Violent History Preserved in Hayabusa's Asteroid Grains

Far from asteroids being quiet, peaceful objects drifting through space, analysis of dust samples from asteroid 25143 Itokawa has shown they are violently sandblasted by micrometeorites.

The world's first asteroid sample return mission is providing some stunning insights to the evolution of the primordial pieces of rock floating around in our solar system.

But far from asteroids being quiet, peaceful objects drifting through space, analysis of the tiny dust samples from the surface of near-Earth asteroid Itokawa has shown they are constantly - and violently - sandblasted by micrometeorites.

It's fair to say that the Hayabusa asteroid sample return mission wasn't the luckiest of spacecraft. It was fried by a huge solar flare shortly after launch in 2003. Mission controllers lost control of the probe during the 2005 rendezvous with its target, asteroid 25143 Itokawa. And then the sample collection mechanism failed when it did finally "land" the 550-meter wide space rock.

But ultimately, the Japanese mission became the symbol of human ingenuity when, on June 13, 2010, the robotic spacecraft limped back to Earth and released its sample return capsule before burning up over the Australian Outback. The capsule landed safely and, after a nervous wait, scientists made the discovery that asteroid dust had miraculously drifted into the sample return capsule despite the failure of the sample return mechanism.

It had been fried, bumped, spun, lost and then found again, but Hayabusa completed its mission and scientists are now uncovering the incredible science locked inside the 1,500 tiny grains of asteroid dust.

Now, by studying just five of these grains, scientists of Okayama University, Japan, have found that Itokawa is constantly shaped by high-speed micrometeorites. The tiny grains, down to the size of 40 microns - or half the width of a human hair - were studied under a microscope and the results were published in Proceedings of the National Academy of Sciences on Feb. 27.

Tiny craters, fractures and even fused micrometeorites embedded in the grains are evident in the samples. The micrometeoroids must measure in the nanometer scale to explain the craters. What's more, they had to be moving at speeds of between 3 to 6 miles per second (5 to 10 kilometers per second). That's 11,000 to 22,000 mph (18,000 to 36,000 kmph)! In other words, even on microscopic scales, the environment asteroids are exposed to is a violent one.

The evidence embedded in these grains of dust, plus the impact of solar wind particles, is collectively called "space weathering" say the researchers:

"Our observations on grain surfaces provide a first demonstration of the sub-micron-scale physical and chemical properties at asteroid exteriors and indicate that space weathering should be regarded as representing a combination of disaggregation, cratering, melting, adhesion, agglutination, and implantation/sputtering."

Asteroids carry critical information about the early history of our solar system and they are the intermediate stage before planetary bodies accumulated enough mass to be considered "planets."

Studying their formation on microscopic scales therefore provides an incredible insight to these primordial bodies - a feat only possible by physically collecting samples from their surfaces, as Hayabusa did when it met Itokawa in 2005. Pristine surface samples cannot be found on meteorites on Earth as their surface layers are burned off during entry through our planet's atmosphere.

But what can explain these high-velocity impacts on the nano-scale? Over millions to billions of years, these space rocks collide with one another. Although the rate of asteroid collision is low, it happens regularly enough over the evolution of the solar system, produce huge quantities of debris. The debris, by its nature, will be very energetic and with no atmosphere to slow the debris down, they eventually hit other space rocks, on the macro-, micro- and nano-scales.

Although the collisions are energetic and damaging, it is this accumulation of primordial debris from the solar system that accumulated to eventually "build" planets, so the Japanese researchers are glimpsing a fundamental mechanism that ultimately created the solar system we live in today.

Like any epic adventure, Hayabusa has a Japanese movie in its honor:

Publication: "Space environment of an asteroid preserved on micrograins returned by the Hayabusa spacecraft," Nakamura et al., PNAS, 2012. doi: 10.1073/pnas.1116236109

Image: A microscopic image of one of the Hayabusa sample grains, exhibiting adhered-to microscopic grains of glass. High-energy impacts caused melting and fusing of the particles to the grains. Credit: Nakamura et al./Okayama University