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We analyze new multi-filter Hubble Space Telescope (HST) photometry of thenormal Type Ia supernova (SN Ia) 2011fe out to $\approx 2400~$days aftermaximum light, the latest observations to-date of a SN Ia. We model thepseudo-bolometric light curve with a simple radioactive decay model and findenergy input from both $^{57}$Co and $^{55}$Fe are needed to power thelate-time luminosity. This is the first detection of $^{55}$Fe in a SN Ia. Weconsider potential sources of contamination such as a surviving companion staror delaying the deposition timescale for $^{56}$Co positrons but thesescenarios are ultimately disfavored. The relative isotopic abundances placedirect constraints on the burning conditions experienced by the white dwarf(WD). Additionally, we place a conservative upper limit of $< 10^{-3}~M_\odot$on the synthesized mass of $^{44}$Ti. Only 2 classes of explosion models arecurrently consistent with all observations of SN2011fe: 1) the delayeddetonation of a low-$\rho_c$, near-$\rm{M}_{\rm{Ch}}$ ($1.2-1.3~M_\odot$) WD,or 2) a sub-$\rm{M}_{\rm{Ch}}$ ($1.0-1.1~M_\odot$) WD experiencing a thin-shelldouble detonation.

The Whisper of a Whimper of a Bang: 2400 Days of the Type Ia SN 2011fe Reveals the Decay of $^{55}$Fe