Nice article today in WaPo:
To get its messages to Earth, Curiosity first sends information to a pair of orbiters, Odyssey and Reconnaissance, that were sent in 2001 and 2005, respectively, to analyze Mars from a distance and are constantly circling the planet. (The Mars Express orbiter, operated by the European Space Agency, is also available if necessary.) The antennae on the orbiters are more than 1,300 times as powerful as the antenna on Curiosity. The rover waits for the orbiters to pass overhead to ship its messages, usually around 3 p.m. and again at 3 a.m.