208996 Achlys

Orbit

Achlys orbits the Sun at a semi-major axis or average distance of 39.5 astronomical units (AU), which places it beyond the orbit of Neptune (30.1 AU) and in the inner reaches of the Kuiper belt, where many other small objects reside. The location of Achlys's orbit makes it a trans-Neptunian object (TNO) and a Kuiper belt object. Achlys completes one orbit about every 247 years, which puts it in a 2:3 mean-motion orbital resonance with Neptune. That is, Achlys completes two orbits around the Sun for every three orbits completed by Neptune. Many other Kuiper belt objects, including the dwarf planet Pluto, share Achlys's 2:3 resonance with Neptune—these objects are classified as plutinos. Achlys is the third largest plutino, after Pluto and Orcus.

The orbit of Achlys is elliptical and inclined with respect to the ecliptic by 13.6°. Unlike some plutinos including Pluto, Achlys does not cross Neptune's orbit: its elliptical orbit brings it as close as 32.4 AU from the Sun (at perihelion) to as far as 46.5 AU from the Sun (at aphelion). Achlys previously passed aphelion in June 1982 and will come to perihelion in March 2107. Even though Achlys's orbit is perturbed by the gravitational influence of other planets, the perihelion of Achlys's orbit is not expected to drop below 31.6 AU in the next 10 million years, according to N-body simulations by the Deep Ecliptic Survey. Achlys's orbit is stable over several billion years and is unlikely to change significantly over the remaining life of the Solar System.

Physical characteristics

Size, shape, density

Observations of stellar occultations from 2012 and 2014 show that Achlys is an elongated object. Under the assumption that Achlys is in hydrostatic equilibrium (that is, its shape is controlled by its own gravity and rotation), its shape can be approximated by a rotationally distorted Jacobi ellipsoid with dimensions of 940 km × 766 km × 490 km (584 mi × 476 mi × 304 mi). With these dimensions, the equatorial diameter of Achlys is roughly twice as long as its polar diameter. This rotationally distorted shape has been seen in other large Kuiper belt objects, like Haumea and Varuna. The approximated dimensions of Achlys translate to an area-equivalent diameter of 772 km (480 mi), which is large enough that astronomers consider Achlys likely in hydrostatic equilibrium and therefore a possible dwarf planet. Achlys is among the top 30 largest known TNOs.

The mass and density of Achlys have not been measured, though the assumption of hydrostatic equilibrium with its ellipsoidal shape predicts that it should have a density in the range of 0.85–1.12 g/cm³, with the most likely value being 0.87 g/cm³. This density indicates Achlys has a mass of around 2×10²⁰ kg. Achlys falls within the 400–1,000 km (250–620 mi) diameter range where TNOs are typically observed with densities lower than that of water ice (1 g/cm³); these objects are theorized to have porous interior structures due to a lack of internal melting, differentiation, and gravitational compression. Achlys's small size also makes it unlikely to hold much internal heat or possess cryovolcanism.

Rotation

Achlys has a rotation period of about 6.79 hours. The apparent brightness of Achlys periodically fluctuates with a small peak-to-peak amplitude of 0.07 magnitudes as it rotates, which could be monitored by telescopes on Earth. The rotation period of Achlys was first measured by Scott Sheppard and David C. Jewitt in 2003, who found a rotation period of either 6.7 and 13.4 hours. The rotation period of Achlys remained ambiguous until 2017, when it was found to have an elongated shape that was best explained by the former period.

Achlys has a highly tilted rotation axis that is pointed somewhat toward Earth, according to observations of Achlys's small rotational brightness variation, constant thermal emission, and variable projected shape in stellar occultations. A 2017 analysis of stellar occultation observations from 2012 and 2014 suggested that Achlys's rotational pole could be oriented at an opening angle of roughly 50° with respect to the plane of the sky and at a position angle of 78° eastward from the celestial north pole.

Surface

Achlys is a dark, icy object with a likely ancient surface devoid of geological activity. Its surface is dominated by water ice and has a low, average geometric albedo of about 10%. Astronomical spectroscopy has shown that both amorphous and crystalline forms of water ice are present on Achlys's surface, which make it highly absorbent in near-infrared wavelengths of light. Crystalline water ice is not commonly found on the surfaces of TNOs due to their cold temperatures, so its presence on Achlys suggests that the object had experienced heating, possibly by an impact event.

Spectroscopic analyses from 2010 and 2011 suggest that Achlys's surface may be darkened by amorphous carbon, which is thought to originate from heavily irradiated organic compounds. These analyses also suggest that water ice and amorphous carbon each comprise major portions of Achlys's surface, with amorphous water ice likely more abundant than crystalline water ice. The surface of Achlys is devoid of volatile ices like nitrogen and carbon monoxide, in contrast to the largest trans-Neptunian objects like Pluto. This is expected because Achlys's gravity is too weak to hold on any atmosphere, especially volatiles after they have sublimated into vapor.

In visible light, Achlys's surface is spectrally neutral with respect to the Sun, which gives it a gray color. Achlys shares its gray color and water ice-rich surface with several TNOs including the large plutino Orcus; these objects are classified as BB ("blue")-type TNOs in terms of visible color and "prominent water" (H
₂O)-type TNOs in terms of spectra. The gray color of Achlys implies that it contains little amounts of tholins—moderately irradiated organic compounds that would otherwise tint the surface red. Spectroscopic analyses suggest that tholins should make up roughly 10% of Achlys's surface composition.

The surface of Achlys is not uniform. The albedo is suspected to vary across Achlys's surface because the object varies in brightness as it rotates. Near-infrared spectroscopy of Achlys shows variable spectral absorption of water ice and tentative signs of methanol ice. Visible-light spectroscopy has detected sporadic hints of hydrated minerals such as silicates, which are suspected to exist in localized deposits on Achlys's surface. The color of Achlys has been observed to change as it rotates, with its visible spectral slope or redness varying between 3.5%/0.1 μm and 8.5%/0.1 μm. This color variation may hint at localized areas of concentrated tholins on Achlys's surface, which could be explained by various phenomena such as an impact crater left by a red TNO.

In November 2014, an observation of a stellar occultation from Yunnan, China revealed that Achlys has a topographic feature located at its limb—the edge of its projected shape. The observation showed a gradual dimming of the occulted star, which has been interpreted as a partial, grazing occultation by Achlys's topographic feature. Achlys is the first TNO whose topographic feature has been observed via stellar occultation. The topographic feature on Achlys could either be a chasm at least 7.7 km (4.8 mi) deep and roughly 22.6 km (14.0 mi) wide, or a shallow-sloped depression roughly 13.4 km (8.3 mi) deep and at least 80 km (50 mi) wide. Such features have been seen on Pluto and its moon Charon.

Satellite

Achlys has one known natural satellite or moon, which has no official name or designation. It was discovered by Michael E. Brown and Terry-Ann Suer in images taken by the Hubble Space Telescope on 2 December 2005. The discovery of Achlys's moon was announced on 22 February 2007 via an International Astronomical Union Circular published by the Central Bureau for Astronomical Telegrams.

The moon of Achlys has not been seen since its discovery. The lack of redetections means that the orbit of Achlys's moon could not be determined, which prevents an accurate determination of Achlys's mass. Nevertheless, some properties of the moon's orbit have been predicted based on its observed separation distance from Achlys at discovery.

In the discovery images, the moon was seen at an angular separation of 0.22±0.01 arcseconds from Achlys, which translates to an apparent distance of at least 7,200 ± 300 km (4,470 ± 190 mi). If this separation distance is the moon's semi-major axis from Achlys, then it would have an orbital period of approximately 12 days. The moon is predicted to have a non-circular orbit due to weak tidal circularization by Achlys. The moon is 5.0±0.3 magnitudes fainter than Achlys, which translates to a diameter of roughly 80 km (50 mi) if it has the same albedo as Achlys. If the moon has an icy composition, its mass would be a few times 10¹⁷ kg. Compared to other TNOs with moons, the moon of Achlys is quite small; this suggests that Achlys's moon formed from a collision with another body.

A 2026 study led by José Gómez-Limón found that Achlys emits more thermal radiation than expected for its size, which could be explained if there is additional thermal radiation coming from its moon. However, to explain Achlys's excess thermal emission, its moon would have to be much larger with a diameter of 236+64
−52 km (147+40
−32 mi). If this large diameter estimate is accurate, then it would imply the moon has significantly different surface properties (such as albedo) from Achlys. On the other hand, if Achlys's moon is much smaller, then the excess thermal emission can be explained if Achlys has either a larger diameter or a second moon.

Proposed exploration

Achlys has not been visited by a space probe, though various studies have found it to be a feasible target for future missions. Planetary scientists advocate for the exploration of large TNOs like Achlys because they can provide insights into the formation and evolution of the Solar System.

A 2019 study by Amanda Zangari and collaborators identified several possible flyby trajectories to Achlys, using different excess launch energies and gravity assists from planets. A spacecraft launched in 2035–2038 could use a single Jupiter gravity assist to reach Achlys in 8.2–11.6 years. Alternatively, a spacecraft launched in 2025–2034 could use a single Saturn gravity assist to reach Achlys in 9.2–18.2 years. A 2037–2038 launch trajectory using gravity assists from both Jupiter and Saturn would be less efficient however, as it would demand higher launch energies with long mission durations of 23.8–23.9 years.

Flyby missions to Achlys using gravity assists from Uranus are also possible. A 2021 study by Bryan Holler and colleagues for the 2023–2032 Planetary Science Decadal Survey identified Achlys as a potential target for a flyby mission to Uranus, which could launch in the 2030s, reach Uranus in 2045, and then reach Achlys in 2049. Zangari and collaborators found that a spacecraft launched in 2034–2038 could reach Achlys in 13.1–13.2 years via gravity assists from both Jupiter and Uranus, whereas a spacecraft launched in 2027–2033 could reach Achlys in 17.7 years via gravity assists from both Jupiter and Uranus.

A 2013 study by the University of Tennessee investigated the possibility of sending a small (100–380 kg) orbiter spacecraft to a TNO and found that Achlys could be a feasible option. An orbiter launched in July 2035 or 2047 could make use of a Jupiter gravity assist to reach Achlys in 10–25 years depending on the excess launch energy, though lower launch energies and longer mission durations are required to make it possible for orbit insertion around Achlys.