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The asteroid explorer Hayabusa finally departed its target, the asteroid Itokawa, in late April
this year and is now heading back home to the Earth. 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). Figure 1 shows the image of Itokawa taken at a wavelength of 7 micrometres
by AKARI. It can be clearly seen that Itokawa moves rapidly on the sky during the
observation of about 12 minutes. Movie 1 demonstrates the motion of Itokawa by animation.
The movie was composed using the images at two wavelengths, 7 and 11 micrometres.

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.(1)

Hayabusa itself is at a distance of around 41 million km from the Earth (it is not possible to
detectit even with AKARI), and continues its journey homeward. In June 2010 after about
two revolutions along its orbit around the Sun, as shown in Figure 2, 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

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 in
frared properties of Itokawa, but also for use as a template and source of comparison with
other asteroids, to improve the estimates of their sizes.

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.

(1) The majority of the sunlight reaching the asteroid Itokawa is absorbed on the surface,
and warm ups the object. Consequently, Itokawa re-emits this heat energy as bright infrared
light which was in turn observed by AKARI. On the other hand, only a small faction of the
incident sunlight is reflected from Itokawa which explains why the asteroid is so very faint in
visible light.

(2) The temperature of asteroids is determined by the balance between the input energy
from the incident sunlight and the output energy from the reprocessed heat energy emitted
as infrared radiation from the asteroid. Computer models that calculate the temperature
distribution in asteroids considering their shapes, rotations, and surface conditions have
been investigated. Observations in the mid-infrared wavelength range provide information
about the infrared light emitted from the asteroids. By comparing the infrared observational
data with that expected from the model calculations, we can derive information about the
asteroid such as its size. Additionally, we can improve the model calculations themselves by
utilizing the infrared observational data of very well studied asteroids such as Itokawa.

The improvement of thermophysical models of asteroids such as Itokawa would be useful
especially for estimating the size of potentially hazardous asteroids which may be discovered
in the future. The new AKARI data will be perfect for improving the model of Itokawa itself
and for increasing the confidence in the model predictions for other near-Earth objects.
After validating current thermophysical models on the well-studied Itokawa, it will now be
possible to determine the sizes of these bodies easily and accurately. We believe the new
AKARI data will also allow us to study in more detail how the surface properties influence
the infrared spectrum of small bodies, or how to derive surface properties from infrared
observations of other targets.

[Higher resolution]

Figure 1:
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. We selected three images from the data taken in the observation period
and composed a single image to show the motion of Itokawa over 12 minutes.
The image covers roughly a 7.4 arcmin x 7.4 arcmin area around the target.

[QuickTime 1.4MB] [MPEG4 0.3MB]

Movie 1:
The motion of Itokawa over the 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 micrometres shown in Figure 1 and also at 11
micrometres. Note that the brightness of stars change at different
wavelengths and this composition causes the stars to appear to twinkle.
The images in this movie are without any cosmetic retouching and are as
raw as those that astronomers themselves use for research. Many
scratches and anomalies from cosmic rays and other effects can be clearly

In fact, there are two more asteroids detected faintly in the image.
Can you find them? The answer is here.


Figure 2:
The positions of Itokawa and the Earth at the time of the observation
(July 26th, 2007) on the inertial (Zodiacal) coordinate frame.
The planets and the asteroid Itokawa move along their orbits in the
direction indicated. After three orbits for the Earth and two orbits for
Hayabusa, the spacecraft will arrive back at the Earth in June, 2010.
(Click to enlarge the image)




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