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The Confusing Lightcurve of 2014 MU69

In the summer of 2017 a team led by PSI Senior Scientist Susan Benecchi collected observations with the Hubble Space Telescope (HST) to help New Horizons mission planners to learn as much as possible about the rotation of Kuiper Belt object (KBO) 2014 MU69 “Ultima Thule” in preparation for spacecraft fly-by on Jan. 1, 2019. The results were accurate when compared to images of the object. 

We strategically spaced 24 HST orbits (96 minutes each in duration) over nine days to investigate rotation periods from approximately 2-100 hours, which tell us how fast the object is spinning, and to better constrain the lightcurve rotation amplitude which tells us about the object’s shape, Benecchi said. This data was added to four years of observations with HST to refine Ultima Thule’s orbit around the Sun. The later data are not ideal for determining rotation information, but they must be consistent with any lightcurve interpretation. Until New Horizons detected MU69 in its optical navigation images beginning in August 2018, this HST lightcurve campaign provided the most accurate photometry to date (Figure 1).

susans fig. 1

Figure 1. Analysis on a combination of the lightcurve (color points) and astrometric (gray points) photometry using a period fitting algorithm designed for minimally sampled datasets such as ours. The lower the dip, the stronger the significance of the period result. We nearly exclusively find a period of 3.38 hours (spherical object), double-peaked period of 6.76 hours (elongated object – the more likely interpretation), although if we exclude periods less than 4 hours we find a period near 21.6 hours (43.2 hours). Unfortunately none of our results are not statistically significant because of the low signal to noise of the dataset. 

The mean variation in our data was small, only 0.15 magnitudes which suggested that Ultima Thule is either nearly spherical (a:b axis ratio of 1:1.15), or its pole vector is pointed near the line of sight to Earth (Figure 2). This later interpretation does not preclude a near-contact binary or bi-lobed object. We were unable to determine an actual rotation period for Ultima-Thule and given the quality of the dataset, this was rather confusing. However, image stacks do conclude that MU69 does not have a binary companion ≥2000 km at the resolution of HST. We reported with confidence that MU69 was not both rapidly rotating and highly elongated.

picture 2 susan 

Figure 2. Model ellipsoidal lightcurve results for the full range of pole inclinations based on the work of Connolley & Ostro 1984. The 14° inclination of Ultima Thule’s pole relative to the Earth is highlighted in red on each of these plots and changes by a factor of 6 between the two configurations (the insert gives a geometric view of this scenario). While these models are simplistic, for a lightcurve amplitude as large as the scatter in our data, 0.15 magnitudes, to be produced would require a contact binary pole inclination of ≥52° or an ellipsoidal pole inclination of ≥40° (gray area in each plot) and outside of the gray oval in the insert. 

The mystery continued until New Horizons provided a resolved image of Ultima Thule just a day or so before fly-by on Jan 1, 2019. These images showed that Ultima Thule is in fact rotating in a face-on/line of sight orientation such that the lobes rotate around a common center point, but the same overall surface-area is more or less always pointed towards Earth (Figure 3). In other words, the spin axis of MU69 lies within the cone within which the brightness variations due to a changing cross-section is smaller than the photometric errors in our measurements. The lightcurve non-detection is consistent with this interpretation. The fact that Ultima Thule is bi-lobed also significantly impacts the lightcurve amplitude that we observed in that a fully elliptical object rotating in the same way would present a lightcurve with a comparatively larger amplitude. The rotation period determined by the fly-by imagery is about 16 hours.

fig 3 susan 

Figure 3. Data from the New Horizons Mission (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute). Raw images showing the propeller-like rotation of Ultima Thule in the nine hours between 20:00 UT (3 p.m. ET) on Dec. 31, 2018, and 05:01 UT (12:01 a.m.) on Jan. 1, 2019, as seen by the Long Range Reconnaissance Imager (LORRI) aboard NASA's New Horizons. 

June 16, 2019
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