1. The problem statement, all variables and given/known data The Star Alpha Centauri is 4.0 light years away. At what constant speed must a spacecraft travel from earth if it is to reach the star in 3.0 years, as measured by the travelers on the spacecraft. 2. Relevant equations Length Contraction: L = Lo*(sqrt(1-(u2/c2))) , Lo = Length proper Time Dilation: T = To / (sqrt(1-(u2/c2))) ,To = Time proper 3. The attempt at a solution Because I'm new to the topic I can only use the formulas for basic length contractions and time dilation. But I am unsure how to use them together. Any help + explanation will be greatly appreciated.