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Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune. [ a ] Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 astronomical units (4.50 × 10 9 km).
The average distance between Neptune and the Sun is 4.5 billion km (about 30.1 astronomical units (AU), the mean distance from the Earth to the Sun), and it completes an orbit on average every 164.79 years, subject to a variability of around ±0.1 years. The perihelion distance is 29.81 AU, and the aphelion distance is 30.33 AU.
S/2021 N 1 is the smallest, faintest, and most distant natural satellite of Neptune known, with a diameter of around 16–25 km (10–16 mi). It was discovered on 7 September 2021 by Scott S. Sheppard, David J. Tholen, Chad Trujillo, and Patryk S. Lykawka using the 8.2-meter Subaru Telescope at Mauna Kea, Hawaii, and later announced on 23 February 2024. [1]
Nereid orbits Neptune in the prograde direction at an average distance of 5,504,000 km (3,420,000 mi), but its high eccentricity of 0.749 takes it as close as 1,381,500 km (858,400 mi) and as far as 9,626,500 km (5,981,600 mi). [4] Nereid's highly eccentric orbit around Neptune.
Naiad / ˈ n eɪ ə d /, (also known as Neptune III and previously designated as S/1989 N 6) named after the naiads of Greek legend, [10] is the innermost satellite of Neptune and the nearest to the center of any gas giant with moons with a distance of 48,224 km from the planet's center. Its orbital period is less than a Neptunian day ...
Hippocamp completes one revolution around Neptune every 22 hours and 48 minutes (0.95 days), corresponding to a semi-major axis, or orbital distance of 105,283 km (65,420 mi). [5] For comparison, this distance is approximately 4.3 Neptune radii, or just over a quarter of the Earth – Moon distance.
Neso / ˈ n iː s oʊ /, also known as Neptune XIII, is the second-outermost known natural satellite of Neptune, after S/2021 N 1. It is a retrograde irregular moon discovered by Matthew J. Holman , Brett J. Gladman , et al. on 14 August 2002, though it went unnoticed until 2003.
Proteus is the second-largest moon of Neptune and is the largest of its regular prograde moons. It is about 420 km (260 mi) in diameter, larger than Nereid, Neptune's third-largest moon. It was not discovered by Earth-based telescopes because Proteus orbits so close to Neptune that it is lost in the glare of reflected sunlight. [15]