S2-6
Ion Engines for Asteroid Explorer Hayabusa2「はやぶさ２」イオンエンジン
西山和孝
Kazutaka Nishiyama
国立研究開発法人

宇宙航空研究開発機構
JAXA

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 Ryugu which
is one of different type of asteroids from Itokawa
and is thought to be rich in organic materials.
Hayabusa2 departed from the Earth on December
3, 2014, will arrive on the asteroid mid-2018 andwill return to Earth at the end of 2020.
This paper overviews the ion engine system(IES) for Hayabusa2 and space operation in the firstyear after launch.
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 (ECR)
at a frequency of 4.25 GHz. The magnetic flux
density of 0.15 T inside the discharge chambers isprovided by SmCo magnets. Microwaves are
generated by commercial traveling wave tube
amplifiers which are widely used on many satellites
and other spacecraft for space telecommunication.
The maximum thrust generated by a single
thruster was 10 mN by 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 four ion thrusters on
the H-IIA launch vehicle No. 26.

Fig.2. Ground operation of one of flight thrusters.

IES specifications and mission requirements aresummarized 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% [2]. The total

impulse required by this new mission is 1.2 MN·s
which is also 20% larger than Hayabusa'srequirement. Estimated total operational hours of all
thrusters are 41100 hours which is almost the sameas Hayabusa achievement of 39637 hours. The
increase of total impulse is due to the increase ofspacecraft mass because required delta-V is almost
the same as that of Hayabusa. This will be achievedby the thrust enhancement with almost the same
thruster operational hours. The xenon load is nearly
the same amount of 66 kg as Hayabusa's 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.

order to update timeline commands stored in thedata handling unit of the spacecraft and to change
spacecraft weekly-fixed attitude, ion thrusters werestopped once per week for approximately threehours. Figure 3 shows time history of total impulses
generated by four ion thrusters. Delta-V in the first year
was executed in separated three terms in March, Mayand June. In addition, trajectory correction maneuvers for
11 hours in total by IES was conducted on September 1and 2. Hayabusa2 has three high voltage power supplies
to accelerate xenon ion beams of four ion thrusters.
Relays are used to change combinations between
thrusters and power supplies. Thrusters A and D werepreferentially used in the first year because these twohave only one high voltage power supply for each, whilethrusters B and C have possible connections with two
power supplies. Three thrusters A, C and D will be usedin the next year and the thruster B will be saved as abackup.
25000

Table I. IES specifications (Middle of life)

Hayabusa
4.4 – 7.6

Hayabusa2
6.3 – 9.0

Thrust perthruster (mN)
Specific impulse (s) 2760 – 3000 2740 – 2890
System power (W) 280 – 1150
380 – 1230
Total operational
39637
41100
hours (h·units)(planned)
Total impulse (MN·s)
1.0 1.2(planned)
Dry mass (kg)5866
Xenon (kg)66
66

3. Space Operation in the First Year
Hayabusa2 was launched from Tanegashima
Space Center in Kagoshima Prefecture by the H-IIA
launch vehicle No. 26 on December 3, 2014.
Launch-locks of the ion engine gimbal mechanism
were released flawlessly during the first tracking
pass. Three months were spent on the initialcheckout of the spacecraft including the IES. Ion
thruster firing tests were conducted from the end of
December to the middle of January. All thrusters
exhibited very frequent high voltage breakdowns
between screen- and accelerator-grids of the ion
optics due to outgassing at first, but automatedrecycle function implemented by the thruster
control unit made continuous operation possible.
The first 24-hour continuous operation of twothrusters at the same time was successfullyachieved on January 20.
Cruise operation in which ion thrusters werecontinuously operated started in March, 2015. In

Total Impulse, Ns

20000

ITR-A
ITR-B
ITR-C
ITR-D

15000

10000

5000

0
Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
2014 2015 2015 2015 2015 2015 2015 2015 2015 2015 2015
MonthFig.3. Time history of total impulses generated by four
ion thrusters (ITR) A – D.

4. Conclusion
The ion engine system for Hayabusa2 asteroid
explorer has been developed and the spacecraft waslaunched on December 3, 2014. During the firstyear in space, two of four ion thrusters have been
operated for about 600 hours to accelerate thespacecraft by 60 m/s. Hayabusa2 is now ready forthe next important event: Earth gravity assist
scheduled in December 3, 2015.
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.