Arrokoth

Arrokoth, Pictured by New Horizons on 2019.

486958 Arrokoth (provisional designation 2014 MU69; formerly nicknamed Ultima Thule) is a trans-Neptunian object located in the Kuiper belt. Arrokoth is a contact binary 36 km (22 mi) long, composed of two planetesimals 21 and 15 km (13 and 9 mi) across, that are joined along their major axes. With an orbital period of about 298 years and a low orbital inclination and eccentricity, Arrokoth is classified as a cold classical Kuiper belt object.

## Discovery

Arrokoth was discovered on 26 June 2014 using the Hubble Space Telescope during a preliminary survey to find a suitable Kuiper belt object for the *New Horizons* spacecraft to fly by. Scientists of the *New Horizons* team were searching for an object in the Kuiper belt that the spacecraft could study after Pluto, and their next target had to be reachable on *New Horizons*'s remaining fuel. Using large ground-based telescopes on Earth, researchers began looking in 2011 for candidate objects and searched multiple times per year for several years. However, none of the objects found were reachable by the *New Horizons* spacecraft and most Kuiper belt objects that may be suitable were just too distant and faint to be seen through Earth's atmosphere. In order to find these fainter Kuiper belt objects, the *New Horizons* team initiated a search for suitable targets with the Hubble Space Telescope on 16 June 2014.

Arrokoth was first imaged by Hubble on 26 June 2014, 10 days after the *New Horizons* team began their search for potential targets. While digitally processing images from Hubble, Arrokoth was identified by astronomer Marc Buie, member of the *New Horizons* team. Buie reported his finding to the search team for subsequent analysis and confirmation. Arrokoth was the second object found during the search, after 2014 MT69. Three more candidate targets were later discovered with Hubble, though follow-up astrometric observations eventually ruled them out.

## Exploration

Arrokoth became the farthest and most primitive object in the Solar System visited by a spacecraft when the NASA space probe *New Horizons* conducted a flyby on 1 January 2019. Of the five potential targets found with Hubble, Arrokoth was deemed to be the most feasible target for the spacecraft as the flyby trajectory required the least amount of fuel compared to that for 2014 PN70, the second most feasible target for *New Horizons*. On 28 August 2015, Arrokoth was officially selected by NASA as a flyby target for the *New Horizons* spacecraft.

## Observations and Occultations

Arrokoth is too small and distant for its shape to be observed directly from Earth, but scientists were able to take advantage of an astronomical event called a stellar occultation, in which the object passes in front of a star from the vantage point of Earth. Since the occultation event is only visible from certain parts of the Earth, the *New Horizons* team combined data from Hubble and the European Space Agency's *Gaia* space observatory to figure out exactly when and where on Earth's surface Arrokoth would cast a shadow. They determined that occultations would occur on 3 June, 10 July, and 17 July in 2017, and set off for places around the world where they could see Arrokoth cover up a different star on each of these dates. Based on this string of three occultations, scientists were able to trace out the object's shape.

### 2017 Occultations

In June and July 2017, Arrokoth occulted three background stars.The team behind *New Horizons* formed a specialized "KBO Chasers" team led by Marc Buie to observe these stellar occultations from South America, Africa, and the Pacific Ocean.On 3 June 2017, two teams of NASA scientists tried to detect the shadow of Arrokoth from Argentina and South Africa. When they found that none of their telescopes had observed the object's shadow, it was initially speculated that Arrokoth might be neither as large nor as dark as previously expected, and that it might be highly reflective or even a swarm. Additional data taken with the Hubble Space Telescope in June and July 2017 revealed that the telescopes had been placed in the wrong location, and that these estimations were incorrect.

On 10 July 2017, the airborne telescope SOFIA was successfully placed close to the predicted centerline for the second occultation while flying over the Pacific Ocean from Christchurch, New Zealand. The main purpose of those observations was the search for hazardous material like rings or dust near Arrokoth that could threaten the *New Horizons* spacecraft during its flyby in 2019. Data collection was successful. A preliminary analysis suggested that the central shadow was missed; only in January 2018 was it realized that SOFIA had indeed observed a very brief dip from the central shadow. The data collected by SOFIA will also be valuable to put constraints on dust near Arrokoth. Detailed results of the search for hazardous material were presented on the 49th Meeting of the AAS Division for Planetary Sciences, on 20 October 2017.

On 17 July 2017, the Hubble Space Telescope was used to check for debris around Arrokoth, setting constraints on rings and debris within the Hill sphere of Arrokoth at distances of up to 75,000 km (47,000 mi) from the main body. For the third and final occultation, team members set up another ground-based "fence line" of 24 mobile telescopes along the predicted ground track of the occultation shadow in southern Argentina (Chubut and Santa Cruz provinces) to better constrain the size of Arrokoth. The average spacing between these telescopes was around 4 km (2.5 mi). Using the latest observations from Hubble, the position of Arrokoth was known with much better precision than for the 3 June occultation, and this time the shadow of Arrokoth was successfully observed by at least five of the mobile telescopes. Combined with the SOFIA observations, this put constraints on possible debris near Arrokoth.

Results from the occultation on 17 July showed that Arrokoth could have had a very oblong, irregular shape or be a close or contact binary. According to the duration of the observed chords, Arrokoth was shown to have two "lobes", with diameters of approximately 20 km (12 mi) and 18 km (11 mi), respectively. A preliminary analysis of all collected data suggested that Arrokoth was accompanied by an orbiting moonletabout 200–300 km (120–190 mi) away from the primary. It was later realized, however, that an error with the data processing software resulted in a shift in the apparent location of the target. After accounting for the bug, the short dip observed on 10 July was considered to be a detection of the primary body.

By combining data about its light curve, spectra (e.g. color), and stellar occultation data, illustrations could rely on known data to create a concept of what it might look like prior to spacecraft flyby.

### 2018 occultations

Path of Arrokoth's shadow on Earth during its 4 August 2018 occultation of an unnamed star in Sagittarius. This event was successfully observed from locations in Senegal and Colombia.
There were two potentially useful Arrokoth occultations predicted for 2018: one on 16 July and one on 4 August. Neither of these were as good as the three 2017 events. No attempts were made to observe the 16 July 2018 occultation, which took place over the South Atlantic and the Indian Ocean. For the 4 August 2018 event, two teams, consisting of about 50 researchers in total, went to locations in Senegal and Colombia. The event gathered media attention in Senegal, where it was used as an opportunity for science outreach. Despite some stations being affected by bad weather, the event was successfully observed, as reported by the *New Horizons* team. Initially, it was unclear whether a chord on the target had been recorded. On 6 September 2018, NASA confirmed that the star had indeed been seen to dip by at least one observer, providing important information about the size and shape of Arrokoth.

Hubble observations were carried out on 4 August 2018, to support the occultation campaign. Hubble could not be placed in the narrow path of the occultation, but due to the favourable location of Hubble at the time of the event, the space telescope was able to probe the region down to 1,600 km (990 mi) from Arrokoth. This is much closer than the 20,000 km (12,000 mi) region that could be observed during the 17 July 2017 occultation. No brightness changes of the target star have been seen by Hubble, ruling out any optically thick rings or debris down to 1,600 km (990 mi) from Arrokoth. Results of the 2017 and 2018 occultation campaigns were presented at the 50th meeting of the American Astronomical Society Division for Planetary Sciences on 26 October 2018.