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Does anyone know why a halo orbit was chosen for the relay sat instead of a regular, lunar orbit?† I know that due to the moon's lumpy gravitational field, ther

Does anyone know why a halo orbit was chosen for the relay sat instead of a regular, lunar orbit?† I know that due to the moon's lumpy gravitational field, there are only a few stable orbits available, but anything orbiting a Lagrangian point is going to require stationkeeping adjustments regardless, right? †I understand that that would put the lander out of contact during that part of the relay-sat orbit, but it would also completely rule out a major source of potential radio interference during observational periods, which strikes me as an even more important consideration. Data can be stored and batched, after all.

snops 8y ago
I think orbiting around L2 would ensure the satellite was at a higher angle in the lunar sky for most of the time, and hence the lander can be in contact for longer, as low angles may be blocked by boulders or crater walls etc. The radio to the satellite would normally use high frequencies so you can have a focused beam for power saving, so it needs to be line of sight. You could transmit faster, but this would take more power. In addition, there is a little rover[1]. NASA rovers since Pathfinder have had some automated driving, I imagine the Chinese can easily do the same, but manually steering is still nice and hence the rover is more useful if you can be in "live" contact with it for longer. [1] https://en.wikipedia.org/wiki/Chang%27e_4#Yutu-2_rover https://en.wikipedia.org/wiki/Chang%27e_4#Yutu-2_rover
- craz8 8y ago
  The article you linked makes it clear why L2 was chosen - this orbit allows continuous communication between Earth and the lander. If the satellite was at L2, this would not be the case, as the moon would be in the way, so it has to be far enough away to see both the lander location and Earth at the same time Also, orbiting the moon would give you coverage much less than half the time, and you’d need to track the satellite across the sky from the lander

snops 8y ago

I think orbiting around L2 would ensure the satellite was at a higher angle in the lunar sky for most of the time, and hence the lander can be in contact for longer, as low angles may be blocked by boulders or crater walls etc. The radio to the satellite would normally use high frequencies so you can have a focused beam for power saving, so it needs to be line of sight. You could transmit faster, but this would take more power. In addition, there is a little rover[1]. NASA rovers since Pathfinder have had some automated driving, I imagine the Chinese can easily do the same, but manually steering is still nice and hence the rover is more useful if you can be in "live" contact with it for longer. [1] https://en.wikipedia.org/wiki/Chang%27e_4#Yutu-2_rover https://en.wikipedia.org/wiki/Chang%27e_4#Yutu-2_rover

craz8 8y ago
The article you linked makes it clear why L2 was chosen - this orbit allows continuous communication between Earth and the lander. If the satellite was at L2, this would not be the case, as the moon would be in the way, so it has to be far enough away to see both the lander location and Earth at the same time Also, orbiting the moon would give you coverage much less than half the time, and you’d need to track the satellite across the sky from the lander