I S A S / Public Outreach: "Ion Engine Operation of Hayabusa
Launched on May 9, 2003, the asteroid explorer �Hayabusa� is now cruising smoothly at a velocity of about 300,000km/day. In other words, the communication delay to the explorer is increasing by two seconds a day. In early July, the distance exceeded 0.1 AU (Astronomical Unit).

Many people are interested in or concerned about the ion engine�s operation. It took a relatively long time to start operation, because we took extensive action to release all possible gas and to prevent large electrical discharges since the explorer uses high-voltage power. We are now confirming the performance and status of the explorer�s four engines, three of which operate simultaneously. We also verified the switch selection function to distribute the three engines� high-voltage power to four engines. These processes took a great deal of time. We first activated the ion engine on May 27, and started acceleration by simultaneous operation of three engines on June 25.

Since the ion engine�s acceleration is too small to measure with an accelerometer, initially we thought that long-duration orbit determination would be the only way to rate acceleration performance. Fortunately, however, it was possible to measure the acceleration value with the so-called Maneuver Monitor display equipment, which indicates the actual time difference of instantaneously measured value and expected value of two-way doppler. The equipment is also used in orbital changes for the Mars explorer �Nozomi�. It allows the measurement and processing of very small acceleration amounts of 4 x10^(-6) G with considerably high accuracy.

So far, ion engine performance has conformed well to the value measured by the ground test and acceleration has progressed smoothly. The acceleration amount expected by the ea"