On 2015-04-06 07:53, Warren D Smith wrote:
Thinking a bit more, I conjecture that a satellite in Earth orbit coplanar with the moon but in 3:2 resonance with it (i.e. 3 periods per 2 moonperiods) would be a stable orbit (i.e. perturbations to the orbit would tend to be damped away).
If so, that'd be Durin-suitable.
Also, KF Lynch was complaining about my argument that the moon has a 2cm layer of dust on top, "therefore" if willing to sacrifice a few outer cm of your satellite, it has a good chance of lasting for Gyears. His complaint was, that dust generated by a blast on the moon, probably will land elsewhere on the moon, but dust blasted off your satellite is gone. Therefore, the moon has more protection. This is true. However, it seems to me nevertheless, that the moon dust thickness implies an upper bound on how much meteoritic dust came at you ever. Which still leads me to about the same conclusion, up to a factor of 2 or so? Also, I believe that when they brought back rocks from the moon they almost all were very old, which seems to prove that plenty of rocks on or very near the surface, survived.
RW Gosper suggested the aliens could leave a message in junk DNA of some lifeform. That had also occurred to me. You'd need error correction techniques of unusual kinds to protect the message. (Block deletions and insertions, block reversals, in addition to the usual model of single-letter changes.)
But... first of all, eukaryotic life containing large amount of junk DNA may not be very old. (Prokaryotes lack junk.) If it is only 1 Gyear old, the aliens could not do it before 1 Gyr ago. Second, you'd have to know more than I to be confident your message would survive. A lot of closely related seeming lifeforms have hugely different amounts (factor 10) of junk. I believe this is because there are mechanisms for quite rapidly generating huge amounts junk, and also mechanisms for quite rapidly removing large amounts junk. The latter could place messages in severe danger. Also, species go extinct.
I think burying a time capsule on moon ala Clarke ought to be safer and longer lasting than a satellite, but then it's much harder to find. Can you invent a trick that makes it easy to find it?
I found it. It was the mere existence of Dorothy Parker. Proof: Any sentience a Gy ahead of us will be a Time Lord, undeterred by the technical challenges we can imagine. That he wants our attention will be staring us in the face. --rwg