Data Rates/Returns
The data rate direct-to-Earth varies from about 500 bits per second to 32,000 bits per second (roughly half as fast as a standard home modem). The data rate to the Mars Reconnaissance Orbiter is selected automatically and continuously during communications and can be as high as 2 million bits per second. The data rate to the Odyssey orbiter is a selectable 128,000 or 256,000 bits per second (4-8 times faster than a home modem).
An orbiter passes over the rover and is in the vicinity of the sky to communicate with the rover for about eight minutes at a time, per sol. In that time, between 100 and 250 megabits of data can be transmitted to an orbiter. That same 250 megabits would take up to 20 hours to transmit direct to Earth! The rover can only transmit direct-to-Earth for a few hours a day due to power limitations or conflicts with other planned activities, even though Earth may be in view much longer.