Japanese Hayabusa mission to rubble pile asteroid Itokawa
The Japanese Hayabusa mission to Itokawa was the first sample return mission to an asteroid. The challenging journey was marked by technical difficulties, but Hayabusa managed to reach its target and return with precious cargo. The success of the mission paved the way for asteroid sample return missions such as OSIRIS-REx and Hayabusa 2.
On 26 September, 1998 astronomers at the Lincoln Near-Earth Asteroid Research (LINEAR) project in New Mexico spotted a tiny blip moving across the starry backdrop of the night sky. The project was part of an initiative by the US Air Force to hunt down Near-Earth Objects, or small Solar System bodies that approach close to the orbit of Earth. Itokawa was flagged by the automated telescopes, and its orbit was confirmed, after which it was catalogued with the number 25143. Asteroids are numbered serially in the order in which their discoveries are confirmed, with the first, 1 Ceres discovered in 1801.
An image captured at a distance of about 4 km from Itokawa. (Image Credit: JAXA).
The unassuming asteroid is barely larger than a city block. It is a time capsule from the infancy of the Solar System, preserving the conditions in which the planets were assembled around a newborn Sun. The asteroid was named after Hideo Itokawa, the man who launched the Japanese Space Programme, where he is known as the ‘Father of Rocketry’. The elliptical orbit of Itokawa ranges from 0.95 to 1.7 AU, or astronomical units. One AU is the distance between the Earth and the Sun. Itokawa orbits the Sun once every 18 months, and occasionally approaches within 1.2 million kilometres to the Earth.
A pile of rubble
Itokawa measures 330 metre long, 180 metres wide and is about 150 metres thick. From the surface, a lumpy, uneven horizon would be visible, a landscape of boulders and dust. The surface is a patchwork of contrasts, dusty plains interspersed with ragged, rock-strewn patches. It was as if nature could not decide whether to polish or pummel it. The surface is not pockmarked with craters, which is unusual for an asteroid. This indicates that the asteroid is either young, or pristine, or is constantly reshaped by its own fragility.
Regions on the asteroid. (Image Credit: JAXA).
ITokawa is an S-type asteroid, rich in silicates such as olivine and pyroxene, and traces of iron. These materials are similar to meteorites that make it to the surface of the Earth. Itokawa is not made up of a single homogenous piece of rock, but is a rubble pile asteroid, a jumble of debris held together by gravity so weak that a grasshopper could escape with a single leap. The density of the asteroid is only about twice that of water, indicating a porous interior. Itokawa is essentially a sponge that is more void than substance. The asteroid tumbles through space, spinning once every 12 hours.
A time capsule
Itokawa was born from the same gas, dust and ice from which the Earth was forged. It is likely to have started off as part of a larger body in the asteroid belt between Mars and Jupiter, about 4.6 billion years ago when the Sun was just born. Catastrophic collisions battered its parent, scattering the fragments across space. The gravitational nudges from Jupiter or Mars moved Itokawa to its present orbit. The rubble pile composition indicates that it is a second-generation asteroid, reassembled from the debris of its ancestors. This very fragility makes Itokawa a time machine. The surface has not been altered by weathering or volcanism, and it preserves the raw material of planetary assembly.
The Hayabusa Mission
Itokawa is dry, and lacks water or carbon. In June 2026, the asteroid will approach within 1.9 million kilometres of the Earth. The Hayabusa spacecraft was launched on 9 May, 2003 by the Japan Aerospace Exploration Agency (JAXA) to study the asteroid Itokawa, and return samples to the Earth. The probe reached Itokawa in September 2005, confirming that the peanut-shaped asteroid was a pile of rubble. For months, the probe orbited the asteroid, mapping its rocky surface strewn with boulders, using spectrometers as well as cameras.
A rubble pile asteroid constantly crumbles. (Image Credit: JAXA).
In November 2005, Hayabusa attempted a pair of daring touchdowns to collect samples, but the effort was plagued by technical glitches, including leaking thrusters, a failure in the sampling mechanism and a loss of contact. Still the probe managed to scoop up tiny grains of dust dislodged during the impact. Hayabusa limped back home with damaged engines and fading power, and reentered the atmosphere of the Earth on 13 June 2010. The capsule used parachutes to shed velocity to gently lower the first samples returned from an asteroid into the Woomera desert in Australia. There were only 1,500 precious grains of sample, measuring about one microgram.