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An Orbital Sciences Pegasus rocket with a NASA satellite nestled in its nose cone will fly away from California's Vandenberg Air Force Base on Saturday, but the winged booster won't be soaring into space.

Slung beneath an L-1011 carrier aircraft, the fully assembled rocket will leave its home port destined for the Kwajalein Atoll in the central Pacific Ocean, part of the U.S. Army's vast missile range, where the launch will occur next weekend.

The "Stargazer" jet should be wheels-up around mid-day local time, beginning the 4,700-mile trip that includes an overnight stop at Hickham Air Force Base in Hawaii.

Arrival at Kwajalein will kick off several days of final tests, rehearsals and reviews before the October 19 launch to deliver NASA's Interstellar Boundary Explorer spacecraft into an extraordinarily high orbit.

The IBEX satellite will observe the interaction between the million mile-per-hour solar wind emitted from the sun and the cold expanse of the galaxy's interstellar medium.

Weighing just 250 pounds, the compact probe targets an orbit looping from 4,400 miles at its closest pass to 200,000 miles at its furthest point from Earth, about 50 times the planet's radius. It will take 8 days to complete just one revolution.

"This is the first time, to my knowledge, that we've used a small launch vehicle to send a spacecraft to such a high orbit. We're really going almost to the moon," said David McComas, the IBEX principal investigator from the Southwest Research Institute.

An artist's concept shows the Pegasus rocket launching IBEX. Credit: NASA

But getting to such a lofty perch will require several steps.

The three-stage Pegasus XL rocket will inject its payload into a 125-mile circular orbit, give it a 60 rpm spin and then let go. A solid-fuel kick motor on the satellite, called a Star 27, then ignites to propel the craft into an egg-shaped orbit with the high point stretching 35 to 40 times Earth's radius. The sequence will take less than 14 minutes.

In the subsequent weeks, hydrazine thrusters on IBEX will be fired to raise the high and low points of the orbit, eventually achieving the desired altitude where the science instruments can perform their observations.

"What we are trying to do is maximize the amount of time we have to collect our science, and we have to be outside the Earth's magnetic shield in order to collect the science," said Mark Phillips, the IBEX deputy project manager from Southwest Research Institute. "So we typically want to be collecting all of the science above 10-Earth radii."

For Pegasus-maker Orbital Sciences, the rocket's job will be complete once the spent third stage releases IBEX and its attached kick motor. The added solid-fuel motor is considered part of the satellite project, not the Pegasus.

An artist's concept shows IBEX separating from the Star 27 after launch. Credit: NASA

The combined satellite and Star 27 weigh 1,009 pounds. That's a hefty sum for Pegasus, the small satellite launcher now making its 40th flight.

"It's pretty big for a Pegasus. In fact, it's the most we've ever thrown," said Eric Denbrook, Orbital's site manager at Vandenberg.

The company has about 75 employees at the California base for Pegasus, Taurus and Minotaur rockets, plus extensive missile defense work.

Assembly and testing of the Pegasus vehicle was conducted in Building 1555, a large hangar with picturesque views of the Pacific and launch pads used by Atlas and Titan rockets.

IBEX arrived at Vandenberg on July 28 and was taken to the Astrotech satellite processing facility where it was loaded with about 78 pounds of hydrazine propellant and underwent spin-balance testing, Phillips said.

The craft and Star 27 later moved to Building 1555 where the two were attached together and then mounted to the front-end of Pegasus. Installation of the two-piece nose cone was completed last week.

The finished product -- a 50,000-pound rocket with its iconic wing, rudder and fins -- was rolled out Monday morning for the 3.8-mile drive to Vandenberg's runway to join the L-1011. The carrier jet, which is stored in Mojave, California, flew in to pick up Pegasus.

The aircraft was jacked up, allowing the trailer hauling the rocket to slide underneath. Ground crews hoisted Pegasus and firmly locked it into place later that same day.

As this week progressed, a comprehensive combined systems test between all of the elements was run, readiness reviews were held and engineers worked to close final technical questions ahead of the ferryflight departure.

The first leg of the journey to Kwajalein takes the team to Honolulu for refueling and sleep. Arrival in the Marshall Islands is expected on Sunday (U.S. time), and officials will use the inbound flight to practice the aircraft's launch day flight path and test links between the rocket and range, NASA Launch Manager Chuck Dovale said.

This map illustrates the L-1011's launch day path and the drop box where Pegasus will be released. Credit: NASA

About 85 people from NASA, Orbital Sciences and the other mission partners are traveling to the remote launch site to support.

Conducting the launch from Florida's Cape Canaveral was once considered. But the payload weight nixed that option, prompting mission designers to select the equatorial site at Kwajalein.

"The initial mass of the spacecraft drove it beyond the limits that a launch from Kennedy (Space Center) could provide," Dovale said.

The air-launched system is highly portable, and Pegasus has been launched from Kwajalein twice before, including an Air Force satellite deployment flight earlier this year.

Moving IBEX from the Cape to Kwajalein ensured the payload size and rocket capabilities were well matched.

An artist's concept shows IBEX in space. Credit: NASA

"You gain about 30 pounds of performance just by going near the equator. You gain the advantage of the Earth's rotation," said John Calvert, the IBEX mission integration manager. "It gave the spacecraft about 30 pounds of mass to utilize for the mission."

The planned launch time on October 19 is 1:48 p.m. EDT (1748 GMT), near the middle of a window extending from 1:44:20 to 1:51:50 p.m. EDT.

Backup launch opportunities are possible the following three days, if needed, Dovale said.

Pegasus has launched more than 70 satellites since 1990 and racked up a string of 25 consecutive successful launches over the past 11 years.

"It's an extremely reliable launch vehicle," Denbrook said.

"I think the entire (NASA) Launch Services Program loves working on the Pegasus program," Dovale said.

"They are Category 3 certified for this vehicle, so they can launch any of NASA's highest-valued spacecraft."