Imagine, it’s next year and a largish alien spacecraft slips into a medium Earth orbit. It orbits the Earth for perhaps a day, then leaves. It’s plainly visible to anyone on the ground with a good pair of binoculars. It does not respond to any attempt at communication, nor does it attempt to communicate. At some point soon after its departure we lose track of it (how is not relevant).
Leaving aside the techie stuff, how will the world react? How will you react?
I think this is one of the few things films like Independence Day get right. I’d expect a wide variety of reaction around the world. Several new religions would be formed overnight, some panic/rioting would occur, some old school religions would preach doom, others would preach salvation, and various other forms of chaos would ensue.
Once the alien ship departed, various religions would claim their prayers spared humanity, but we must repent, etc. As noted above NASA and the U.S. Military would merge, and we’d get serious about weaponizing space/planetary defense. Might see some movement towards a stronger U.N. Security Council, more international cooperation in space efforts.