What's exploding all the time?

I'm playing my fourth survival game on the Earthlike start as I learn to play this game. Now and again I've heard a distant explosion.

For my first two games, I assumed these were unannounced meteors, since I was playing with meteors on, and I noticed the "Meteor Storm Incoming" alert was buggy.

Except now I'm playing with meteors off, and I'm still hearing it. Moreover, I have now witnesses the explosions twice (my current base is atop a a hill with excellent viewshed over the surrounding terrain). Once I caught the residual smoke from the explosion, and just this morning the whole thing - fireball and all - erupted in front of me, probably about 750m-1.5km distant. (There's no distancing laser, so I can't be certain).

These explosions leave no craters in the voxel material as would a crash, so I don't think they're unmarked "unknown signals" meeting unfortunate ends... but I can't imagine what else these could be?