CALET docked on the ISS

On 19 August, with a spectacular launch on board the Japanese H2-B rocket operated by the Japan Aerospace Exploration Agency (JAXA), the CALorimetric Electron Telescope (CALET) left the Tanegashima Space Center to reach the International Space Station five days later.


After berthing with the ISS, CALET was extracted by a robotic arm from the Japanese HTV-5 transfer vehicle and installed on the Japanese Exposure Facility (right) where it will start its first data-taking. (Image: NASA/JAXA.)


CALET is a space mission led by JAXA with the participation of the Italian Space Agency (ASI) and NASA. It is a CERN-recognised experiment and the second high-energy astroparticle experiment to be installed on the International Space Station (ISS) after AMS-02, which has been taking data since 2011. Designed to be a space observatory for long-term observations of cosmic radiation aboard the external platform JEM-EF of the Japanese module (KIBO) on the ISS, CALET aims to identify electrons, nuclei and gamma-rays coming from space and measure their energies with high-resolution. 

“One of the main scientific objectives of CALET is to measure the detailed shape of the electron spectrum above 1 TeV,” explains Shoji Torii of Waseda University in Tokyo, Principal Investigator (PI) of CALET. “This unexplored region is attracting growing interest from the scientific community as it might be able to show for the first time the smoking gun of the presence of nearby astronomical source(s) where electrons are accelerated. We know that electrons cannot travel for long distances as they quickly lose their energy. Therefore, they are expected to originate relatively near to Earth – about 1 kiloparsec.”

Using a deep homogeneous calorimeter preceded by an imaging pre-shower calorimeter, CALET will perform accurate measurements of the electron energy spectrum from 1 GeV to 20 TeV. “The high end of the spectrum could be particularly interesting as it could help resolve the controversial interpretation of the electron and positron spectra measured by AMS-02 and could provide a clue on possible signatures of dark matter,” says John Wefel of Louisiana State University, Co-PI of the CALET project and leader of the American team participating in CALET.

CALET could also help explain the deviation from a pure power law that was recently observed by the AMS-02 collaboration in the energy spectra of light nuclei. “Thanks to its excellent energy resolution and ability to identify cosmic nuclei from hydrogen to iron and above,” explains Pier Simone Marrocchesi, Co-PI of the CALET collaboration and head of the Italian team, “CALET will be able to extend the present data to higher energies, and measure accurately the curvature of the spectrum and the position of the spectral break-point for individual nuclear species. The calibration of the two calorimetric instruments is the key to controlling the energy scale and this is why we performed several calibration tests at CERN.”

After berthing to the ISS, CALET was extracted by a robotic arm from the Japanese H-II transfer vehicle (HTV5) and installed on the JEM-EF, where it will start a first data-taking period of five years.

by Antonella Del Rosso