Portia (moon)
| Portia | |
|---|---|
 |
|
| Voyager 2 shot of Portia, Cressida and Ophelia | |
| Provisional or systematic name | S / 1986 U 1 |
| Central body | Uranus |
| Properties of the orbit | |
| Major semi-axis | 66,097.265 ± 0.050 km |
| Periapsis | 66,093.960 km |
| Apoapsis | 66,100.570 km |
| eccentricity | 0.00005 ± 0.00008 |
| Orbit inclination | 0.05908 ± 0.039 (equatorial plane) ° |
| Orbital time | 0.5131959201 ± 0.0000000093 d |
| Mean orbital velocity | 9.3663 km / s |
| Physical Properties | |
| Albedo | 0.08 ± 0.01 |
| Apparent brightness | 20.42 ± 0.05 mag |
| Medium diameter | 135.2 ± 8 (156 × 126 × 126) km |
| Dimensions | ≈ 1.6815 · 10 18 kg |
| surface | ≈ 57,000 km 2 |
| Medium density | ≈ 1.3 g / cm 3 |
| Acceleration of gravity on the surface | ≈ 0.0246 m / s 2 |
| Escape speed | ≈ 57.64 m / s |
| Surface temperature | ≈ −184 to −209 ° C / 64–89 K |
| discovery | |
| Explorer | |
| Date of discovery | January 3, 1986 |
| Remarks | Physical data relatively imprecise. |
Portia (also Uranus XII ) is the seventh innermost and one of the medium-sized of the 27 known moons of the planet Uranus . It is the largest moon in the Portia group and the eighth largest moon in the Uranus system.
Discovery and naming
Portia was discovered on January 3, 1986 by the astronomer Stephen P. Synnott together with Juliet on photographs taken by the Voyager 2 space probe . The discovery was announced on January 16, 1986 by the International Astronomical Union (IAU); the moon was initially given the provisional designation S / 1986 U 1 .
Portia is a female character from William Shakespeare's comedy The Merchant of Venice . Bassanio, who wants to marry the rich noblewoman Portia, receives the money he needs from his friend Antonio, the merchant from Venice , who is in debt with Shylock. As a condition, Shylock demands "a pound of meat" from Antonio's body instead of interest if he does not repay the money in time. Meanwhile, Bassanio has to pass an exam from Portia's late father in Belmont. When it looks like Antonio won't be able to repay the debt on time, Portia appears disguised as Balthasar's lawyer and resolves the situation at the last minute.
All the moons of Uranus are named after characters from Shakespeare or Alexander Pope . The first four Uranus moons discovered ( Oberon , Titania , Ariel , Umbriel ) were named after suggestions by John Herschel , the son of the Uranus discoverer Wilhelm Herschel . Later the tradition of naming was retained.
Track properties
Orbit
Portia orbits Uranus on a prograde , almost perfectly circular orbit at an average distance of around 66,097 km (approx. 2,586 Uranus radii) from its center, i.e. 40,538 km above its cloud ceiling. The orbital eccentricity is 0.00005, the web is 0.05908 ° to the equator of Uranus inclined .
Portia is the first discovered, fifth innermost and largest moon of the Portia group , which also includes Bianca , Cressida , Desdemona , Juliet , Rosalind , Cupid , Belinda and Perdita . These moons have similar orbits and similar spectral properties.
The orbit of the next inner moon Juliet is on average 1,739 km away from Portia's orbit, that of the next outer moon Rosalind 3,830 km.
Portia is located in the middle of two rings of Uranus; the inner ε (epsilon) ring is on average around 14,948 km from the Portia orbit, and the inner edge of the outer ν (Ny) dust ring is almost exactly on the orbit of Portia.
Portia orbits Uranus in 12 hours, 19 minutes and 0.13 seconds. Since this is faster than the rotation of Uranus, Portia rises in the west and sets in the east as seen from Uranus.
It moves in a descending orbit within the synchronous orbit around the planet and will at some point be torn apart as a result of tidal forces to form a ring or fall or burn up in Uranus' atmosphere.
rotation
It is believed that Portia rotates synchronously and its axis has an inclination of 0 °.
Physical Properties
Portia has a mean diameter of 135.2 km. In the recordings of the Voyager-2 - probe appeared Portia as an elongate object having dimensions of 156 × 126 × 126 km, whereby the longitudinal axis is aligned with Uranus.
Its mean density of 1.3 g / cm 3 is significantly lower than the density of the earth and indicates that the moon is mainly composed of water ice . Spectral analyzes from the Hubble Space Telescope and Earth-based telescopes confirm this assumption.
Portia has a very low albedo of 0.08; That is, 8% of the incident sunlight is reflected from the surface. It is therefore a very dark celestial body.
On its surface, the acceleration due to gravity is 0.0246 m / s 2 , which corresponds to about 2 ‰ of that on earth.
The mean surface temperature of Portia is estimated to be between −184 and −209 ° C (89–64 K ).
In the spectrum , the surface of Portia appears gray.
Otherwise, not much is known about this moon, as the probe images were taken at a great distance and therefore have a low resolution.
exploration
Since the Voyager 2 space probe's flyby, the Uranus system has been studied intensively by both Earth-based observations and the Hubble Space Telescope . The orbit parameters of Portia could be specified more precisely.
Web links
- IAUC 4164: Satellites of Uranus January 16, 1986 (discovery)
- IAUC 4165: Satellites of Uranus January 17, 1986 (Correction to IAUC 4164)
- IAUC 4609: Satellites of Saturn and Uranus June 8, 1988 (numbering and naming)
- Polish moon page: Portia description and further links (English)
- Satellite Viewer Orbit simulation of the Uranus moons