Twilight phenomenon is produced when unburned particles of missile or rocket propellant and water left in the vapor trail of a launch vehicle condenses, freezes and then expands in the less dense upper atmosphere. The exhaust plume, which is suspended against a dark sky is then illuminated by reflective high altitude sunlight, which produces a spectacular, colorful effect when seen at ground level. The phenomenon typically occurs with launches that take place either 30 to 60 minutes before sunrise or after sunset when a booster rocket or missile rises out of the darkness and into a sunlit area, relative to an observer's perspective on the ground.
This phenomenon usually produces a cloud of green, blue, white and rose colored hues which takes on a corkscrew appearance as it is whipped around by wind currents. It is seen within two to three minutes after a launch has occurred. Depending on weather conditions, it could remain in the sky for up to half an hour before dispersing.
Pre-dawn launches are probably less spectacular than their dusk counterparts. During dusk launches, the sunlight shines through the exhaust plume. Pre-dawn launches, on the other hand, produce a more subtle display because the sunlight directly reflects off the plume.
At Vandenberg AFB in California, more than 1,800 missiles and space boosters have been launched from the central California coastline in northern Santa Barbara County since December 1958. However, only a small percentage of these launches have created the twilight phenomenon.
Some observers have assumed the missile creating the aerial spectacle must have malfunctioned or been destroyed while in flight. That belief stems from the appearance of the missile's contrail as it becomes twisted into knots by upper altitude air currents or wind shear.
No malfunctioning missile has ever created the phenomenon. On the rare occasions when a missile does malfunction, it is destroyed by a Range Safety Officer before reaching the altitudes where twilight phenomena occur.
The phenomenon's appearance and intensity varies with viewer location, but can usually be seen throughout the state of California, and as far away as Arizona, Nevada and Utah. On the East Coast, similar sightings can be observed during twilight launches of the space shuttle and other expendable launch vehicles from NASA's Kennedy Space Center and the U.S. Air Force's launch complexes at Cape Canaveral in Florida.
- In 2010 a contrail from an airplane in California had people believing they saw a missile launch.
- On July 7, 2010, reports of a "UFO" sighting forced Xiaoshan Airport in Hangzhou, China to temporarily cease operations. The event appears to have been sparked by a flight crew preparing for descent and detected the object—suspected to be a Chinese rocket test—as a bright twinkling light ‘above’ the runway and notified the air traffic control department. ATC could not locate it on radar and prudently waved off landing flights. Eighteen flights were affected. Though normal operations resumed four hours later, the incident captured the attention of the Chinese media and sparked a firestorm of speculation on the UFO's identity.
- McKee, Maggie (2010-11-09). "Mystery 'missile' likely a jet contrail, says expert". New Scientist. Archived from the original on 2010-11-09. Retrieved 2010-11-10.
- "China Airport UFO - Mystery or Military?". National Ledger. July 10, 2010. Retrieved July 11, 2010.
- UFO in China's Skies Prompts Investigation
- Video of "UFO" over Xiaoshan Airport in Hangzhou, China (Later attributed to Twilight phenomema caused by a Progress-M launch on June 30, filmed from Almaty, Kazakhstan)[Link is dead]
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