When a spacecraft enters a planet's atmosphere from space, the friction resulting from the passage of the spacecraft through the relatively thick atmosphere creates friction, converting the kinetic energy of the spacecraft's motion (which is typically many kilometers per second, provided to the spacecraft by some other propulsion system or rocket) into heat energy, thus slowing the spacecraft.1 The space shuttle performs aeroentry every time it returns from orbit around the Earth, as did the Apollo capsules that sent men to the Moon. The space shuttle, which orbits the Earth approximately every 90 minutes as it travels through space at more than 17,000 miles per hour, enters the Earth's atmosphere and uses it to slow down to zero miles per hour as it lands.

The primary technology required for aeroentry is the heat shield. Without some sort of shield, the enormous heat generated during atmospheric entry would quickly melt the spacecraft, or at least critical parts of it. To provide this protection, families of Thermal Protection Systems (TPS) were developed and have been used successfully on multiple vehicles, including the space shuttle as well at the Mercury, Gemini, and Apollo spacecraft.

The maneuver is not without risk, as was tragically seen in the destruction of space shuttle Columbia in 2003. Without its highperformance heat-resistant materials protecting the lightweight aluminum skin of the orbiter, the hot gasses created during its passage through the

1 Regan, F.J., Reentry Vehicle Aerodynamics AIAA Education Series, 1984.

atmosphere acted like a blowtorch, damaging the vehicle and ultimately causing it to come apart high above the Earth.

Aeroentry has also been used at Mars. The Viking missions used aeroentry on their way to the surface of Mars in 1976, as did the Mars Pathfinder mission in 1997 and the Mars Exploration Rovers in 2004.2 In these missions, aerodynamic forces alone were not sufficient to land the spacecraft safely on the surface of Mars. Other systems, like a rocket engine in the last moments of the descent (Viking) or air bags (Mars Pathfinder and Mars Exploration Rovers) were also used. However, if they had not been able to use the Martian atmosphere as a brake, and had been required to perform the descent using only a rocket, the total spacecraft mass at launch might have made the missions impossible to carry out. Simply too much rocket propellant would have been required to accelerate the ship on its journey to Mars and slow it down for landing. Unfortunately, some airless destinations, such as the Moon, provide us with only one option—a rocket-based landing.

In our exploration of the solar system, it will be possible to use aeroentry at any destination that has an atmosphere. This includes Earth, for use when returning from some deep-space destination, Mars, Venus, Jupiter, Saturn, Uranus, Neptune, and Titan—and any other moon blessed with this natural resource.

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