The 139th running of the Kentucky Derby at Churchill Downs will have a field of 19 horses breaking from the starting gates at 6:24 p.m. EDT on Saturday.
   Just over two minutes later, a winner will emerge and thoroughbred racing will have its newest contender for a Triple Crown. The Preakness follows in two weeks and the Belmont Stakes is June 8. The last horse to sweep all three races was Affirmed in 1978.
   Black Onyx was scratched from the race on Friday.
   NBC televises the 1 1/4-mile race from 4 p.m.-7 p.m. The purse is $2,199,800, with $1,439,800 to the winner.