Last week I wrote briefly about the Japanese Hayabusa mission to the asteroid Itokawa. This mission was pretty cool because we went to this piece of space rock instead of waiting a piece of it to come to us. In spite of the mechanical failures that nearly scuttled the mission, the Hayabusa probe was able to collect dust from Itokawa and bring it home for us to study.
I wanted to write a bit about this mission because meteorites come from asteroids. This is the equivalent of studying not only pumice from Mt. Hood, but the origin of the volcano as a whole and its relation to the Cascades. The collected dust particles tell us the story of how Itokawa formed and the types of meteorites that could come from it. From this evidence we can also gain more insight into the formation of the solar system.
The dust particles tell us that Itokawa is not a single chunk of space rock. It’s actually a patchwork collection of various asteroids that were broken apart and coalesced into a single piece of rock (1). In geology parlance we call this a breccia– or a big pile of rubble that cemented together over time. Sometimes this breccia is made up of the same rock type, other times it’s not.
In the case of Itokawa, we learned that it’s composed of asteroids with different thermal characteristics. Some of the breccia pieces have seen little heating and contain glass and clinopyroxene- a mineral common to type 3 and 4 meteorites. Others contain minerals such as diopside and large plagioclase crystals- these are indicators of the type 5 and 6 meteorites that have seen a higher degree of heating. (Nakamura et al., 2011)
From this information Nakamura et al., were able to construct a history for Itokawa. They suggest that Itokawa was large enough to undergo a high degree of thermal metamorphism, but was subsequently broken apart by collisions with other asteroids. These pieces fused together into the much smaller Itokawa that is present today. They also confirm that Itokawa is a member of the S-type asteroid family and that a direct link exists between it and ordinary chondrites.
1. Nakamura, Tomoki, et al., Itokawa Dust Particles: A Direct Link Between S-Type Asteroids and Ordinary Chondrites. Science 333, 1113 (2011). DOI: 10.1126/science.1207758
2. For more close-ups of Itokawa, check out the image archive from JAXA.