The Space System Engineering Section, which I work in, is engaged in flight analysis and planning of exploration and flying vehicles using navigation, guidance, and control strategies backed by the orbital/attitude motion (astrodynamics) and automatic control theories. Looking back, the motivation that influenced my current research field was mainly attributed to the accomplishments of NASA's Mars explorers, Viking-1 and 2, in 1975 and 1976 when I was at university. Since then, the terms "automatic control" and "orbit correction", which recently are more commonly used, have had a decisive influence on directing my research career. In relation to these words, I would like to introduce these two aspects of the astrodynamics research applied to the actual "HAYABUSA" project.

Figure 3 Current flight plan of "HAYABUSA" (in Sun-Earth line fixed coordinate)

The first case is an application of orbital optimization. Concerning the flight plan of "HAYABUSA," we are frequently asked why we use the earth swingby (see Fig. 3). Launch of "HAYABUSA" was delayed due to the M-V Rocket problem found in the previous launch. We were compelled to change the asteroid target for the exploration, but the spacecraft, including the fuel tank, had already been designed and built as well as the mission having originally been planned to use the maximum capability of the M-V vehicle. Therefore, the flight to other possible asteroids begged the problems that the vehicle capability would be insufficient or the spacecraft weight would be too heavy. Fortunately, the propellant tank of the ion engine system had a little margin so that we had a sole measure to augment and compensate for the vehicle transportation capability. The ion engine is a propulsion system to get thrust from the reaction force of very high-speed exhaust gas, and it features its very high fuel-to-speed-up efficiency. On the other hand, the thrust is very small and difficult to use for the launch phase that requires impulsive acceleration in a short period against the gravity. Once the spacecraft is put into an interplanetary space, there is naturally no need to worry about "dropping to the ground," owing to the gravity and it is possible to obtain sufficient speed-up (acceleration) with steady-but-slow manipulation such as only one steering operation for the thrust direction per day. The actual departure of "HAYABUSA" from Earth bound for the asteroid will be May 2004. "HAYABUSA" therefore has one year from its launch in May 2003 until May 2004 to be accelerated in interplanetary space in order to compensate for the insufficient launcher capability. It may be difficult to understand intuitively, but the efficient method for accelerating the explorer in interplanetary space is to combine the ion engine's' acceleration with the Earth swingby. According to calculations, we can ideally obtain twice the acceleration amount achieved by the ion engines in interplanetary space by combining the swingby. For "HAYABUSA," due to the reasons such as limited thrust, its efficiency is a little low at around 1.3. Converting this value to the spacecraft mass, it becomes 25 to 30 kg. This means that we can save the chemical propellant mass taking up equivalently 5 to 6% of the total mass of the spacecraft . The orbital motion is the simplest case of nonlinear dynamics. This means that the result of combining the original velocity at launch with the acceleration amount from the ion engines in interplanetary space is not simply the sum of them together, but provides a surplus. This technology is very efficient, and it is the trump card that enables drastically improving the small launcher's capability without increasing its own capability. This technology can apply widely for future missions requiring high-energy launches or planetary explorations where encounter points are very far away such as outer planets or comets.