(118378) 1999 HT11

(118378) 1999 HT₁₁ (provisional designation 1999 HT₁₁) is a trans-Neptunian object from the outermost region of the Solar System, locked in a 4:7 orbital resonance with Neptune. It was discovered on 17 April 1999, by astronomers at the Kitt Peak Observatory, Arizona, in the United States.^[2] The very red object measures approximately 134 kilometers (83 miles) in diameter. As of 2021^[update], it has not been named.

1999 HT₁₁ was first observed on the night of 17 April 1999, by astronomers using the 4-meter Nicholas U. Mayall Telescope at Kitt Peak National Observatory in Arizona. Five additional objects were discovered on the same weekend: 1999 HR₁₁, 1999 HS₁₁, 1999 HU₁₁ (Deucalion), 1999 HV₁₁ and 1999 HW₁₁. The observing astronomers were Robert Millis, James Elliot, Matthew Holman, Mark Wagner as well as Kim Falinski. Follow-up observations with the Nordic Optical Telescope at the Roque de los Muchachos Observatory, La Palma, Spain, were made three weeks later. The body's observation arc begins with its official discovery observation at Kitt Peak on 17 April 1999.^[1][2]

This minor planet orbits the Sun at a distance of 38.7–49.0 AU once every 290 years and 7 months (106,124 days; semi-major axis of 43.87 AU). Its orbit has an eccentricity of 0.12 and an inclination of 5° with respect to the ecliptic.^[3] As of 2021^[update], it is at 39.7 AU from the Sun with an apparent magnitude of 23.42,^[8] and will come to perihelion in 2047.^[3]

1999 HT₁₁ is a resonant trans-Neptunian object that stays in a 4:7 mean-motion orbital resonance with Neptune, orbiting exactly four times the Sun for every seven orbits Neptune does and are therefore protected from the planets scattering effect. The classification is deemed secure.^[4][5] The 4:7 resonance is located in the midst of the classical objects of the Kuiper belt, a circumstellar disc of otherwise non-resonant bodies, contrary to the more prominent resonant plutinos (2:3) and twotinos (1:2) which form the inner and outer rim of the Kuiper belt, respectively.

This minor planet was numbered by the Minor Planet Center on 16 November 2005, receiving the number (118378) in the minor planet catalog (M.P.C. 55526).^[9] As of 2021^[update], it has not been named.^[2] According to the established naming conventions, it will be given a mythological name associated with the underworld or with creation.^[10]

1999 HT₁₁ has a very red surface color (RR) in the visible part of the spectrum, with B−V and V–R color indices of 1.150±0.064 and 0.670±0.040, respectively, for a combined B−R magnitude of 1.820±0.050.^[4][6][7] A red surface color is typically associated with the presence of tholins, polymer-like organic compounds, formed by long exposures to solar and cosmic radiation.

Based on a generic magnitude-to-diameter conversion, 1999 HT₁₁ measures approximately 134 kilometers (83 miles) in diameter, for an assumed albedo of 0.9 and an magnitude of 7.6.^[4][11] According to Mike Brown, who estimates a mean-diameter of 137 km (85 mi), the object is too small for being considered a dwarf planet candidate ("probably not").^[12] As of 2021, no rotational lightcurve for this body has been obtained from photometric observations. Its rotation period, pole and shape remain unknown.^[3]

• List of Transneptunian Objects, Minor Planet Center
• (118378) 1999 HT11, Small Bodies Data Ferret
• Discovery Circumstances: Numbered Minor Planets (115001)-(120000), Minor Planet Center
• (118378) 1999 HT11 at AstDyS-2, Asteroids—Dynamic Site
  • Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info
• (118378) 1999 HT11 at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit viewer · Orbit parameters · Physical parameters