Figure 3.
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.,
2018, Icarus, 305: 174.)
NASA’s planned Psyche Discovery Mission,
scheduled for launch in 2022 and orbital
insertion four years later, will be the first to
visit an M-class asteroid.
A team of astronomers led by Jack Drum-
mond of the Starfire Optical Range at Kirt-
land Air Force Base in New Mexico has car-
ried out a new analysis, published earlier this
year in Icarus, of a comprehensive set of 25
images taken with adaptive optics (AO) on
six different nights spanning four opposi-
tions of Psyche from June 2004 through De-
cember 2015. (Because the rotational period
of Psyche is 4.2 hours, observations from the
same night can sample significantly differ-
ent orientations.) The data were acquired
using the Near-InfraRed Imager and spec-
trometer (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 deconvolved im-
ages were then fitted simultaneously using
July 2018
a triaxial ellipsoidal model incorporating the
known orbit and rotation of Psyche.
Figures 3 and 4 show the 25 deconvolved AO
images and the best-fit model as it would
have appeared at the time of each observa-
tion. Psyche has an obliquity of 95°, meaning
that it rotates “on its side,” and its shape is dis-
tinctly non-spherical. The analysis yields triax-
ial ellipsoid dimensions of (a, b, c) = (274 ± 9,
231 ± 7, 176 ± 7) km and leads to an estimat-
ed density of 4.2 ± 0.6 grams per cubic centi-
meter, where the large part of the uncertainty
comes from the mass. This density is consid-
erably 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. In-
stead of a solid iron core, it may be a disrupted
and re-assembled heap of scrap metal. Poros-
ities of some “rubble pile” asteroids are known
to be this large, but none have such high met-