Australasian Science: Australia's authority on science since 1938

Extraterrestrial Body Brought Back to Earth

 Retrieval of the capsule at Woomera. Credit: JAXA/ISIS

Retrieval of the capsule at Woomera. Credit: JAXA/ISIS

By David Reneke

What do the samples from the Itokawa asteroid tell us about the history of our solar system?

The space and astronomy world held their collective breaths on 13 June 2010 as an ion-powered Japanese spacecraft named Hayabusa (literally “peregrine falcon”) successfully returned the first samples gathered from the surface of an asteroid. At last, the impossible had become possible. Scientists hoped they had captured those long-awaited pristine pieces of our solar system.

Hayabusa was launched on 9 May 2003 and rendezvoused with asteroid Itokawa in mid-September 2005. Over the next two-and-a-half months, Japanese Aerospace Exploration Agency (JAXA) scientists and engineers performed an analytical study of the asteroid’s shape, spin, colour, mineral composition, density, gravity and the way it reflected the Sun’s rays.

Then, on 25 November of that year, Hayabusa briefly touched down on the surface of Itokawa. It collected samples in the form of tiny grains of asteroidal material and returned them to Earth in a small capsule aboard the spacecraft on that historic day in 2010.

Other spacecraft sent by NASA, notably Galileo and NEAR Shoemaker, have visited asteroids before but the Hayabusa mission was the first time that an attempt was made to return an asteroid sample to Earth for analysis.

Until now, the only extraterrestrial celestial body from which we have gathered samples is the Moon, but time has changed their material structures. Asteroids, on the other hand, are believed to be small enough to have preserved the state of the early solar system and are sometimes referred to as celestial “flotsam and jetsam”. A soil sample from an asteroid can give us clues about the raw materials that made up planets and asteroids in their formative years, and about the state of the inside of a solar nebula around the time of the birth of the planets.

The 510 kg mother ship Hayabusa had logged about 2 billion kilometres since its launch in May 2003. Now it was time to come home The capsule landed in the Woomera prohibited area in Australia and was flown back to Japan for analysis.

JAXA scientists knew there were particles of material inside, but were unsure if they were actual pieces of an asteroid, cosmic dust or contamination from Earth. “About 1500 grains were identified as rocky particles, and most of them were judged to be of extraterrestrial origin, and definitely from Asteroid Itokawa,” JAXA said in a statement.

The analysis of the Itokawa asteroid sample illustrates the wealth of information that can be obtained, even from very small samples, and sets the stage nicely for NASA’s OSIRIS REx mission, which is to sample a more primitive asteroid that will help us to understand the role asteroids played in the origin of the Earth’s oceans and life.

After a series of comprehensive analyses, four papers published in Science have confirmed that the Hayabusa spacecraft came home with samples of an asteroid. Japanese researchers along with their western counterparts were anxious for the first preliminary test to begin. And the wait was worthwhile.

The Hayabusa samples are pristine grains collected from the surface of Itokawa, and seem to be essentially different from other extraterrestrial materials. Using a scanning electron microscope to study the samples, researchers identified minerals such as olivine, pyroxene and others that matched remote-sensing observations taken by Hayabusa when it visited the asteroid.

Samples were handled in the clean chambers of the JAXA curation facility under conditions that minimised contamination. What scientists found in the Itokawa sample is unequivocal evidence that this type of asteroid is the parent of ordinary chondrites – the most common type of meteorites found on earth.

Mineral chemistry indicates that the majority of regolith surface particles suffered long-term thermal annealing and subsequent impact shock, suggesting that Itokawa is an asteroid made of reassembled pieces of the interior portions of a once-larger asteroid.

Space weather morphs asteroid fragments, and when they enter the Earth’s atmosphere they burn up, changing their chemical nature a bit. The pristine sample collected from Itokawa has helped distinguish the original material on the rock and how it changed when it entered the Earth’s atmosphere. This helps us to understand the origin and evolution of the planet and the solar system.

The samples also indicated a history and time frame of space weathering. Itokawa is continuously losing its surface materials into space at a rate of tens of centimetres per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

All this is probably true for all or most of the other asteroids in that belt of rubble orbiting between Mars and Jupiter. Unfortunately, we’ll have to wait for a return visit to find out for sure.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com