Castor and Pollux (French satellites)
Castor and Pollux were a pair of French microsatellites, also known as D5A and D5B, launched unsuccessfully on 21 May 1973, by a Diamant B from Kourou.
Castor was to have studied interstellar space and collected aeronomy and geodesic data; Pollux was to have tested a hydrazine propulsion system.