S2-5
Development of Ion Engines for Hayabusa2 Spacecraft「はやぶさ２」イオンエンジンの開発
Kazutaka Nishiyama, Satoshi Hosoda, Ryudo Tsukizaki and Hitoshi Kuninaka
西山和孝, 細田聡史, 月崎竜童, 國中均Japan Aerospace Exploration Agency
3-1-1, Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan252-5210
3-1-1

宇宙航空研究開発機構 〒相模原市中央区由野台

Hayabusa2 will be the second asteroid sample return mission by JAXA. The ion engine system (IES) for
Hayabusa2 is based on that developed for Hayabusa with modifications necessary to improve durability, toslightly increase thrust, and to reflect on lessons learned from Hayabusa mission. Hayabusa2 willrendezvous with a near-earth asteroid 1999 JU3 and will take samples from its surfaces. More scientificinstruments than Hayabusa including an impactor to make a crater and landers will be on board thanks to
the thrust enhancement of the IES. An improved neutralizer with stronger magnetic field for longer life has
been under endurance test in diode mode since August 2012 and has accumulated the operational hours of
19,000h (> mission requirement: 14,000h) by the middle of October 2014. Unified tuning parameters forfour thrusters such as grid gaps, geometry of microwave tuning elements including antennas, waveguides
and stub tuners improved performance variation. The spacecraft is scheduled to be launched fromTanegashima Space Center in Kagoshima Prefecture on-board an H-IIA rocket on 30 November.

1. Introduction
Japan’s first asteroid explorer “Hayabusa”
came back to the earth on June 13, 2010.
Hayabusa completed more than 7-year space
mission, and finally Hayabusa could return thecapsule to the earth, although Hayabusa had a lotof troubles and difficulties [1]. After carefulinspections of about 1,500 grains found in thesample container, it turned out that we actuallyobtained rocky particles from the surface ofAsteroid Itokawa. In 2011, Japan Aerospace
Exploration Agency (JAXA) started the second
project “Hayabusa2” for the asteroid sample
return mission to another Asteroid 1999 JU3
which is one of different type of asteroids fromItokawa. The launch is scheduled for November
30, 2014, arriving on the asteroid mid-2018 andreturning to Earth at the end of 2020.
This paper briefly summarizes the developmentof the ion engine system (IES) for Hayabusa2.

consuming a small flow rate (of order 0.3 mg/s) of
xenon gas and an electric power of 420 W for
plasma generation and ion beam acceleration.

Fig.1. Hayabusa2 spacecraft with ion engines

2. Ion Engine System for Hayabusa2
The ion engine system for Hayabusa2 have beendeveloped based on that for Hayabusa. Hayabusa2
spacecraft has four µ10 ion thrusters on a single
plate called “IES plate” which was mounted on topof a two-axis pointing gimbal mechanism as shownin Fig. 1. An ion thruster consists of an ion source
and a neutralizer both of which utilize microwave
discharge with electron cyclotron resonance at afrequency of 4.25 GHz. The maximum thrustgenerated by a single thruster was 10 mN by

IES specifications and mission requirements are
summarized in Table I. Ion sources were slightly
modified in that their propellant injectors wereadded in different places in the discharge chamber
and ion optics thicknesses and aperture diameters
were changed so that the maximally available thrustincreases by approximately 20%. The total impulse
required by this new mission is 1.2 MN·s which isalso 20% larger than Hayabusa's requirement.
Estimated total operational hours of all thrusters are41100 hours which is almost the same as Hayabusa
achievement of 39637 hours. The increase of total
impulse is due to the increase of spacecraft mass
because required delta-V is almost the same as thatof Hayabusa. This will be achieved by the thrustenhancement with almost the same thruster
operational hours. The xenon load is nearly thesame amount of 60 kg as Hayabusa's 65 kg because
both mission needs extra propellant for extended
missions after return to the earth. Neutralizers havebeen improved for longer life according to lessons
learned in the Hayabusa mission and groundexperiments. Subsystem architecture of Hayabusa2
ion engines is almost identical to Hayabusa's
although many components such as valves, pressure
sensors, microwave components, and so on werediscontinued in production and were replaced by
new alternatives.
Table I. IES specifications (Middle of life)

Hayabusa
Hayabusa2
Thrust per
4.4 – 7.6
6.3 – 9.0
thruster (mN)
Specific impulse (s) 2760 – 3000 2740 – 2890
System power (W) 280 – 1150
380 – 1230
Total operational39637
41100
hours (h·units)(planned)
Total impulse (MN·s)
1.0 1.2(planned)
Dry mass (kg)58
66
Xenon (kg)65
60
3. Ion Source Development
The original well-tuned µ10 ion thruster can
generate 8 mN at a specific impulse (includingneutralizer flow) of 3000 s with consuming 32 W of
microwaves and 2.35 sccm of xenon flow. Research
effort after Hayabusa’s launch indicated that designchange of gas injector layout had the large impact
on thrust enhancement [2]. The highest thrust of 10
mN is generated when the xenon flow is divided at
the ratio of 1:2 between the original injector deep inthe waveguide and newly added injectors between
magnets, respectively, when the total flow rate is
3.5 sccm. Mass utilization efficiencies aredecreased from the original but can be improved byusing a small hole accelerator grid whose aperture
diameter was decreased from 1.8 to 1.5 mm.
Decrease of screen grid thickness from the originalvalue of 0.95 mm to 0.8 mm also helped the massutilization recovery. The combination of above
mentioned modifications (gas distributors, small
hole accelerator grid and thinner screen grid)achieved the maximum thrust enhancement shownin Table I and required flow rate can be saved to 3.2

sccm as for flight model thrusters. Figure 2 shows
plasma luminosity during the ground operation of
one of flight thrusters.

Fig.2. Ground operation of one of flight thrusters

4. Neutralizer Development
Lifetimes of Hayabusa neutralizers ranged
between 9579 and 14830 hours which were shorter
than expected from the ground test's achievement of
20,000 hours by a prototype model. Most
remarkable difference between in-space andon-ground conditions would be the temperature
range and cycles. We have analyzed the stored
prototype neutralizer and found many magnetized
metal flakes. These may be immersed into thedischarge plasma, may become a source of surface
contamination of the dielectric part of microwave
launcher by ion sputtering and sputtered metaldeposition. Minimization of the operating voltage toimprove the neutralizer durability were achieved byincreasing the magnetic field inside the dischargechamber and by more intensive microwave
impedance matching effort using a stub tuner. As ofOctober 2014, still continuing endurance test of anengineering model of new neutralizer, including
once-per-week on-off cycles like in orbit,
accumulated the operational hours of 19,000h
without any performance degradations.
5. Conclusion
The ion engine system for Hayabusa2 asteroid
explorer has been successfully developed and is
now ready for launch in Tanegashima Space
Center.
References[1] K. Nishiyama, S. Hosoda, H. Koizumi, Y. Shimizu, I.
Funaki, H. Kuninaka, M. Bodendorfer, and J.
Kawaguchi: Proc. 46th AIAA/ASME/SAE/ASEE
Joint Propulsion Conference & Exhibit, 2010,
AIAA-2010-6862.
[2] K. Nishiyama, S. Hosoda, M. Usui, R. Tsukizaki, H.
Hayashi, Y. Shimizu and H. Kuninaka: SpaceTechnology Japan, 7 (2009) 113.