AKARI observes asteroid Itokawa

Last month, about three months after Hayabusa's encounter, on the 26th of July, 2007, the infrared astronomical satellite AKARI also succeeded in observing Itokawa with its onboard instrument, the Infrared Camera (IRC).
This is the motion of Itokawa over 12 minutes of observation by AKARI. The image size is about 7.4 arcmin x 7.4 arcmin. This movie is composed from the images taken at 7 and 11 micrometres. Credit: JAXA

At the time of the observation, Itokawa was in the constellation of Scorpius and was about 19 magnitudes bright in visible light. The asteroid and the Earth were closest to each other, lying at a distance of about 0.28 Astronomical Units apart (AU; 1 AU corresponds to the distance from the Earth to the Sun, about 150 million km, i.e., Itokawa was 42 million km away). Due to its proximity, Itokawa moved a significant distance on the sky over the short time of the observation. AKARI detected the target very clearly at infrared wavelengths. The asteroid is hardly visible using telescopes of similar sizes from the ground.

Hayabusa itself is at a distance of around 41 million km from the Earth (it is not possible to detect it even with AKARI), and continues its journey homeward. In June 2010 after about two revolutions along its orbit around the Sun, Hayabusa will arrive back home to the Earth, after which time the Earth will also have made three orbits around the Sun. As of August 22nd, 2007, the spacecraft is about 50 million km away from the Earth.

What is the meaning of this new observation of Itokawa after Hayabusa previously explored it in detail? When we study asteroids, their size is one of the most sought after pieces of information. For asteroids that are not explored directly, we can estimate their sizes based on various observations from the Earth. In fact, before Hayabusa arrived at Itokawa, many observations to determine the asteroid's approximate size had already been attempted. Among the many different measurement methods, the most accurate estimate was achieved by mid-infrared observations. This time, with AKARI, we observed Itokawa at several different wavelengths in the mid-infrared range obtaining a much more comprehensive set of data. This data is very important, not only for the study of the infrared properties of Itokawa, but also for use as a template and source of comparison with other asteroids, to improve the estimates of their sizes.

Image Credit: JAXA
High resolution image

The asteroid Itokawa observed by the Infrared Camera (IRC) onboard AKARI at 7 micormetres. The observation was carried out at 11:23--11:35 (UT) on July 26th, 2007.



Image Credit: JAXA
High resolution image

There are two more asteroids detected faintly in the image, 2003 CO9 and 2000 JS33.

AKARI has also made observations of possible candidates for future asteroid exploration and we expect that this detailed information will help us to greatly further our knowledge of these interesting relics of our Solar System.

The observation of Itokawa was carried out as a part of the asteroids observation programme by Dr. Suano Hasegawa (ISAS&JSPEC/JAXA) and colleagues of the AKARI Solar-System Object working group.

Source: JAXA / Institute of Space and Astronautical Science
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