Ersa (moon)

Ersa /ˈɜːrsə/, also designated Jupiter LXXI, is a small outer natural satellite of Jupiter discovered by Scott S. Sheppard on 11 May 2018, using the 4.0-meter Víctor M. Blanco Telescope at Cerro Tololo Observatory, Chile. It was announced alongside nine other Jovian moons on 17 July 2018 and was provisionally designated S/2018 J 1 by the Minor Planet Center, after observations were collected over a long enough time span to confirm the satellite's orbit.^[1] The satellite has been found in precovery observations as early as 6 August 2000.^[3]

Ersa

Precovery image of Ersa taken by the Canada-France-Hawaii Telescope in February 2003
Discovery[1][2]
Discovered by	Scott S. Sheppard
Discovery site	Cerro Tololo Obs.
Discovery date	11 May 2018
Designations
Designation	Jupiter LXXI
Pronunciation	/ˈɜːrsə/
Named after	Ἔρσα Ersa
Alternative names	S/2018 J 1
Orbital characteristics
Epoch 1 January 2000 (JD 2451545.0)
Observation arc	21.08 yr (7,701 d)
Earliest precovery date	6 August 2000[3]
Satellite of	Jupiter
Group	Himalia group
Proper orbital elements[4]
Proper semi-major axis	11,401,000 km (0.07621 AU) AU
Proper eccentricity	0.116
Proper inclination	29.1° (to ecliptic)
Proper mean motion	527.587231 deg / yr
Proper orbital period	0.68235 yr (249.229 d)
Precession of perihelion	8994.503 arcsec / yr
Precession of the ascending node	4232.058 arcsec / yr
Physical characteristics
Mean diameter	≈3 km[5]
Albedo	0.04 (assumed)[5]
Apparent magnitude	22.9[5]
Absolute magnitude (H)	15.9[1]

Ersa is part of the Himalia group, a tight cluster of prograde irregular moons of Jupiter that follow similar orbits to Himalia at semi-major axes between 11–12 million km (6.8–7.5 million mi) and inclinations between 26–31°.^[5] With an estimated diameter of 3 km (1.9 mi) for an absolute magnitude of 15.9, it is one of the smallest known members of the Himalia group.^[5]

Name

Animation of Ersa precovery images by the CFHT on 24 February 2003

The moon was named in 2019 after Ersa, the Greek goddess of dew, daughter of Zeus and Selene: Herse (Jupiter L) is also named for this goddess.^[6] The name was suggested in a naming contest held by the Carnegie Institute on Twitter,^[7] where more than twenty tweets suggested the name, including Aaron Quah (@8603103) who submitted the name first, StSauveur_MoonsProject (@StSauMoons) that are the 12th grade students of Saint Sauveur High School in Redon, France, the fifth grade at Hillside Traditional Academy in British Columbia, Canada (submitted on their behalf by @mrgrouchypants), and a 4-year-old child who sang a song about Ersa (submitted on his behalf by @Thoreson).^[8]

It belongs to the prograde Himalia group which are given names ending in a.

Orbit

On average, Ersa orbits Jupiter at a semi-major axis of about 11,401,000 km (7,084,000 mi) at an inclination of about 29.1° with respect to the ecliptic. Like all of Jupiter's irregular moons, Ersa orbits far enough away that it is highly subject to gravitational perturbations by the Sun and other planets, which makes its orbit highly variable over time.^[4]