Empl, A.

Koontz, S. L.

Lawrence, D. J.

Miller, R. S.

Mittlefehldt, D. W.

Nolan, M. C.

Pinsky, L. S.

Prettyman, T. H.

Redell, B. D.

Sykes, M. V.

NASA Technical Reports Server

Muon Imaging of Asteroid and Comet Interiors 
T. H. Prettyman,1 S. L. Koontz,2 R. S. Miller,3 M. C. Nolan,4 L. S. Pinsky,5 M. V. Sykes,1 A. Empl,5      
D. J. Lawrence,6 D. W. Mittlefehldt,2 B. D. Redell2 
1Planetary Science Institute (prettyman@psi.edu),  2NASA Johnson Space Center,  3University of Alabama in Huntsville, 
4National Astronomy and Ionosphere Center,  5University of Houston,  6Johns Hopkins University Applied Physics Laboratory. 
Particle kinetic energy (MeV)
Le
ak
ag
e 
cu
rr
en
t (
pa
rt
ic
le
s/
(c
m
2
s M
eV
)
S
p
P
K
Muons transmitted 
through a 50 m 
asteroid
1/(U+ k)
Nucleonic o Electromagnetic
n
n
p
p
n
n
D
n
n
(Ŷ͕Ŷ഻J)
(n,J)
Galactic Cosmic Ray
J J
K
Radioelements
J
H, A, 6eff
Discrete J-rays (O, Mg, Si, Fe) 
and J continuum (Z,A)
Samples 
decimeter 
to meter 
depths
n
Electromagnetic Mesonic
S
Pr
er
e
e
J
J S
r
J continuum 
(Z, A)
J
K
S
S
J
e-
J J
e-
Pr (muons) can penetrate to km depths
Solid 
regolith
Pr
1 m
2 km
<1.75 >1.96
Density (g/cm3)
~0
.5
 k
m
Muon telescope (“hodoscope”) 
~500 m
1
2
3
4
Pr
T0
1 2
3
What if we could look inside an asteroid or comet?  
Information about their internal density structure 
and macroporosity would provide powerful 
constraints on their formation and evolution as well 
as information needed for planetary defense, 
mining, and in situ resource utilization.   At present, 
the internal structure of small bodies must be 
inferred from surface morphology (e.g. by optical 
imaging or radar) and indirectly from other 
observations.  We are investigating new methods to 
directly map the deep interior structure of small 
solar system bodies using secondary particles, such 
as muons, produced by galactic cosmic ray (GCR) 
showers within the body itself.  
1) Artist’s conception of a rubble-pile 
cometary  nucleus (from Weissman et al., 
2004); 2) Surface of C-G 67P 
(ESA/Rosetta); 3) Breakup of Shoemaker-
Levy 9 (NASA/HST)  suggestive of rubble-
pile  structure. 
Internal structure of the Satsuma-
Iojima volcano using  muon 
radiography by Tanaka et al., 2010. 
A B C Muons are  charged particles, similar to electrons, 
but much more massive. Muons are made by the 
decay of mesons in galactic cosmic ray showers (A, 
B).  Relativistic muons can penetrate large distances  
through rock.  Muons produced in Earth’s 
atmosphere have been  detected in deep mines.  
Muons are made in the top meter of the regoliths of 
asteroids and comets.   In solid surfaces, mesons 
often collide before decaying to produce muons.  
Thus, the muon source is suppressed relative to 
Earth’s thin atmosphere and muon production 
depends  on the density of surface materials (C). 
D E F 
Spherical 
asteroid, with 
inclusion 
Reconstructed, 
uniform 
Reconstructed, 
with inclusion 
Reconstructed 
axial slices, 
with inclusion 
D) The path of a high energy muon (Pr) through the 
asteroid is illustrated: 1) incident GCR; 2) initial direction 
of the muon; 3) final direction of the muon as it exits the 
asteroid; 4) detection of the muon by a telescope 
deployed on an orbiting spacecraft.  The deflection of the 
muon is exaggerated (T0 is less than a few 10s of 
milliradians for muons that can readily penetrate the 
asteroid).  E) The flux of transmitted muons arriving at the 
spacecraft is sensitive to the density of intervening 
materials.      F) A simulated tomograph of a small, 
spherical asteroid reconstructed from orbital muon 
radiographic data reveals asteroid interior structure.   
Muon production in regolith materials 
Data acquisition and reconstruction Rubble-pile asteroid  
Itokawa by Hyabusa 
Et 
(threshold for muon detection) 
G H 
Et 
G) Conceptual diagram of a hodoscope consisting of two, 
position-sensitive, scintillating layers.  A muon (Pr) that 
transects the hodoscope will produce two flashes of 
sensible light in coincidence.  The incident direction of 
the muon is determined from the positions of the 
interactions.   If a Cherenkov detector or transition 
radiation detector is used, then the telescope will only 
record muons with energies greater than a threshold (Et).   
The selection Et is critical to separate muons transmitted 
through the asteroid from other “upward-going” ionizing 
particles made on the same side of the asteroid as the 
hodoscope. 
Telescope design 
Preliminary results indicate that interior contrast can be 
detected using a compact muon telescope with integration 
times ranging from hours to weeks for 100-m to 1-km 
asteroids, with intrinsic spatial resolution is on the order of 
meters. Practical limits for resolution and contrast 
sensitivity depend on integration time and telescope 
design. Regolith density within the top meter of an asteroid 
can be determined from radar observations.  A pilot 
mission would combine remote radar measurements with 
muography of a near-Earth asteroid.  Alternatively, the flux 
of prompt muons produced by the decay of relatively rare 
charmed mesons is insensitive to regolith density and may 
also provide information on interior structure.  
Summary 
We are investigating compact, low-cost  designs 
that can be flown on a planetary mission. 
Upward-going particles 
6013 
https://ntrs.nasa.gov/search.jsp?R=20150001654 2019-08-31T14:08:26+00:00Z


Muon Imaging of Asteroid and Comet Interiors

Muon Imaging of Asteroid and Comet Interiors

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