Samples returned from asteroid Itokawa reveal history of violent impacts

Feb 28, 2012 by Bob Yirka report
A scanning ion image of a grain specimen. Image (c) PNAS, doi: 10.1073/pnas.1116236109

(PhysOrg.com) -- In June of 2010, Japanese researchers launched a probe called Hayabusa into space on a mission to study the asteroid Itokawa (25143). It managed to collect dust specimens from the asteroid’s surface which were subsequently (for the first time ever) brought back to Earth when the probe parachuted into the Australian outback. Since that time, the specimens have been studied in every imaginable way resulting in several papers being published in several journals. Now, in a paper published in the Proceedings of the National Academy of Sciences, the research team is reporting on its findings after placing several specimens under an electron microscope.

To study the dust particles more closely the team cut five of the grains into smaller pieces and placed them under an electron microscope. What they found were dust particles that very clearly displayed a violent history. The grains were pitted, or cratered, indicating they had been struck by other particles at speeds high enough to cause them to melt on impact. They also found other particles stuck to the grains that had struck hard enough to embed themselves; to cause such affects, the researchers estimate that the smaller particles must have been traveling at speeds between ten and twenty two thousand miles per hour - particles that were micro to micron sized. In studying the particles that had stuck, the researchers found several that were apparently made of glass.

Because the were taken from the surface of the , they believe they represent the low gravity environment that the asteroid would have encountered as it journeyed through space. Thus, the outside layer is made up mostly of ordinary chondrite material.

Itokawa is a 500 meter long asteroid that is believed to be a fragment of a much larger rock, some suggest one as large as 20 kilometers long, and because it has such a low density, many in the research community refer to it as more of a rubble pile than a true asteroid. It was first discovered in 1998 and named for famed Japanese scientist Hideo Itokawa. Hayabusa brought back more than fifteen hundred particle samples.

The team’s findings suggest that rather than existing as static remnants created during the formation of the solar system, as many in the past have assumed, asteroids appear to have undergone almost constant change as they have struck and been struck by other space particles altering their exterior and perhaps their interior as well.

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More information: Space environment of an asteroid preserved on micrograins returned by the Hayabusa spacecraft, PNAS, Published online before print February 27, 2012, doi: 10.1073/pnas.1116236109 (OPEN ACCESS ARTICLE)

Abstract
Records of micrometeorite collisions at down to submicron scales were discovered on dust grains recovered from near-Earth asteroid 25143 (Itokawa). Because the grains were sampled from very near the surface of the asteroid, by the Hayabusa spacecraft, their surfaces reflect the low-gravity space environment influencing the physical nature of the asteroid exterior. The space environment was examined by description of grain surfaces and asteroidal scenes were reconstructed. Chemical and O isotope compositions of five lithic grains, with diameters near 50 μm, indicate that the uppermost layer of the rubble-pile-textured Itokawa is largely composed of equilibrated LL-ordinary-chondrite-like material with superimposed effects of collisions. The surfaces of the grains are dominated by fractures, and the fracture planes contain not only sub-μm-sized craters but also a large number of sub-μm- to several-μm-sized adhered particles, some of the latter composed of glass. The size distribution and chemical compositions of the adhered particles, together with the occurrences of the sub-μm-sized craters, suggest formation by hypervelocity collisions of micrometeorites at down to nm scales, a process expected in the physically hostile environment at an asteroid’s surface. We describe impact-related phenomena, ranging in scale from 10-9 to 104 meters, demonstrating the central role played by impact processes in the long-term evolution of planetary bodies. Impact appears to be an important process shaping the exteriors of not only large planetary bodies, such as the moon, but also low-gravity bodies such as asteroids.

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barakn
5 / 5 (4) Feb 28, 2012
The quality of the writing in the first paragraph is abysmal.
julianpenrod
1 / 5 (11) Feb 28, 2012
Right wing "nay-sayers" constantly condemn "conspiracy theories" as having no basis \in circumstances, as being nothing more than guesses. The claims about Hayabusa's claimed trip to asrteroid Itokawa, in fact, do lend themselves to the eminent suspicion upon which conspiracy theories rest.
Interesting, among other things, that no attempts seem to have been made to actually photograph the collection of samples from Itokawa. One of the few widely disseminated images purports to show the "shadow" of Hayabusa on Itokawa as it "approached". But look at the photograph and how large the "shadow" appears. Itokawa has a width from 200 to 300 meters, but Hayabusa is only about 6 meters wide with solar panels fully deployed. But the "shadow" is comparatively about 30 meters wide! The sun's rays going around Hayabusa are approximately parallel, so any "shadow" should be the same size as Hayabusa!
julianpenrod
1 / 5 (7) Feb 28, 2012
And the appearance of the asteroid is questionable in the extreme. The general "explanation" for boulder fields on asteroids is rubble from collisions. With the energy claimed for most collisions and a suggested escape velocity of 8 inches per second, it seems unreasonable it could collect such mounds of boulders. Indeed, Itokawa is about the size of the Enpire State Building. If it could hold mounds of rubble so well, the Empire State Building should have gravity that significantly deviates objects dropped near it.
And the very description of the mission, to fly low over the asteroid's surface and lower a scoop to gather soil. The smallest obstruction could cause torquie that would cause the entire probe to pivot over and crash. And, if not, it should have raised a cloud of dust ahead of it that would move with it, possibly penetrating its systems.
julianpenrod
1 / 5 (9) Feb 28, 2012
Add things like that the picture of the Hayabusa "shadow" are generally available only on sites written in foreign languages looks like there is an initiative to keep Americans who only speak one language from examining the photo. And the curious case of the Japanese examining only a Japanese discovered asteroid, as if they feel now they can say whatever they want without fear of international repercussions taking liberties with other countries' asteroids.
d_robison
5 / 5 (4) Feb 28, 2012
@julianpenrod

In the well-spoken words of Professor Farnsworth from Futurama, "I don't wish to live on this planet anymore." Unfortunately, I don't have the luxury or technology to do so.
Lurker2358
not rated yet Feb 28, 2012
And the appearance of the asteroid is questionable in the extreme. The general "explanation" for boulder fields on asteroids is rubble from collisions. With the energy claimed for most collisions and a suggested escape velocity of 8 inches per second, it seems unreasonable it could collect such mounds of boulders. Indeed, Itokawa is about the size of the Enpire State Building. If it could hold mounds of rubble so well, the Empire State Building should have gravity that significantly deviates objects dropped near it.


While I agree with what you are saying here, you are not technically correct. The Empire State building is mostly empty space, surrounded substantial by walls, floors, and ceilings.

Surface gravity of this asteroid should be close to 0.00038 m/s^2.

Escape velocity is near 0.43m/s...

You could jump and easily obtain several times escape velocity, even in a space suit...

The supposed 11 to 17 km/s impact velocities should disintegrate objects this size.
lomed
not rated yet Feb 29, 2012
The sun's rays going around Hayabusa are approximately parallel, so any "shadow" should be the same size as Hayabusa!
Looking at the orbit of the asteroid on wikipedia (ref. http://en.wikiped..._Itokawa ), one sees that its distance from the Sun varies from 0.953 to 1.695 AU. At these distances the Sun has a non-negligible angular diameter. So, just like on Earth the farther an object is above a surface illuminated by the Sun the larger its penumbral shadow (ref. http://en.wikiped.../Umbra ).
julianpenrod
1 / 5 (1) Feb 29, 2012
The orbit for Itokara varies between 90,000,000 and about 160,000,000 miles. When Hayabusa was hovering near Itokawa, it was no more than a couple dozen miles high. That is so small an angular difference that changes in the size of the shadow should be miniscule. Certainly, the shadow should not be ten times the size of the probe. Unfortunately, there are so many determined to promote "the official story" that they will rabidly defend lomed's "explanation".
And, with respect to Lurker's comment, even if the Empire State Building is "mostly empty space", the density of the iron in its design is so much greater than the substance of Itokawa that its mass can be greater than that of the asteroid.