  On 15 February 2013, the asteroid 367943 Duende (2012 DA14) experienced a
near-Earth encounter at an altitude of 27,700 km or 4.2 Earth radii. We present
here the results of an extensive, multi-observatory campaign designed to probe
for spectral and/or rotational changes to Duende due to gravitational
interactions with the Earth during the flyby. Our spectral data reveal no
changes within the systematic uncertainties of the data. Post-flyby lightcurve
photometry places strong constraints on the rotation state of Duende, showing
that it is in non-principal axis rotation with fundamental periods of P_1 =
8.71 +/- 0.03 and P_2 = 23.7 +/- 0.2 hours. Multiple lightcurve analysis
techniques, coupled with theoretical considerations and delay-doppler radar
imaging, allows us to assign these periods to specific rotational axes of the
body. In particular we suggest that Duende is now in a non-principal, short
axis mode rotation state with a precessional period equal to P_1 and
oscillation about the symmetry axis at a rate equal to P_2. Temporal and
signal-to-noise limitations inherent to the pre-flyby photometric dataset make
it difficult to definitively diagnose whether these periods represent a change
imparted due to gravitational torques during the flyby. However, based on
multiple analysis techniques and a number of plausibility arguments, we suggest
that Duende experienced a rotational change during the planetary encounter with
an increase in its precessional rotation period. Our preferred interpretation
of the available data is that the precession rate increased from 8.4 hours
prior to the flyby to 8.7 hours afterwards. A companion paper by Benson et al.
(2019) provides a more detailed dynamical analysis of this event and compares
the data to synthetic lightcurves computed from a simple shape model of Duende.
(abbreviated abstract)
