A SpaceX rocket soared into the early morning sky over the California coast Thursday after liftoff from Vandenberg Air Force Base.
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The Falcon 9 launch had been scheduled for last Saturday, then it was pushed to Sunday, then it was delayed again until the planned Wednesday morning liftoff, which was postponed to Thursday at 6:17 a.m. The delays were due to strong winds.
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Southern Californians hoping for a repeat of the spectacular Dec. 22 launch of a Falcon 9 rocket from Vandenberg were left somewhat disappointed. That launch occurred during clear twilight conditions, but Thursday's sky was much brighter -- making the plume less brilliant.
The Falcon 9 rocket's primary payload is a Spanish-government Paz satellite, but documents cited by multiple media outlets indicated that the rocket will also be deploying a pair of demonstration satellites that are part of Musk's vision to create a space-based broadband network providing worldwide affordable internet access.
According to a communication between Hawthorne-based SpaceX and the Federal Communications Commission, the mission will deploy satellites known as Microsat-2a and Microsat-2b. Those satellites were described by the company in an earlier license application as part of a test of a "broadband antenna communications platform."
Documents cited in media reports identify the satellites as part of a SpaceX program known as "Starlink," which envisions an array of nearly 12,000 satellites circling Earth within about six years, creating a worldwide internet system.
A December launch of a SpaceX Falcon 9 rocket from Vandenberg created an evening visual spectacle over the Southland, with hundreds of people snapping photos of the light show and posting them online, with some even suggesting it was an alien invasion.
The SpaceX Falcon 9 is a two-stage launch vehicle, meaning it is made of up two parts that carry the satellite payload into orbit. The first stage is pretty visible during dusk, dawn and night launches due to the nine rocket engines that fire for the first 162 seconds of the flight.
After the main engines cut off, the second stage (upper portion of the vehicle) separates and fires a single engine to continue the flight into space. During the highly visible launch in late December, the first stage created a spiral-like pattern in the exhaust of the second stage.
Most of the launches out of Vandenberg put satellites into a polar orbit, meaning that the satellites are launched toward the south. The rocket parallels the coast, increasing the visibility for many in SoCal. The launches become spectacles just after sunset, or just before sunrise, because the exhaust plume becomes illuminated by sunlight as the rocket goes higher.
Thanks to the curvature of the Earth, sunrise and sunset times change with altitude. The general rule of thumb is that for roughly every 5,000 feet of altitude, sunset or sunrise changes by one minute. So for a rocket at 40 miles up, sunset is about 40 minutes later than at the ground. As the rocket gains altitude, it will eventually be back into sunlight, which is when the exhaust plume becomes a bright cloud in the night sky.
You may notice the plume spreading out as the rocket continues its climb. This effect is caused by the decrease in air pressure as you get higher. At the surface, pressure is high enough to keep the exhaust from spreading out.