We present photometric observations of an apparent Type Ia supernova (SN Ia) at a redshift of ~1.7, the farthest SN observed to date. The supernova, SN 1997ff, was discovered in a repeat observation by the Hubble Space Telescope (HST) of the Hubble Deep Field-North (HDF-N) and serendipitously monitored with NICMOS on HST throughout the Thompson et al. Guaranteed-Time Observer (GTO) campaign. The SN type can be determined from the host galaxy type: an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse supernovae. The classification is further supported by diagnostics available from the observed colors and temporal behavior of the SN, both of which match a typical SN Ia. The photometric record of the SN includes a dozen flux measurements in the I, J, and H bands spanning 35 days in the observed frame. The redshift derived from the SN photometry, z = 1.7 ± 0.1, is in excellent agreement with the redshift estimate of z = 1.65 ± 0.15 derived from the U300B450V606I814J110J125H160H165Ks photometry of the galaxy. Optical and near-infrared spectra of the host provide a very tentative spectroscopic redshift of 1.755. Fits to observations of the SN provide constraints for the redshift-distance relation of SNe Ia and a powerful test of the current accelerating universe hypothesis. The apparent SN brightness is consistent with that expected in the decelerating phase of the preferred cosmological model, ΩM ≈ 1/3,ΩΛ ≈ . It is inconsistent with gray dust or simple luminosity evolution, candidate astrophysical effects that could mimic previous evidence for an accelerating universe from SNe Ia at z ≈ 0.5. We consider several sources of potential systematic error, including gravitational lensing, supernova misclassification, sample selection bias, and luminosity calibration errors. Currently, none of these effects alone appears likely to challenge our conclusions. Additional SNe Ia at z 1 will be required to test more exotic alternatives to the accelerating universe hypothesis and to probe the nature of dark energy.
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