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Meanwhile, three secondary payloads are scheduled to separate from the H-2A rocket's upper stage in the next few minutes. They include a student-built amateur radio satellite, a "deep space sculpture" conceived in a marriage of art and space exploration, and a suitcase-sized probe designed to fly by another asteroid in 2016.
This rocket burn is required to put Hayabusa 2 at the correct speed to break free of Earth's gravitational pull and travel into interplanetary space.
The burn will last about 4 minutes, 1 second.
Separation of Hayabusa 2 is scheduled for 0609 GMT (1:09 a.m. EST), followed a few minutes later by release of three piggyback payloads carried on today's launch.
An H-2A rocket has never attempted such a lengthy coast period during any of the booster's 25 previous flights.
"In this launch of the H-2A rocket, we will execute a difficult operation called a long coast operation," said Hitoshi Kuninaka, JAXA's Hayabusa 2 project manager, in an interview with Spaceflight Now. "For most H-2A rocket launches, the satellite is separated about 30 minutes after the launch, but for this mission, we have a long coast operation and the H-2A rocket will do one orbit around Earth and when the rocket comes back over Japan, we will turn on the second stage engine again. We accelerate the spacecraft away from Earth and separate."
A second burn of the second stage engine is scheduled to begin at T+plus 1 hour, 39 minutes, 23 seconds.
Rocketing east across the Pacific Ocean, the second stage LE-5B engine will fire for more than five minutes during this first burn of the mission to place the vehicle into a parking orbit.
A second burn will come later to put Hayabusa 2 on track to break free of Earth's gravity and escape into the solar system.
The ignition sequence of the first stage engine begins 5.2 seconds before liftoff. Solid rocket booster ignition occurs at T-zero.
The automatic sequence will begin at T-minus 4 minutes, 30 seconds and computers will pressurize the H-2A's propellant tanks for flight at about T-minus 4 minutes, 20 seconds.
At about T-minus 3 minutes, the launcher will transition to internal battery power and remove external power.
Water will be released onto the launch pad deck beginning at T-minus 73 seconds to help suppress sound and acoustics during the ignition and liftoff. The vehicle's pyrotechnic and ordnance systems will be armed at T-minus 30 seconds and the rocket's guidance system initializes at T-minus 18 seconds. Batteries controlling solid rocket booster ignition are activated at T-minus 15 seconds.
Sparklers underneath the rocket's main engine ignite at T-minus 11.7 seconds to burn off residual hydrogen that could be an explosive hazard at main engine start.
At liftoff, the H-2A will be propelled upward from the launch pad on the power of a single hydrogen-burning LE-7A main engine and two solid rocket motors. The total liftoff thrust of the rocket is approximately 1.6 million force pounds.
The rocket will pitch southeast from Tanegashima, racing through the speed of sound in less than a minute and reaching an altitude of more than 30 miles in the first two minutes of flight.
After consuming their pre-packed solid propellant, the two strap-on boosters will jettison at about the two-minute point. The 13.1-foot diameter payload shroud will separate at T+plus 4 minutes, 10 seconds after the H-2A rocket ascends above the discernable traces of Earth's atmosphere.
First stage main engine cutoff, stage separation and ignition of the second stage's LE-5B engine will occur nearly seven minutes into the mission. The second stage will burn for more than five minutes before shutting down at T+plus 11 minutes, 18 seconds.
After a nearly 90-minute coast, the second stage will reignite for a 4-minute, 1-second burn.
Deployment of the Hayabusa 2 spacecraft is expected at T+plus 1 hour, 47 minutes, 15 seconds.
Three secondary payloads -- Procyon, Shin'en 2 and Artsat 2 -- will separate from the H-2A rocket a few minutes later.
Officials just gave the "go" to enter the terminal count, which began at 0322 GMT. The final hour of the countdown will prepare the rocket, the payloads, and ground systems for flight.
The H-2A rocket for today's launch is flying in the "202" configuration with two large 15-meter-long (49-foot) solid rocket boosters and without any of the smaller strap-on boosters sometimes used to augment the launcher's thrust.
It also features a standard four-meter (13.1-foot) diameter nose shroud, which encloses the Hayabusa 2 spacecraft for the first few minutes of flight through the lower atmosphere.
Japanese engineers added thermal paint to a section of the rocket's upper stage for today's launch, which will mark the first time the rocket will fly a lengthy hour-and-a-half coast phase before restarting the second stage engine.
The long coast is necessary to inject Hayabusa 2 on the proper trajectory.
The lead contractor for the H-2A rocket is Mitsubishi Heavy Industries.
Hayabusa 2 weighs about 600 kilograms, or approximately 1,320 pounds, at the time of launch. It was built by NEC Corp. of Japan.
The spacecraft will be deployed on a trajectory into deep space to begin a six-year mission to gather samples from a distant asteroid and return the material to Earth.
Hayabusa 2 will fly by Earth again in December 2015 to receive a speed boost from Earth's gravity. The probe will reach asteroid 1999 JU3 in June 2018 for one-and-a-half years of detailed study.
During its stay near the asteroid, the spacecraft will conduct a series of touch-and-go maneuvers to collect bits of rock and dust from its surface. It will also release four landers -- three from Japan and one from Europe -- to bounce around the asteroid, take pictures, and provide ground truth for the Hayabusa 2 mothership's observations.
Hayabusa 2 will leave the asteroid in December 2019 and return to Earth in December 2020, when scientists will recover the samples for analyses in laboratories.
Asteroid 1999 JU3 is about six-tenths of a mile across and completes one rotation every seven hours and 38 minutes. Scientists believe the object is a "C-type" asteroid rich in carbon, organic molecules and water.
Another "go/no go" decision point is coming up soon, before the countdown enters the terminal phase at T-minus 60 minutes.
The Hayabusa 2 spacecraft is heading for an interplanetary journey. The H-2A rocket's upper stage LE-5B engine will fire two times to accelerate the probe to escape the grasp of Earth's gravity.
The next few hours of the countdown will be spent activating and checking out a variety of rocket systems, including radio frequency links with tracking stations. Another steering check of the rocket's main engine is also planned.
After testing is completed, officials will give the go-ahead for the terminal countdown scheduled to start at 0322 GMT. Liftoff remains set for 0422 GMT (11:22 p.m. EST), or 1:22 p.m. local time at the Tanegashima Space Center in southern Japan.
The pads are positioned on a rocky outcrop on the southeastern flank of Tanegashima Island, which lies about 80 miles south of Kagoshima, Japan.
The complex features two pads for the H-2A and H-2B rockets. Launch Pad No. 2 has been used for four flights of the H-2B rocket with the H-2 Transfer Vehicle, an unmanned cargo ship for the International Space Station. In all, 36 rockets have departed Earth from the Yoshinobu complex since 1994. The most recent flight was an H-2A rocket launch in October 2014.
The LE-7A and LE-5B engines on each stage burn the super-cold propellants during the flight. Because the propellant is stored under cryogenic conditions, it must be gradually replenished throughout the countdown to ensure proper levels of fuel are inside the rocket at liftoff.
The 500-meter rollout from the Vehicle Assembly Building occurs about 12 hours before liftoff. The trip follows rail tracks and takes about a half-hour to complete.
JAXA and Mitsubishi Heavy Industries, the H-2A rocket's prime contractor, report all systems are functioning well at this point in the countdown.
The 17-story rocket is set for liftoff at 0422:04 GMT Wednesday (11:22:04 p.m. EST Tuesday) with Hayabusa 2, a robotic spacecraft embarking on a six-year roundtrip journey to collect samples from and asteroid and return the materials to Earth.
Fitted with two solid-fueled boosters and hydrogen-burning first stage and second stage engines, the H-2A launcher will roll out on rails to Launch Pad No. 1 at Tanegashima. Rollout is expected about 12 hours before liftoff.
The launch base is located on Tanegashima Island, which lies off the southern coast of Kyushu at the southwest end of the Japanese main islands.
Once the rocket arrives at the launch pad, technicians will connect the vehicle to the facility's electrical and propellant supplies before fueling of the launcher with liquid hydrogen and liquid oxygen.
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