Samos 3 was an American reconnaissance satellite which was lost in a launch failure in 1961.^[1] It was an early electro-optical reconnaissance spacecraft, meaning that it transmitted images to receiving stations on Earth rather than returning them in a film capsule, and was to have been operated as part of the Samos programme. Samos 3 was the only Samos-E2 spacecraft to be launched. Samos-E2 satellites were based on an Agena-B, and carried a camera with a focal length of 91 centimetres (36 in), and a resolution of 6 metres (20 ft).^[2]

The launch of Samos 3 occurred at 19:28 UTC on 9 September 1961. An Atlas LV-3A Agena-B rocket was used, flying from Launch Complex 1-1 at the Point Arguello Naval Air Station.^[3] At the moment of liftoff, one of the launch tower umbilicals detached 0.21 seconds late. This tripped a switch in the Atlas and caused it to change from internal to external power. The booster's engines shut off and it fell back onto the pad and exploded, destroying the satellite and causing extensive pad damage.^[4] Samos 3 was to have operated in a Sun-synchronous low Earth orbit, at an altitude of 480 kilometres (300 mi) and with 83 degrees of inclination.^[4] The satellite had a mass of around 1,900 kilograms (4,200 lb),^[4] and was designed to operate for around four months.^[2] Damage to SLC3 mainly affected electrical and plumbing components and repair work began almost immediately. The pad was back online to host the launch of Samos 4 in November. Samos 3 was the last DOD-related launch to be unclassified, and afterwards, much greater secrecy would be put around such flights.