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Modern theories of galaxy formation predict that the Galactic stellar halowas hierarchically assembled from the accretion and disruption of smallersystems. This hierarchical assembly is expected to produce a high degree ofstructure in the combined phase and chemistry space; this structure shouldprovide a relatively direct probe of the accretion history of our Galaxy.Revealing this structure requires precise 3D positions (including distances),3D velocities, and chemistry for large samples of stars. The Gaia satellite isdelivering proper motions and parallaxes for >1 billion stars to G~20. However,radial velocities and metallicities will only be available to G~15, which isinsufficient to probe the outer stellar halo (>10 kpc). Moreover, parallaxeswill not be precise enough to deliver high-quality distances for stars beyond~10 kpc. Identifying accreted systems throughout the stellar halo thereforerequires a large ground-based spectroscopic survey to complement Gaia. Here weprovide an overview of the H3 Stellar Spectroscopic Survey, which will deliverprecise stellar parameters and spectrophotometric distances for 200,000 starsto r=18. Spectra are obtained with the Hectochelle instrument at the MMT, whichis configured for the H3 Survey to deliver resolution R~23,000 spectra coveringthe wavelength range 5150A-5300A. The survey is optimized for stellar haloscience and therefore focuses on high Galactic latitude fields (|b|>30 deg.),sparsely sampling 15,000 sq. degrees. Targets are selected on the basis of Gaiaparallaxes, enabling very efficient selection of bone fide halo stars. Thesurvey began in the Fall of 2017 and has collected 88,000 spectra to-date. Allof the data, including the derived stellar parameters, will eventually be madepublicly available via the survey website: h3survey.rc.fas.harvard.edu.

Mapping the Stellar Halo with the H3 Spectroscopic Survey