The gamma ray large area space telescope - an astro-particle mission to explore the high energy gamma ray sky

The Large Area Telescope (LAT), the main instrument on the Gamma-ray Large Area Space Telescope (GLAST) mission under development for launch by NASA in 2006, will survey the sky in the rich yet poorly explored high-energy band between 20 MeV and 1 TeV with unprecedented resolution and sensitivity. The LAT, which benefits from the application of particle physics detection technologies to astrophysics instrumentation, has a pair-conversion silicon-strips tracker and an imaging CsI calorimeter to measure the direction and energies of gamma rays. The relatively very intense background of charged particles will be rejected primarily by an anticoincidence shield of segmented plastic scintillators. In this paper we give an overview of the many physics goals and potential reach of the LAT, describe the instrument design and performance, and report on the status of the tracker construction