## Glow of Space Shuttles

#### Learning Objective

• Recall that excited-state nitrogen dioxide is responsible for the glow observed as space shuttles re-enter Earth’s atmosphere.

#### Key Points

• When space shuttles return from space, there is often a phenomenon observed as they begin to re-enter the Earth’s atmosphere: a glow, especially around the tail end of the shuttle, is usually seen.
• The glow around the space shuttle is due to excited-state nitrogen dioxide returning to its ground state and emitting photons or light; the excited-state nitrogen dioxide forms from nitric oxide (found on the surface of the space shuttle) and atomic oxygen (found in the atmosphere).
• Atomic oxygen forms from the interaction of the sun’s high-energy light with molecular oxygen; the light has sufficient energy to break the oxygen-oxygen bond in diatomic oxygen, producing single oxygen atoms (O2 goes to O).

#### Terms

• ground statethe lowest energy state of a particle or system of particles
• excited stateany state of a particle (often an electron) or system of particles that has a higher energy than that of its ground state

When space shuttles return from space and being to re-enter the Earth’s atmosphere, a glow, especially around the tail end of the shuttle, can often be observed. This phenomenon has to do with the composition of the atmosphere at these altitudes and the lack of shielding from the sun.

## Where Does the Glow Come From?

You may have heard of the ozone layer, which protects the Earth from ultraviolet radiation. This protection is not present higher up in the atmosphere, however, where the high-energy light from the sun interacts with molecular oxygen, breaking it apart into atomic oxygen. This highly-reactive species interacts with almost any molecule with which it comes in contact.

The lining of the space shuttle is no exception; specifically, the glow on the space shuttle forms when atomic oxygen reacts with the nitric oxide (NO) on the exterior of the shuttle. The origin of the NO is unclear, although it may be collected from the atmosphere or be a byproduct of the fuel consumption from the shuttle thrusters. This interaction produces NO2 according to the equation:

$O+NO\rightarrow NO_2^*\rightarrow NO_2+light$

where NO2* represents the excited state of electrons in NO2. It is the relaxation of these electrons from the excited state back to the ground state that produces the glow that is visible around the space shuttle (see the concept about the emission spectra for more information).