By 1959, some American observers had predicted that the Soviet Union would be the first to get a human into space because of the time needed to prepare for Mercury's first launch. The Soviets designed their first human spaceflight capsule using the same spacecraft bus as their Zenit spy satellite, forcing them to keep the details and true appearance secret until after the Vostok program was over. The craft consisted of a spherical descent module with a mass of and a diameter of , which housed the cosmonaut, instruments, and escape system; and a biconical instrument module with a mass of , long and in diameter, containing propellant and the engine system. On reentry, the cosmonaut would eject from the craft at about and descend via parachute, while the capsule would land separately. The reason for this was that the Vostok descent module made an extremely rough landing that could have left a cosmonaut seriously injured.