Complex sentence: SpaceX plans to begin launches of research spacecraft to Mars prior to the completion, and first launch, of the Interplanetary Transport System, using Falcon Heavy launch vehicles and specialized modified Dragon spacecraft.
Split into simpler sentences: SpaceX plans to fly its earliest missions to Mars using its Falcon Heavy launch vehicle prior to the completion, and first launch, of any ITS vehicle. The company is planning for launches of research spacecraft to Mars using Falcon Heavy launch vehicles and specialized modified Dragon spacecraft.