In a show seen across the southwest, a Minotaur rocket painted the sky with colors Thursday night while streaking along a path to put a spacecraft with a mysterious mission into orbit.

The booster, assembled by Orbital Sciences Corp. from Minuteman missile stages and Pegasus rocket parts, blasted off at 7:24 p.m. from Space Launch Complex-8 on south Vandenberg Air Force Base.

Speculation about the mission's purpose took a temporary back seat as the blastoff's "twilight phenomenon" stole the spotlight. Central Coast residents watched the skies. Vehicles stopped along a roadside in San Diego to look at the spectacle. In Glendale, Ariz., one person reported seeing it from football stadium bleachers.

"It seems like everyone has seen the launch and it was absolutely beautiful," said Tech. Sgt. Rebecca Danét, a Vandenberg spokeswoman. "Some folks were alarmed by it because they didn't know what it was."

Callers throughout California and from as far away as Utah and Arizona flooded the Vandenberg Public Affairs Office Thursday night to inquire about the sight often mistaken for a failed launch or an unidentified flying object.

In reality, the twilight effect happens under certain conditions - sunset or sunrise plus clear skies - after a launch. Sunlight reflects off unspent fuel particles that freeze. The result is an unintentionally colorful contrail of rose, blue, green, orange and other shades.

Thursday's mostly clear skies created the perfect palette as the rocket dropped off its used stages and fired motors to carry the payload higher.

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Minotaur ferried a classified satellite for the Defense Advanced Research Projects Agency. Late Thursday night, DARPA officials confirmed the spacecraft had separated from the rocket.

Officially called STP-R1 for its role in the Space Test Program of small research satellites, the satellite was dubbed "Streak" by the military because of the visible path it cut across the sky.

But that's about all DARPA or other defense officials will say about the satellite. Even its cost remains hush-hush.

"Streak is a technology demonstrator whose objective is to demonstrate rapid response, short mission life, low-earth-orbit space technologies and gather information about the low-earth-orbit environment," DARPA officials said.

The craft sports two environmental measurement instruments, an ion gauge and an atomic oxygen sensor.

During its one-year mission in a sun-synchronous orbit expected from 345 miles above Earth, the vehicle "will characterize the orbital regime, demonstrate operational feasibility from a command and control standpoint and also from a platform perspective for future DoD missions," according to an official statement.

Defense analysts suspect the satellite's purpose is to collect data to help build spacecraft quickly in the future.

"What this suggested to me is they're thinking of satellites that can be put up quickly in a high-stress defense situation, in an emergency, if you will," said Philip E. Coyle III, senior advisor with the Center on Defense Information, a think tank.

That rapid response could be employed during a time of heightened tension between the United States and a hostile nation, such as North Korea.

"They don't require it to last forever ... but they want it to last long enough to figure out what Iran or North Korea or whoever is doing," Coyle said.

Most of today's operational satellites took decades to build and feature increased safeguards against ionized gases and other harsh aspects of space that can interfere with electronics.

"They don't want to build a satellite that takes years to build that is very heavily protected," Coyle said. "What they want is something they can build quickly and yet still survive long enough to do the job."

Most of today's military satellites are huge, expensive constellations made to be robust. They're akin to a "space tank," he said.

"If you're going to have a space tin can, so to speak, you're going to need to understand the environment more carefully," Coyle elaborated.

This was the year's second Minotaur launch from Vandenberg, and is the first of three rocket launches planned from mid-September to mid-October from the base.

"Vandenberg is committed to supporting America's objective of providing assured access to space," said Col. Jack Weinstein, 30th Space Wing commander at Vandenberg. "Our partnerships with defense and civil space agencies like Orbital Sciences Corp. and DARPA are key to meeting that objective."

Janene Scully can be reached at 739-2214 or janscully@santamariatimes.com.