Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 48 (2015)

Measures for Micro Meteoroids and Orbital Debris in Cooler Driver
Harnesses of the Soft X-ray Spectrometer onboard ASTRO-H
H. Noda∗1 and T. Tamagawa∗1 on behalf of the SXS teamThe Soft X-ray Spectrometer (SXS)1) onboard the
6th Japanese X-ray satellite ASTRO-H2) covers an
0.3–12 keV band with an unprecedentedly high energy resolution of 7 eV (the target value is 4 eV). Theperformance of X-ray spectroscopy is achieved using
micro-calorimeter technology at a low temperature of∼ 50mK, which is maintained through the following
cooling chain. Stirling coolers, called Shield Coolers(SCs) and Pre-Coolers (PCs), decrease the temperature from ∼ 290K to ∼ 20K, and subsequently, a JouleThomson cooler (JT) further decreases the temperature to ∼ 4.5K. Then, the temperature is cooled downto ∼ 1.2K with superﬂuid helium and/or an Adiabatic
Demagnetization Refrigerator (ADR), and ﬁnally thetemperature is reduced to ∼ 50mK using two-stage
ADRs at the front end of the chain.
Electric power derived by a satellite bus is supplied
to SCs, PCs, and the JT via cooler drivers named
SC Driver (SCD), PC Driver (PCD), and JT Driver(JTD), respectively1) . The cryo-coolers and cooler
drivers are connected by multiple harnesses, and thenumbers of particularly important harnesses for driving the cryo-coolers are 4 in SCD, 4 in PCD, and 2in JTD (10 in total). Because a part of the dewar
surface is placed in front of a window of the satellite
panel (Fig. 12) ), the harnesses are partly exposed tospace, and hence, Micro Meteoroids and Orbital Debris(MMODs) of various sizes can collide with the harnesses. If one of the important harnesses is destroyed
because of collision with an MMOD, the cooling chain
fails to work as required, signiﬁcantly degrading theSXS performance. Thus, the design needs to satisfy
the requirement that the number of MMODs passing
through an important harness per year must be lower
than 0.1. In the present document, we roughly estimate the probabilities with 1-digit accuracy in caseswith and without a Kevlar measure3) , and we determine whether the requirement can be satisﬁed.
The probability that a harness is penetrated by an
MMOD can be estimated by multiplying an area exposed to space, a solid angle against space, and ﬂuxof MMODs. In each harness, the part exposed tospace commonly has a length of ∼ 2 m and a solid
angle of ∼ 0.5π against space, while the sum of widthsof important parts driving the cryo-coolers is ∼ 1–3mm (for simplicity, we hereafter employ a width of
3 mm for all the harnesses). According to a debris
ﬂux model, MASTER20094) , developed by the European Space Agency (ESA), the MMOD ﬂux drasticallychanges with their sizes. Because the critical MMOD
size is determined as the smallest diameter penetrating a harness, the ﬂux depends on the toughness of theharnesses. According to the JAXA space debris protection manual3) , the critical MMOD size for a commonlyused harness cable is ∼ 0.2 mm, while that for the samecable covered by a 1-layer Kevlar ﬁber is calculated tobe ∼ 0.4 mm. Therefore, the MMOD ﬂuxes can be
determined to be ∼ 10 and ∼ 1 collisions/m2 /year forharnesses without and with the 1-layer Kevlar coverage, respectively.
We calculated the probabilities that one of the important harnesses is penetrated per year. The casewithout Kevlar coverage has the probability
0.5π
10 pieces × 2 m × 0.003 m ×× 10 collisions/m2 /year
2π≈ 0.2 collisions/year,
while the probability for the case with the 1-layer
Kevlar ﬁber decreases to0.5π
× 1 collisions/m2 /year
10 pieces × 2 m × 0.003 m ×2π≈ 0.02 collisions/year.
Thus, we successfully conﬁrmed that the design withthe 1-layer Kevlar ﬁber coverage is eﬀective to satisfy
the requirement, and we have employed the measures
for all the SCD, PCD, and JTD important harnesses.

Fig. 1. Flight model of the ASTRO-H satellite body in a vibration test, including the engineering model of the SXS
dewar2) on the inside. The red dotted square shows awindow at the satellite panel, through which the harnesses are exposed to the space and MMODs can enter.
∗1

RIKEN Nishina CenterReferences
1) K. Mitsuda et al. SPIE 7732, 773211 (2010)2) T. Takahashi et al. SPIE, 9144, 914425 (2014)
3) JAXA space debris protection manual, 2009, JERG-2144-HB001
4) ESA MASTER: http://www.master-model.de- 181 -完全版2014_本文.indd 181

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