That's the question. Short and simple.
Having spent time on each lately, I am left wondering if this would help build overall numbers and participation rates.
Clearly both sites are not generally driven by professional driven questions, but rather public curiosity questions, with more informed serious amateurs or enthusiasts providing the answer side of things.
Currently public participation gets split across the two groups. When questions are migrated, the original poster needs to join and sign up to the other group just to follow up on their question if they want to post comments etc and get involved in the process.
Signing up to a new group with ones current account is not that clear on SE - or it was not to me - you put in the same details, and the connection as same account happens automatically. I found I had to reload the page for it to take hold for some reason. It took a few explorations to find out how, and have it sorted. A few hours of time.
The time and 'work out how' barriers to sign up to another group to add more to a question you've asked and had migrated may seem trivial to the experienced users, but I think they are significant barriers to complete new comers. (It is one of the reasons why I think new comers questions should not be migrated unless they really need to be.) The difference for new comers is post, and find following up on the response to your question a bit difficult and don't come back, or post, enjoy the answers where they posted (they took the time to learn to post the question and bookmarked it to come back), no need to work out how the site works, and with a few more questions and positive outcomes, are then likely to become more involved.
I am not sure if there are stats available to test this perception. Stats like, how many people post once and don't return, vs post and become involved.
Merging the two groups means both groups, Astronomy and Space Exploration, and the public that come along for the first time, are regularly seeing the questions of both groups. I wonder if the synergy of interest may be significant in this regard, and increased participation a possible consequence.
Counter to all this is: Does it matter? Sure groups participation is currently where it is, ticking along, but is there really a problem? Possibly not! Some seem to hold a degree of concern, while others do not.
I raise the question to learn in more succinct form if it has been considered more recently, what the pros and cons would be, and if a merger of the two groups would be seen as a positive from both sites.
"Astronomy and Space Exploration" Acronym, ASE Site! I can see it working.
If I had been working on one of these groups, and someone new came along to suggest this (as I do now) I would feel pretty annoyed. The work involved, time to debate, etc .. . I get that. So apologies there. Its just that it strongly presents to me that the question does need asking. Not suggesting it be done now, or at all, but asked all the same, possibly this not being the first time.
If it has been discussed, possibly once again in the current state of things and context, then its on the shelf so to speak should it prove a necessary consideration at a future time.