Suborbital Space Environment
In theory, an object can be in orbit around the Earth at any altitude, as long as it is imparted with enough velocity. The term "suborbital" refers to an object that is not imparted with enough energy (and hence enough velocity) to reach orbit. For various reasons beyond the scope of this discussion, 115 miles (approximately) is the minimal altitude at which objects are placed in orbit. At this altitude, the velocity required to achieve orbit is roughly 30,100 feet per second, or 20,500 mph. Suborbital launch vehicles are not designed to achieve these speeds, and are generally much smaller than orbital vehicles since they carry less propellant. The altitude at which "space" begins is still the topic of much debate, but many consider space to begin at an altitude of 50-miles, since the U.S. Air Force grants astronaut "wings" for any altitude achieved beyond 50 miles. Just beyond the edge of space, at 62 miles (100 km), is where the X-prize competition has set its goal for sending civilian passengers to space. As a comparison, civilian aircraft operate at altitudes below 18 miles. Even high performance military aircraft and high-altitude weather balloons do not travel past the upper stratosphere (approximately 34 miles). Figure 1 displays the operational altitudes of various aircraft and spacecraft, and illustrates that there is a significant portion of the space environment served only by sounding rockets.
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