Figure 10.
Deconvolved Gemini/
NIRI and Keck/NIRC2
images of asteroid 16
Psyche. Each image is
labeled with the initial
of the observatory
and the two-digit
year in which it was
taken; the rotational
phase and sub-Earth
latitude during each
observation are shown
in brackets. Note that
the sub-Earth latitudes
are negative in all cases.
The black outlines show
the best-fit ellipse for
each image.
[Figure reproduced
from Drummond, et al.
Icarus, 305: 174, 2018.]
year in Icarus (viewable here), of a compre-
hensive set of 25 images taken with adap-
tive optics (AO) on six different nights span-
ning four oppositions of Psyche from June
2004 through December 2015. (Because the
rotational period of Psyche is 4.2 hours, ob-
servations from the same night can sample
significantly different orientations.) The data
were acquired using the Near-InfraRed Im-
ager and spectrometer (NIRI) with the Altair
AO system at Gemini North and the NIRC2
camera with the AO system on the Keck II
telescope; all images were processed using
parametric blind deconvolution. The de-
convolved images were then fitted simul-
taneously using a triaxial ellipsoidal model
incorporating the known orbit and rotation
of Psyche.
Figures 10 and 11 (on next page) show the
25 deconvolved AO images and the best-fit
model as it would have appeared at the time
of each observation. Psyche has an obliquity
of 95°, meaning that it rotates “on its side,”
and its shape is distinctly non-spherical. The
analysis yields triaxial ellipsoid dimensions of
January 2019 / 2018 Year in Review
(a, b, c) = (274 ± 9, 231 ± 7, 176 ± 7) km and
leads to an estimated density of 4.2 ± 0.6
grams per cubic centimeter, where the large
part of the uncertainty comes from the mass.
This density is considerably less than that of
pure nickel-iron and would require a porosity
of 47% if the bulk composition is the same as
its surface. That is to say, Psyche appears to
be full of holes. Instead of a solid iron core, it
may be a disrupted and re-assembled heap of
scrap metal. Porosities of some “rubble pile”
asteroids are known to be this large, but none
have such high metal contents. Alternatively,
Psyche could be a stony-iron asteroid with
low porosity and an interior much more sili-
cate-rich than its surface, but such an invert-
ed structure would be difficult to understand.
The study also derives an improved determi-
nation for the asteroid’s rotational pole, with
an uncertainty radius of 3 degrees. This is
useful in operations planning for the Psyche
Mission, but the precision is currently limited
by the restricted range of orientations avail-
able for the modeling. Remarkably, Psyche
has an orbital period of 5.00 years, which
GeminiFocus
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