S/2021 N 1

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S/2021 N 1
Discovery
arcsec / yr
Physical characteristics
Mean diameter
16–25 km[a]
14 km[5][6]
27 (average)[5]
12.1[1]

S/2021 N 1 is the smallest, faintest, and most distant

retrograde
direction at an average distance of over 50 million km (31 million mi) and takes about 27 Earth years to complete its orbit—the largest orbital distance and period of any known moon in the Solar System.

Discovery

S/2021 N 1 was first observed on 7 September 2021 by Scott S. Sheppard and collaborators, during their search for Neptunian irregular moons with the 8.2-m

Mauna Kea, Hawaii.[1] Sheppard's team was able to detect this faint moon through the shift-and-add technique, in which they took many long-exposure telescope images, aligned and shifted them to follow Neptune's motion, and then added them together to create a single deep image that would show Neptunian moons as points of light against trailed background stars and galaxies.[6] Applying the shift-and-add technique to very large aperture telescopes like Subaru enabled Sheppard's team to probe deeper than previous Neptunian irregular moon surveys.[6]

From September 2021 to November 2023, Sheppard's conducted follow-up observations of S/2021 N 1 using other large-aperture telescopes around the world, which included the 6.5-m

Mauna Kea Observatory to determine the moon's orbit and ensure it would not be lost.[1][6] S/2021 N 1 and S/2002 N 5, another Neptunian irregular moon discovered by Sheppard's team, were both confirmed and announced by the Minor Planet Center on 23 February 2024, bringing Neptune's number of known moons from 14 to 16.[1]

Orbit

semi-major axis) in the horizontal axis and orbital inclination in the vertical axis. The semi-major axis values are expressed as a fraction of the planet's Hill sphere's radius, while the inclination is expressed in degrees from the ecliptic
. The relative sizes of moons are indicated by the size of their symbols, and the Sao and Neso groups of Neptunian moons are labeled. Data as of February 2024.

S/2021 N 1 is an

Keplerian elliptical orbit cannot accurately describe the long-term orbital motions of irregular moons. Instead, proper or average orbital elements are used to describe the long-term orbits of irregular moons more accurately, since these are calculated by averaging out the perturbed orbit over a long period of time.[7]
: 4 

Over an 800-year time span from 1600 to 2400, S/2021 N 1's average

Earth years.[3] Both of these orbital properties are greater than those of Neptune's moon Neso (49.9 million km; 26.8 yr),[3] which makes S/2021 N 1 hold the record for the largest orbital distance and period of any known moon in the Solar System.[8] For comparison, the planet Mercury has a semi-major axis of 57.9 million km (36.0 million mi; 0.387 AU) from the Sun,[9] which is about 14% larger than S/2021 N 1's average semi-major axis from Neptune. S/2021 N 1's extreme orbital distance is possible thanks to the large size of Neptune's Hill sphere of gravitational influence, which spans about 115 million km (71 million mi; 0.77 AU) in radius.[10]: 262  S/2021 N 1's average semi-major axis takes up about 44% of Neptune's Hill radius, although the Jovian moons of the Carme and Pasiphae groups orbit at a greater percentage of their primary's Hill radius.[5]

S/2021 N 1 has an average

retrograde orbit, meaning it orbits in the opposite direction of Neptune's orbit around the Sun.[5] Due to perturbations, S/2021 N 1's orbital elements fluctuate over time: its semi-major axis can range from 49 to 53 million km (30 to 33 million mi), eccentricity from 0.32 to 0.70, and inclination from 129° to 139°.[11]

S/2021 N 1 last passed

periapsis, or its nearest point to Neptune in its orbit, in September 2017 at a distance of approximately 27.1 million km (16.8 million mi; 0.181 AU).[12] The moon is moving away from Neptune until it reaches apoapsis, its farthest point from the planet, in March 2032 at a distance of approximately 74.8 million km (46.5 million mi; 0.500 AU).[13] S/2021 N 1 last passed apoapsis in November 2002 at a distance of approximately 76.6 million km (47.6 million mi; 0.512 AU)—the difference in distance is due to perturbations on the moon's orbit.[14]

S/2021 N 1's orbit exhibits

argument of pericenter of S/2021 N 1's orbit periodically librates around 90°, which is a behavior shared by Neso and Sao.[3][7]: 7  This behavior is due to the Kozai–Lidov resonance, where perturbations by the Sun and Neptune are periodic. The Kozai–Lidov resonance causes periodic exchanges between eccentricity and inclination: for example, as S/2021 N 1's orbit becomes more eccentric, its orbit becomes less inclined and vice versa.[7]
: 1 

S/2021 N 1 is part of the Neso group, a cluster of distant retrograde irregular moons of Neptune that includes Psamathe and the group's namesake Neso.[5] The moons of the Neso group have orbital elements that are clustered with semi-major axes between 46–51 million km (29–32 million mi), eccentricities between 0.4 and 0.5, and inclinations between 125° and 140°.[5] Like all other irregular moon groups, the Neso group is thought to have formed from the destruction of a larger captured moon of Neptune due to asteroid and comet impacts, which left many fragments in similar orbits around Neptune.[6]

Physical characteristics

S/2021 N 1 is extremely faint with an average apparent magnitude of 27, which is near the detectability limits of some the largest telescopes on Earth like the Subaru Telescope.[5][10]: 266  It is the faintest moon of Neptune discovered as of 2024.[8] Nothing is known about S/2021 N 1's physical properties other than its absolute magnitude of 12.1, which can be used to estimate the moon's diameter.[1] Assuming a geometric albedo range of 0.04–0.10 that is typical for most irregular moons,[15] S/2021 N 1 has a diameter between 16–25 km (10–16 mi).[a] Sheppard estimates the diameter to be 14 km, which if correct would make S/2021 N 1 the smallest known satellite orbiting Neptune.[5][6]

Notes

  1. ^ a b The diameter (in km) is calculated from absolute magnitude (H) and geometric albedo (p) according to the formula .[4] Given H = 12.1 and assuming an albedo range of 0.04–0.10, the diameter range is 16–25 km.

References

  1. ^ a b c d e f g "MPEC 2024-D112 : S/2021 N 1". Minor Planet Electronic Circulars. Minor Planet Center. 23 February 2024. Retrieved 23 February 2024.
  2. ^ "Planetary Satellite Discovery Circumstances". JPL Solar System Dynamics. NASA. Retrieved 23 February 2024.
  3. ^ a b c d e f "Planetary Satellite Mean Elements". JPL Solar System Dynamics. NASA. Retrieved 28 February 2024.
  4. ^ "Asteroid Size Estimator". Center for Near Earth Object Studies. NASA. Retrieved 23 February 2024.
  5. ^ a b c d e f g h Sheppard, Scott S. "Moons of Neptune". Earth & Planets Laboratory. Carnegie Institution for Science. Retrieved 23 February 2024.
  6. ^ a b c d e f "New Uranus and Neptune Moons". Earth & Planetary Laboratory. Carnegie Institution for Science. 23 February 2024. Retrieved 23 February 2024.
  7. ^
    S2CID 248458067
    . 241.
  8. ^ a b Crane, Leah (23 February 2024). "Tiny new moons have been spotted orbiting Neptune and Uranus". NewScientist. Retrieved 28 February 2024.
  9. ^ Williams, David R. (11 January 2024). "Mercury Fact Sheet". NASA Goddard Space Flight Center. NASA. Retrieved 28 February 2024.
  10. ^
    S2CID 13282788
    .
  11. ^ "JPL Horizons On-Line Ephemeris for 2021N1 Osculating Orbit (1600-Feb-01 to 2399-Dec-01)". JPL Horizons On-Line Ephemeris System. Jet Propulsion Laboratory. Retrieved 1 March 2024. Ephemeris Type: Elements. Center: 500@8 (Neptune Barycenter).
  12. ^ "JPL Horizons On-Line Ephemeris for 2021N1 from 2017-Sep-01 to 2017-Oct-01". JPL Horizons On-Line Ephemeris System. Jet Propulsion Laboratory. Retrieved 28 February 2024. Ephemeris Type: Observer. Center: 500@899 (Neptune body center) Periapsis occurs when radial velocity (deldot) changes from negative to positive. Distance from Neptune (delta) is given in AU.
  13. ^ "JPL Horizons On-Line Ephemeris for 2021N1 from 2032-Mar-01 to 2032-Apr-01". JPL Horizons On-Line Ephemeris System. Jet Propulsion Laboratory. Retrieved 28 February 2024. Ephemeris Type: Observer. Center: 500@899 (Neptune body center) Apoapsis when radial velocity (deldot) changes from positive to negative. Distance from Neptune (delta) is given in AU.
  14. ^ "JPL Horizons On-Line Ephemeris for 2021N1 from 2002-Nov-01 to 2002-Dec-01". JPL Horizons On-Line Ephemeris System. Jet Propulsion Laboratory. Retrieved 28 February 2024. Ephemeris Type: Observer. Center: 500@899 (Neptune body center) Apoapsis when radial velocity (deldot) changes from positive to negative. Distance from Neptune (delta) is given in AU.
  15. S2CID 264819644
    . 223.

External links
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