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Paper text:
the extreme universe space observatory on board the jem exposure facility of the iss ( jem  euso ) is a detector concept for the study of uhecrs @xcite .
this mission focuses on the spectrum above 5 @xmath0 ev where the flux is extremely low and the required detector areas are extremely large .
the detector consists of a wide fov ( @xmath1 30 deg ) downward  looking uv camera orbiting at 400 km altitude which monitors a @xmath2 @xmath3 km@xmath4 area of atmosphere .
euso aims at the detection of the fluorescence light emitted by eas in the atmosphere ( see fig . [ fig1 ] ) .
in such a way we can reconstruct the direction , energy and x@xmath5 of the showers at the most extreme energies .
the identification of the direction of arrival jointly with a proper energy reconstruction is mandatory for anisotropy studies , while the possibility to measure the longitudinal profile of the shower is needed to constrain the average mass at 10@xmath6 ev .
another big advantage of this technique with respect to ground based detectors is the uniformity of the sky coverage : jem  euso at 400 km can orbit every 90 minutes the entire earth covering both hemispheres with good uniformity .
the main scientific objectives of the project are : the study of the anisotropies at the extreme energies with unprecedented statistics , the identification of the sources ( and possibly the reconstruction of their spectra ) and the high statistics measurement of the trans  gzk spectrum @xcite .
we define moreover several other exploratory objectives : the search of uhe neutrinos , the search of uhe gamma photons and the study of the galactic and extragalactic magnetic fields through the measurement of the magnetic point spread function of a source .
the nature of the detector allows also to study other phenomena like tles , lightnings , the airglow , auroras , meteors , space debris , bioluminescence in the oceans and to search ( or put limits on the flux of ) hypothetical phenomena like nuclearites .
jem  euso is a refractor consisting of a system of 3 fresnel lenses focusing the light on the focal surface @xcite .
the focal surface is made by @xmath2 5000 multi anode photomultipliers ( mapmt ) produced by hamamatsu ( r11265m64 ) capable of single photon counting with a double pulse resolution of the order of @xmath2 10 ns .
the chosen pmt is 2.7 cm side and has 64 pixels each of 3 @xmath7 3 mm .
four mapmts are organized in the so called elementary cells ( ec ) while 2 ecs ie .
a block of 6@xmath76 mapmts is making a photo detection module ( pdm ) .
the electronics is modular in order to guarantee high redundancy and organized in several levels : the spaciroc asic for the front end electronics , an fpga for the pdm  level electronics , the cluster control board and the central cpu .
the front end electronics counts the photoelectrons within a gate time unit ( gtu ) window of 2.5 microseconds
. the trigger is organized hierarchically and has the challenging task to reduce the trigger rate from @xmath2 100 gb / s down to the 3 gb / day allowed by the telemetry , still keeping the physical events .
the high complexity and risk of this mission demands the development of several pathfinders .
we will present here all of them : euso  ta
which is taking data at the black rock mesa ( brm ) site of the telescope array ( ta ) observatory , the euso  balloon ( operated by cnes ) which flew in august 2014 , euso  spb which is foreseen to fly in spring 2017 on a nasa super pressure balloon ( spb ) to detect crs and finally mini  euso , the first space prototype to be launched on the iss within 2017 , whose main goal is to measure the airglow and several atmospheric phenomena visible from space . to conclude
, we will present the k  euso detector , which is capable of doing uhecr science from space and is foreseen to fly on the iss by the year 2020 .
euso  ta is a pathfinder of the jem 
euso detector @xcite which is taking data at the brm site of the ta observatory in utah ( see fig .
[ fig3 ] ) .
this detector consists of two square fresnel lenses which measure @xmath2 1 m side and a single pdm of @xmath2 16 cm side .
the detector is pointing to the sky , overlapping with the ta field of view .
each of the 2304 pixels is covering @xmath2 0.18@xmath70.18 degrees@xmath8 .
the elevation of the telescope can be changed from 0 to 25 degrees .
the purpose of this pathfinder is the detection of crs in coincidence with ta .
the trigger signal of ta is therefore used as an external trigger and the acquisition of a packet of 128 gtus is started in coincidence with the ta trigger .
ta is detecting in this way several cr events ( one example in fig .
[ fig4 ] ) .
another purpose was the calibration of the detector , also relative to ta .
we used the ta central laser facility ( clf ) and other dedicated mobile laser sources to test the technology with events reproducing the kinematics of cr events .
the laser is shot up in the sky and the scattered light was detected by euso 
ta in such a way as to mimic the signal of a cosmic ray shower .
we also used flashers , flat screens , leds to perform a complete characterization of the instrument .
ev . the event is falling 2.6 km away and euso 
ta sees with magnified resolution a part of the shower.,width=245 ]    we also detected meteors , lightnings , planes and clouds and therefore we tested the detection and the analysis of such events for jem  euso .
ta detector could be also used to develop the jem 
euso autonomous trigger .
indeed euso 
ta data have been used offline to test and optimize the trigger algorithms . as a further step ,
the pdm board with the autonomous trigger was installed on the euso 
ta detector and the trigger has been tested online on laser events .
the trigger performances are compliant with the expectations .
euso  balloon is a cnes stratospheric balloon pathfinder of the jem 
euso mission , which flew on august 25@xmath9 2014 from timmins canada ( see fig . [ fig5 ] ) .
the purpose of this pathfinder was to test the jem 
euso technology in space environment and to test the response of the detector with respect to several artificial cr events @xcite .
the detector consisted of a system of two square , 1 m side fresnel lenses and of a single pdm .
the flight lasted one night and has successfully proven the capability of the detector to operate in stratospheric conditions ( 38 km altitude ) .
unexpectedly the detector landed in water also proving the water tightness of the instrument .
the fov was 8 @xmath7 8 km@xmath8 and each single pixel covered a projected area 120 @xmath7 120 m@xmath8 on the ground .
the balloon covered the distance of roughly 100 km passing over different landscapes including forest lakes , cities , clear and cloudy sky .
2014 . in the box : the integrated image of a laser shot together with the flasher and led.,width=321 ]    during the flight the balloon was followed at 3,000 m altitude by an helicopter which was equipped with a flasher , a led and a laser to generate artificial light events ( see one example in fig .
[ fig5 ] ) .
we could therefore detect several hundreds artificial events which were used to test the electronics .
we used such data to test the algorithms for the reconstruction of the direction of the laser events @xcite .
the rms of the angular reconstruction was estimated in @xcite to be few degrees . a map of the background in the uv range has been produced and is shown in fig .
thanks to the present study we could give a preliminary estimate ( except the direct airglow component ) of the uv background which will be seen by jem  euso on orbit both in clear sky and cloudy conditions .          in spring 2017 the euso  spb prototype will be launched from wanaka , new zealand @xcite .
the detector will consist of a system of two fresnel lenses of 1 m side and one single pdm .
the balloon flight will make use of a super pressure balloon ( spb ) , a new technology currently developed by nasa .
the test flights performed by nasa in 2015 and 2016 achieved a 32 and 45 days duration .
the detector is assembled and currently ( feb .
2017 ) in new zealand in the launch preparatory phase .
the detector has now to be fully autonomous and must therefore be provided with solar panels , rechargeable batteries , antennas for data and command transfer .
the main objective of euso 
spb is the autonomous detection of cr events for the first time from above using the fluorescence technique .
the electronics is therefore equipped with a fully autonomous trigger which has been already tested on the ta site in october 2016 .
we show in fig .
[ fig9 ] the image of a laser shot which autonomously triggered euso 
the collaboration estimated the expected rate of cr to be of the order of few events during the entire flight .
mini  euso will be launched in 2017 and consists of a system of circular fresnel lenses and one single pdm to be accommodated inside the iss @xcite .
the detector will be placed in the russian segment of the iss behind a uv  transparent downward  looking window and will monitor the atmosphere from 400 km height in the same condition as jem  euso
. the main objective of this prototype is the measurement of the atmospheric uv emission from space .
given the small diameter of the lens ( 25 cm ) , no detection of crs can be expected . despite that ,
the collaboration plans to use artificial sources ( with a luminosity equivalent to a 10@xmath10 ev cr ) as in the balloon missions in order to mimic a cr signal .
other targets of the missions are the study of tles , meteors and auroras .
we see in fig .
[ fig10 ] the image of a simulated meteor . an interesting application of this prototype is the detection of space debris during twilight , which will fly on a lower orbit with respect to the iss .
k  euso is the first large size detector being developed in the framework of the jem  euso project @xcite .
the optical design follows a so  called schmitt optics , namely a combination of a mirror and of a corrector lens .
the detector is exploiting the experience gathered in the jem  euso and the klypve collaborations .
the detector is planned to fly in the early 2020s and to be attached to the russian section of the iss .
the detector will be the first one capable of doing full scale uhecr science from space through fluorescence .
we see in fig .
[ fig11 ] the comparison between the exposure curves of several existing and planned detectors compared to k  euso . the main goal of this detector is the study of both small and large scale anisotropies , in particular the investigation of the ta hot spot , and of the differences between north and south hemisphere , thanks to its roughly uniform full sky coverage .
this represents a major issue and challenge of the current research in the field of uhecrs .
we gave a brief introduction to the jem 
euso program .
all the pathfinders were presented and for the already existing ones a brief summary of their main results has been given .
the collaboration is on its path for the development of space  based uhecr detectors .
the capability of the jem  euso electronics to trigger cr  like events has been proven together with the technological readiness of the detector .
the triggered events are used to develop the reconstruction of their direction .
the response of the jem 
euso detector with respect to the background is being studied .
the instrument is being calibrated with respect to ground based observatories . within 2017
the collaboration will perform autonomous fluorescence cr detection from balloon and a complete mapping of the uv background from space .
such achievements will be useful to build a detector capable of detecting cr from space .
k  euso should mark the first significant step , providing for the first time a full  sky coverage with a single instrument , with auger  like statistics in the gzk energy range .
jem  euso is an international collaboration for the development of space  based ultra high energy cosmic ray ( uhecr ) detectors . 
 the instrument consists of a wide field of view ( fov ) camera for the detection of the uv light emitted by extensive air showers ( eas ) in the atmosphere . within the jem  
 euso framework several pathfinders have been developed or are in course of development : euso  ta , euso  
 balloon , euso  spb and mini  euso . for the near future the k  
 euso detector is foreseen to detect cosmic rays from space . in this paper 
 we present the jem  euso project and give an overview of the pathfinders and of their results .