Polarimeter to UNify the Corona and Heliosphere (PUNCH): Science, Status, and Path to Flight

PUNCH is a Small Explorer constellation mission in development for NASA&#x0027;s Heliophysics Division. PUNCH will image the transition zone between the outer reaches of the solar corona and the solar wind in the inner heliosphere, helping to unify the fields of solar physics and solar wind (space) physics. A constellation of four microsatellites (microsats) in Sun-synchronous LEO will produce deep field, continuous, 3D visible-light images of the corona and young solar wind from 6R<inf>s</inf> to 180R<inf>s</inf> in polarized visible light. A single Narrow Field Imager (NFI) on one microsat captures the outer corona from 6 R<inf>s</inf> to 32 R<inf>s</inf>, and three Wide Field Imagers (WFIs) on the remaining microsats capture from 20R<inf>s</inf> to 180R<inf>s</inf>. The instruments are matched and synchronized to operate as a single &#x201C;virtual observatory&#x201D;, with a 90-degree field of view centered on the Sun. The instruments use conventional lens optics and deep baffles to image the faint traces of visible sunlight that are Thomson-scattered by free electrons in the tenuous plasma of the outer corona and young solar wind. PUNCH includes polarized optics to produce 3D images using the polarization physics of the scattering. By bringing imaging science outward from the Sun and into the heliosphere, PUNCH fulfills its science objectives to (1) understand how coronal structures become the ambient solar wind, and (2) to understand the dynamic evolution of transient structures (such as CMEs) in the young solar wind. We briefly introduce the PUNCH science, describe fundamental trades that enabled the mission, and report current development status and the steps ahead toward on-orbit science operations.