I believe they should be made as synonyms. Thinking of previous arguments brought forward in the Use of the word “Space” in a tag, Should [space-law] be just [law]?, Can we make the [space-sickness] and [motion-sickness] tags synonyms?, and other similar threads, there seems to be the prevailing thought that the use of "space" in tags is synonymous with the scope of this website and as such (mostly) unneeded. But on the other hand, also hard to get rid of, so tags with "space" in them might persist, no matter how hard we try.
So I'm all for it to make these two tags synonyms, and simply let posters decide which one they prefer, and at the same time indicate, that we mean space-probes with probe (on input of "probe", all tags with this keyword and all synonyms would display as suggestions to use instead), and if they had any other probes in mind (atmospheric-, seismic-, proximity-,...), they can create a new tag to further differentiate between them.