Which Mars mission carried the first interplanetary CubeSats, MarCO-A and MarCO-B? InSight carried them.
NASA launched the InSight lander and the two Mars Cube One spacecraft together on 5 May 2018. MarCO-A and MarCO-B were compact 6U CubeSats designed to demonstrate that small spacecraft could navigate and communicate beyond Earth orbit.
The pair separated from the launch vehicle soon after liftoff and flew independently toward Mars. During InSight’s entry, descent and landing on 26 November 2018, they relayed telemetry directly to Earth in near real time. They were flyby spacecraft, not Mars orbiters, and their communications experiment was supplemental to the main lander mission.
The common confusion is to treat MarCO as part of Mars Odyssey or MAVEN. Those were separate NASA orbiters with different missions. MarCO’s success helped establish CubeSats as practical tools for deep-space technology demonstrations, although both spacecraft eventually went silent after their Mars encounter.