Orbited 1965–68, three on test flights of the UR-500 ICBM and one on a Proton-K rocket, all four satellites completed their missions successfully, the last reentering the Earth's atmosphere in 1969.

The Proton satellites were heavy automated laboratories launched 1965–68 to study high energy particles and cosmic rays.

[5] These satellites were built to utilize the test launches of the UR-500, a heavy two-stage ICBM designed by Vladimir Chelomey's OKB-52 to carry a 100-megaton nuclear payload.

[6] Protons 1–3 were largely identical craft massing 12,200 kg (26,900 lb), with scientific packages developed under the supervision of Academician Sergey Nikolayevich Vernov of Moscow State University's Scientific-Research Institute of Nuclear Physics.

Though the equipment had been developed eight years earlier (by Professor N. L. Grigorov), the UR-500 was the first booster powerful enough to orbit a satellite carrying the sensitive particle counter.

[18] The success of Proton afforded Chelomey a status in the Soviet rocket industry equal to that of Sergei Korolev of OKB-1 (developer of Sputnik, Vostok, and Voskhod) and Mikhail Yangel of OKB-456 (an important designer of military missiles).

The UR-500, originally named "Gerkules" (Russian: Геркулес) ('Hercules'), was renamed "Proton" when news reports conflated the launcher and its payload.