Linerider/Hamrah are right; It's a rocket launch, complete with staging (saw one like this whilst airborne a few years back - very unsettling when it happens unannounced and you are only 200 km from the launch site).
FWIW comets don't move at anything like that rate across the sky, don't change their appearance that rapidly and more importantly there hasn't been a comet visible from Earth that bright in several years.