Since EP Grondine has posted that
enticing post without giving any details, I might as well make some educated guesses as to what it can be...or can't be.

[1] I'm sure it's not the shrinking
GRS on Jupiter: That could take centuries to disappear.

[2] It's not the latest Eiffel-Tower sized NEO asteroid missing Earth by two or three lunar-orbit radii.

[3] It's not the humungous solar-flare
I've heard about lately.

[4] It's probably not a volcano on Mars
starting to belch and smoke.

[5] It might be a 'nearby' star [within
2000-3000 parsecs] about to go supernova.

[6] Or it might be...It might be...
another comet on an impact course
with Jupiter's cloudtops!!
I think it was Yogi Berra who said, "That would be deja vu all over again".

cheers

Jacob