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.
Use of Langmuir probes in the atmospheres of planets is complicated by oxidation of the probe surface when high‐density oxygen atoms/molecules and/or ions are present. Oxidation of most materials creates an electrically resistive layer on the probe surface that reduces the current collected at a given bias voltage, changing the probe's current‐voltage (I‐V) curves and consequently the measured plasma parameters. TiN (Titanium Nitride), DAG (a graphite coating), or Gold are currently used Langmuir probe coatings, yet they all have issues when exposed to oxygen‐rich environments. Iridium and Rhenium are selected as new coating candidates because of the high conductivity of their oxidized forms and high hardness. Here we present the oxidation effect on the measurements of probes made of current coating materials (DAG, Gold, and TiN) and new coating materials (Iridium and Rhenium) against controls (Copper and Nickel) in the laboratory. The oxidation process is performed by bombarding oxygen ions on the probe surface with energies of 1.5–10 eV. The probe's I‐V curves taken in an argon plasma are compared before and after oxidation. Our results show that the TiN, Gold, and DAG probes show significant to small changes in their I‐V curves, while Iridium outperforms all the other testing materials with almost unchanged I‐V curves after the oxidation process. Additionally, this new coating can be applied for other plasma instruments in which surface oxidation may pose an issue. However, for the application of Iridium to electric field probes, future work must be carried out to determine photoemission characteristics.
Surface oxidation of Langmuir probes is an important issue for probe measurements in space environments. Followed by our previous work about the oxidation effect on the collection of ambient plasma electrons and ions, here we present its effect on photoemission from the probe surface of various materials. Photoemission is either a contamination for traditional Langmuir probes or a necessity for electric field probes in low-density plasma. Our results show that all materials after oxidation have a varying degree of reduction in photoemission. The photoemission of copper, gold, and niobium drops most significantly followed by DAG213 (a resin-based graphite coating), TiN (titanium nitride), and rhenium. Iridium, DAG213, and AquaDAG (graphite coating) have the largest photoemission after oxidation, making them appropriate coating candidates for electric field probes. Both DAG materials show a large photoemission enhancement after the oxidation products are cleaned off the surface. A long exposure test shows that the photoemission from iridium slowly degrades. Due to the high surface conductivity of oxidized iridium shown in previous work, it is suggested that iridium can be oxidized before flight to minimize the photoemission when being used as a coating for Langmuir probes. Overall, iridium is found to be a coating material appropriate for both electric field probes and Langmuir probes.
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