MarCO or “Mars Cube One” is an experimental mission that will accompany InSight to Mars. It consists of two identical spacecraft, each 36.6 by 24.3 by 11.8 centimeters in size. If successful, they will relay real-time telemetry from InSight to Earth during the landing. MarCO costs $18.5 million. (…)
The MarCO satellites will perform bent-pipe relay of InSight’s transmissions during landing: they will receive UHF transmissions from the lander, decode them, and send the decoded data on to Earth. If successful, they will provide mission controllers and Mars fans around the world with live information on the status of the spacecraft during landing. Meanwhile, Mars Reconnaissance Orbiter will record all of the transmissions from the lander without decoding them, saving them for later transmission. If something goes wrong on landing day, it will be the data recordings from Mars Reconnaissance Orbiter that help NASA figure out what went wrong. If everything goes right, we’ll learn about it first through MarCO. [More at link]