  We analyze new multi-filter Hubble Space Telescope (HST) photometry of the
normal Type Ia supernova (SN Ia) 2011fe out to $\approx 2400~$days after
maximum light, the latest observations to-date of a SN Ia. We model the
pseudo-bolometric light curve with a simple radioactive decay model and find
energy input from both $^{57}$Co and $^{55}$Fe are needed to power the
late-time luminosity. This is the first detection of $^{55}$Fe in a SN Ia. We
consider potential sources of contamination such as a surviving companion star
or delaying the deposition timescale for $^{56}$Co positrons but these
scenarios are ultimately disfavored. The relative isotopic abundances place
direct 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 are
currently consistent with all observations of SN2011fe: 1) the delayed
detonation 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-shell
double detonation.
