Japan prepares for second asteroid sample return
BY STEPHEN CLARK
Posted: March 27, 2013
HOUSTON -- Japanese engineers hurriedly redesigned the rock-collector and science payloads on the Hayabusa 2 spacecraft set to launch on an asteroid-sampling mission in late 2014, hoping to trump a problem which limited the load of asteroid rock fragments brought home by a preceding mission.
The 1,320-pound Hayabusa 2 probe, slightly larger than the preceding Hayabusa spacecraft, is due for launch on a Japanese H-2A rocket in December 2014, and its destination is asteroid 1999 JU3, an object about 3,000 feet in length. Hayabusa 2 will arrive at 1999 JU3 in 2018 and loiter around the asteroid for about 18 months. Hayabusa spent about three months near asteroid Itokawa, a smaller rock than 1999 JU3.
After up to three close approaches to acquire samples, Hayabusa 2 will depart the asteroid and deploy a sample-bearing re-entry capsule into Earth's atmosphere in December 2020.
Although the first Hayabusa mission made history, it collected substantially fewer samples than expected because the spacecraft's rock-gathering device failed to function when the probe approached Itokawa.
Hayabusa also suffered from a crippling fuel leak, ion engine failures, reaction wheel problems, battery issues and a break in communications with Earth for two months, and Japanese officials grew concerned they would lose the spacecraft.
But controllers delayed Hayabusa's return to Earth by three years, buying time for engineers to devise a method of using the craft's remaining ion propulsion engines to control its orientation and guide the probe home.
When scientists opened Hayabusa's re-entry capsule after it landed in Australia, they found more than 1,500 tiny rock and soil grains, most of which were confirmed to be from Itokawa.
Launched in May 2003, Hayabusa made two descents toward Itokawa in late 2005 to collect samples, and the spacecraft was supposed to fire a tantalum pellet into the asteroid and scoop up bits of rock blasted away by the projectile.
The tantalum bullet did not fire on either sampling attempt, and scientists feared the mission was a failure.
But an analysis of telemetry from Hayabusa showed the probe inadvertently landed on the asteroid for up to a half-hour, leaving officials hopeful some surface specimens made their way through the craft's sampling horn and into a collection chamber.
It turns out that is what happened.
But with the mission slated to launch in late 2014, engineers had to prioritize what upgrades to make to the spacecraft, which will use much the same technology flown on Hayabusa.
Like its predecessor, Hayabusa 2 carries four ion engines, but the efficient electrically-propelled engines will generate more power on the new spacecraft. And Hayabusa 2 will have a Ka-band communications antenna to beam data and imagery back to Earth at higher speeds than Hayabusa, according to Makoto Yoshikawa, Hayabusa 2's project manager at the Japan Aerospace Exploration Agency.
NASA's Deep Space Network will support communications with the Japanese probe.
And with the benefit of a decade's progress in computer development, Hayabusa 2 will use upgraded software to keep the probe resilient to faults caused by radiation and other threats.
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.
Led by the DLR Institute of Space Systems in Bremen, Germany, with support from the French space agency, the Mobile Asteroid Surface Scout will measure the asteroid's magnetic field, surface temperature, rock composition, and take pictures during descent and after landing, according to a presentation by Ralf Jaumann, a planetary scientist at DLR.
Scientists will compare data from MASCOT and the Japanese lander to analyses of samples returned to Earth by Hayabusa 2.
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.
"These C-type asteroids could be the bridge from the start of the evolution of the solar system to the beginning of life on Earth," Tachibana said in an interview at the 44th Lunar and Planetary Science Conference near Houston.
Two of the sampling attempts will focus on regions identified to be rich in hydrated, water-rich minerals and organic molecules. Hayabusa 2's sampling system will fire a tantalum projectile into the asteroid to kick up rocks into the 5.5-inch opening of a funnel leading into a storage container.
The sampling gun did not fire on Hayabusa's visit to asteroid Itokawa, limiting the probe's haul of samples for return to Earth to tiny grains of dust. Engineers blamed the failure on a software coding error.
Tachibana is confident the tantalum bullet will fire on Hayabusa 2, but even with another glitch, engineers devised a backup method to ensure the probe comes home with more samples than Hayabusa. The end of the new sampling horn has teeth like a comb to dig into the asteroid's gravely surface to pick up some material even if the sampling gun does not fire, according to Tachibana.
"Even if the bullet does not shoot, as long as we touch down, we will get samples," Tachibana said.
Hayabusa 2 will collect a third sample with the help of a small copper impactor, which was not aboard the Hayabusa mission to Itokawa.
Slamming into the asteroid at more than 4,000 mph, the grapefruit-sized impactor will blast a crater and excavate material from beneath the rock's surface. After the collision, Hayabusa 2 will glide to a soft landing at or near the impact site, collecting material from the newly-formed crater.
Scientists say the subsurface samples are more pristine than surface rocks, which are exposed to weathering from radiation and the solar wind.
Each sample will be funneled into three separate holding chambers to separate the specimens, then the containers will be sealed to trap volatile compounds like hydroxides.
Depending on the texture of the rocks on 1999 JU3, Hayabusa 2 should pick up between a gram and several grams of samples.
That is much less than the 60-gram, or 2.1-ounce, sampling goal of NASA's OSIRIS-REx asteroid mission scheduled for launch in September 2016. Some scientists say OSIRIS-REx could corral more than 4 pounds of asteroid material under the right conditions.
"We get much less samples than OSIRIS-REx, but we think we can do a lot of science with our samples," Tachibana said. "Of course, we want to have more."
OSIRIS-REx is targeting another primitive carbon-rich asteroid similar to Hayabusa 2's target. It will arrive at asteroid 1999 RQ36 in 2020 and return to Earth in 2023.
Tachibana said OSIRIS-REx and Hayabusa 2 are complementary.
"They get more samples, but we get samples with better quality," Tachibana said, adding that Hayabusa 2's sample container has the advantage of differentiating each batch of asteroid material from three sampling attempts.