Earth- and space-based observations provide synergistic inFormation for space mission encounters by providing data over longer timescales, at different wavelengths and using techniques that are impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P/Hartley 2. The nucleus is small and dark, and exhibited a very rapidly changing rotation period. Prior to the onset of activity, the period was similar to 16.4 hr. Starting in 2010 August the period changed from 16.6 hr to near 19 hr in December. With respect to dust composition, most volatiles and carbon and nitrogen isotope ratios, the comet is similar to other Jupiter-family comets. What is unusual is the dominance of CO2-driven activity near perihelion, which likely persists out to aphelion. Near perihelion the comet nucleus was surrounded by a large halo of water-ice grains that contributed significantly to the total water production. ; Swedish National Space Board ; Canadian Space Agency ; National Technology Agency of Finland ; Centre National d'Etudes Spatiales (CNES, France) ; Belgian Fund for Scientific Research (FNRS) ; Swiss National Science Foundation ; European Community 229517 ; Spanish Ministerio de Ciencia e Innovacion AYA2008-01839/ESP, AyA2009-08011 ; Junta de Andalucia PE2007-TIC 02744 ; NASA NNM07AA99C ; Aerospace Corporation ; NASA through NASA Astrobiology Institute NNA04CC08A ; McDonald Observatory