Gold dust collection in the sky: the World's largest asteroids database made by AKARI
The AKARI asteroid catalogue is constructed by laborious processes from the enormous quantity of data of the AKARI All-Sky Survey observations, like collecting gold dust. AKARI is the infrared astronomical satellite, which is a JAXA mission with ESA participation. The new AKARI asteroid catalogue contains 5120 asteroids and becomes the world's largest one. The catalogue will significantly contribute to progress of the asteroid research.
Recently, planetary science, especially research of asteroids, has dramatically progressed. Nowadays in-situ observations can be made by flyby and rendezvous missions, and moreover, sample return from asteroids can be achieved as the Japanese asteroid explorer Hayabusa did successfully. Even so, the importance of astronomical observations cannot still be overlooked to investigate the properties of each asteroid, since we cannot visit all asteroids, whose number is known to be more than half a million up to now. Most asteroids are known only as their orbits and their properties are poorly studied; in particular, their sizes which are one of the most basic physical properties are not known for most of them. This is because asteroids are very tiny and their sizes cannot be measured directly even by the largest telescope.
To understand the properties of asteroids, the research team led by Fumihiko Usui at JAXA Institute for Space and Astronautical Science intends to use the All-Sky Survey data obtained by AKARI. Instruments on board AKARI are suitable for detecting asteroids in the infrared. The All-Sky Survey data exactly fit an unbiased search for numerous celestial objects. It is a wonderful opportunity also for the study of asteroids.
The infrared catalogue based on the AKARI All-Sky Survey was open to the public on 2010 March (catalogue data access via http://darts.isas.jaxa.jp/ir/akari/ for researchers). The catalogue contains information of nearby stars and galaxies. Solar system objects are, on the other hand, moving on the celestial sky and these objects were intentionally discarded by the process of generating the catalogue since it used multiple detections at the same position as a confirmation of the object. In this time, the research team developed a special method to focus on the infrared signals from asteroids by comparing the discarded data in the All-Sky Survey and the positions of the asteroids with known orbits. This is just like collecting gold dust.
The brightness of an asteroid that reflects the sunlight depends on not only the size but also the reflectance of its surface. Thus the size estimation by only the brightness results a large uncertainty. A large amount of the sunlight received on the surface of the asteroid is absorbed and heats the asteroid and is emitted as the infrared radiation. Based on the observed infrared flux, the size of the asteroid can be estimated with high accuracy.
Now the AKARI asteroid catalogue is constructed; it contains a dataset for 5120 asteroids. This is the world's largest asteroid catalogue ever made. The AKARI catalogue contains almost all asteroids larger than 10km in diameter. This catalogue will be significant for various fields of solar system science. For example, combining the obtained size data of the asteroids with the orbital information, we can investigate the origin and evolution of the solar system.
The new asteroid catalogue is publicly available on the data archive at JAXA, and can be freely used by everyone in the world. The first result with this catalogue is published on the 2011 October 25 issue of the Publications of the Astronomical Society of Japan.
The AKARI asteroid catalogue is constructed in collaboration with the researchers affiliated in JAXA Institute of Space and Astronautical Science (ISAS), National Astronomical Observatory of Japan, Max Planck Institute for Extraterrestrial Physics (Germany), Seoul National University (Korea), Tohoku University, Nagoya University, and the University of Tokyo.
Figure 1 (click to play mp4 video): This movie shows the orbital motions of 5120 asteroids detected with AKARI during the All-Sky Survey observation, which was made from 2006 April 24 to 2007 August 26. The size and surface's reflectance (albedo) are distinguished by different sizes and colors of dots. The positions of Sun, Earth, Mars, and Jupiter, and their orbits are also shown. Their positions are calculated separately by using their known orbital elements.
Figure 2: Distribution of 5120 asteroids detected by AKARI as of 2007 August 26. The distribution of asteroids is not uniform, but is split into various groups due to the motion of Jupiter. There are a number of groups of asteroids; the main belt asteroids that shows the most dense population between Mars's and Jupiter's orbit; the Jovian Trojans that swarm around the Jupiter's orbit; the near Earth asteroids that have the possibilities to come close to the Earth's orbit. Asteroids of each group have different properties and are thought to have different origins. The near Earth asteroids tend to be detected close to Earth, since AKARI observed in the direction perpendicular to the Sun-Earth line.
- The visible light reflectance of asteroid is typically 10%, while the value of reflectance is different from object to object. This implies the difference in the composition and condition of the materials on asteroids.
- This is the world's largest publicly opened asteroid catalogue that records the largest number of asteroids with the measured sizes as of 2011 October. The second one in a previous work is of the Infrared Astronomical Satellite (IRAS) launched in 1983 by a joint project of the United States, United Kingdom, and the Netherlands, which contains about two thousand asteroids. Asteroid Itokawa, of which the asteroid explorer Hayabusa successfully returned with grains, and asteroid 1999 JU3, which is considered as the target of the next Japanese asteroid explorer Hayabusa-2, were observed with AKARI not in the All-Sky Survey, but in the pointed observation mode (see, a previous article on August 22, 2007: The Infrared Astronomy Satellite AKARI observes the asteroid Itokawa, the target of the Hayabusa explorer).