Is reagan's statement that tragedies like the challenger explosion are ''all part of the process of exploration and discovery" valid?

For me, YES. 

Tragedies like the challenger explosion are ''all part of the process of exploration and discovery" because we are exploring horizons that are beyond our experiences. We only prepare for things that we have already encountered in life. When we go beyond our borders, we need to prepare for risks and dangers that we will face. As we explore, we discover new things and we learned new things. We can only apply our learning in future endeavors after we have encountered them. 

There is a phase called "Trial and Error." We try and try and we encounter errors along the way but we do not give up trying until we achieve our desired output. We learn from our errors, we examine the cause, the situation it is in and we apply these learning to ensure that no such scenario will happen again.