I. Introduction
The Hayabusa space mission is focused on demonstrating the technology needed for a sample return
from an asteroid, using electric propulsion, optical navigation, material sampling in a zero gravity field,
and direct re-entry from a heliocentric orbit. Four ƒĘ10, the cathode-less electron cyclotron resonance ion
engines, propelled the Hayabusa asteroid explorer, launched in May 2003, aiming at rendezvous, softlanding,
lift-off and Earth return. It succeeded in rendezvousing with the asteroid Itokawa in September
2005 after a 2-year flight, producing a delta-V of 1,400 m/s, while consuming 22 kg of xenon propellant
and operating for 25,800 hours. After a series of scientific observations the Hayabusa landed on and lifted
off the asteroid in November 2005. Malfunctions of onboard equipment seriously damaged the Hayabusa
spacecraft and delayed Earth return in 2010 from the original plan in 2007. Reconstruction on the
operational scheme using remaining functions and newly unloaded control logic made the Hayabusa leave
for Earth in April 2007. This paper reports the recent status of the Hayabusa space mission.
II. Hayabusa Asteroid Explorer
The Hayabusa space mission aims to retrieve surface material of the asteroid to Earth. Total launch
mass of the spacecraft is 510 kg including hydrazine fuel 67 kg. Figure 1 shows its configuration,
including the pair of stowed solar cell paddles (SCP), which can generate 2.6 kW electrical power at 1
1 Professor, Department of Space Transportation Engineering, kuninaka@isas.jaxa.jp
2 Associate Professor, Department of Space Transportation Engineering, nishiyama@ep.isas.jaxa.jp
AU from Sun. The high gain antenna (HGA) is mounted on the upper surface of the body. The SCP and
HGA have no rotational or tilt mechanisms. The ion engine system (IES) is mounted on the side panel
perpendicular to the z-axis, with which the HGA aperture is aligned. At high bit rate communication with
8 kbps, the spacecraft orientates the HGA towards Earth without IES firing. In cruise mode, the spacecraft
orients the SCP face toward Sun in order to generate electrical power and rotates its attitude around the
solar direction to steer the thrust direction of the IES. Three reaction wheels (RW) control the attitude of
spacecraft in the original design.
Ion
Magnets
Permanent
Microwave
Power
Beam
Electrostatic
Grids
Microwave
Neutralizer
Fig.1 Hayabusa spacecraft in clean room. Fig.2 Cross section of the microwave discharge
ion engine without solid electrodes.
III. Cathode-less Microwave Discharge Ion Engines
The cathode-less microwave discharge ion engines have the technological features as follows:
1) Xenon ions are generated using ECR (electron cyclotron resonance) microwave discharge without
solid electrodes, which in conventional ion engines are the critical parts and the cause of flaking
leading to electrical grid shorts. Thus, the elimination of the solid electrodes makes the ion engine
more durable and highly reliable.
2) Neutralizers are also driven using ECR microwave discharge. The removal of the hollow cathodes
releases IES from heater failures and hollow cathode emitter performance degradation due to oxygen
contaminating the propellant, as well as air exposure during satellite assembling.
3) A single microwave generator simultaneously feeds the ion generator and the neutralizer. This feature
reduces the system mass and simplifies control logic.
4) DC power supplies for ion acceleration have been reduced to 3. This feature also has the advantage of
making the system lighter and requiring simpler operational logic.
5) The electrostatic grid system is fabricated from a carbon-carbon composite. The clearance between the
grids is kept stable regardless of the temperature since there is no thermal expansion. This prolongs
the life of the acceleration grid due to the low sputtering rate against the xenon ions. Low wettability
of carbon seldom causes electrical shorts between the grids.
The cross section of the microwave discharge ion engine is illustrated in Fig.2. The ƒĘ10 ion engine with
10 cm effective diameter was developed for in order to dedicate to the Hayabusa space mission. The
ground qualification schemes are described in detail in Refs. 1, 2, 3, and 4. Four ƒĘ10 are installed on the
Hayabusa spacecraft, and three of them can generate thrust simultaneously. The dry mass of IES is 59 kg
including a gimbal and a propellant tank, which was filled with xenon propellant 66 kg. A single ƒĘ10 is
rated at 8 mN thrust, 3,000 sec Isp, and 350 W electrical power consumption so that the Hayabusa
spacecraft is accelerated 4 m/s per a day by the maximum thrust 24 mN.
IV. Flight Chronology in Outward Journey
A. Outward Journey and Proximity Operation
The Hayabusa asteroid explorer was launched in May 2003. Since July IES have been continuously
accelerating the Hayabusa, which reached a distance of 0.86 AU from Sun in February 2004 and 1.7 AU
from Sun in February 2005. These distances are the farthest that an electric propulsion system has yet
attained in the solar system. Depending on the solar distance IES was operated between 250W and 1.1kW
in electrical power. The Hayabusa succeeded in rendezvousing with the asteroid Itokawa in September
2005 after a 2-year flight, producing a delta-V of 1,400 m/s, while consuming 22 kg of xenon propellant
and operating for 25,800 hours. Reference 5 reports the details of the space operation on IES. The
Hayabusa executed the scientific observation6 staying around the asteroid in September and October 2005.
And in December it succeeded twice touchdowns on the asteroid.
B. Rescue
During the proximity operation with the asteroid the Hayabusa lost the functions of two of three
reaction wheels. And just after the lift-off from the asteroid a fuel leak disabled the function of the RCS
thrusters and disturbed to control the attitude of spacecraft. Then the Hayabusa was missing on December
8, 2005. It is believed that the Hayabusa without electrical power and all of active controls caused serious
nutation motion, which was attenuated to simple spinning by fluid friction of liquefied xenon in the main
tank. Revolution motion in the heliocentric space gradually made Sun shine on SCP so that the Hayabusa
recovered electrical power as seen in Fig.3. A set of commands from Earth initiated the Hayabusa again
and established microwave communication on January 23, 2006 fortunately. The cold gas jets from the
canted neutralizers generated enough torque over several tens micro newton meter to control the attitude
of spacecraft due to long torque arm as seen in Fig. 4 and rescued the Hayabusa. Combination and timing
of cold gas jets from four neutralizers during spinning enabled to change the attitude of spacecraft on each
axis as seen in Fig.5. From January to June in 2006 the rescue operations consumed 9 kg xenon propellant
in the cold gas jets.
Engine
Neut
Gimbal
-ralizer
Ion
Fig.3 Revival of Hayabusa due to the orbital motion. Fig.4 Torque of the xenon gas jets.
Fig.5 Torque of the xenon gas jets.
Spin
Momentum
Vector
Photon
Pressure
Torque
Solar
Tracking
Spin
Momentum
Vector
Ecliptic
Point of
Action
Orbital
Motion
Fig.6 Torque of the xenon gas jets.
In April 2006 some of IES were turned on again and exhausted the ion beams. The spacecraft was
spun down from 1 rpm to 0.2 rpm by the ion jets with three-day continuous operation. And then it was
operated under hibernation from May 2006 to February 2007, in which the solar tracking of the spin axis
was controlled by the photon pressure torque so as to save the propellant consumption. The mechanism
on the photon pressure torque is illustrated in Fig.6.
C. Homeward Journey
In order to execute the homeward journey the non-spin attitude control scheme using the RW-Z and
IES was established. The only available reaction wheel RW-Z, which is set along the z-axis as seen in
Fig.7, takes a biased momentum of the spacecraft. The thrust vector control of IES by the gimbal can
actively generate torques around the y- and z-axes in order to cancel disturbance torques. The following
section describes in detail the IES torque force around the thruster along the x-axis of the spacecraft.
Because the center of gravity on the spacecraft does not meet with the action point of the solar pressure,
the lean attitude against Sun results in the torque, which is devoted to track the Z axis to Sun. The torques
around three axes are adjusted independently by the thrust vector control of IES and the solar pressure.
After the test operation the Hayabusa spacecraft left the asteroid for Earth in April 2007. By the end of
July IES achieve the total accumulated operational time 29,000 hours and total delta-V 1,500 m/s. One of
four thrusters, which has been most frequently used, reaches 13,000 hours in space operation. The
Hayabusa will come back Earth in 2010 after two revolutions around Sun as seen in Fig.9. For the return
trip IES is requested 10,000-hour operation, 700 m/s delta-V, and 10 kg propellant. Hayabusa still keeps
enough propellant 30 kg.
Z
X
Y
RW
Fig.7 Attitude stabilization using a reaction wheel, thrust vector control of IES, and solar pressure torque.
Asteroid
Itokawa
Hayabusa
Spacecraft
Fig.8 Histories on accumulated operational time and Fig.9 Return orbit to Earth in the rotational
propellant consumption. coordinate system. Red line shows the planned
orbit of Hayabusa. Blue means the trace of
asteroid Itokawa.
V. Conclusion
The Hayabusa space mission is focused on demonstrating the technology needed for a sample return
from an asteroid and was launched in May 2003. Four ƒĘ10, the cathode-less electron cyclotron resonance
ion engines, which were developed by the Electric Propulsion Laboratory ISAS/JAXA, propelled the
Hayabusa asteroid explorer. It reached a distance of 0.86 AU from Sun in February 2004 and 1.7 AU
from Sun in February 2005. These distances are the farthest that an electric propulsion system has yet
attained in the solar system. Depending on the solar distance IES was operated between 250 W and 1.1
kW in electrical power. It succeeded in rendezvousing with the asteroid Itokawa in September 2005 after
a 2-year flight, producing a delta-V of 1,400 m/s, while consuming 22 kg of xenon propellant and
operating for 25,800 hours. After a series of scientific observations the Hayabusa landed on and lifted off
the asteroid in November 2005. Though the spacecraft was seriously damaged after the successful
proximity operation, the xenon cold gas jets from the ion engines rescued the Hayabusa. The new attitude
stabilization method using a single reaction wheel, the ion beam jets, and the solar pressure was
established and enabled the homeward journey aiming the Earth return on 2010. The total accumulated
operational time of the ion engines reaches 29,000 hours at the end of July 2007.
References
1 Kuninaka, H. and Satori, S., "Development and Demonstration of a Cathode-less Electron Cyclotron Resonance Ion
Thruster", Journal of Propulsion and Power, Vol.14, No.6, November/December 1998, pp.1022-1026.
2 Funaki, I., Kuninaka, H., Toki, K., Shimizu, Y., and Satori, S., "Development of Microwave Discharge Ion Engine
System for Asteroid Sample and Return Mission MUSES-C", Journal of Space Technology and Science, Vol.13,
No.1, 1999, pp.26-34.
3 Funaki, I., Kuninaka, H., Toki, K., Shimizu, Y., Nishiyama, K., and Horiuchi, Y., gVerification Tests of Carbon-
Carbon Composite Grids for Microwave Discharge Ion Thrusterh, Journal of Propulsion and Power, Vol.18, No.1
January/February 2002, pp.169-175.
4 Kuninaka, H., Nishiyama, K., Funaki, I., Shimizu, Y., Yamada, T., and Kawaguchi, J., gAssessment of Plasma
Interactions and Flight Status of HAYABUSA Asteroid Explorer Propelled by Microwave Discharge Ion Enginesh,
IEEE Transaction of Plasma Science, Vol.34, No.5, October 2006, pp.2125-2132.
5 Kuninaka, H., Nishiyama, K., Funaki, I., Yamada, T., Shimizu, Y., and Kawaguchi, J., gPowered Flight of Electron
Cyclotron Resonance Ion Engines on Hayabusa Explorerh, Journal of Propulsion and Power, Vol.23, No.3, May-June
2007.
6 Fujiwara, A., Kawaguchi, J., Yeomans, D.K., Abe, M., Mukai, T., Okada, T., Saito, J., Yano, H., Yoshikawa, M.,
Scheeres, D.J., Barnouin-Jha, O., Cheng, A.F., Demura, H., Gaskell, R.W., Hirata, H., Ikeda, H., Kominato, T.,
Miyamoto, H., Nakamura, A.M., Nakamura, R., Sasaki, S., and Uesugi, K., gThe Rubble-pile Asteroid Itokawa as
Observed by Hayabusa,h Science, Vol.312, No.5778, June 2006, pp.1330-1334.