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The Science of Shooting Stars
-Exploring the formation of the solar system through comet dust-

Mutsumi Komatsu
Assistant Professor, Waseda Institute for Advanced Study

The news that shocked the world about a meteorite crashing into the Chelyabinsk region of Russia in February 2013 is likely fresh in the memories of a great many people. Such a meteor impact is often regarded to cause havoc that threatens all forms of life on earth, as can be seen in the notion of mass dinosaur extinction being ascribed to an asteroid impact, besides the number of disaster movies dealing with this topic. But is a meteor impact, or a shooting star, really nothing but dangerous?

A comet or a meteor?

Shooting stars are small particles present in the solar system. When they enter the earth’s atmosphere, they burn and glow, becoming observable either with the naked eye or through telescopes. Most of shooting stars originate from either comets or asteroids. Particles originating from comets are less than a few millimeters in size, while larger particles, called meteorites, are mostly of the asteroid origin surviving the passage through the atmosphere.

What are the differences between comets and meteorites? Comets are celestial objects coming from the Kuiper Belt or the Oort Cloud, far beyond Pluto’s orbit. So, comets were previously thought to have been formed by low-temperature materials and ice. In contrast, meteorites were considered to have no association with comets, originating in the asteroid belt between the orbits of Mars and Jupiter and being composed of high-temperature materials.

They all turned out not to be true, however, following the analysis of comet dust. In 2006, NASA’s spacecraft, Stardust, successfully obtained dust particles emitted from the coma of Comet Wild 2, the first successful sample return mission of this kind (Figure 1). Unexpectedly, the dust samples revealed the presence of high-temperature minerals (formed at >1,300°C). Such high-temperature minerals could not be formed in the outer solar system, leading to a new interpretation that these minerals were formed in the inner regions of the solar nebula before somehow being transported to the outer solar system, where comets originate. In other words, this is evidence that there was large-scale transport of materials, at a scale beyond our imagination, during the early solar system. Furthermore, there is some indication that comets and asteroids are composed of common materials, despite their differences in relative abundance of ice and volatile elements. Our research team has been investigating comet dust in order to better understand the physical conditions of the solar system at the time of its creation (Figure 2).

Figure 1. An image of the Stardust spacecraft (left) and the actual particle samples collected. The object located at the top of the spacecraft, resembling a tennis racket, is the particle catcher. Particles captured onto the silica aerogel tiles are separated into each individual particle before being distributed among researchers (photos from NASA).

Figure 2. An electron micrograph image of comet dust. On the right is a composite map showing magnesium, iron, and chromium. Two minerals, i.e., magnesium-rich olivine and iron-chromium-rich chromite, coexist here. The chemical and isotopic compositions of the sample indicate that it was formed approximately 4.56 billion years ago. 1 μm is 1/1,000 mm.

Shooting stars: The ‘cradle of life’

Whenever a comet nears the sun, it emits dust particles that then start to follow behind along its orbital path. As the dust tail of a comet approaches the earth, a huge amount of dust hits the atmosphere, causing a meteor shower.

Recent studies have shown that comet dust, meteors and other cosmic dust contain a wide range of organic matter, presumably formed soon after the birth of the solar system. During the early solar system, some meteors were likely to have reached the surface of the earth without experiencing high-temperature environments, providing organic matter to the pre-biotic earth. This may indeed have given rise to the appearance of early life forms on earth.

Searching for the origin of life

In 2014, the Japanese asteroid explorer, Hayabusa 2, is set to depart for Asteroid 1999 JU3. 1999 JU3 is a C-type asteroid considered to contain organic matter and water. If Hayabusa 2 manages to analyze the surface of the asteroid and complete return of collected samples, it would no doubt provide us with a further opportunity to uncover the detailed processes of planet formation in the early solar system as well as the evolution of organic matter and other solar system materials.

Also of interest are active asteroids, or extinct comets, which exhibit properties intermediate between those of comets and asteroids. The asteroid Phaethon, the parent body forming the Geminids, is in fact likely to be an extinct comet having expelled most of its volatile components and, hence, its surface materials have been the focus of a planned analytical mission. Analysis of such Phaethon samples, when realized, will certainly unveil much-awaited information on the evolution of asteroids and comets in the solar system, since they should be an abundant source of key data.

Figure 3. An image of the Hayabusa 2 spacecraft. Hayabusa 2 is planned to probe and conduct a return of samples from a more primordial asteroid than in the preceding Hayabusa mission (photo from JAXA).

2013 could be the Year of the Comet

The Perseids, one of the three most prominent annual meteor showers, peaks in mid-August every year, but this year is said to be the best over the last several years to observe them in Japan because of the age of the moon around the peak time and also the estimated timing of the peak. Additionally, Comet ISON, already attracting great attention as possibly the Greatest Comet in our history, is predicted to become a naked eye object in November 2013. Comet ISON will make its closest approach to the sun at approximately 1.9 million km from the center of the sun. Accordingly, it is anticipated to become brighter even than the full moon around its perihelion (the orbital point closest to the sun).

Shooting stars light up the night sky. Although they may be a potential risk to living organisms on earth, at the same time they may well hold vital clues to the evolution of those organisms. Why not bear this in mind next time you look up at the stars?

Mutsumi Komatsu
Assistant Professor, Waseda Institute for Advanced Study

[Profile]
Graduated from the Department of Earth Sciences, School of Education, Waseda University; completed a Doctor of Science at the Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo. Holds a Doctor of Science degree. Her academic and professional career includes the NASA Johnson Space Center Summer Intern, Visiting Researcher at the Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i, and JSPS Research Fellow before taking up the current position in 2011.