The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

proton
A positively charged subatomic particle found, together
with neutrons, in the nuclei of atoms. Its mass is 1,836.12
times that of the electron.
Proton
A large launch vehicle that has been the heavy-lift workhorse
of the Russian space program for over three
decades and today plays a key role in building the International
Space Station (ISS). Capable of delivering 21
tons to low Earth orbit, it typically uses three stages, all
powered by storable liquid propellants, although a fourth
stage can be added for higher-altitude missions. The first
stage layout is unusual, comprising a central oxidizer
tank with six fuel tanks and engines arranged in what
appear to be, but are not, jettisonable boosters clustered
around the base.
Proton 331
The Proton originated as the UR-500—a design for a
two-stage intercontinental ballistic missile capable of
lofting the heaviest warheads in Russia’s arsenal. But in
1961, it received approval from the Kremlin to be developed
into a space launcher for a manned flight around
the Moon. When this concept was abandoned, the Proton
found alternative use as the launch vehicle for the
main components of the Salyut and Mir space stations.
It has also launched large payloads into geosynchronous
orbit and interplanetary missions, including the Vega
probes to Halley’s Comet.
In 1997, Lockheed Martin signed an agreement with
Russia to market the Proton internationally as part of their
International Launch Services subsidiary. Today it continues
its workhorse role by delivering the largest Russian elements
of the ISS. A new version, the Proton-M, is
planned, which will use a Breeze-M fourth stage—an
Proton A Proton launch vehicle being erected on its launch pad at Baikonur Cosmodrome. Lockheed Martin/Russ Underwood