TOP > Report & Column > The Forefront of Space Science > 2012 > Planetary Plasma Environment and Atmospheric Outflow to be Elucidated by Extreme Ultraviolet Spectroscopy
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Is the space environment of our earth and environs, which hosts life forms including mankind, consequential or incidental? Have you not wondered about this fundamental question? Of the eight planets in our solar system, the existence of life is only confirmed on earth. Many planets have been discovered outside the solar system, however, including planets similar in distance from a central star and in size to the earth. There may be environments similar to the earth elsewhere. The human eye cannot see usually plasma and atmosphere, but their composition and distribution become visible when observed with extreme ultraviolet. What is revealed by observing the planetary environment with extreme ultraviolet? This article introduces a planetary observation mission by extreme ultraviolet spectroscopy (EXCEED), one project on a small scientific satellite SPRINT-A to be launched by a new solid rocket Epsilon in the summer of 2013. Outlines of a comprehensive test of SPRINT-A and the next-generation power-supply system’s equipment mission NESSIE are reported separately. What is extreme ultraviolet? First I would like to explain extreme ultraviolet, which is used by SPRINT-A to perform spectroscopic observation. There are several definitions on the term because of wavelength-boundary overlap. In this article, I use the definition from the standpoint of ultraviolet spectroscopy. Visible light is that with colors visible to the human eye, i.e., the colors of the rainbow from violet to red. Its wavelength is about 380 to 760nm (nano meter, 1nm= 1/1billion meter). For light invisible to the human eye, shorter wavelength light is called ultraviolet because it is light beyond violet while longer wavelength light is called infrared because it is light beyond red. The infrared wavelength is approx. 700nm to 1mm, and its energy is almost the same as the vibration energy of molecules making up matter and of black-body radiation. Accordingly, by investigating infrared spectra radiated from or absorbed by matter, we can estimate its chemical composition and temperature. The ultraviolet wavelength is approx. 10 to 400nm with a strong chemical action. For example, it can cause color fading, or sunburn or freckling on skin, and it can also be used to sanitize or disinfect. As wavelength in the ultraviolet region becomes shorter, it becomes harder to transmit in the atmosphere. Ultraviolet wavelength shorter than 315nm is absorbed by ozone, and below 200nm, it is absorbed by oxygen and nitrogen molecules in the atmosphere and, thus, becomes light that cannot propagate through the atmosphere. To handle light of approx. 10 to 200nm wavelength that cannot propagate though the atmosphere, we need to eliminate the atmosphere to make a vacuum. Ultraviolet can only be handled in a vacuum known as vacuum ultraviolet. In addition, at the boundary of 105nm wavelength, the technology to handle vacuum ultraviolet differs largely. Some materials do not absorb light longer than 105nm but permit their transmission, whereas no material permits the transmission of light shorter than 105nm. This makes a big difference. Specifically, it allows use of transmission-type optical components such as a lens for the measurement system, or limits use to reflection-type optical components. For this reason, different names are given to light at the boundary of 105nm wavelength. Light shorter than that is called extreme ultraviolet. This is the light that SPRINT-A will use for observation and it is seen only in outer space. Many nuclei and molecules absorb specific-wavelength vacuum ultraviolet to emit light. Thus, we can identify nuclei and molecules by measuring wavelength in spectroscopic observation and estimate their quantity by measuring emission intensity.
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