6.1 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| 486958 Arrokoth | 4/8 | https://en.wikipedia.org/wiki/486958_Arrokoth | reference | science, encyclopedia | 2026-05-05T13:09:12.451441+00:00 | kb-cron |
=== Internal structure === Topography variations at the limb of Arrokoth suggest that its interior is likely composed of mechanically strong material consisting of mostly amorphous water ice and rocky material. Trace amounts of methane and other volatile gases in the form of vapors may also be present in Arrokoth's interior, trapped in water ice. Under the assumption that Arrokoth has a low comet-like density of around 0.5 g/cm3, its internal structure is expected to be porous, as volatile gases trapped in Arrokoth's interior are thought to escape from the interior to the surface. Assuming that Arrokoth may have an internal heat source caused by the radioactive decay of radionuclides, the trapped volatile gases inside Arrokoth would migrate outward and escape from the surface, similarly to the scenario of outgassing of comets. The escaped gases may subsequently freeze and deposit on Arrokoth's surface, and could possibly account for the presence of ices and tholins on its surface.
== Orbit and classification ==
Arrokoth orbits the Sun at an average distance of 44.2 astronomical units (6.61×10^9 km; 4.11×10^9 mi), taking 294 years to complete a full orbit around the Sun. Having a low orbital eccentricity of 0.04, Arrokoth follows a nearly circular orbit around the Sun, only slightly varying in distance from 42.6 AU at perihelion to 45.9 AU at aphelion. Because Arrokoth has a low orbital eccentricity, it does not approach close enough to Neptune for its orbit to become perturbed. (Arrokoth's minimum orbital intersection distance from Neptune is 12.7 AU.) Arrokoth's orbit appears to be stable over long time scales; simulations by the Deep Ecliptic Survey show that its orbit will not significantly change over the next 10 million years. At the time of the New Horizons flyby in January 2019, Arrokoth's distance from the Sun was 43.28 AU (6.47×10^9 km; 4.02×10^9 mi). At this distance, light from the Sun takes over six hours to reach Arrokoth. Arrokoth is generally classified as a distant minor planet or trans-Neptunian object by the Minor Planet Center as it orbits in the outer Solar System beyond Neptune. Having a non-resonant orbit within the Kuiper belt region 39.5–48 AU from the Sun, Arrokoth is classified as a classical Kuiper belt object, or cubewano. Arrokoth's orbit is inclined to the ecliptic plane by 2.45 degrees, relatively low compared to other classical Kuiper belt objects such as Makemake. Since Arrokoth has a low orbital inclination and eccentricity, it is part of the dynamically cold population of classical Kuiper belt objects, which are unlikely to have undergone significant perturbations by Neptune during its outward migration in the past. The cold classical population of Kuiper belt objects are thought to be remnant planetesimals left over from the accretion of material during the formation of the Solar System.
== Rotation and temperature ==
Results from photometric Hubble Space Telescope observations show that the brightness of Arrokoth varies by around 0.3 magnitudes as it rotates. Though the rotation period and light curve amplitude of Arrokoth could not be determined from Hubble observations, the subtle brightness variations suggested that Arrokoth's rotational axis is either pointed toward the Earth or is being viewed at an equator-on configuration with a nearly spherical shape, with a constrained a/b best-fit aspect ratio around 1.0–1.15. Upon the New Horizons spacecraft's approach to Arrokoth, no rotational light curve amplitude was detected by the spacecraft despite Arrokoth's irregular shape. To explain the lack of its rotational light curve, scientists surmised that Arrokoth is rotating on its side, with its rotational axis pointing nearly directly at the approaching New Horizons spacecraft. Subsequent images of Arrokoth from New Horizons upon approach confirmed that its rotation is tilted, with its south pole facing towards the Sun. The rotational axis of Arrokoth is tilted 99 degrees to its orbit. Based on occultation and New Horizons imaging data, Arrokoth's rotation period is determined to be 15.938 hours. Due to the high axial tilt of its rotation, the solar irradiance of the northern and southern hemispheres of Arrokoth varies greatly over the course of its orbit around the Sun. As it orbits around the Sun, one polar region of Arrokoth faces the Sun continuously while the other faces away. The solar irradiance of Arrokoth varies by 17 percent due to the low eccentricity of its orbit. The average temperature of Arrokoth is estimated to be around 42 K (−231.2 °C; −384.1 °F), with a maximum of around 60 K on the illuminated subsolar point of Arrokoth. Radiometric measurements from the New Horizons REX instrument indicate that the mean surface temperature of Arrokoth's unilluminated face is about 29±5 K, higher than the modeled range of 12–14 K. The higher temperature of Arrokoth's unilluminated face as measured by REX implies that thermal radiation is emitted from Arrokoth's subsurface, which was predicted to be intrinsically warmer than the exterior surface.
== Mass and density == The mass and density of Arrokoth are unknown. A definitive mass and density estimate cannot be given as the lobes are in contact rather than orbiting each other. Although a possible natural satellite orbiting Arrokoth could help determine its mass, no such satellites were found. Under the assumption that both lobes are bound by self-gravity, with the mutual gravity of the two overcoming centrifugal forces that would otherwise separate them, Arrokoth is estimated to have a very low density similar to that of comets, with an estimated minimum density of 0.29 g/cm3. In order to maintain the shape of the neck, the density of Arrokoth must be less than the maximum possible density of 1 g/cm3, otherwise the neck would be excessively compressed by the mutual gravity of the lobes such that the entire object would gravitationally collapse into a spheroid.
== Formation ==