What lies beyond...

Astronomy

What lies beyond...

Eight years ago, a tiny Japanese spacecraft left the earth to venture into an uncharted territory. Its mission was to land on a near-earth asteroid, collect samples and bring them home.

After almost two years, the Hayabusa spacecraft gently landed on the target asteroid – 25143 Itokawa, named after Hideo Itokawa, a Japanese radio scientist. But much to the disappointment of the scientific community, the main sample collection instrument of the probe malfunctioned due to a technical glitch and scientists were not sure if it could gather asteroid samples.

Many of them lost hope. But the probe returned to earth after five years, June 13, 2010, to be precise, not with a large sample collection but with a small amount of materials that gave researchers their first up-and-close look at dust from the surface of the small, stony asteroid. This incidentally is the second set of extra-terrestrial rock samples returned to the earth, the first one being moon rocks brought back by the Apollo and Luna missions.

Chondrites, born from asteroids?
Analysis of the asteroid dust particles confirms a long-standing suspicion, the most common meteorites found on earth, called chondrites, are born from these stony asteroids. Because chondrites are among the most primitive objects in the solar system, the discovery also means that these asteroids have been recording a long and rich history of early solar system events.

Six papers published in the August 26 issue of Science showed the complex formation history of the near-earth asteroid and the impact of space weathering on its surface. Near-earth asteroids are subjects of intense research because they are large rocks floating in space, and have the maximum chances of colliding with the earth.

The asteroid sampled by Hayabusa is a rocky asteroid with the appearance of a rubble pile. Based on ground observations, researchers believed that similar asteroids, located in our solar system’s inner and middle asteroid belt, are responsible for most of the small meteorites that regularly strike the earth. But the visible spectra of these asteroids have never precisely matched those of ordinary chondrites, a fact that has left researchers suspicious of their actual affiliation.

The only way to confirm a direct relationship between meteorites and these asteroids was to physically sample the regolith (loose material) from an asteroid’s surface. “Our study demonstrates that the rocky particles recovered from the asteroid are identical to ordinary chondrites, which proves that asteroids are indeed very primitive solar system bodies,” said Tomoki Nakamura from Tohoku University in Japan who, along with colleagues, was among the first to analyse this regolith brought back by Hayabusa.

The researchers used powerful electron microscopes and X-ray diffraction techniques to study the mineral chemistry of Itokawa’s dust particles. They noticed Itokawa’s regolith went through significant heating and impact shocks. They deduced that the asteroid is actually made up of small fragments of a much bigger asteroid.

Insights into Itokawa’s composition
“The particles recovered from the asteroid have experienced long-term heating at about 800 degrees Celsius,” said Nakamura. “But to reach 800 degrees, an asteroid would need to be about 20 kilometers in diameter. The current size of Itokawa is smaller than that so it must have first formed as a larger body, then been broken by an impact event and reassembled in its current form.”

The thermal metamorphism possibly happened by decay of a short-lived radionuclide (Aluminium 26) during the first 10 million years of solar system evolution. It was subsequently catastrophically disaggregated by impacts into small pieces, some of which re-accreted into the present rubble-pile asteroid, said Alexander N Krot from the University of Hawaii at Honolulu who wrote a perspective article in Science.

Launched by the Japan Aerospace Exploration Agency (JAXA) in 2003, Hayabusa probe was to sample the surface of the near-earth asteroid, 25143 Itokawa, which is close to the earth-moon system rather than being a part of the asteroid belt between the orbits of Mars and Jupiter. When the unmanned vessel reached its destination a little more than two years later, it made two separate touchdowns on the asteroid’s surface.

Although its primary sampler malfunctioned, the spacecraft struck the asteroid’s surface with an elastic sampling horn to catch a small amount of dust particles that were kicked up. The results from JAXA Hayabusa; NASA Stardust (returned samples of Jupiter family comet Wild2) and Genesis (returned samples of solar wind) missions demonstrated that solar system samples are crucial in understanding the origin and evolution of the solar system.

“When researchers analysed regolith from the moon, they needed kilogram-sized samples. But, for the past 40 years, experts have been developing technologies to analyse extremely small samples. Now, we’ve gained all this information about Itokawa with only a few nano-grams of dust from the asteroid,” said Michael Zolensky at the NASA Johnson Space Centre in Texas, a co-author.

More missions
The success of Hayabusa-1 has led to two more sample-returning missions. The JAXA Hayabusa-2 spacecraft will be launched in 2014 to another asteroid 1993 JU3. A NASA mission, OSIRIS-Rex, scheduled for 2016 will explore another asteroid 1996 RQ6. These explorations would expand our knowledge of near-earth space as it currently exists and contribute significantly to humanity’s understanding of earth’s most distant past, added Krot.

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