One of the most challenging issues in Astroparticle Physics is represented by the observation of the energy spectrum of the EECRs, Cosmic Rays of Extreme Energy (E > 1019 eV). The very existence of such energetic particles and of neutrinos of comparable energy raises fundamental scientific questions in connection with their origin and propagation in the interstellar/intergalactic space. 

The EECR particles can be detected through the EAS (Extensive Air Shower) they produce along their path in the Earth Atmosphere. The shower development is accompanied by emission of fluorescence, in particular that induced in nitrogen with characteristics spectral lines in the UV band. Viewed at some instant from a distance, an EAS appears as a relatively small disc-shaped luminous object. When it is viewed continuously, the object moves on a straight path with the speed of light. As it does so, the disc luminosity changes continuously from so faint to be undetectable, up to a maximum followed by a gradual fading. Because the amount of UV light generated from an EAS is relatively faint, the collecting area of the optical system must be correspondingly large. Furthermore, air showers occur at unpredicted locations and with extremely low rate in the atmosphere; a wide field of view is then necessary in order to observe a statistically significant number of events.

Following a first suggestion by Linsley in the early 1980`s, taken over by Takahashi, the fluorescence observation can be advantageously carried out by space. By using wide angle optics with large collecting surface, we can monitor a target area of atmosphere of the order of millions km2 sr, and corresponding mass above 1013 tons, allowing the detection of the very small flux values typical of the EECRs and making possible the search of the elusive high energy neutrinos.

JEM-EUSO, once the acronym for “Extreme Universe Space Observatory on the Japanese Experiment Module” (today: “Joint Experiment Missions for Extreme Universe Space Observatory”, see the project page here) on board the International Space Station, is the first space mission devoted to the exploration of the outermost bounds of the Universe through the detection of the ultra high energy (above 1020 eV) cosmic rays and neutrinos.

JEM-EUSO will utilize the Earth atmosphere as a giant detector of the EECRs, the most energetic particles coming from the Universe. Looking downward the Earth from Space, JEM-EUSO will detect the extreme energy cosmic rays particles observing the fluorescence signal produced during their pass in the atmosphere; the Cherenkov signal, diffused when the shower hits ground or the top of a cloud, will also be imaged.

The main objective of JEM-EUSO is doing astronomy and astrophysics through the particle channel with extreme energies so extending, with a significant statistical evidence, the measurement of the energy spectrum of the cosmic radiation beyond the Greisen-Zatsepin-Kuzmin (GZK) cut-off. Moreover, using the atmosphere as a giant detector, JEM-EUSO could observe extremely high energy neutrinos, so opening the field of high energy neutrino astronomy. Furthermore, JEM-EUSO will contribute to the investigation of phenomena intrinsic to the Earth’s atmosphere or induced by the flux of meteoroids incoming from Space.

Firstly proposed as a free-flyer in the framework of the AirWatch program, the observatory was selected by the European Space Agency (ESA) as a mission attached to the Columbus module of the ISS. The phase-A study for the feasibility of that observatory, named EUSO, was successfully completed in July 2004. Nevertheless, because of financial problems in ESA and European countries, the green-light to start the EUSO phase-B was postponed for a long time.

In 2006, Japanese and U.S. teams redefined the mission as an observatory attached to the Japanese Experiment Module/Exposure Facility (JEM/EF) of the International Space Station (ISS). They renamed it as JEM-EUSO and started with a renewed two-year-long phase-A study, whose results were firstly reported in December 2008. The phase-A study has then been continued with extensive simulations, design, and prototype hardware developments that have significantly improved the JEM-EUSO mission profile, targeting the launch in the framework of a next phase of JEM/EF utilization.

JEM-EUSO is designed to operate for more than 3 years on board the ISS.

JEM-EUSO in brief

The JEM-EUSO instrument of Flight Segment basically consists of an EECR telescope assisted by an atmosphere monitoring device and controlled by a calibration system.

The JEM-EUSO telescope has a super-wide (60o) full Field-of-View with optics composed by Fresnel lenses; the telescope records the track of an EAS with a time resolution of 2.5 microseconds and a spatial resolution of about 0.75 km (corresponding to 0.1o) in nadir mode. These time-segmented images allow determining energy and direction of the primary particles. The focal surface of the JEM-EUSO telescope is formed by about 6000 multi-anode photomultipliers; the number of pixels is of the order of two hundred thousand. 

With respect to the original EUSO, first design of such an observatory, JEM-EUSO reduces the threshold energy down to around 1019 eV and increases the effective area by means of advances in technology and taking advantage of specific features of the JEM/EF module. The reduction in the threshold energy is realized thanks to new lens material and improved optical design, detectors with higher quantum efficiency, and improved algorithm for event trigger. The increase in effective area, as schematized in the figure representing JEM-EUSO passing over Japan, is realized by inclining the telescope from nadir; under this so-called “tilted mode”, the threshold energy gets higher since the mean distance to EAS and atmospheric absorption both increase. First few years of the mission lifetime will be devoted to observe lower energy region in “nadir mode” and then later to observe high energy region by “tilted mode”. 

The JEM-EUSO telescope can reconstruct the incoming direction of the EECRs with accuracy better than few degrees. Its observational aperture of the ground area is a circle with 250 km radius, and its atmospheric volume above it, with a 60o FoV, is about 1 Tera-ton or more. The target volume for upward neutrino events exceeds 10 Tera-tons. The instantaneous aperture of JEM-EUSO is larger than the Pierre Auger Southern Observatory by a factor ranging from 65 to 280, depending on its observation mode (nadir or tilted).

As concerns the atmosphere monitoring, JEM-EUSO will use an infrared camera and a Lidar with ultraviolet laser to observe the conditions of the atmosphere in the field of view of the main EECR telescope, with the objective of determining effective observation time, and of increasing the reliability of the events around the energy threshold.

JEM-EUSO will be calibrated through instrumentation both onboard and on ground. The onboard calibration system will be composed of a set of LEDs with different wavelengths (from 300 to 500 nm) to be located in the telescope cylinder as diffusive light sources. Moreover, Xenon flasher lamps will be installed in a dozen of sites on the ground and, when JEM-EUSO passes over them, once a day or so, it will detect such lights and measure the total atmospheric UV absorption and, therefore, calibrate the device. In order to estimate the systematic error in the energy and arrival direction of the primary cosmic rays, JEM-EUSO telescope observes the ultraviolet laser from ground Lidar stations. This also will allow to estimate the transmittance of the atmosphere as a function of the altitude.

The design and the construction of the JEM-EUSO telescope is a real technical challenge, as it involves the use of new technologies from the laboratories of both industrial and research laboratories in areas as diverse as optical large and accurate Fresnel lenses, a technique of photo-detection highly sensitive and good resolution, and very innovative analog and digital electronics. In view of  the launch, when JEM-EUSO should be accommodated on the ISS, a reduced version of the EECR telescope will be tested through specific experiment, both on ground (EUSO-TA) and on board of stratospheric balloons (EUSO-Balloon). Moreover, a Mini-EUSO would be mounted inside the pressurized ISS, with access from astronauts.