The largest and most powerful rocket ever built took off from Texas but exploded within minutes, in a test flight that its creators, SpaceX, hope will be the first step in a human journey to Mars. After a canceled launch earlier this week due to a pressurization issue, the Starship 120-meter rocket system lifted off at 8:33 a.m. local time Thursday. It picked up speed but then began to spin in altitude before exploding about four minutes after leaving the ground. It appeared that the two sections of the rocket system, the booster and the cruiser, were unable to separate properly after liftoff, possibly causing the spacecraft to fail.