29 Amphitrite

29 Amphitrite is one of the largest S-type asteroids, approximately 200 kilometers (120 miles) in diameter, and probably fifth largest after Eunomia, Juno, Iris and Herculina.

29 Amphitrite

VLT image of Amphitrite
Discovery[1]
Discovered by	A. Marth
Discovery site	London
Discovery date	1 March 1854
Designations
MPC designation	(29) Amphitrite
Pronunciation	/æmfɪˈtraɪtiː/[2]
Named after	Amphitrite[3] (Greek mythology)
Alternative designations	A899 NG
Minor planet category	main-belt[1][4] · (middle) background[5][6]
Adjectives	Amphitritean /ˌæmfɪtrɪˈtiːən/, /ˌæmfɪˈtraɪtiən/
Symbol	(historical)
Orbital characteristics[4]
Epoch 17.0 October 2024 (JD 2460600.5)
Uncertainty parameter 0
Observation arc	169.16 yr (61,784 d)
Aphelion	2.7425 AU
Perihelion	2.3664 AU
Semi-major axis	2.5544 AU
Eccentricity	0.0736
Orbital period (sidereal)	4.08 yr (1491 d)
Mean anomaly	48.40°
Mean motion	0° 14m 29.04s / day
Inclination	6.0772°
Longitude of ascending node	356.26°
Argument of perihelion	62.01°
Earth MOID	1.38454 AU
Jupiter MOID	2.48544 AU
TJupiter	3.427
Physical characteristics
Dimensions	233 km × 212 km × 193 km[7] 222 km × 209 km × 183 km (± 6 km × 6 km × 5 km)
Mean diameter	204±2 km[8] 189.6±1.1 km[9] 196±22 km[10] 206.86 km[11] 212.22±6.8 km[12] 227.1±4.0 km[13]
Flattening	0.18[a]
Mass	(12.7±2.0)×1018 kg[8] (11.8 ± ?)×1018 kg[7]
Mean density	2.86±0.45 g/cm3[8] 2.36±0.26 g/cm3[7]
Synodic rotation period	5.3921 h[14][6][4] 5.390119 ± 0.000001 h[8]
Axial tilt	116°[8]
Pole ecliptic longitude	323°±2°[8]
Pole ecliptic latitude	−29°±2°[8]
Geometric albedo	0.194[8] 0.157±0.035[13] 0.1793±0.012[12] 0.195[11] 0.216±0.043[9][4]
Spectral type	S[6][4]
Absolute magnitude (H)	5.85[1] 5.98[4] 6.0[5]

Discovery

Amphitrite was discovered by Albert Marth on 1 March 1854, at the private South Villa Observatory, in Regent's Park, London. It was Marth's only asteroid discovery. Its name was chosen by George Bishop, the owner of the observatory, who named it after Amphitrite, a sea goddess in Greek mythology.^[3] Its historical symbol was a shell and star; it is in the pipeline for Unicode 17.0 as U+1CECF 𜻏 ().^[15][16]

Characteristics

Lightcurve-based 3D-model of Amphitrite

Amphitrite's orbit is less eccentric and inclined than those of its larger cousins; indeed, it is the most circular of any asteroid discovered up to that point. As a consequence, it never becomes as bright as Iris or Hebe, especially as it is much further from the Sun than those asteroids. It can reach magnitudes of around +8.6 at a favorable opposition, but usually is around the binocular limit of +9.5.

In 2007, James Baer and Steven R. Chesley estimated Amphitrite to have a mass of 1.9×10¹⁹ kg.^[17] A 2008 estimate by Baer suggests it has a mass of 1.18×10¹⁹ kg.^[7]

A satellite of the asteroid is suspected to exist, based on lightcurve data collected by Edward F. Tedesco.^[18][19] In 1988 a search for satellites or dust orbiting this asteroid was performed using the UH88 telescope at the Mauna Kea Observatories, but neither were found.^[20]

Proposed exploration

In 1984, while determining the Galileo mission's prelaunch trajectory to Jupiter, JPL engineers found out that if the spacecraft launched on May 1986, it could perform a flyby of 29 Amphitrite at a distance of 6,200 mi (10,000 km) on December 6, 1986. However, with the Challenger disaster delaying the launch to October 1989, the flyby was ultimately cancelled.^[21]

Notes

1. Flattening derived from the maximum aspect ratio (c/a): ${\displaystyle f=1-{\frac {c}{a}}}$, where (c/a) = 0.82±0.03.^[8]