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A little over a week ago, I showed you a pretty cool picture: the LG and Lunar spacecraft of the Chinese Space Agency, Chang – e – 4 and Yutu – 2, seen from the orbit by the Lunar Reconnaissance Orbiter of NASA.
On this image, LRO's orbit placed it about 330 kilometers east of the LG (plus tens of kilometers above the surface), so it had to switch way on the side to see it, and the resulting shot was taken at a rather steep oblique angle. Because it was so far, even the powerful LRO cameras could not see the material as much as a tiny dot.
However, while LRO surrounds the Moon, the territory he sees changes – I'll explain it in a moment, because it's really cool – and therefore its distance from the LG. This previous image had been taken on January 30, 2019. A week or so later, on February 7, LRO passed directly over Chang-e-4 and Yutu-2, sliding to the landing site at an altitude of 82 kilometers.
It changes things a lot. Here is what LRO saw while looking closely at the Chinese material:
Ah, it's better.
Although you can not see the finer details – 82 klicks remains a good hike – the lander appears much larger, covering several pixels. At this height, each LRO pixel is about 0.85 meters (about the size of a standard lawnmower, it's hard to find analogies that are easy to grasp at times!), And the lander is about two meters (I Actually, I can not find good numbers, so if someone knows, please let me know!) The rover is smaller, about 1.5 meters by 1 x 1, and can be seen in the upper left corner of the undercarriage. This image was taken just under 30 meters from the LG.
I have to see again how amazing it is to be able to get this kind of pictures. The lunar landscape is vast and does not have the same type of landmarks we have on Earth. This area located on the other side of the Moon has craters of a size ranging from too small to several thousand meters. There are rocks, mounds and huge hills, and … well, that's about it. If you are not familiar with the field* so it's easy to lose yourself desperately.
Do not believe me? I went to LRO's blog and I downloaded the huge, high-resolution mosaic that LRO took while flying over the Chang-e-4 site and rummaging through to try to find the LG … then j & rsquo; Laughed at seeing that LRO people had already pointed out the site put a square around it. If they had not done it, I never I would have found it.
Here is the picture with the landing site described. I have cropped the close up from above to match the square and I inserted it in the shot:
Yes, do you see what I mean? And it's even worse: I reduce the original high resolution image five times to create this image.. The original contains even more details, which makes it even more difficult to search for an individual grain. This band shows a region of the Moon about 8.5 kilometers wide, and before I cropped it was about 50 kilometers long. Imagine finding something a few meters in it all!
Now, there are some tips. On the one hand, we already knew where the lander was, which greatly reduced the area of interest. But there is something else too: the shadows.
Note the shadow on the craters. Because the sun comes from the upper right corner of the image, the craters are shaded at the top right and lit at the bottom left. But now let's look at the LG: it's shiny up and shaded down! This distinguishes it more easily from craters. Otherwise, yuck. Good luck finding it.
So, why does the angle change between LRO and Chang? E-4? This is because of the orbit. When a satellite gravitates around a world, it gravitates around it center of this world. All that matters to him is the gravity he feels, and that comes from the mass of the object in orbit. It does not matter if the object is rotating or not.
The plane of this orbit is almost fixed in space. So, if it is, for example, full north / south of the Moon, it will stay that way. If the moon did not turn, LRO would pass on the exact same places of the moon again and again. But the moon Is turn, about once a month, so that each orbit around the LRO passes over different places as the Moon turns below. The calculation is a bit complicated, because the ORB's orbit is influenced by many other factors, but in the end, it means that, over time, the LRO passes over the entire surface of the Moon, a band at the time.
That's why there was a Chang-e-4 side in late January, and then directly in early February. To date, LRO is far to the west of this site and it will take some time before the Moon turns enough for the Chinese site to be back under its direct gaze. At that time, the lighting will also have changed, so that the shadows will also be different. It should be interesting in a few weeks to see the new pictures!
I will note that LRO also creates mineralogical maps of the moon, which helps science to place the measurements taken by the lander and the rover in a broader geological context, in the image of the L & # 39; image itself. That's why we send different missions to other worlds. everyone does his own thing, but when you add them up, you get more than the sum of their parts.
You understand a world. I think it's really worth it.
*I use this word loosely because it is based on the word terrafor the Earth. But lunain seems strange.
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