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The Forefront of Space Science

The True Face of Comets Illuminated by AKARI
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The special feature article “Infrared Astronomical Satellite AKARIEin ISAS News (April, 2009 - Japanese only) included a report on AKARI’s observations of asteroids and zodiacal light (interplanetary dust), which are typical small solar system bodies, under the title “AKARI from the Solar System.EThe article introduced the results of observations mainly in mid-infrared (i.e., 5 to 25μm) conducted while AKARI (meaning “lightEin Japanese) was cooled by liquid helium).

Since the liquid helium ran out in August, 2007, the telescope and observation instruments onboard AKARI have been cooled by mechanical coolers, allowing AKARI to continue observing in the near-infrared wavelength (i.e., 2 to 5μm). This article reports on current, active near-infrared observation of comets.

Snowball or dirtball?

Readers of this article must know comets. As they trail their long tails of various shapes in the night sky, surely comets are some of the most beautiful objects in the universe. Also known as a “dirty snowball,Ea comet’s main body (cometary nucleus) is composed of ice and dust (e.g. grains of sand, small rocks). As a comet from a distant place in the universe approaches the Sun, its ice sublimates and changes to gas. This gas and dust form a “comaEthat thinly spreads around the cometary nucleus and forms a long tail. In this way, the familiar shape of the “broom starEis produced. There are still many questions about the ratio of ice and dust in comets. One theory in the latest studies suggests that comets contain much more dust than previously thought, making them more like “icy dirtballEthan “dirty snowball.EUsing such humorous terms, researchers conduct serious discussions.

It is thought that ice, the main constituent of cometary nucleus, is comprised of about 80% water and the remaining 20% carbon dioxide (i.e., dry ice), carbon monoxide, etc. In addition, it is thought that it contains trace matters: hydrocarbons such as methane and ethane, ethanol (alcohol!), and ammonia. “Snowball (or dirtball)Emade of ice and dry ice including other trace materials are flying across the solar system. They are comets.

Comet Lulin and AKARI

By the way, which comet is most vivid in your memory? Until now many comets have become popular news topics, including comets Bennett and West in the 1970s, Halley returning after 76 years in the 80s, and Hale-Bopp and Hyakutake in the 90s. The comet Shoemaker-Levy that broke up and collided with Jupiter was very hot news at that time. You may guess a person’s age by the name of the comet most familiar to them. In early 2009, it was news that comet Lulin (C/2007 N3) would become bright. The actual brightness of the comet was at a level barely visible to the naked eye, so few people watched it directly. However, the pictures of the comet with its greenish blue-white body and long tail were impressive.

Comet Lulin was discovered in July, 2007, by the Lulin Observatory, an observation site managed by the Graduate Institute of Astronomy, National Central University in Taiwan. Comet Lulin is the first comet discovered by a Taiwanese observatory. Daisuke Kinoshita, a member of the AKARI solar system team, is at the Institute and we have been exchanging information about the comet with the Institute astronomers. Small- to medium-aperture telescopes and observations by amateur astronomers around the world are very important for research on small solar system bodies. We expect partnerships like this one in Asia will advance further.

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