Short answers, pulled from the story.
The first human-made object reached space on the 20th of June 1944 when a V-2 rocket launched from Peenemünde, Germany crossed the Kármán line at 189 kilometers. Wernher von Braun co-designed the V-2 rocket and his work later defined the American space program.
Yuri Gagarin became the first human to orbit the Earth on the 12th of April 1961 aboard Vostok 1. He completed a single orbit in 108 minutes and ejected from the capsule for the final seven miles of his descent to parachute to the ground.
The Kessler syndrome is a cascading collision of space debris that poses a threat to future spaceflight by creating hundreds of small fragments traveling at thousands of kilometers per hour. These fragments can destroy functional satellites and create chain reactions that render certain orbits inaccessible for generations.
Konstantin Tsiolkovsky published the Tsiolkovsky rocket equation in 1903 to describe the change in a rocket's velocity based on exhaust velocity and the ratio of initial to final mass. Engineers still use this equation today to calculate the total potential change in velocity required for a mission.
Long-term exposure to microgravity leads to bone loss, some of which is permanent, and significant deconditioning of muscular and cardiovascular tissues. The human body also faces radiation threats from the Van Allen belts, solar radiation, and cosmic radiation that increase the chances of cancer over decade exposure.
The Space Shuttle was retired in 2011 after the Challenger was lost in January 1986 and the Columbia broke up during reentry in February 2003. Six orbiters were built in total, with the Enterprise used only for approach and landing tests and the other five flying in space.