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Special Feature

Research in the Asteroid 'Sample & Return' Explorer, "Hayabusa"  Junichiro Kawaguchi, Space System Engineering, ISAS
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Next is the optical navigation. Literally, this is a method where the camera on the spacecraft photographs the target asteroid object and estimates the relative position and velocity between the spacecraft and the asteroid based on the directional information. Thinking this over, it seems to be matter of course because we always do this to avoid crowds in our daily lives. Let's assume that we watch a distant lighthouse from a train running through a plain, and suppose we measure the angle of the lighthouse to the railroad. Where we are measuring at two points, we may mistakenly think that we can estimate the distance to the lighthouse by using triangular surveying. If we know the speed of the train, we can estimate it. If we don't know the speed, we cannot know the distance to the lighthouse. If we can change the train speed by a known amount, then we can estimate the distance to the lighthouse even if we don't know the original speed. Baseball outfielders do this when catching fly balls. In order to catch a ball flying in front of him, an outfielder moves right and left to estimate the falling point of the ball. This is the same method. The optical information is two-dimensional angle information. The information is therefore incomplete, and there are not enough elements necessary for observation. By intentionally inputting a movement element, we can fulfill the required elements for observation. By making observations while accelerating the spacecraft with the ion engines, "HAYABUSA" can approach optically and rendezvous with the asteroid. When "HAYABUSA" arrives at the asteroid, the distance from Earth is about 300 million km. Estimating the spacecraft position by radio wave leaves an error of about 300km, so it is difficult to rendezvous with the object of 500m in size. Thus, by observing the asteroid based on the image information, "HAYABUSA" can reach the asteroid.

"HAYABUSA" is an engineering experiment explorer. The mission is also considered a good case for superb blending the two factors of realizing a new scientific observation purpose, i.e., sample & return and developing new engineering technologies. Synergetic efforts of science and engineering bring about this good result in promoting strategically the country's deep space exploration. The "HAYABUSA" mission has reaffirmed that we are in a well-balanced environment provided by the unique position and stance at ISAS. We have many controversial issues concerning the purpose of space development, but, through launching "HAYABUSA," I can perceive some of what we are expected in terms of socio-cultural aspects. To our delight, a new suite of jazz was presented to us for our mission. We also learned that science and engineering is a cultural activity having contact with various fields. In this way, we may regard and position our activity as one of the wide cultural activities including human and social sciences as well as natural sciences. As one of the research themes of "HAYABUSA," we can take up this matter.

Fortunately, we were able to conduct the first acceleration test by inputting high voltage power into the ion engine on May 27 and 28, 2003. Although the test was done just for one single engine and at 80% of full power, we confirmed the planned acceleration amount observed through reading the doppler measurement (see Fig. 4), making a significant step toward future missions. It remains a very far off road until the arrival and landing on the asteroid. The spacecraft has to fly 2 billion km to return to Earth. We would like to ask for the support of all people for our mission.

Figure 4 Doppler Observation-Calculation change chart by showing ion Ion Engines drive

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