Communicating with spacecraft at Mars always involves a wait. Depending on how far apart the planets are, it can take up to 21 minutes to get a signal from Earth to the red planet, resulting in a round-trip time of more than 40 minutes. The lag can be agonizing for an engineer trying to steer a surface probe or debug a software problem. On Aug. 27, when Mars is closer to Earth than ever in human history, the one-way travel time of light and radio signals will be just 3 minutes and 6 seconds. Astronomers love to measure cosmic distances in light-years. In this case, you can think of the distance between the two planets as being 186 light-seconds.