Size and Shape Constraints of (486958) Arrokoth from Stellar Occultations

Abstract

We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by SOFIA with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 Aug 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 ± 0.0005 hours. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies.

The geology and geophysics of Kuiper Belt object (486958) Arrokoth

“…No periodic brightness variation due to rotation was detected in Hubble Space Telescope (HST) photometry before the flyby, with an upper limit amplitude of about 0.15 magnitudes (10). Stellar occultations in July 2017 and August 2018 showed that Arrokoth had an elongated, possibly contact-binary shape (11). The elongated shape and the low lightcurve amplitude implied that Arrokoth's rotational pole was roughly aligned with the direction of the Sun and Earth.…”

Orbits and Occultation Opportunities of 15 TNOs Observed by New Horizons

“…It is a bilobate body with a larger lobe measuring approximately 20 × 20 × 10 km and a smaller lobe of 15 × 14 × 10 km, connected by a very thin neck (Spencer et al 2020). Arrokothʼs contact-binary nature was first determined by a stellar occultation and then confirmed with resolved New Horizons images (Stern et al 2019;Buie et al 2020). Because the Kuiper Belt is a population of objects that have been mostly isolated since they first formed from the protoplanetary disk (Levison et al 2008;Nesvorný et al 2010), small TNO shapes must illustrate fundamental accretionary processes.…”

“…Gaia EDR3 (Lindegren et al 2020) provides star positional knowledge to a precision of a few milliarcseconds per occultation star, equating to a few kilometers of uncertainty in the ground track of an occultation. Arrokoth is the only small TNO that has been observed with a high-cadence occultation campaign, which revealed its contact-binary shape (Buie et al 2020). But this powerful technique can and should be extended to determine the nature and origin of other small TNOs.…”

“…To fit the orbits, we used the same orbital fitting and uncertainty process as was successfully used to estimate the orbit of Arrokoth prior to its flyby in -19 (Porter et al 2018Buie et al 2020). We used the astrometric PDFs described above to create a function that estimated the log probability of a given orbit state vector and then used that to populate an orbital PDF with emcee (Foreman-Mackey et al 2013).…”

The Sizes and Albedos of Centaurs 2014 YY49 and 2013 NL24 from Stellar Occultation Measurements by RECON

“…As a part of the Canadian extension to the RECON network, the (C-06) Anarchist Mountain Observatory team recorded data for the January event with a QHY174M-GPS. This is the same camera used to obtain the occultation measurement on Arrokoth (Buie et al 2020b). As opposed to the video data recorded by the standard RECON setup, the QHY system writes each single integration directly to a FITS frame, with GPS location and timing data written into the image header.…”