GeminiFocus July 2018 - Page 15

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-