(363027) 1998 ST27

Near-Earth asteroid May 2026 yes (363027) 1998 ST 27 #FFC2E0 1998ST27 annotated.png Radar echo of 1998 ST 27 and its two moons LINEAR Lincoln Laboratory ETS 24 September 1998 NEOAtenPHA 21 November 2025 (JD 2461000.5) 0 1.2541 AU 0.38550 AU 0.81982 AU 0.8188 21.0507° 197.515° 322.439° 88.4052° 1.3278° / d 271.127 d (0.7423 yr) 2 0.009813 AU 6.975 ~800–900 m 0.059 0.066 ~2.7 h C? 19.58 (363027) 1998 ST 27 is a triple near-Earth asteroid (NEA) in the inner Solar System. It was discovered on 24 September 1998 by the LINEAR program at Lincoln Laboratory ETS in New Mexico, United States. It is about 800 to 900 m ft in diameter and rotates once every 2.7 hours. 1998 ST 27 has two unnamed moons. Its inner and outer moon were discovered by radar observation in 2024 and 2001, respectively. The very small inner moon is no larger than 50 m ft. The larger, 100 m ft on outer moon lies on a wide orbit, with an orbital period of at least seven days. Its rotation period is shorter, so it is not tidally locked. Orbit 1998 ST 27 orbits the Sun at an average distance—its semi-major axis—of 0.82 astronomical units, taking 271.1 Earth days to complete one orbit. It has an orbital eccentricity of 0.82, causing its distance from the Sun to range from 0.39 AU at perihelion to 1.25 AU at aphelion. Since it is an Earth-crossing near-Earth asteroid with a semi-major axis of less than 1 AU, it is categorized as an Aten asteroid. Its orbit is inclined by 21° with respect to the ecliptic. It crosses the orbits of every terrestrial planet,15 and it frequently has close encounters with Mercury and Earth. Physical characteristics Radar observations of 1998 ST 27 show that it is about 800 to 900 m ft in diameter. It is oblate in shape, with a relatively smooth surface. It has a low3146 albedo of 0.059, and its surface color properties are consistent with a C-type classification. Its rotation period is approximately 2.7 hours long, and it spins in a prograde direction. Moon system 1998 ST 27 has two unnamed moons. Its outer moon may have formed in a fissioning event, with its orbit raised by other dynamics after formation,12 such as very close encounters with the planets.15 Upon the outer moon's discovery, the 1998 ST 27 system was the first wide NEA binary known with an asynchronously rotating secondary.13 Inner moon The inner moon was discovered using Goldstone Observatory radar observations on 11 October 2024, as 1998 ST 27 was undergoing a close encounter with Earth. It orbits 1998 ST 27 "much more closely" than the outer moon, and it appears to be smaller than 50 m ft in diameter. Outer moon The outer moon was discovered by a team of astronomers led by Lance A. M. Benner. They used Arecibo delay-Doppler radar observations taken on 7 and 9 October 2001. The satellite's discovery was announced in an International Astronomical Union Circular on 9 October. It is on an unusually wide orbit, with a separation of at least 5 km mi and an orbital period greater than seven days. It is asynchronously rotating—that is, not tidally locked—spinning faster than its orbital period.2 It is about 100 m ft in diameter. See also Other triple NEAs: 3122 Florence (153591) 2001 SN 263 (136617) 1994 CC References (363027) = 1998 ST27 https://www.minorplanetcenter.net/db_search/show_object?object_id=363027 Minor Planet Center 2026-05-22 (1048 obs) 2024-10-14 last obs. JPL Small-Body Database Lookup: 363027 (1998 ST27) https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=363027 Jet Propulsion Laboratory 2026-05-22 Benner L. A. M. etal 1998 ST_27 October 2001 International Astronomical Union Circular 7730 2 2001IAUC.7730....2B Fang Julia Margot Jean-Luc 2012-01-01 NEAR-EARTH BINARIES AND TRIPLES: ORIGIN AND EVOLUTION OF SPIN-ORBITAL PROPERTIES https://iopscience.iop.org/article/10.1088/0004-6256/143/1/24 The Astronomical Journal 143 1 24 10.1088/0004-6256/143/1/24 free 0004-6256 1111.2794 Jacobson Seth A. Scheeres Daniel J. McMahon Jay December 11, 2013 FORMATION OF THE WIDE ASYNCHRONOUS BINARY ASTEROID POPULATION https://iopscience.iop.org/article/10.1088/0004-637X/780/1/60 The Astrophysical Journal 780 1 60 10.1088/0004-637X/780/1/60 0004-637X 1311.4887 Hromakina T etal February 2023 NEOROCKS project: surface properties of small near-Earth asteroids https://academic.oup.com/mnras/article/520/2/3143/7017844 Monthly Notices of the Royal Astronomical Society en 520 2 3143–3150 10.1093/mnras/stad330 free 0035-8711 2302.01165 1998 ST27, 2006 UY64, 2020 WG, and 1998 TU3 https://echo.jpl.nasa.gov/asteroids/oct2024.goldstone.planning.html 2024 2026-05-22 echo.jpl.nasa.gov Petrescu Elisabeta etal Lightcurves of Near-Earth Asteroids 2024 ON, 2024 MK and 1998 ST27 As Observed From Trappist The Minor Planet Bulletin April 2025 52 2 150–152 2025MPBu...52..150P NEO Groups NASA/JPL Near-Earth Object Program Office http://neo.jpl.nasa.gov/neo/groups.html https://web.archive.org/web/20020202160655/http://neo.jpl.nasa.gov/neo/groups.html dead 2002-02-02 2012-06-04 External links 363027 363027 1998 ST27 363026 1998 SK19 363027 363028 1998 TQ24 363027 363027 363027 363027 19980924