Since this thread is being necroed yet again, I'll make a contribution I made in
another thread on the topic.
The Thetis is in Nefertem now, but it is extremely unlikely that the event that killed off the crew, happened in Nefertem. So searching for the source planet 15 LY from Nefertem is futile.
The ship would have travelled more or less in a straight line from Earth, so the event had to have happened somewhere along that line. The source planet is going to be not far from that straight line. And there are really only two options for systems in a straight-ish line between Sol and Nefertem:
- Gendalla.
- the Abrogo cluster, comprising three star systems: Abrogo, LHS 2149, and Yin Sector GW-W c1-26.
Which one of these two is more likely depends on how long you define a "generation". First, some definitions: the "first generation" of crew were the people who boarded the ship. The first "second generation babies" would have been born not long (probably within a year) after ship launch - they might even have encouraged pregancies to happen while the ship was still accelerating. As the logs record the birth of "the first" ninth-generation baby, this would logically have been on the early end of possible procreation for each generation; I guessed an average of 21 years between generations. So, eight generations at 21 years apart gives 168 years.
Generation ships cruised at about 0.2 c. So, 168 years after launch puts the Thetis at 33.6 LY from Earth. Searching for a system 15 LY "behind" them at 33.6 LY distance along the Earth-Nefertem line gives only one possible result: Gendalla. Gendalla is currently inhabited, but would have been uncolonized back in the generation-ship era as there's no ELWs or terraformables in the system. The ship would have been just past the Abrogo cluster when the event happened, meaning the dead Thetis then coasted at 0.2 c for an extra 100 years / 20 LY before the ship's autopilot decides to stop in Nefertem - which was presumably not the original intended destination of the ship, given that Nefertem doesn't have any ELWs or terraformables in it either.
As for "whodunnit", I have my own theory: the signal came from another, crashed, generation ship.
Hypothesis: an early generation ship (not the Thetis, but one of the 70,000 other generation ships launched from Earth) is launched with a lethal flaw: a death-meme message, planeted in its computer systems. Who put it there we don't know, maybe a rival generation-ship-launching company trying to cause a horrible accident. This generation ship goes ghost not long after launch, as the meme kills everyone aboard. The derelict ship drifts outwards at 0.2 c and eventually crashes into one of the planets in the Gendalla system. An automated distress beacon activates, but the meme has buried itself in the beacon's signal.
Some years later, the Thetis passes by Gendalla and picks up the wreck's signal with its buried death-meme, which gets stored in the comms computers. 75 years later, the message escapes, and wreaks havoc on the Thetis.
Invoking aliens seems out of place; if they wanted to kill everybody aboard, surely just blowing up the ship would have been much more certain than transmitting a signal and hoping that one day everybody dies because of that signal. Blowing up a generation ship is ridiculously easy: just park a few big rocks in its flightpath.
Not that we're likely to find any wreckage, ruins or signal transmitter in Gendalla, or anywhere else. I think it unlikely that FD's story writers who wrote the Thetis script would have had any idea where in the bubble their ship was going to be placed, when they wrote it. Equally unlikely that FD's coders would have said "Hey, this story says there should be some kind of signal generator planted on a planet somewhere, we'd better give ourselves extra work and go create that".