51, and the moonset at 14:23.
Perfect timing for the ground stations.

Some (dis-)agreement here?

Best
Thorsten

[Bernard]

No disagreement.
This looks brilliant to me.

[nprev]

Very impressive orbital mechanical detective work, Thorsten! It will be fascinating to see how this plays out.

[charborob]

1- Is there an official Chang'e 4 website?
2- Do we know if the landing will be transmitted live?

[nogal]

1- Is there an official Chang'e 4 website? I've been combing the web and couldn't find any
2- Do we know if the landing will be transmitted live? All mentions I've seen say it will not

Andrew Jones of the gbtimes has been providing coverage of the mission. On that page is this interesting link
The Sun is reporting a Jan 3 landing at about 1:00 UTC, which matches with a tweet I've seen (but lost the reference to).
EDIT: techradar has a story about "how to watch live online" by visiting the CGTN site. But this site is unavailable to several European countries, including mine.

[Thorsten Denk]

Very impressive orbital mechanical detective work, Thorsten! It will be fascinating to see how this plays out.

Thanks nprev.
But apparently something is wrong:
(1) The LOI plane was not perpendicular to Earth
(2) The orbital plane did not remain constant
(3) They will land somewhere else (not Von-Kármán)
(4) They already have landed but didn't tell
(5) I did a mistake somewhere
Will be interesting to see what was the reason...
Thorsten

[Phil Stooke]

What are we about to see?

Almost everywhere i look in Von Karman southeast of the central peak the surface is characterized not only by the expected craters but also by a multitude of small troughs or valleys. This scene is about 4 km wide:



Hazard avoidance should allow CE4 to land in a level spot, but traverse planning may be interesting. I will not speculate as to the origin or depth of these features at the moment.

Phil

EDIT - 7 January - These grooves are further west than the actual landing site. Our area is free of them.

[Explorer1]

Unconfirmed social media reports of a successful landing at 2:26 UTC!
Presumably downloading the landing footage before any official announcement on state media (what I would do to maximize public outreach).

[Hungry4info]

Official Chinese media are reporting successful landing.

[charborob]

Landing announced on Xinhuanet link

[Hungry4info]

Attached thumbnail(s)

 

[nprev]

Remarkable. Congratulations to China!

[Explorer1]

First surface image from the far side. A historic achievement!

Nearby crater and distant hills, should make finding the location easy enough; LRO should pass over the area on January 4th from what I've read.

Another image of the landing legs on social media; I can't find the original so I won't link it yet.

Video here (skip to 1:11 for apparent real time descent imagery, should make it easy for our resident cartographer to pinpoint the area )

https://krcom.cn/2656274875/episodes/104219...324354331200641

[Sym05]

Smartphone screenshot of CCTV 13 news cannel. 14:00 Beijing time edition. Screen in the control room shows the expected landing zone and expected landing point (only one decimal) 177.6 E, 45.5S

[Phil Stooke]

Okey-dokey, here we go.

These are the views from that landing video:



This is an overlay of the landing site image posted just above this, overlain on a Quickmap image of the area:



Zooming in, the area looks like this:



And here two frames from the video are located.



That points to the approximate site. No doubt more tomorrow.

Phil