The 20th scientific satellite, the MUSES-C, was renamed "HAYABUSA" after the launch on May 9, 2003 and is now operating its ion engines toward the Earth swingby in May 2004. The "HAYABUSA" project is an experimental engineering mission , whose objectives are to develop and demonstrate many kinds and a variety of new technologies, and it is a research activity positioned in the Integrated Engineering . On the other hand, the core objectives of "HAYABUSA" in terms of planetary science are to extend the synoptic perspective for solar system exploration to the primitive celestial bodies, and to demonstrate the new exploration technique of "Sample and Return" applied to them. But, in this article I would like to concentrate the discussion to introducing the engineering aspect of "HAYABUSA."

The study on the asteroid sample & return mission at ISAS was actually initiated long ago. Chaired by Dr. Tsuruta, current Director General of ISAS, it was started in June 1986 when the first interplanetary explorer "SAKIGAKE" had been successfully launched and the "SUISEI" satellite was ready for launch. The following year, a scenario of sample & return targeting "Anteros" using a chemical propulsion system was drawn up as an engineering mission case, which would be executed during the 1990s (see Fig. 1). Still, it was not officially proposed since it was too early to realize it. What surprises us, looking at the plan, is that the orbit plan happens to be very similar to that in the mission targeting "Itokawa" (1998SF36 was renamed recently after the founder of the rocket research in Japan). It is also worthy mentioning that the plan already reported and proposed that the direct entry from an interplanetary orbit is an essential technology.

The four new technology elements that we intend to test and demonstrate via the "HAYABUSA" mission are: Interplanetary cruising by employing the ion engines as the primary propulsion system; Autonomous navigation and guidance technique by using optical observation: Sample collection technology from the asteroid surface; and Direct reentry into the atmosphere from an interplanetary orbit for recovery. A variety of new satellite/explorer technologies are also introduced in the mission, although they are not greatly emphasized. The examples include: Bi-propellant small thrust chemical propulsion system; Duty guaranteed heater control with the total power consumption constrained; Wheel unloading incorporating the ion engines into a closed loop; PN-code for long-distance ranging (PN-code is a Pseudo Noise code added onto the transmission radio signal. Range is given by measuring the round-trip time obtained via correlation of the sent and received signals); and adopting a Lithium-ion rechargeable battery. "HAYABUSA" is just a high-tech robotic spaceship (see Fig. 2). Each of these technological elements requires a high degree of expertise to discuss, so the relevant professionals in charge should provide detailed explanations. They are simply listed in this article and I will introduce the flight plan and future operation of those elements in relation to the research subjects as far as I am able to.

	Figure 2 Onboard instruments aboard the spacecraft