Smallest interplanetary spacecraft

The smallest spacecraft launched from Earth to another planet were the twin spacecraft of the Mars Cube One (MarCO mission), launched on 5 May 2018 along with the InSight Mars lander. They traveled to Mars independently of InSight, under their own power, with their own power, propulsion, telecommunication, and navigation systems. When packed for launch, each MarCO was a box measuring 30 by 20 by 10 cm and weighing 13.5 kilograms. They are a special class of small satellites called CubeSats, spacecraft designed to be launched from standard-sized "pods" based on the number of unit "cubes" the spacecraft volume occupies. A one-unit CubeSat is 10 cm on a side; the MarCOs were 6-unit CubeSats. They were designed to perform radio relay between the InSight lander and Earth during a period of InSight's landing when the spacecraft would be over the horizons as seen from Earth. The mission was successful, and took several photos of Mars as they flew past. They were nicknamed "WALL-E" and "EVE" for the characters in the Disney movie. WALL-E was last heard from on 29 Dec 2018; EVE, on 4 Jan 2019.

Note that this is distinct from the "smallest subsatellite," another record. These are the smallest spacecraft that traveled independently from Earth to another planet.