Acta Polytechnica


doi:10.14311/AP.2013.53.0811
Acta Polytechnica 53(Supplement):811–813, 2013 © Czech Technical University in Prague, 2013

available online at http://ojs.cvut.cz/ojs/index.php/ap

THE JEM-EUSO MISSION

Mario Bertainaa,b, Toshikazu Ebisuzakic, Piero Galeottia,b,∗,
Fumiyoshi Kajinod, on behalf of the JEM EUSO Collaboration

a Department of Physics, University of Torino, Torino, Italy
b National Institute of Nuclear Physics (INFN), Sez. di Torino, Italy
c RIKEN Advance Science Institute, Tokyo, Japan
d Department of Physics, Konan University, Kobe, Japan
∗ corresponding author: piero.galeotti@unito.it

Abstract. The JEM-EUSO mission explores the origin of extreme energy cosmic rays (EECRs)
above 50 EeV and explores the limits of fundamental physics, through observations of their arrival
directions and energies. It is designed to open a new particle astronomy channel. This super-wide-field
(60 degrees) telescope with a diameter of about 2.5 m looks down from space onto the night sky to
detect near UV photons (330 ÷ 400 nm, both fluorescent and Cherenkov photons) emitted from the
giant air showers produced by EECRs. The arrival direction map with more than five hundred events
will tell us the origin of the EECRs and will allow us to identify the nearest EECR sources with
known astronomical objects. It will allow them to be examined in other astronomical channels. This is
likely to lead to an understanding of the acceleration mechanisms, perhaps producing discoveries in
astrophysics and/or fundamental physics. The comparison of the energy spectra among the spatially
resolved individual sources will help to clarify the acceleration/emission mechanism, and also finally to
confirm the Greisen–Zatsepin–Kuz’min process for the validation of Lorentz invariance up to γ ∼ 1011.
Neutral components (neutrinos and gamma rays) can also be detected, if their fluxes are high enough.
The JEM-EUSO mission is planned to be launched by a H2B rocket in about 2017 and transferred to
ISS by the H2 Transfer Vehicle (HTV). It will be attached to the Exposed Facility external experiment
platform of KIBO.

Keywords: cosmic rays, neutrino, Lorentz invariance, International Space Station.

1. Introduction
The Extreme Universe Space Observatory – EUSO
is the first space mission devoted to exploring the
Universe through the detection of the extreme energy
(E > 50 EeV) cosmic rays (EECRs) and neutrinos [1–
5]; it looks downward from the International Space
Station (ISS). It was first proposed as a free-flyer, but
was selected by the European Space Agency (ESA) as
a mission attached to the Columbus module of ISS.
The phase A study for the feasibility of this obser-
vatory (which we will refer to here as ESA-EUSO)
was successfully completed in July 2004. However,
because of financial problems at ESA and in the Euro-
pean countries, together with the logistic uncertainty
caused by the Columbia accident, the start of phase
B was put back. In 2006, Japanese and U.S. teams
redefined the mission as an observatory attached to
KIBO, the Japanese Experiment Module (JEM) of
ISS. They renamed it JEM-EUSO and started with a
renewed phase A study.
JEM-EUSO is designed to achieve our main scien-

tific objective: astronomy and astrophysics through
the particle channel to identify sources by arrival di-
rection analysis and to measure the energy spectra
from individual sources, with an overwhelmingly high
exposure, approaching 1 × 106 km2 sr yr at energies

above 300 EeV (see Fig. 3). This will allow the explo-
ration of an energy region beyond any other previous
or planned experiment (in the case of space-based
telescopes, the observation area of the Earth’s surface
is essentially determined by the projection of the field
of view of the optics). It will constrain the acceler-
ation or emission mechanisms, and will also finally
confirm the Greisen–Zatsepin–Kuz’min process [6] for
the validation of Lorentz invariance up to γ ∼ 1011.

2. Scientific objectives
The scientific objectives of the JEM-EUSO mission are
divided into one main objective and five exploratory
objectives. The main objective of JEM-EUSO is to
initiate a new field of astronomy that uses the extreme
energy particle channel (5 × 1019 eV < E < 1021 eV).
JEM-EUSO has the critical exposure ((0.1 ÷ 1 ×

106) km2 sr yr depending on energy) to observe all the
sources at least once inside several hundred Mpc, and
makes possible the following:
• Identification of sources with high statistics by ar-
rival direction analysis.

• Measurement of energy spectra from individual
sources to constrain the acceleration or the emission
mechanisms.

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Mario Bertaina et al. Acta Polytechnica

Figure 1. Principle of the JEM-EUSO telescope for
detecting extreme energy cosmic rays.

We have set five exploratory objectives:
• Detection of extreme energy gamma rays.
• Detection of extreme energy neutrinos.
• A study of the Galactic magnetic field.
• Verification of relativity and quantum gravity effects
at extreme energies.

• A global survey of nightglows, plasma discharges,
and lightning and meteors.
See [7–9] for detailed discussions of the scientific

objectives. The criteria for the success of the mission
involve achieving these scientific objectives.

3. Instrument
The JEM-EUSO instrument consists of the main tele-
scope, the atmosphere monitoring system, and the
calibration system [10]. The main telescope of the
JEM-EUSO mission is an extremely-fast (∼ µs) and
highly-pixelized (∼ 3 × 105 pixels) digital camera with
a large diameter (about 2.5 m) and a wide-FoV (±30°).
It works in near-UV wavelength (330 ÷ 400 nm) with
single-photon-counting mode. The telescope consists
of four parts: the optics, the focal surface detector
and electronics, and the structure. The optics focuses
the incident UV photons on to the focal surface with
an angular resolution of 0.07 degree [11]. The focal
surface detector converts the incident photons to pho-
toelectrons and then to electric pulses [12, 13]. The
data electronics issues a trigger for an air-shower event
or an other transient event in the atmosphere, and
sends necessary data to the ground for further analysis.
The atmosphere Monitoring System (AMS) monitors
the earth’s atmosphere continuously inside the FoV
of the JEM-EUSO telescope [14]. AMS uses an IR
camera, Lidar, and the slow data of the main tele-
scope to measure the cloud-top height with accuracy
better than 500 m. The calibration system measures
the efficiencies of the optics, the focal surface detector,
and the data acquisition electronics [15].

Figure 2. Arrival time distribution of photons at
the telescope per m2 from an EAS of E = 1020 eV
and Θ = 60° with various cloud conditions. Dashed
and dotted lines correspond to the case of cirrus-like
and stratus-like test clouds, along with a solid line for
the clear atmosphere case. The peak at ∼ 150 µs in
a clear atmosphere and cirrus-like cloud is due to the
Cherenkov light reflected from the ground, and it is
fainter for cirrus. In case of stratus-like clouds, such
reflection occurs at cloud-top; this peak is therefore
closer to the fluorescence shower maximum.

4. Observational merit
In comparison with ground-based observatories, the
space-based telescope may provide various merits in
observations of EASs induced by EECR. One of the
substantial differences is that the signals of EAS from
higher altitudes are efficiently observed with no at-
tenuation or with limited attenuation in cloudy cases,
either if the cloud lies at lower altitudes or if optically
thin clouds lie at high altitude. In order to determine
the primary energy of EECRs, it is necessary to mea-
sure the shower development, including the signature
around the maximum of the shower development.
Figure 2 compares the behavior of typical EAS

developping inside clouds and without clouds. In the
case of optically thick clouds that lie at altitudes lower
than the shower maximum, e.g. stratus, the main part
of the shower development is well measured and this
allows the energy deposit in the atmosphere to be
reconstructed. Moreover, the diffusively reflecting
Cherenkov light enhances the total intensity from
the shower, which helps to increase the efficiency of
triggering the showers at nearly threshold energies. In
the presence of the optically thin clouds that lie at
high altitudes, e.g. those categorized as cirrus, most
EAS signals penetrate the layer of the clouds and are
attenuated partly and may be recognized as a lower
energy event. In such a case, however, the geometry of
the shower axis is properly determined by the analysis
of the angular velocity of the EAS signal.
Figure 3 shows the evolution of the exposures of

the past and future missions devoted to research on
the extremely high energy cosmic rays. At the high-
est energies, JEM-EUSO can achieve more than one

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vol. 53 supplement/2013 The JEM-EUSO Mission

Figure 3. Expected highest cumulative exposure, in
km2 sr yr or Linsley units, of JEM-EUSO. The two
thick red curves correspond to pure nadir mode and
pure tilted mode; the actual exposure will depend on
the final operating mode adopted and will lie between
the two curves. For comparison, the evolution of ex-
posure by other retired, running and proposed EECR
observatories is shown.

Figure 4. Relative deviation from uniformity of
the aperture as a function of the sine of declination.
Dashed curves show the cases for a selection of events
with different zenith angle. The pure isotropic expo-
sure to the solid angle is defined as 0. The horizontal
axis on the bottom denotes the corresponding declina-
tion.

order of magnitude larger exposure than the Auger
experiment or the Telescope array experiment.
Figure 4 demonstrates the uniformity of exposure

expected in the JEM-EUSO mission as a function
of the sine of declination (solid angle). This is not
the case for ground-based experiments, such as Auger
and Telescope Array. In addition to the significant
increase in the overall exposure by about one order
of magnitude compared with Auger as of today, the
orbiting JEM-EUSO telescope will cover the entire
Celestial Sphere. Moreover, the cumulative exposure
results in a high degree of uniformity, thanks to the
inclined ISS orbit. This advantage is more pronounced
if the EECRs from the single source are observed with
angular spread. If this is the case, the gradient of
exposure distributions in the Celestial Sphere may

sweet over the real signals from the sources.
With the wide FoV of JEM-EUSO telescopes ob-

serving from Space, the measurements of the entire
profile of the shower development is simpler than
with ground-based experiments with relatively small
FoV. JEM-EUSO will survey an atmospheric mass on
the Earth in excess of 1012 tons, and will be more
sensitive to showers with larger zenith angles. This
property allows effective measurements of neutrino-
induced showers.

5. Conclusions
JEM-EUSO is a scientific mission looking downward
from ISS to explore the extremes in the Universe and
fundamental physics through the detection of extreme
energy (E > 5 × 1019 eV) cosmic rays. It is the first
instrument that has full-sky coverage and achieves
exposure comparable to one million km2 sr yr. The
JEM-EUSO mission is planned to be launched by an
H2B rocket in about 2017. It will be transferred to
ISS by the H2 transfer vehicle (HTV), and attached
to the external experiment platform of KIBO.

Acknowledgements
This work has been partially supported by the Italian
Ministry of Foreign Affairs, General Direction for Cultural
Promotion and Cooperation.

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	Acta Polytechnica 53(Supplement):811–813, 2013
	1 Introduction
	2 Scientific objectives
	3 Instrument
	4 Observational merit
	5 Conclusions
	Acknowledgements
	References