You see, I've been on the base for a launch, and I guess you haven't. (I've also seen a couple of others go up from off base.) The initial ascent of the Shuttle appears to be quite slow.
ML/NJ
The exhaust plume changes color as the shuttle ascends through the atmosphere.
The contrail in question here doesn't.
Ergo, the missile launch theory isn't supported by the evidence, once it's considered.
Well, I wouldn't rule out a throttle back for visual effect but doubt it.
I too have watched a shuttle launch, STS-101 Atlantis. As a flight goes down-range they look like they are traveling slow but you know they hauling butt.
I can see the Vandenbereg launches from my living room and while I didn’t witness this incident it sure looks like a launch to me.