Ion Engine Operation of Hayabusa - July 25, 2003

http://www.isas.ac.jp/e/new/release/2003/07_02.html

Ion Engine Operation of Hayabusa
ISAS
July 25, 2003

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 earth swing-by date (planned for 2004) is a little
less than 500m/sec. We expect that this will be accomplished by the end of
2003 or early 2004 .