If they place a giant sheet in space with a hole in it, as Prof. Webster Cash suggests, will it be possible to observe planets outside the solar system?
Uriel Brizon
The diameter of the sheet will be several hundred meters. In the center there will be a hole with a diameter of ten meters. The telescope will be located at a distance of about 200,000 kilometers. Imaging: Prof. Webster Cash
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Discovering in space a planet similar to the Earth, where life can exist - this is the vision that motivates many astronomers. According to researchers, the technology that will allow this vision to be realized will mature in the next two decades. But University of Colorado professor Webster Cash isn't willing to wait. The system he designed for photographing distant worlds is based on a technology that had already been invented, not recently but thousands of years ago: it is mentioned in the writings of Aristotle and in the lists of the sages of ancient China from the fifth century BC.
The principle is extremely simple: a small hole in the side of a box causes an image to be projected onto the corresponding side. This is one of the basic phenomena in optics, which since the 17th century is known as "camera obscura". From this concept is derived the name of the modern device that makes use of the phenomenon: the camera. The first cameras were closed boxes with a small hole on one side and a photographic plate on the back wall on which the picture was taken.
Prof. Cash proposes to build a huge version of these simple cameras, which will consist of two satellites that will be transported in space. The first satellite will deploy a thin, opaque sheet with a diameter of several hundred meters that will serve as a screen. In the center of this "screen of the stars", as Cash calls it, there will be a hole ten meters in diameter through which the light will pass. At a distance of about 200 thousand kilometers from the screen, the second satellite will move, on which a telescope will be installed, and it will watch through the hole in the screen. According to Cash's calculations, the telescope will be able to distinguish planets at a distance of billions of kilometers from Earth.
With the methods that exist today, it is almost impossible to observe distant planets, and this is due to their proximity to the mother star, which shines with a strong light and interferes with observations. To date, about 150 planets orbiting distant suns have been discovered. This is done with the help of indirect observation methods, which measure changes in the movement of the mother star and the light coming from it. Various research teams are trying to design advanced systems that will allow direct observation of planets and examination of their components. The idea presented by Cash is different from those of the major research groups. He calls for using the simple optical technique of the camera obscura and taking advantage of the large spaces where satellites can be placed in space instead of trying to develop very precise and complex optical devices.
According to Cash, when the star screen is aimed at a distant star, the light from this star will pass through the hole in the screen and cause an image of the star and its immediate surroundings to be created. If we launched a huge photographic board into space and placed it behind the screen of stars, an image of the star would be obtained on it - like in a camera. The further the photographic plate is from the screen, the larger the resulting image. In order to allow the identification of a planet, the light of the star around it must be overcome, and therefore a very large image must be created, in which it will be possible to distinguish between the star and the planet.
But the photographic board that needs to be launched into space to do this must be thousands of square meters in size, and this is impossible. The alternative that Cash suggests is to place a satellite at the point in space where we would place the board. When the telescope in this satellite is aimed at the hole in the screen, part of the large image projected onto its surroundings is placed on it. According to Cash's plan, the telescope would move on the giant image plane as if it were an ant walking on a photographic plate on the back of a camera. Through a scanning movement in the plane of the image in space, the large image can be assembled - the one that would have been received on the giant imaginary photographic board.
Prof. Cash is very experienced in designing research satellites. He led research teams in several NASA projects and currently heads the Astrophysics Institute at the University of Colorado. Cash is aware of the engineering problems involved in his plan, those related to the layout of the star screen, maintaining the exact angle between the screen and the telescope, and even the design and manufacture of the screen and the telescope. Despite the difficulties, he is convinced that the optical principles underlying the program are solid and that the engineering obstacles can be overcome. However, when he presented his plan to NASA management, he encountered a skeptical attitude. Despite his reputation as a leading scientist, his ideas were perceived as problematic - according to him, due to their innovation and due to being the result of "thinking outside the box".
Fortunately for Cash, the NASA Institute for Advanced Concepts (NIAC) works at NASA, whose entire mission is to leave a window open for ideas and proposals that the main door has slammed in their face. It seems that the American space agency knows that sometimes breakthroughs lie precisely in ideas that seem strange and far-fetched. After years of limited activity, the institute recently received additional funding, in light of the new space vision presented by President Bush last year, which talks about renewed landings on the moon and planning a manned mission to Mars.
In September 2004, Cash's research proposal received a limited initial funding of about 75 thousand dollars from the institute. This budget will be used by him to carry out preliminary tests and to show that his ideas do meet scientific and engineering feasibility tests. According to the findings of the initial research phase, the management of NIAC will decide whether to grant Kash additional funding of 300 thousand dollars. This is a budget that will enable the preparation of detailed plans for the satellites and may pave the way for the realization of the mission. The results will be submitted in March this year and the decision should be made a few weeks later.
According to Cash, using the space screen and one satellite telescope it will be possible to distinguish distant planets and determine the composition of their atmospheres. To produce a detailed image of distant planets, it will be necessary to combine two space screens, two telescopes and an additional satellite to combine the observations. This is the advanced stage of the project, and its realization will depend on the success of the first pair of satellites. The image obtained from the advanced system will make it possible to distinguish details on the face of the planet, such as oceans and continents, clouds, snowy peaks and valleys. Cash claims that if we invest the required resources it will be possible to build a system that will allow us to see even smaller details on the surface of the ground of a distant world. Maybe even settlements.
Know planets outside the solar system
The article was published in "Haaretz"
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