1. Berlin
MESSENGER-BepiColombo Joint Science Meeting
Mercury Sodium Atmosphere Spectral Imager (MSASI) Onboard BepiColombo/MMO
Go Murakami1, Shingo Kameda2, Ichiro Yoshikawa3, and Oleg Korablev4
1ISAS/JAXA, 2Rikkyo University, 3The University of Tokyo, 4IKI

2. Scientific objectives
Source mechanism of Mercury’s sodium exosphere
Photon-stimulated
desorption
Charged-particle
sputtering
Micro-meteoroid
Impact/vaporisation
Still open issue
[Yoshioka et al., 2008]
MESSENGER/MASCS observation
[Ronald et al., 2010]
Spectral resolution: 0.6 nm
-> Impossible to separate solar reflections from the disc and emissions from the exosphere
Key observation:
-Imaging the global distribution of the sodium exosphere
-Long-term and continuous monitoring
[Cassidy et al., 2015]

3. Mercury Sodium Atmosphere Spectral Imager（MSASI）
Fabry-Perot interferometer
Detector unit:
I. I. + coupling lens + CMOS
Band-pass filter
Light entrance
Scanning mirror:
1-axis scan of FOV
MSASI flight model (FM)

4. Mercury Sodium Atmosphere Spectral Imager（MSASI）
MMO spacecraft (FM)
Electronics
(MSASI-E)
Sensor (MSASI-S)
Hood (MSASI-H)
Field of view

5. Mercury Sodium Atmosphere Spectral Imager（MSASI）
Solar reflection from Mercury’s surface
0.6 nm
0.005 nm
Specifications
Na D2
MMO/MSASI
MESSENGER/MASCS
Spectral range
± nm (Na D2 line) nm
Spectral resolution
0.009 nm
0.6 nm
Spatial resolution
0.18° x 0.18°: 30 km (1/80 Rm) (nominal) 3 km (1/800 Rm) (near Aphelion)
25-km (height) km (latitude/longitude)
Field of view
1.8° x 1.8° Scan: 30° (lat) x 45°-180° (lon)
1° x 0.04°
Exposure time
2 msec
1-2 sec
Sensitivity
10 kR (S/N = 5)
100 R
Mass
3.48 kg
3.1 kg
Power
15.2 W (nominal)
6.7 W
-Na D2 only
-Resolution > 60,000
-> Exosphere on dayside disc can be seen
-High spatial resolution
-Wide FOV (by scan)

6. MSASI observation: 2-D scanning
“Fast imaging”
Spin
Spectral image (1 shot)
Spin scan (45°)
Mirror scan (30°)
・Spin scan: 0.18 deg (= 1 pix)/2 msec
Nominal: 256 shots/spin (max: 1024 shots/spin)
　 →FOV: 45 deg (max: 180 deg)
・Mirror scan: 1.44 deg/step, 21 steps
→FOV: ~30 deg

7. Observation plan: nominal
Nominal observation geometry
FOV: 30° (latitudinal) x 45°(longitudinal)
8 iamges / orbit (9.3 h) (~30% will be downlinked)
-> At least one image every orbit will be available in almost all season
MSASI can monitor the temporal variation of exosphere every 9.3h

8. Observation plan: perihelion
#Thermal limitations exist in this season
Nominal
FOV: 30° (latitudinal) x 45°(longitudinal)
8 iamges / orbit (9.3 h) (~30% will be downlinked)
MSASI can observe the dawn and dusk sides at the same time

9. Observation plan: aphelion
#No observation during the eclipse
Nominal
Wide FOV
FOV: 30° (latitudinal) x 45°(longitudinal): 2 images +
Wide FOV: 30° (latitudinal) x 90°(longitudinal): 3 images
The highest spatial resolution (3 km) of dayside will be achieved

10. Observation plan: tail observation
Nominal + wide
Wide FOV
FOV: 30° (latitudinal) x 45°(longitudinal): 1 image +
Wide FOV: 30° (latitudinal) x 90°(longitudinal): 2 images
The longest sodium tail can be observed in this season

11. Conjunction observations
MPO/PHEBUS (UV obs.)
-Other exospheric compositions
-Relations of temporal and spatial variations
MPO/MIXS and MGNS
-Relations with surface compositions
MPO/SIMBIO-SYS and MERTIS
-Relations with surface structures
MMO/MDM
-Dust monitoring
MMO/MPPE and MPO/SERENA
-Ionized particles
MMO/MPPE and MGF
-Solar wind monitoring

12. Summary MSASI: 2D + spectral imaging of Na D2 line
Long-term and continuous monitoring of Mercury’s sodium atmosphere (every 9.3 h)
Spectral resolution: nm
Spin scan + mirror scan
-> Global images of Mercury can be obtained

13. From MESSENGER to BepiColombo
Thank you!
Nicholas Piccillo - Fotolia
From MESSENGER to BepiColombo