In this incredible video we can see Chang'e-4 in it's final approach and landing on the moon from the point of view of the spacecraft.

The view is horizontal as the spacecraft seems to stop and hover over the chosen site, and then the view pitches forward until it is looking straight down. Then the spacecraft descends and lands. Specifically 00:50 to 01:10 in the video.

The speeds are slow and the altitudes are low so I assume that the engine should be pointed mostly down the whole time, but that conflicts with the view.

Can someone explain to me how the spacecraft hovers so nicely and yet appears to pitch forward by 90 degrees so gracefully?

  • $\begingroup$ Does the spacecraft really pitch forward by 90 degrees during the first phase of landing? It is hard to tell without knowing the viewing angle of the camera. But the pitch angle may be about 60 degrees for combined hovering and slowing down. $\endgroup$ – Uwe Jan 11 at 14:01
  • $\begingroup$ Here's a graph of the powered descent phase. As for how it does this so 'nicely' as you put it, I think the answer is as simple as "it has a very good control system". I'm not at all an expert in control systems so concerning the specifics I have no clue what kind of control architecture they use. $\endgroup$ – Alexander Vandenberghe Jan 11 at 14:10
  • $\begingroup$ @AlexanderVandenberghe please go ahead and post that as a partial answer. With the video and that image shown together it becomes more obvious just how much vertical descent is happening. Watching again I can see how craters too small to be visible in the beginning get much larger before touch down. I think in this particular case, an incomplete answer is fine, there may be additional answers to fill in the gaps. $\endgroup$ – uhoh Jan 11 at 14:14
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    $\begingroup$ I think @RusselBorogrove 's answer is already clearer than what i commented. It would be nice if the video included an altimeter so it is clear what part of the graph we're actually seeing and how fast it's descending. Also i checked for more information on the control system but the only papers I found were in Chinese, which I unfortunately don't know. $\endgroup$ – Alexander Vandenberghe Jan 11 at 14:33

The view is horizontal as the spacecraft seems to stop and hover over the chosen site, and then the view pitches forward until it is looking straight down.

enter image description here

According to the graph linked by @AlexanderVandenberghe, that pitchover is happening at 6-8km altitude, so I think what you're interpreting as a "stop and hover" is not a hover. It's falling, but the moon's surface is far enough away that you can't easily see the fall rate. Once it's more or less nulled the horizontal velocity, it pitches to vertical and begins to seek out its final landing site.

It's especially hard to judge the altitude and vertical speed throughout most of this video because the terrain is uniformly fractal -- it looks basically the same at a wide range of scales. 6km altitude and coming down at 100 m/s looks much the same as 600m coming down at 10 m/s.

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    $\begingroup$ yes, fractal is a good way to explain it in a word. I see a pattern of big and small craters, 30 seconds later I see "the same" pattern of big and small crater pattern, so I don't recognized I'm 5x closer. $\endgroup$ – uhoh Jan 11 at 14:34

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