Bringing bits of an asteroid back to Earth
DOI: 10.1063/PT.5.010140
The kidney-shaped asteroid 25143 Itokawa (shown below) has a length of 535 m and a mass of 3.5 × 1010 kg. Its eccentric 556-day orbit takes it just within Earth’s orbit and just beyond Mars’s orbit.
On 25 November 2005 the Japanese space probe Hayabusa touched down on Itokawa, scooped up a sample of the dusty surface, and headed back to Earth. On 13 June last year Hayabusa reentered Earth’s atmosphere. Before the probe burned up, it jettisoned its sample return capsule, which made a safe parachute landing at the Woomera Test Range in the South Australian outback.
Today, in a series of six papers that appear in Science, the Hayabusa science team reports what it learned from subjecting the Itokawa dust grains to a series of physical and chemical tests. The team’s main finding—that asteroids and meteorites are made of the same, ancient material—confirmed prevailing theory. What came as a surprise—at least to me—is how much the asteroid has changed, and continues to change, even as it sits in a seemingly passive orbit.
And of course, bringing back a sample from any solar system object represents a stunning technological feat—and for Japan’s space agency, JAXA, a triumphant milestone in the history of space exploration.
Asteroids and meteorites
Asteroids are rocky objects of various types and sizes that orbit the Sun in a wide belt that lies mostly between Mars and Jupiter. Meteorites are rocky objects of various types and sizes that fall to Earth. Both classes of object are thought to be made up of ancient material that did not end up in Mercury, Venus, Earth, or Mars.
By tracing meteorite trajectories, astronomers had already deduced that meteorites mostly likely come from the asteroid belt. Comparing asteroids’ remotely sensed spectra with meteorites’ chemical compositions had suggested that the two classes of space rock are closely related.
The Hayabusa team has proven directly that Itokawa, which belongs to one of the most common classes of asteroid, the stony S-class, has the same composition as the most common type of meteorite, the chondrite. Evidently, meteorites are asteroids or bits of asteroid that are knocked off their orbital perches and make their way to Earth.
But the team could do more than clinch the case for kinship. From the composition and structure of some of the grains, the researchers deduced that the grains had been heated in the past to 800 °C and had cooled to 600 °C at a rate of 0.5 °C per millennium. If, as seems likely, the heat source was internal and originated in the decay of aluminum-26, then the slow cooling rate implies that Itokawa was once 20 km in diameter—40 times its current size! Writing in one of the papers , Tomoki Nakamura of Tohoku University and his collaborators concluded that
the Itokawa parent S-class asteroid was originally much larger, experienced intense thermal metamorphism, and was then catastrophically disaggregated by one or more impacts into many small pieces, some of which re-accreted into the present greatly diminished, rubble-pile asteroid.
Further evidence of catastrophic impact comes from cracks in some of the grains. But, according to the team’s analysis of the grains’ three-dimensional structure, the impacts, presumably from meteorites, were not energetic enough to melt the grains.
Itokawa also shows evidence of another kind of bombardment: from particles in the solar wind and from cosmic rays. The solar wind contains helium, neon, argon, and other noble gases. When those atoms strike Itokawa, they become implanted at depths of about 1 μm. The more energetic cosmic rays not only penetrate more deeply, they also transmute sodium, potassium, and other elements into noble gases.
The two sources of implanted nobel gases—the solar wind and cosmic rays—have different isotopic compositions. By analyzing the noble gas content of a grain, it’s therefore possible to deduce where a given isotope originated. When the researchers detected in a micron-sized gathered from Itokawa’s surface an isotope created by a cosmic ray at an original depth of 1 m, they reached a remarkable conclusion. To quote from the paper written by Tohoku University’s Keisuke Nagao and his collaborators:
Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.
The Hayabusa team is made up of researchers from about 20 different institutions, mostly in Japan. But the lead authors of the six papers all come from Tohoku University in Sendai. Although Sendai is far enough inland to have escaped the 11 March tsunami, the accompanying earthquake damaged many buildings there. The prompt publication of the Hayabusa results, despite the earthquake and its continuing aftermath, represents another triumph.