chang'er 4 landing

scheduled to soft land on  Von Kármán crater on 3 Jan, 2019

Von Kármán will offer the chance to analyse the variations of composition of the lunar surface and SPA melt sheet with the Visible and Near-Infrared Imaging Spectrometer (VNIS), while also returning unprecedented images with cameras on both the lander and rover.

A number of subsequent impact craters in the surrounding region, such as Finsen, are expected to have delivered ejecta to the floor of Von Kármán, potentially providing a rich sample of the SPA basin and far side geologic history.

The Lunar Penetrating Radar (LPR) and low‐frequency radio spectrometer (LFS) payloads will also return unique data from the landing region, while the Lunar Lander Neutrons and Dosimetry (LND) instrument and Advanced Small Analyser for Neutrals (ASAN) will provide insight into the space environment and interactions with the surface.

The CNSA administration said Chang’e 4’s scientific tasks included astronomical observation using low-frequency radio, surveying the terrain and landforms, detecting the mineral composition and shallow lunar surface structure, and measuring neutron radiation and neutral atoms, to build up an understanding of the environment on the moon’s far side.

与嫦娥三号相似，嫦娥四号探测器也经历了点火准备、主减速段、快速调整段、接近段、悬停段、避障段以及缓速下降段等动力下降过程，实现从距离月面15公里高度安全下降至月球表面的软着陆。全过程用时约690秒。不同之处在于，嫦娥四号在15公里到8公里高度为倾斜下降，8公里之后就改为垂直向下，引入相对于月面的测量导航。孙泽洲解释：“这样就可以克服着陆区周边地形起伏的问题。”

　　为了增加着陆的安全性，嫦娥四号探测器会通过接力避障的方式。下降至2公里左右，探测器会做一次光学的初避障，主要识别大障碍。下降至100米左右，探测器会做一个悬停，利用激光敏感器实现精避障，识别0.2米障碍、坡度等，通过地形的最优识别方法找到安全区域降落。嫦娥四号的自主能力有很大提升，“如果找不到安全区域，嫦娥四号会选择相对次优的 区域，还可以做水平机动调整。”

planetary.org.  

miniature model 

鵲橋