Why Do Lunar Satellites Eventually Crash into the Moon? [video] | Hacker News Reader

Why Do Lunar Satellites Eventually Crash into the Moon? [video]

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56 pointsColinWright8 comments

Even after watching the video I didn't really get it. One of the comments made it more clear though:

On Earth, the orbits decay because the satellite hits the atmosphere and slows down, losing energy to drag. On the moon, satellites don't lose energy, but their orbits change. The perturbations of the gravity field cause the satellite to be moving faster but lower on one side of the orbit and higher and slower on the other side. Taken far enough, the low side is lower than the mountain the satellite slams into.

Mass concentrations ("mascons") perturb orbits of lunar satellites. The Lunar Orbiter is in a special "frozen" orbit in which the various perturbations cancel each other, with the tradeoff that this eccentric orbit is not optimal for photography.

EDIT: The auto-generated transcript feature enables one to read enough to decide whether or not to watch the video.

TL;DW from the video

>Spacecraft orbiting the Earth have remained in space since the 1950's and may last for centuries more, but spacecraft orbiting close to the moon can crash within weeks. The reason for this is the lunar gravity field is lumpy, with geological features making the smooth orbits of space probes distort until they hit the surface.

One of my favorite papers asks the opposite question: Why do Earth satellites stay up?

https://arxiv.org/abs/1309.5244

It turns out that the orbit of most satellites would be unstable were it not for the quadrupole moment of the Earth's gravitational field which detunes certain orbital resonances.

GRACE is/was the Earth-centric counterpart. I believe there may be a more modern variant as well.

https://www.nasa.gov/mission_pages/Grace/overview/index.html

Somehow I knew before clicking that this was going to be a Scott Manley video. I love that guys channel, it's always so good. Even his less good stuff is still good!

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