15810 Arawn
A plutino observed closely by New Horizons in 2016.
15810 Arawn, originally designated 1994 JR1, is a Kuiper belt object located in the inner region of that distant zone, with a diameter of roughly 133 kilometers. It belongs to the plutinos, the most populous class of resonant TNOs, and it stands out because the New Horizons spacecraft captured images of it from a far closer range than is typical for such bodies. Discovered on 12 May 1994 by astronomers Michael Irwin and Anna Żytkow using the 2.5-meter Isaac Newton Telescope at the Roque de los Muchachos Observatory in Spain’s Canary Islands, the object was named after the mythological figure Arawn. Its orbit lies entirely beyond Neptune, with a semi-major axis of 39.4 AU, a perihelion of 34.7 AU, and an aphelion of 44.1 AU. It has an eccentricity of 0.12 and an inclination of 4° relative to the ecliptic, completing one orbit around the Sun every 247.5 years. Like Pluto, Arawn is locked in a 2:3 mean motion resonance with Neptune. Based on an absolute magnitude of 7.6 and an estimated albedo of 0.1, its diameter is about 133 kilometers. Hubble Space Telescope observations reveal a very red surface. In April 2016, its rotation period was determined to be 5.47 hours. In 2012, researchers proposed that Arawn might enter a quasi-satellite loop around Pluto every 2 million years, remaining in that state for nearly 350,000 years. Data from New Horizons in 2015 refined the calculations and confirmed the general dynamics, though whether Arawn qualifies as a true quasi-satellite of Pluto remains debated, since Neptune primarily governs its orbit, with only occasional minor perturbations from Pluto. Arawn’s orbit is highly stable, comparable to Pluto’s, suggesting it is a primordial plutino that formed around the same time as Pluto and Charon, rather than being recent collision debris or a captured object. As of 2017, Arawn was only 2.7 AU from Pluto. Before the discovery of 486958 Arrokoth in 2014, it was the best-known candidate for a New Horizons flyby after the Pluto encounter in 2015. New Horizons first imaged Arawn on 2 November 2015 using its LORRI instrument, at a distance 1/15 that of any previous observation of a Kuiper belt object outside the Pluto–Charon system.
Quick Facts
- Minorplanet
- yes
- Background
- #C2E0FF
- Discoverer
- M. J. Irwin / A. Żytkow
- Discovery Site
- La Palma Obs.
- Discovered
- 12 May 1994
- Mpc Name
- (15810) Arawn
- Pronounced
- ˈ · ɑ: · r · aʊ · n, with the PASTA vowel
- Named After
- Arawn (Welsh mythology)
- Mp Category
- TNOplutino
- Epoch
- 16 February 2017 (JD 2457800.5)
- Uncertainty
- 2
- Observation Arc
- 21.91 yr (8,002 days)
Facts from the source article.
Lore & Background
Arawn was discovered on 12 May 1994 by astronomers Michael Irwin and Anna Żytkow using the 2.5-metre Isaac Newton Telescope at Roque de los Muchachos Observatory in the Canary Islands, Spain. It was named after the mythological figure Arawn. It is a plutino, trapped in a 2:3 mean motion resonance with Neptune, similarly to dwarf planet Pluto. Its orbit has a semi-major axis of 39.4 AU, an eccentricity of 0.12, and an inclination of 4° with respect to the ecliptic. It measures approximately 133 km in diameter, based on an absolute magnitude of 7.6 and an estimated albedo of 0.1. Observations by the Hubble Space Telescope show that Arawn has a very red surface. In April 2016, its rotation period of 5.47 hours was determined. In 2012, Arawn was hypothesized to be in a quasi-satellite loop around Pluto, becoming a Plutonian quasi-satellite every 2 Myr and remaining in that phase for nearly 350,000 years. Measurements by New Horizons in 2015 increased the accuracy of calculations, confirming the general dynamics, but whether Arawn should be classified as a quasi-satellite of Pluto remains debated because its orbit is primarily controlled by Neptune with only occasional smaller perturbations caused by Pluto. Arawn is moving in a very stable orbit, likely as stable as Pluto's, suggesting it might be a primordial plutino formed around the same time Pluto and Charon came into existence.
Reader's Guide
Arawn's significance stems from its close observation by the New Horizons spacecraft, which imaged it from a distance of 111 million km in April 2016, allowing its rotation period to be precisely determined. Before the discovery of 486958 Arrokoth in 2014, Arawn was the best known target for a flyby by New Horizons after its Pluto flyby in 2015. It was one of the first objects targeted for distant observations by New Horizons, imaged on 2 November 2015 from a distance 1/15 that of the previous nearest observation of a Kuiper belt object other than the Pluto–Charon system. The April 2016 images allowed the science team at Southwest Research Institute to pinpoint Arawn's location to within 1000 kilometers. Arawn occulted a star on 25 August 2022, and on 15 April 2024, the James Webb Space Telescope conducted an observation of Arawn using its NIRCam instrument in Moving Target mode, with published findings expected at a later date. Its debated quasi-satellite relationship with Pluto adds to its scientific interest, though its orbit is primarily controlled by Neptune.
Discovery and Physical Profile
First identified on 12 May 1994 by astronomers Michael Irwin and Anna Żytkow using the 2.5-metre Isaac Newton Telescope at Roque de los Muchachos Observatory in the Canary Islands, Spain, the object initially carried the provisional label 1994 JR1 before receiving its permanent designation 15810 Arawn, a name drawn from Celtic mythology. Situated in the inner reaches of the Kuiper belt, Arawn is a plutino locked in the same 2:3 mean-motion resonance with Neptune that defines Pluto's own orbit, making it part of the most numerous family of resonant trans-Neptunian objects. Its path around the Sun spans a semi-major axis of 39.4 AU, with a full revolution taking roughly 247 years and six months. The orbit is moderately eccentric (0.12) and tilted only 4 degrees from the ecliptic, swinging between 34.7 AU at perihelion and 44.1 AU at aphelion. Based on an absolute magnitude of 7.6 and an estimated albedo of 0.1, Arawn measures approximately 133 kilometres across. Hubble Space Telescope imaging revealed that its surface is remarkably red in colour, a trait that distinguishes it from many of its Kuiper belt neighbours.
New Horizons and Modern Observations
Although it orbits far beyond Neptune, Arawn has been studied from far closer ranges than most of its Kuiper belt counterparts, largely thanks to the New Horizons spacecraft. On 2 November 2015, the probe's LORRI camera captured one of the first distant observations of Arawn, placing it at roughly one-fifteenth the distance of any prior close imaging of a Kuiper belt object outside the Pluto–Charon system. A more detailed pass followed on 7–8 April 2016, when New Horizons photographed Arawn from a then-record distance of about 111 million kilometres (0.74 AU). The science team at Southwest Research Institute in Boulder, Colorado, used those images to refine Arawn's position to within 1,000 kilometres and to calculate a rotation period of approximately 5.47 hours. Before the 2014 discovery of 486958 Arrokoth, Arawn had been the leading candidate for a post-Pluto flyby by New Horizons. More recently, on 15 April 2024, the James Webb Space Telescope spent just over a minute observing Arawn with its NIRCam instrument in Moving Target mode, with results still to be published.
The Pluto Quasi-Satellite Question
In 2012, researchers proposed that Arawn periodically enters a quasi-satellite loop around Pluto, a recurring dynamical pattern in which the object becomes a Plutonian quasi-satellite roughly every two million years and remains in that configuration for nearly 350,000 years at a time. The 2015 measurements made by New Horizons sharpened the precision of Arawn's orbital calculations, and the refined numbers broadly confirmed the general dynamics outlined in the original hypothesis. Nevertheless, the classification question remains contested. Because Arawn's orbit is governed primarily by Neptune's gravitational pull, with Pluto exerting only occasional, comparatively small perturbations, some astronomers hesitate to label it a true quasi-satellite of the dwarf planet. The debate highlights how subtle gravitational interactions in the outer Solar System can produce ambiguous dynamical states that resist simple categorisation, and it underscores the need for continued long-term tracking of objects in the plutino population.
Primordial Origins and Orbital Longevity
Arawn's orbit is described as exceptionally stable, likely as enduring as Pluto's own, a fact that carries significant implications for the object's history. Rather than being a fragment produced by a relatively recent collision within the Pluto system or a body captured from elsewhere, Arawn is thought to be a primordial plutino that formed in the same broad epoch in which Pluto and its companion Charon came into existence. Its membership in the 2:3 resonance with Neptune, combined with its low eccentricity of 0.12 and modest 4-degree inclination, suggests it has maintained a coherent orbital architecture for billions of years. The fact that it resides in the inner Kuiper belt and belongs to the most populous class of resonant trans-Neptunian objects further supports the view that it is a long-lived survivor from the early Solar System. In 2017, Arawn passed within 2.7 AU of Pluto, a proximity that offered a natural laboratory for studying the subtle gravitational interplay between the two bodies over time.
Frequently Asked Questions
What is 15810 Arawn?
It is a Kuiper belt object roughly 133 kilometers across that orbits the Sun at a semi-major axis of about 39.4 AU. Originally tracked as 1994 JR1, it resides in the inner stretch of the Kuiper belt and was later named after the mythological figure Arawn.
Who discovered 15810 Arawn and when?
Astronomers Michael Irwin and Anna Żytkow made the detection on 12 May 1994 using the 2.5-metre Isaac Newton Telescope at the Roque de los Muchachos Observatory in Spain's Canary Islands. The object received its permanent number 15810 and its mythological name in subsequent years.
Why is 15810 Arawn considered special among trans-neptunian objects?
The New Horizons spacecraft imaged Arawn from a far closer range than is typical for Kuiper belt flybys during its 2016 encounter. That unusually tight pass gave scientists a detailed look at the surface and shape of a plutino that would otherwise remain just a point of light.
What orbital class does 15810 Arawn belong to?
It is a plutino, locked in a 2:3 mean-motion resonance with Neptune so that it completes two orbits for every three of the giant planet's. Plutinos make up the largest resonant family of known trans-neptunian objects.
How does 15810 Arawn's size compare to other TNOs?
At roughly 133 kilometres (83 miles) in diameter, it is a mid-sized body in the Kuiper belt. It is far smaller than giants like Pluto or Eris but noticeably larger than many of the tiny scattered-disk objects catalogued beyond Neptune.
More in Trans-Neptunian Objects 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced