Boundary of space in the context of "Star system"

A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, peaking in number at an altitude around 800 km (500 mi), while the farthest in LEO, before medium Earth orbit (MEO), have an altitude of 2,000 kilometers, about one-third of the radius of Earth and near the beginning of the inner Van Allen radiation belt.

The term LEO region is used for the area of space below an altitude of 2,000 km (1,200 mi) (about one-third of Earth's radius). Objects in orbits that pass through this zone, even if they have an apogee further out or are sub-orbital, are carefully tracked since they present a collision risk to the many LEO satellites.

Joe Henry Engle (August 26, 1932 – July 10, 2024) was an American pilot, aeronautical engineer, and NASA astronaut. He was the commander of two Space Shuttle missions including STS-2 in 1981, the program's second orbital flight. He also flew two flights in the Shuttle program's 1977 Approach and Landing Tests. Engle was one of twelve pilots who flew the North American X-15, an experimental spaceplane jointly operated by the Air Force and NASA.

As an X-15 pilot, Engle made three flights above 50 miles (80 km), thus qualifying for astronaut wings under the American convention for the boundary of space. In 1966, he was selected for NASA's 5th Astronaut Group, joining the Apollo program. He was backup Lunar Module Pilot (LMP) for Apollo 14 and was originally scheduled to walk on the Moon as LMP for Apollo 17. However, cancellation of later flights prompted NASA to select geologist-astronaut Harrison Schmitt as the Lunar Module Pilot, displacing Engle.