Proton () ('proton') was a Soviet series of four cosmic ray and elementary particle detecting satellites. 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.
Background
The Proton satellites were heavy automated laboratories launched 1965–68 to study high energy particles and cosmic rays. 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. Each Proton was housed in a purpose-built third stage added to the UR-500 stack.
Spacecraft design
thumb|150px|Schematic of the Proton satellite|left
Protons 1–3 were largely identical craft massing , with scientific packages developed under the supervision of Academician Sergey Nikolayevich Vernov of Moscow State University's Scientific-Research Institute of Nuclear Physics. The counters were able to determine the total energy of each super-high energy cosmic particle individually, a capability no prior satellite had possessed. 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. The counters could measure cosmic rays with energy levels up to 100 million eV.
Proton 3 also was equipped with a gas-Cerenkov-scintillator telescope to attempt to detect the newly postulated fundamental particle, the quark. The entire experiment package massed and was composed of metal, plastic, and paraffin blocks. though the launch was threatened by a leak in the oxidizer pipeline resulting in nitrogen tetroxide spilling on electrical wires. Early in the flight, launch specialists only received signals indicating that the satellite was functioning. Eventually, however, Proton 1 performed normally, returning physics data on ultra-high-energy cosmic particles. Its mission lasted 45 days,
Proton 2
The virtually identical Proton 2 was launched 2 November 1965 12:28 UTC, also from Baikonur Cosmodrome Site 81/23.
Proton 3
thumb|150px|alt=Proton 3 as seen from [[Gemini 11, 13 September 1966|Proton 3 as seen from Gemini 11, 13 September 1966]]
After an unsuccessful launch of the third test UR-500 on 24 March 1966 14:39 UTC, Proton 3 was successfully launched into Earth orbit from Baikonur Cosmodrome Site 81/23 on 6 July 1966 12:57 UTC reentering Earth's atmosphere on 16 September 1966.
Proton 4
After the end of the run of UR-500 test launches, the rocket (now designated Proton) and its successors were largely employed in the launch of the Zond lunar spacecraft. However, on 16 November 1968 11:40 UTC, the final and much larger Proton 4 was launched into orbit via Proton-K rocket from Baikonur Cosmodrome Site 81/24 to continue the search for the quark and supplement the earlier Proton satellites' cosmic ray measurements. 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" () ('Hercules'), was renamed "Proton" when news reports conflated the launcher and its payload. Though the Proton was never used in the ICBM role it had been built for, the rocket became an extraordinarily successful booster for commercial satellites, serving well into the 1990s.