The 5VK Balloon is the balloon portion of the Lavochkin-Babakin 5VK. It is a robotic aircraft design designed for the Venusian atmosphere. Two examples have been lofted extraterrestrially. They are the only Earthly extraterrestrial aircraft ever used in an atmosphere off-Earth. They are a Soviet robotic atmospheric probe designed by the Babakin Space Centre, and built by the Lavochkin Factory, and operated by the Soviet Space Agency.
However, several other designs for non-terrestrial aircraft exist, and some have even been tested on Earth. None of these have flown in non-Earthly atmospheres except the 5VK design.