This is the third time that I have written the lead article for ISAS News. The first was "Electric Propulsion/Ion Engine" in No. 230 (May 2000 , Japanese only), when I was engaged in the manufacturing of MUSES-C, later renamed HAYABUSA. The second article was "High Hopes for the Age of Epic Space Voyages - Asteroid Explorer HAYABUSA and Ion-Engine Technology" in No. 276 (March 2004). I believe it was when HAYABUSA's ion engine was operating at full capacity toward the asteroid Itokawa.
Taking into account the above articles, I would like to give my comments and opinions on electric rockets as of May 2011 when the round-trip cruise between earth and the asteroid was completed.
On the completion of the deep-space round-trip cruise
Japan has very earnestly pursued electric-rocket R&D to improve the range capability of rockets, even weaker power ones, to provide round-trip capability, and to prolong the life-span of satellites. Japan's attempt in the 1990s to use the ion engine for north/south station keeping of geosynchronous satellites was a timely, pioneering experiment that led the world. Because of a lack of power, however, Japan was unable to take the initiative in the world at that time. According to the organizational framework of space development in Japan at the time, the National Space Development Agency of Japan (NASDA) was responsible for application satellites while the Institute of Space and Astronautical Science (ISAS) was responsible for scientific satellites. As I belonged to the latter, there was no chance for me to take part in the actual-use program of the electric rocket and I was forced to be an observer only. The next chance was to use the rocket as the propulsion system of a deep-space maneuver. I tried aggressively and confidently to introduce our ion engine in asteroid/comet missions. Twenty years passed between the original idea and actual use. However, we successfully returned with samples from the asteroid, so I believe that we pioneered the electric-rocket field, and were not behind Europe, the U.S. and Russia.
For the HAYABUSA round-trip voyage, we developed an ISAS original - a microwave discharge-type ion engine in which plasma is produced using radiowave without electrodes. The requirement for the engine was in-space operation of 14,000 hours per thruster and a total of 40,000 hours by all thrusters. We fulfilled this with 10,000 hours of thruster B, 12,000 hours of thruster C, 15,000 hours of thruster D, and 3,000 hours of cross-operation by neutralizer A plus ion source B. On the ground, albeit in a controlled environment, durability of 20,000 hours per thruster was confirmed. It must be said, however, that the neutralizer lacked enough durability for practical operation in space, even though the ion engine was forced to operate above its specifications. Through actual demonstration, we discovered a performance degradation mechanism peculiar to the ion engine and gained a great deal of information about electric-rocket technology. Meanwhile, we achieved our original target. This was mainly because our redundant design to prepare four thrusters for the round trip - in place of the plan to use three thrusters - worked effectively. Thus, apart from the lessons learned above, it should be emphasized that adequate and sufficient crisis management was performed for HAYABUSA in terms of space technology.
Since the thrust of an electric rocket is small, continuous operation is required. Therefore, it was believed that the long-duration rocket operation would place a burden on satellite operation. Conversely, their low thrust and continual acceleration can offer not just one but multiple routes to reach a destination. This allows us to offset yesterday's unattained acceleration with tomorrow's acceleration. Although we had to postpone the return for three years, we completed the round-trip cruise between earth and asteroid Itokawa after overcoming many difficulties. The success was due entirely to the freedom of trajectory design using the electric rocket, and it is a remarkable accomplishment in space technology proving the effectiveness of trajectory-design freedom. We also demonstrated a variety of the rocket's skills and versatility including: variable thrust; electric-power throttling; combined operation; thrust-direction control; cold gas jet; coordinated operation with other sub-systems; and precise guidance to the landing point.