Vostok 4 launched one day after Vostok 3 and entered a similar orbit very close to Vostok 3. The dual mission marked the first time two human space missions were in orbit at the same time. The two craft passed as close as 6.5 km from each other, allowing the two cosmonauts to perform the first-ever two-way communication between spacecraft in orbit. The mission, along with Vostok 3, was the first to seriously study the effects of microgravity on the body (including on sleep) by studying differing reactions of two cosmonauts in two spacecraft flying in similar orbits. The Vostok craft was modified to allow collection of more flight data. The mission was brought to an early end when mission control thought Popovich gave them the codeword requesting to come home immediately. He had in fact not said the codeword.