F-15s and other modern USAF problems

Just wondering hat the present situation is with the F-15 Eagle
groundings?
I also have a few other questions concerning upcoming
retirements that I wonder anyone can help with:

1) When is the S-3 Viking due for retirement?

2) When is the A-6 Prowlers due for retirement?

3) Any rumors about he U-2 being retired?

4) Did NASA recently retire their NB-52H?

Cheers
S.Beck