  Recent developments of transition-edge sensors (TESs), based on extensive
experience in ground-based experiments, have been making the sensor techniques
mature enough for their application on future satellite CMB polarization
experiments. LiteBIRD is in the most advanced phase among such future
satellites, targeting its launch in Japanese Fiscal Year 2027 (2027FY) with
JAXA's H3 rocket. It will accommodate more than 4000 TESs in focal planes of
reflective low-frequency and refractive medium-and-high-frequency telescopes in
order to detect a signature imprinted on the cosmic microwave background (CMB)
by the primordial gravitational waves predicted in cosmic inflation. The total
wide frequency coverage between 34GHz and 448GHz enables us to extract such
weak spiral polarization patterns through the precise subtraction of our
Galaxy's foreground emission by using spectral differences among CMB and
foreground signals. Telescopes are cooled down to 5Kelvin for suppressing
thermal noise and contain polarization modulators with transmissive half-wave
plates at individual apertures for separating sky polarization signals from
artificial polarization and for mitigating from instrumental 1/f noise. Passive
cooling by using V-grooves supports active cooling with mechanical coolers as
well as adiabatic demagnetization refrigerators. Sky observations from the
second Sun-Earth Lagrangian point, L2, are planned for three years. An
international collaboration between Japan, USA, Canada, and Europe is sharing
various roles. In May 2019, the Institute of Space and Astronautical Science
(ISAS), JAXA selected LiteBIRD as the strategic large mission No. 2.
