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New Horizons Confirms the Red Color of 2014 MU69 Proposed by HST Measurements

fig 1

Figure 1: Sample Hubble space Telescope images in the F606W (visible wavelength) filter for each long-range observation Kuiper Belt Object presented in Benecchi et al. 2019a. These images show the quality of the data and give the reader a sense of what small, faint objects at the edge of our solar system look like.


New Horizon’s fly-by of Kuiper Belt object (KBO) 2014 MU69 “Ultima Thule” on Jan. 1, 2019 confirmed the accuracy of Hubble Space Telescope (HST) observations showing that the object is red in color (slightly redder than Pluto), according to research by PSI Senior Scientist Susan Benecchi. 

In preparation for the New Horizons fly-by of the Kuiper Belt Object 2014 MU69 “Ultima Thule” a team led by Benecchi collected observations with the Hubble Space Telescope (HST) to help provide context for the mission and to provide comparison of mission data with other ground and space-based datasets afterwards. Ultima Thule is faint enough that we have only had success observing it from Earth with HST – this being both the discovery and follow-up telescope for all of our measurements until New Horizons was in range to observe it with its panchromatic LOng Range Reconnaissance Imager (LORRI). 

In the fall of 2015 the team observed one object, 15810 Arawn (1994 JR1) simultaneous with New Horizons and HST. In the summer of 2016 we observed seven more objects, inclusive of Ultima Thule (HST GO-14092, PI Benecchi; Figure 1). We collected data in two different filters, F606W and F814W (effectively a green and red filter) using the high resolution Wide Field Camera 3 (WFC3) with the goal of determining the colors of the objects and to see if they were consistent with the apparent dynamical classifications of the objects. 

“Effectively we were looking to determine if high resolution measurements, as well as phase angle measurements which can tell us about surface roughness (unobtainable from Earth-based locations), with New Horizons could be extrapolated to characterize those dynamical groupings of objects as a whole,” Benecchi said. “We were also looking to determine if any of the objects were resolved binaries or objects with unusually large and rapid rotations. The key feature about these objects other than the fact that they are being observed by the New Horizons spacecraft is that they are some of the smallest and faintest objects ever observed from the Earth.” 

Figure 2 shows the resultant colors of our objects in comparison with other objects that have been observed with HST using the same instrument and filter configuration. Our main conclusions of this work are: (1) that Ultima Thule as well as all the other KBOs we measured are red (not as red as Mars, but a bit redder than Pluto). The objects we will be observing with New Horizons in the Cold Classical dynamical population appear to be representative of the larger population, in particular Ultima Thule is representative. This allows us to use the high resolution “in-situ” images of Ultima Thule to hypothesize what other objects in this part of the Kuiper belt are like. (2) We found that none of our KBOs are be binary within the resolution of HST. And (3) A few of our objects show large brightness changes, but our observations are not of sufficient signal-to-noise or time duration for further interpretation. 

The beauty of this story is that when New Horizons flew past Ultima Thule on 1 January 2019 the MVIC spectroscopic results revealed that our HST measurements are identical within the error bars of the disk integrated color. This gives us great confidence in our HST results for our larger dataset and will allow us to more completely interpret remote observations of these objects.

fig 2 

Figure 2: Color results for all long-range New Horizons targets with HST colors compared with the colors of all other KBOs that have been measured in the same HST filters with respect to their direct F606W filter magnitudes. The top axis gives an estimate of the object sizes after assuming some basic material properties about the object surfaces. Objects plotted as red diamonds are in the same orbital space around the Sun as Ultima Thule. The New Horizons objects observed in this program are plotted as black triangles and 2014 MU69 is a solid black star. These objects are fainter and comparatively redder than many KBOs.

March 3, 2019
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