The third Vostok mission became the first multi-day human spaceflight in history, lasting nearly 4 days. It marked the first time two human space missions were in orbit at the same time when Vostok 4 launched one day later. The two craft passed as close as 6.5 km from each other, allowing the two cosmonauts to perform the first-ever direct communication between spacecraft in orbit. The mission, along with Vostok 4, was the first to seriously study the effects of microgravity on the body (including on sleep) by studying the different effects on two cosmonauts in two spacecraft flying in similar orbits. The Vostok craft was also modified to allow collection of more flight data. During the mission, Nikolayev became the first person to unstrap from his seat and float freely in space.