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The Forefront of Space Science

Mysteries of Comets and the Solar System revealed by the Deep Impact mission
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On July 4, 2005, the copper-based projectile released from NASA’s Deep Impact spacecraft hit comet 9P/Tempel 1 and successfully excavated the internal materials of the comet, which had been a mystery in the past. The materials excavated from the interior were examined in detail by both instruments onboard the spacecraft and many telescopes on the ground. We also participated in the ground-based observation campaign using the Subaru telescope of the National Astronomical Observatory of Japan (NAOJ). Analysis of the acquired data is still continuing, and new facts and insights are reported in science conferences.

In this article, I would like to summarize the scientific results obtained so far from the Deep Impact mission, including our own observation results.

Mysteries of comets

Before discussing the results of the mission, I would like to discuss briefly the importance of comet research. As is widely known, when comets approach the Sun, solar radiation heat causes intensive evaporation on the comet and forms extremely large coma and tail. Spectroscopic analysis of the comas and tails shows that comets contain much organic material and volatile constituents such as water and carbon dioxide. These substances are the main elements of the atmosphere and oceans of terrestrial planets, although less abundant in the solid parts of planets. They are also the building blocks of life on the Earth. It is also known that terrestrial planets have experienced a large number of comet collisions through their histories, and it is believed that these collisions have strongly affected the origin and evolution of the atmosphere, oceans, and life.

Since the comet’s interior maintains an extremely cold environment since its formation, thermal metamorphism of the interior is expected to be very small. Thus, the record of the solar system 4.5 billion years ago, just after its formation, is thought to have been preserved there to date. Accordingly, research into comets’ interiors would link directly to an understanding of the formation process of the solar system. Because of such importance, space telescopes orbiting the earth and a number of large ground-based telescopes, as well as the scientific instruments on the spacecraft, were directed to comet 9/P Tempel 1 to conduct observations of the event.

Observations by ground-based telescopes and their results

Figure 1
Figure 1. Comet 9/P Tempel 1 before the impact.
The x mark in the photograph indicates the impact point of the spacecraft. (Photo credit: NASA)

The main spacecraft of the Deep Impact mission can make, of course, observations of the comet with the highest spatial resolution. In fact, the camera onboard the spacecraft revealed a quite different figure of Tempel 1 compared to other comets’ nuclei observed in the past (Fig. 1). Since the spacecraft moves away from the comet at a relative velocity of about 10km/s, however, the apparent diameter of the comet seen from the spacecraft decreases to less than 1/100 deg. in an hour. This advantage of proximity of the onboard cameras is lost. Under such circumstances, the merit of ground-based telescopes or space telescopes orbiting around the earth increases because they can observe a broader band of wavelengths for a longer time using various instruments. Thus, ground-based telescopes played a significant role in the mission in that they could observe wavelengths not covered by the spacecraft for a longer time from several hours to several weeks.

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