Final answer:
The events in space exploration in chronological order are: Sputnik I was launched, Explorer I was launched, Humans orbited Earth, and the first human stepped onto the moon.
Step-by-step explanation:
- Sputnik I was launched into space in 1957.
- Explorer I was launched in 1958.
- Humans orbited Earth in 1961.
- The first human stepped onto the moon in 1969.