New Horizons Arrokoth flyby

Color composite image of Arrokoth compiled from data obtained by NASA's New Horizons spacecraft. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute//Roman Tkachenko 

The New Horizons spacecraft flew by the Kuiper belt object (KBO) (486958) Arrokoth on January 1, 2019. The closest approach, being 3,538 km, occurred at 05:33 UTC. At 43.4 au from the Sun, it was the farthest object in the Solar System ever to be visited by a spacecraft.

   Shortly after the Pluto flyby, in August 2015, the New Horizons team chose a Kuiper Belt Object 2014 MU69 (officially named "Arrokoth" in November 2019) as the next flyby target. In early November 2015 New Horizons fired its thrusters to alter the direction of its trajectory by 1/4 of a degree, to set a course toward a close flyby of Arrokoth.

   The approach to Arrokoth began in August 2018, when the spacecraft took its first pictures of its target, which appeared as a faint dot barely visible against a crowded field of background stars. Through fall 2018 the spacecraft continued to take regular images of Arrokoth as it got closer and brighter, using them to check that it was on the right course, and firing its thrusters to make course corrections as necessary. In early December, the spacecraft performed an intensive campaign of imaging to look for any dangerous rings or moons around Arrokoth — which it did not find.

   In the last few days of the approach, the navigation team analyzed the latest images of Arrokoth taken by New Horizons to refine estimates of the KBO's position relative to the spacecraft. The team uplinked the updated information to New Horizons, so that the spacecraft could more accurately time its observations and point its cameras.

   Intensive science observations began 24 hours before the flyby. The spacecraft took frequent grayscale, color, near-infrared and ultraviolet observations of Arrokoth as it rotated, to investigate its shape, composition, and any possible degassing, on all sides of the object. Long-exposure images of the space surrounding Arrokoth searched for rings or moons and determine their orbits. The closest approach observations, taken during the hour or so nearest closest approach, needed to account for the fact that Arrokoth's position was uncertain. Observations thus consisted of a series of long scans to obtain color and grayscale images, and infrared spectra, of all the possible places where the KBO might have been.

   After the closest approach, New Horizons pointed its ultraviolet instrument at the Sun to look for absorption of ultraviolet light by any gases being released by Arrokoth (though detection of outgassing was unlikely). It also made additional searches for rings around the KBO. Four hours after the flyby, the spacecraft turned briefly to Earth to report that the flyby was successful. A few hours after that it began downlinking the roughly seven gigabytes of data acquired during the flyby.

 

Artist's impression of the New Horizons spacecraft. Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

   Arrokoth is the first unquestionably primordial contact binary ever explored. Initial images hinted at a reddish, snowman-like shape, but further analysis of images taken near the closest approach revealed just how unusual the KBO’s shape really is. End to end, the overall shape of Arrokoth measures about 35 kilometers long. It’s about 20 kilometers wide, by 10 kilometers thick. The larger lobe was found to be "lenticular," which means it's flattened and shaped like two lenses placed back to back. It has dimensions of approximately 22 × 20 × 7 kilometers. The smaller lobe is more rounded and is approximately 14 × 14 × 10 kilometers in its dimensions.

   Because it is so well preserved, Arrokoth offered our clearest look back to the era of planetesimal accretion and the earliest stages of planetary formation. Apparently the two lobes once orbited each other, like many so-called binary worlds in the Kuiper Belt, until something brought them together in a "gentle" merger.

   In color and composition, New Horizons data revealed that Arrokoth resembles many other objects found in its region of the Kuiper Belt. Consistent with pre-flyby observations from the Hubble Telescope, Arrokoth is very red — redder even than Pluto, which New Horizons flew past on the inner edge of the Kuiper Belt in 2015 — and about the same color as many other so-called "cold classical" KBOs. ("cold" referring not to temperature but to the circular, uninclined orbits of these objects; "classical" in that their orbits have changed little since forming, and represent a sample of the primordial Kuiper Belt.)

 

New Horizons Arrokoth flyby trajectory

Animation of images of Arrokoth taken during the flyby. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/National Optical Astronomy Observatory

 

References:

Johns Hopkins Applied Physics Laboratory: New Horizons mission — Arrokoth
NASA Science: New Horizons

 

 

© 2025, Andrew Mirecki