Twilight phenomenon lights up sky
By Airman 1st Class Stephen Cadette, 30th Space Wing Public Affairs
/ Published June 21, 2006
VANDENBERG AFB, Calif. -- Prismatic, corkscrew-shaped clouds over the Pacific Ocean could be seen shortly after sunset Sept. 22 from far away as Arizona, Nevada and Utah.
The rare occurrence, called twilight phenomenon, resulted from Vandenberg's Minotaur launch.
Vandenberg has launched more than 1,800 missiles and space boosters since December 1958, and only a small number of these launches have created twilight phenomenon. This occurs when unburned missile propellant particles and water left in the wake of an intercontinental ballistic missile or space booster freeze in the less-dense upper atmosphere. These frozen fragments reflect high-altitude sunlight, typically producing a green, blue, white and rose-colored luminescence.
For conditions to be right, skies must be clear and dark, and the sun must be below the horizon. A twilight phenomenon may result as the missile rises out of the darkness into sunlight.
Some observers incorrectly assume the missile creating the aerial spectacle must have malfunctioned because the missile's contrail becomes twisted into giant white pretzel shapes by high-altitude air currents.
"The twisting motion of the clouds is a function of the wind speed and direction at various altitudes," said Ron Cortopassi, 30th Space Wing Safety chief of engineering.
No malfunctioning missile has ever created the phenomenon.
"Vandenberg goes to great extremes to ensure all launches are accomplished safely," Mr. Cortopassi said. "In fact, in the history of Vandenberg, no one has been injured by a nominal or errant launch."
On the rare occasion a missile or rocket does malfunction, it's destroyed by the range safety officer before reaching the altitude at which twilight phenomenon occurs.
"The safety office sets limits to terminate the mission if the missile approaches populated areas," said Mark Gotfraind, 30th SW Safety chief of engineering assessment. "If the vehicle was hazarding the public, radio commands would have been sent to the vehicle that would have initiated onboard explosives to terminate its flight."