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Curiosity carries two radio systems: an X-band system that it can use to communicate directly with Earth, and a UHF system that requires less power, but also necessitates relaying the data via the two Mars orbiters.
One of the orbiters, the Mars Reconnaissance Orbiter (pictured above) has its own X band antenna and can manage up to 6Mbps — it launched in 2005 and has been used for relaying data in this way from the now-defunct Spirit rover.
The older orbiter, named 2001 Mars Odyssey, is also being used to relay UHF signals from Curiosity.
Image credit: NASA