The Hayabusa 2 asteroid probe, on track for liftoff this winter, will be shipped to its island launch base at the end of September for final preparations to start the most audacious space exploration mission ever attempted by Japan.

The Hayabusa 2 spacecraft inside a clean room at JAXA's Sagamihara campus. Credit: Koumei Shibata
 
The mission will take off on top of an H-2A launcher as soon as December, fly to an asteroid scientists believe is a relic from the genesis of the solar system, drop a European-built lander, and return to Earth in 2020 with extraterrestrial rock samples.

The Japan Aerospace Exploration Agency revealed Hayabusa 2 to media Sunday as it neared the finish line in a four-year effort to design, construct and test the spacecraft.

Japanese officials have not announced the target launch date, but they say the mission is on schedule to lift off as soon as December in a narrow window when Earth and Hayabusa 2's target asteroid are properly positioned to make the journey possible.

Backup launch windows are available in June and December 2015.

The spacecraft, now almost fully assembled for flight, will soon wrap up testing at JAXA's Sagamihara campus near Tokyo, according to Hitoshi Kuninaka, Hayabusa 2's project manager.

"At the end of September, the spacecraft will be transported to Tanegashima," Kuninaka said.

Hayabusa 2's launch is next in line for liftoff from the Tanegashima Space Center -- located on Tanegashima Island in southwestern Japan -- after an Oct. 7 launch of the Himawari 8 weather satellite.

Once the spacecraft arrives at the launch site, Kuninaka said technicians will install pyrotechnic charges for its mission, which include explosives to excavate material from beneath the asteroid's surface. Ground crews will also add the mission's flight batteries and fill the probe with xenon gas and hydrazine propellant.

JAXA says the Hayabusa 2 mission's cost is 28.9 billion yen, or about $275 million.

Hayabusa 2's launch follows four years after its namesake -- the hard-luck Hayabusa mission -- returned to Earth with microscopic specimens collected from asteroid Itokawa.

Engineers designed Hayabusa with upgrades to expand its scientific payoff and increase its chance for success.

Hayabusa 2 carries four xenon-fueled ion thrusters for the voyage to asteroid 1999 JU3, an object with a diameter of about 3,200 feet that researchers believe is made of primitive rock left over from the ancient solar system.

Artist's concept of the Hayabusa 2 spacecraft at asteroid 1999 JU3. Credit: Akihiro Ikeshita/JAXA
 
After a swingby of Earth in late 2015 to get a gravity boost, the 1,320-pound craft will arrive at 1999 JU3 in June 2018 and loiter around the asteroid for about 18 months.

Once it arrives at asteroid 1999 JU3, Hayabusa 2 will survey the rock with an array of instruments, including imagers, a spectrometer, and a terrain-mapping altimeter.

The craft will also release a small Japanese rover named MINERVA to hop across the surface of the asteroid and deploy the MASCOT lander developed by the German Aerospace Center, or DLR.

Hayabusa spent about three months exploring Itokawa, an asteroid about half the size of 1999 JU3.

Hayabusa 2's destination is a different type of miniature world than Itokawa. Asteroid 1999 JU3 is a C-type asteroid, a classification of primitive objects made of organic and hydrated minerals.

Itokawa is an S-type asteroid composed of rocks and metals heated and modified over the solar system's 4.5 billion year history, causing the material to lose chemical markers left over from the dawn of the solar system.

Scientists expect the Hayabusa 2 samples to hold a record of the tumultuous early phases of the solar system's formation, including the basic building blocks of life such as amino acids.

Hayabusa 2 will collect up to three samples from 1999 JU3, including material blasted from beneath the asteroid's surface by a explosive grapefruit-sized copper impactor released from the mothership.

Depending on the texture of the rocks on 1999 JU3, Hayabusa 2 should pick up between a gram and several grams of samples.

After up to three close approaches to acquire samples, Hayabusa 2 will depart the asteroid in December 2019 and deploy a sample-bearing re-entry capsule into Earth's atmosphere in December 2020.

Follow Stephen Clark on Twitter: @StephenClark1.