Pioneer 5 (also known as Pioneer P-2, and Able 4, and nicknamed the “Paddle-Wheel Satellite”) was a spin-stabilized space probe in the NASA Pioneer program used to investigate interplanetary space between the orbits of Earth and Venus. It was launched on 11 March 1960 from Cape Canaveral Air Force Station Launch Complex 17A at 13:00:00 UTC with an on-orbit dry mass of 43 kg. It was a 0.66 m diameter sphere with 1.4 m span across its four solar panels and achieved a solar orbit of 0.806 × 0.995 AU (121,000,000 by 149,000,000 km).
Data was received until 30 April 1960. Among other accomplishments, the probe confirmed the existence of interplanetary magnetic fields. Pioneer 5 was the most successful probe in the Pioneer/Able series.
The original mission plan was for a launch in November 1959 where Pioneer 5 would conduct a flyby of Venus, but technical issues prevented the launch from occurring until early 1960 by which time the Venus window for the year had closed. Since it was not possible to send the probe to Venus, it would instead merely investigate interplanetary space and an actual mission to the planet would have to wait another three years.