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2011 HM₁₀₂
Discovery
Discovered by	New Horizons KBO Search
Discovery site	Las Campanas Obs.
Discovery date	29 April 2011; (first observed only)
Designations
MPC designation	2011 HM102
Minor planet category	Neptune trojan; L5; TNO; distant;
Orbital characteristics
	Epoch 4 September 2017 (JD 2458000.5)
	Uncertainty parameter 6
Observation arc	(354 days)
Aphelion	32.778 AU
Perihelion	27.660 AU
Semi-major axis	30.219 AU
Eccentricity	0.0847
Orbital period (sidereal)	166.12 yr (60,675 days)
Mean anomaly	33.571°
Mean motion	0° 0m 21.24s / day
Inclination	29.348°
Longitude of ascending node	100.97°
Argument of perihelion	151.24°
Physical characteristics
Dimensions	100 km (est. at 0.10)
Absolute magnitude (H)	8.1

2011 HM₁₀₂ is the ninth Neptune trojan discovered. It was first observed on 29 April 2011, during the New Horizons KBO Search (268) using the Magellan II (Clay) Telescope at Las Campanas Observatory in Chile.^[2]^[5] It has the same orbital period as Neptune and orbits at the L₅ Lagrangian point about 60° backwards of Neptune.^[3]

Orbit and classification

Neptune trojans are resonant trans-Neptunian objects (TNO) in a 1:1 mean-motion orbital resonance with Neptune. These Trojans have a semi-major axis (a) very similar Neptune's (30.10 AU).

2011 HM₁₀₂ orbits the Sun with a semi-major axis of 30.219 AU at a distance of 27.7–32.8 AU once every 166 years and 1 month (60,675 days). Its orbit has an eccentricity of 0.08 and an inclination of 29° with respect to the ecliptic.^[1] Among the first 17 Neptune trojans discovered so far, it is the one with the highest inclination.^[3]

Physical properties

Diameter and albedo

Based on a generic magnitude-to-diameter conversion, it measures approximately 100 kilometers in diameter using an absolute magnitude of 8.1 and an assumed albedo of 0.10.^[4] It is an averaged-sized body among the first 17 Neptune trojans discovered so far, which measure between 60 and 200 kilometers (for an absolute magnitude of 9.3–6.6 and an assumed albedo of 0.10).^[3]^[4]

Surface

Observations with NIRSpec showed that the spectrum of 2011 HM₁₀₂ does not exhibit clear Triton-like ice tholin features. The surface is likely covered in a thin layer of water ice and CO₂ ice. Due to the variable color classification of this object, it is suspected that inhomogeneities in the surface composition exist.^[6]

Numbering and naming

Due to its orbital uncertainty, this minor planet has not been numbered and its official discoverers have not been determined.^[1]^[2] If named, it will follow the naming scheme already established with 385571 Otrera, which is to name these objects after figures related to the Amazons, an all-female warrior tribe that fought in the Trojan War on the side of the Trojans against the Greek.^[7]

Exploration

In October 2012, 2011 HM₁₀₂ was the closest known object of any kind to the New Horizons spacecraft.^[8] In mid- to late 2013, New Horizons passed within 1.2 AU of 2011 HM₁₀₂, where it would be detectable with one of the onboard instruments.^[5] An observation from New Horizons would measure the phase curve of 2011 HM₁₀₂ at phase angles unobtainable from Earth. The New Horizons team eventually decided that they would not target 2011 HM₁₀₂ for observations because the preparations for the Pluto approach took precedence.^[9]