A Tuesday test of SpaceX’s reusable launch system was aborted at the last minute due to a problem with the Falcon 9 rocket’s second stage. Multistage rockets shed stages as they climb, to lighten the load by detaching segments whose fuel has burned out, and to clear the way for different types of motors that are suited to various atmospheric conditions. The SpaceX system’s stages are being designed to find their way back down to Earth’s surface for reuse as soon as a few hours later, which could drastically reduce the cost of space travel. Tuesday’s would have been the first test for bringing down the first stage on a solid surface. It may now be conducted as soon as Friday.