In reality, it’s not smoke, but water vapor. The Mobile Launch Platforms that they use to launch these shuttles now come equipped with hundreds of thousands of gallons of water that they dump into the exhaust.
They didn’t always do this, as it wasn’t always necessary. It became clear, however, that to continue launching shuttles form these platforms, they would need a way to lessen the acoustic shockwave the launch causes.
The shockwave could actually off the platform and hit the shuttle as it lifts off.
Starting 6.6 seconds before engine ignition, a 300,000 gallon water tank at the launch site began dumping water down a pipeline and into the exhaust vents of the MLP. Next, six 12 foot-high towers known as "rainbirds" begin to spray water over the MLP and into the flame deflector trenches below it.
The water absorbed some of the bruising forces of the acoustic waves, and discouraged fires that might be caused by the rocket exhaust.
The whole system is emptied in about 41 seconds. This causes huge quantities of water to evaporate in the extreme heat of the exhaust, creating the giant, billowing clouds of water vapor we see.
