 Achlys andits moon (upper right) imaged by theHubble Space Telescope in December 2005 | |
| Discovery[1] | |
|---|---|
| Discovered by | |
| Discovery site | Palomar Obs. |
| Discovery date | 13 January 2003 |
| Designations | |
| (208996) Achlys | |
| Pronunciation | /ˈækləs/[2] |
Named after | Achlys (Ἀχλύς)[3] |
| 2003 AZ84 | |
| TNO[4] · plutino[5] · distant[1] | |
| Adjectives | Achlyan |
| Orbital characteristics (barycentric)[6][a] | |
| Epoch 21 November 2025 (JD 2461000.5) | |
| Uncertainty parameter 1[4] | |
| Observation arc | 29+yr[4] |
| Earliestprecovery date | 19 March 1996 |
| Aphelion | 46.536AU |
| Perihelion | 32.403 AU |
| 39.470 AU[a] | |
| Eccentricity | 0.1790 |
| 247.81yr (90,511 d) | |
| 242.166° | |
| 0° 0m 14.319s / day | |
| Inclination | 13.565° |
| 252.029° | |
| ≈ 27 March 2107[7] ±0.14 days[4] | |
| 15.117° | |
| Knownsatellites | 1 |
| Physical characteristics | |
| Dimensions | (940±40) × (766±20) × (490±16) km[8][b] |
| 772±12 km (area equivalent)[8][b] | |
| Mass | ≈2×1020 kg[c] |
Meandensity | 0.87±0.01 g/cm3[8][b] |
| 6.7874±0.0002h[9] | |
| 0.097±0.009[8][b] | |
| 20.3[13] | |
| 3.760±0.058 (V band)[9] | |
208996 Achlys (provisional designation2003 AZ84) is a largetrans-Neptunian object orbiting the Sun in theKuiper belt, a region of icy bodies beyondNeptune. It was discovered on 13 January 2003 byChad Trujillo andMichael E. Brown atPalomar Observatory. Achlys has an elongated shape that is believed to be distorted by its rapid 6.8-hour rotation. Its diameter is estimated to be roughly 940 km (580 mi) across its equator to 490 km (300 mi) across its poles. AfterPluto andOrcus, Achlys is the third largest member of theplutinos—a population of Kuiper belt objects following a 2:3orbital resonance with Neptune, in which they complete two orbits for every three orbits completed by Neptune.
The surface of Achlys is dark gray and mostly composed of waterice. Observations ofstellar occultations show that Achlys's surface bears achasm ordepression between 8 and 13 km (5.0 and 8.1 mi) deep, similar to those seen on Pluto and its moonCharon. Achlys has one knownmoon that is about 80 km (50 mi) in diameter. The mass and density of Achlys has not been measured, though predictions based on its shape and rotation suggest that it has a density lower than that of water ice.[8]
Achlys was discovered on 13 January 2003 by astronomersChad Trujillo andMichael Brown[d] atPalomar Observatory inSan Diego County, California.[1] At the time, Trujillo and Brown were searching the sky for bright and largetrans-Neptunian objects—Solar System objects beyondNeptune—using the Palomar Observatory's 1.22-meter (48 in)Samuel Oschin telescope, as part of their "Caltech Wide Area Sky Survey" project.[14]: 100, 103 Theirsky survey had been operating jointly with Palomar'sNear Earth Asteroid Tracking (NEAT) program since 2001 and was responsible for the discovery of several other large trans-Neptunian objects includingQuaoar.[14]: 99–100
Achlys was found through manual vetting of potential moving objects identified by Trujillo and Brown's automated image-searching software.[14]: 101 It was detected at ared-filterapparent magnitude of 20.2.[15] The discovery of Achlys was announced byMinor Planet Center (MPC) on 26 January 2003.[15] Since then, numerousprecovery observations of Achlys were identified from Palomar Observatory, with observations from 2001–2002 reported in April 2003[16] and a single observation from 19 March 1996 reported in December 2007.[17] This 1996 observation is the earliest known precovery of Achlys, predating its discovery by over six years.[1]
This object is named afterAchlys, the goddess of sorrow and grief in theancient Greekepic poemShield of Heracles. InHomer'sIliad, "achlys" also refers to the mist that covers the eyes of the dying.[18]: 16 This name follows the theme of mythological figures related to theunderworld, which is required forplutinos by theInternational Astronomical Union (IAU).[19]: 8 The object was officially named Achlys on 30 June 2025 by the IAU'sWorking Group for Small Body Nomenclature.[18]: 24
Before Achlys was named, it was known by itsprovisional designation2003 AZ84, which was given by the MPC in its discovery announcement.[15] This provisional designation encodes the year and half-month of Achlys's discovery date.[20] Achlys'sminor planet catalog number of 208996 was given by the MPC on 11 March 2009.[21]
While moving across the sky, Achlys occasionally passes in front of a background star and briefly blocks out its light from Earth, resulting in astellar occultation. When observed at different locations, stellar occultations by Achlys can reveal fine details that are unresolvable to telescope imaging, such as its size, shape, and potential surrounding features such asrings andmoons.[22]: 2 [8]: 2 The first successful detection of a stellar occultation by Achlys was reported by a single observer from Chile on 8 January 2011.[23][8]: 2 Later observations of stellar occultations on 3 February 2012 and 15 November 2014 were successfully detected by multiple observers at different locations, which revealed the elongated shape andtopography of Achlys for the first time.[8]: 2 No rings and moons were detected around Achlys in these observations.[8]: 8
Achlys orbits theSun at asemi-major axis or average distance of 39.5 astronomical units (AU),[a] which places it beyond the orbit ofNeptune (30.1 AU) and in the inner reaches of theKuiper belt, where many other small objects reside.[25] The location of Achlys's orbit makes it atrans-Neptunian object (TNO) and a Kuiper belt object.[4][26] Achlys completes one orbit about every 247 years, which puts it in a 2:3mean-motion orbital resonance withNeptune.[5][8]: 2 That is, Achlys completes two orbits around the Sun for every three orbits completed by Neptune.[27] Many other Kuiper belt objects, including thedwarf planetPluto, share Achlys's 2:3 resonance with Neptune—these objects are classified asplutinos.[27] Achlys is the third largest plutino, after Pluto and Orcus.[28]: 2, 7
The orbit of Achlys iselliptical andinclined with respect to theecliptic by 13.6°.[a] Unlike some plutinos including Pluto,[e] Achlys does not cross Neptune's orbit: its elliptical orbit brings it as close as32.4 AU from the Sun (atperihelion) to as far as46.5 AU from the Sun (ataphelion).[a] Achlys previously passed aphelion in June 1982[30] and will come to perihelion in March 2107.[7] Even though Achlys's orbit isperturbed by the gravitational influence of other planets,[31] the perihelion of Achlys's orbit is not expected to drop below31.6 AU in the next 10 million years, according toN-body simulations by theDeep Ecliptic Survey.[5] Achlys's orbit is stable over several billion years and is unlikely to change significantly over theremaining life of the Solar System.[31]: 7, 11
Observations of stellar occultations from 2012 and 2014 show that Achlys is an elongated object.[8] Under the assumption that Achlys is inhydrostatic equilibrium (that is, its shape is controlled by its own gravity and rotation), its shape can be approximated by a rotationally distortedJacobi ellipsoid with dimensions of 940 km × 766 km × 490 km (584 mi × 476 mi × 304 mi).[8]: 1 With these dimensions, the equatorial diameter of Achlys is roughly twice as long as its polar diameter.[32]: 3 This rotationally distorted shape has been seen in other large Kuiper belt objects, likeHaumea andVaruna.[8]: 13 The approximated dimensions of Achlys translate to anarea-equivalent diameter of 772 km (480 mi),[8]: 10 which is large enough that astronomers consider Achlys likely in hydrostatic equilibrium[f][9]: 10 and therefore apossible dwarf planet.[33]: 245 [34]: 7, 13 Achlys is among the top 30 largest known TNOs.[31]: 2
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 of0.85–1.12 g/cm3, with the most likely value being0.87 g/cm3.[8]: 9–10 This density indicates Achlys has a mass of around2×1020 kg.[c] 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 haveporous interior structures due to a lack of internal melting,differentiation, and gravitational compression.[35]: 1, 8 [36]: 5 Achlys's small size also makes it unlikely to hold much internal heat or possesscryovolcanism.[37]: 10
Achlys has arotation period of about 6.79 hours.[9]: 10 The apparent brightness of Achlys periodically fluctuates with a small peak-to-peakamplitude of 0.07magnitudes as it rotates, which could be monitored by telescopes on Earth.[9]: 10 The rotation period of Achlys was first measured byScott Sheppard andDavid C. Jewitt in 2003, who found a rotation period of either 6.7 and 13.4 hours.[38]: 217 [39]: 7 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.[8]: 9
Achlys has a highlytilted rotation axis that is pointed somewhat toward Earth, according to observations of Achlys's small rotational brightness variation, constant thermal emission, and variableprojected shape in stellar occultations.[8]: 9–10 [9]: 11 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 roughly50° with respect to the plane of the sky and at aposition angle of78° eastward from thecelestial north pole.[8]: 9–10
Achlys is a dark, icy object with a likely ancient surface devoid of geological activity.[41]: 1507 Its surface is dominated by waterice and has a low, averagegeometric albedo of about 10%.[9]: 10 Astronomical spectroscopy has shown that bothamorphous and crystalline forms of water ice are present on Achlys's surface, which make it highlyabsorbent innear-infraredwavelengths of light.[42][40][37]: 9 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 animpact event.[43]: 946, 953
Spectroscopic analyses from 2010 and 2011 suggest that Achlys's surface may be darkened byamorphous carbon, which is thought to originate from heavilyirradiatedorganic compounds.[43]: 953 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.[42]: 301 [43]: 949 The surface of Achlys is devoid ofvolatile ices likenitrogen andcarbon monoxide, in contrast to the largest trans-Neptunian objects like Pluto.[37]: 9 This is expected because Achlys's gravity is too weak to hold on anyatmosphere, especially volatiles after they havesublimated intovapor.[41]: 1502, 1507
Invisible light, Achlys's surface is spectrally neutral with respect to the Sun,[g] which gives it a gray color.[44][11]: 181, 188 Achlys shares its gray color and water ice-rich surface with several TNOs including the large plutino Orcus;[45] these objects are classified asBB ("blue")-type TNOs in terms of visible color[11] and"prominent water" (H
2O)-type TNOs in terms of spectra.[10]: 6 The gray color of Achlys implies that it contains little amounts oftholins—moderately irradiated organic compounds that would otherwise tint the surface red.[43]: 949, 953 Spectroscopic analyses suggest that tholins should make up roughly 10% of Achlys's surface composition.[42]: 301 [43]: 949
The surface of Achlys is not uniform.[37]: 9 The albedo is suspected to vary across Achlys's surface because the object varies in brightness as it rotates.[8]: 10 Near-infrared spectroscopy of Achlys shows variablespectral absorption of water ice and tentative signs ofmethanol ice.[37]: 9 Visible-light spectroscopy has detected sporadic hints ofhydrated minerals such assilicates, which are suspected to exist in localized deposits on Achlys's surface.[46]: 464 [37]: 9 The color of Achlys has been observed to change as it rotates, with its visiblespectral slope or redness varying between 3.5%/0.1 μm and 8.5%/0.1 μm.[37]: 9 This color variation may hint at localized areas of concentrated tholins on Achlys's surface, which could be explained by various phenomena such as animpact crater left by a red TNO.[37]: 9
In November 2014, an observation of a stellar occultation fromYunnan, China revealed that Achlys has atopographic feature located at itslimb—the edge of its projected shape.[8]: 1–2 The observation showed a gradual dimming of the occulted star, which has been interpreted as a partial, grazing occultation by Achlys's topographic feature.[8]: 10 [32]: 24 Achlys is the first TNO whose topographic feature has been observed via stellar occultation.[47][8]: 2, 13 The topographic feature on Achlys could either be achasm at least 7.7 km (4.8 mi) deep and roughly 22.6 km (14.0 mi) wide, or a shallow-slopeddepression roughly 13.4 km (8.3 mi) deep and at least 80 km (50 mi) wide.[8]: 12–13 Such features have been seen on Pluto and its moonCharon.[48][8]: 12
| Discovery | |
|---|---|
| Discovered by | Michael E. Brown Terry-Ann Suer[32]: 24 |
| Discovery date | 2 December 2005[49] |
| Orbital characteristics[50] | |
| 7200±300 km[50] or ~10000 km[8]: 2 | |
| ~12 days[h] | |
| Satellite of | 208996 Achlys |
| Physical characteristics | |
| 72±12 km (calculated)[50] or ~80 km (same albedo as Achlys)[8]: 2 | |
| Mass | few 1017 kg[i] |
| 25.1[51] | |
| 8.76[j] | |
Achlys has one knownnatural satellite or moon, which has no official name or designation.[4][50] It was discovered by Michael E. Brown and Terry-Ann Suer in images taken by theHubble Space Telescope on 2 December 2005.[32]: 24 The discovery of Achlys's moon was announced on 22 February 2007 via anInternational Astronomical Union Circular published by theCentral Bureau for Astronomical Telegrams.[50][49]
The moon of Achlys has not been seen since its discovery.[32]: 24 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.[8]: 2 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 anangular separation of0.22±0.01arcseconds from Achlys,[49] which translates to an apparent distance of at least 7,200 ± 300 km (4,470 ± 190 mi).[50][k] If this separation distance is the moon'ssemi-major axis from Achlys, then it would have anorbital period of approximately 12 days.[50] The moon is predicted to have a non-circular orbit due to weaktidal circularization by Achlys.[52]: 10 The moon is5.0±0.3magnitudes fainter than Achlys, which translates to a diameter of roughly 80 km (50 mi) if it has the same albedo as Achlys.[50][8]: 2 If the moon has an icy composition, its mass would be a few times 1017 kg.[8]: 9 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.[52]: 12
Achlys has not been visited by aspace probe, though various studies have found it to be a feasible target for future missions.[53][54]: 1506 Planetary scientists advocate for the exploration of large TNOs like Achlys because they can provide insights into theformation and evolution of the Solar System.[55][56]: 5
A 2019 study by Amanda Zangari and collaborators identified several possibleflyby trajectories to Achlys, using differentexcess launch energies andgravity 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.[53]: 922 Alternatively, a spacecraft launched in 2025–2034 could use a single Saturn gravity assist to reach Achlys in 9.2–18.2 years.[53]: 923 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.[53]: 923
Flyby missions to Achlys using gravity assists fromUranus are also possible. A 2021 study by Bryan Holler and colleagues for the2023–2032 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.[56]: 5 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.[53]: 926, 928
A 2024 study by theUniversity 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.[54]: 1506 A spacecraft 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 the spacecraft to enterorbit insertion around Achlys.[54]: 1500
| Consensus | _(cropped).jpg)  | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Candidate (for TNOs, D+1σ ≥ 700 km or H ≤ 4.0) |
| ||||||||||||
| Planetary satellites of |    | |
|---|---|---|
| Dwarf planet satellites of | ||
| Minor-planet moons | ||
| Ranked by size | ||
