Discovery | |
---|---|
Discovered by | John Russell Hind |
Discovery date | August 13, 1847 |
Designations | |
(7) Iris | |
Pronunciation | /ˈaɪrɪs/ [2] |
Named after | Īris |
Main belt | |
Adjectives | Iridian /ɪˈrɪdiən,aɪ-/ [3] |
Symbol | (historical) |
Orbital characteristics [4] | |
Epoch 13 September 2023 (JD 2453300.5) | |
Aphelion | 2.935 AU (439.1 million km) |
Perihelion | 1.838 AU (275.0 million km) |
2.387 AU (357.1 million km) | |
Eccentricity | 0.22977 |
3.69 a (1346.8 d) | |
Average orbital speed | 19.03 km/s |
207.9° | |
Inclination | 5.519° |
259.5° | |
4 April 2025 | |
145.4° | |
Earth MOID | 0.85 AU (127 million km) [4] |
Proper orbital elements [5] | |
Proper semi-major axis | 2.3862106 AU |
Proper eccentricity | 0.2125516 |
Proper inclination | 6.3924857° |
Proper mean motion | 97.653672 deg / yr |
Proper orbital period | 3.6865 yr (1346.493 d) |
Precession of perihelion | 38.403324 arcsec / yr |
Precession of the ascending node | −46.447128 arcsec / yr |
Physical characteristics | |
Dimensions | 268 km × 234 km × 180 km ± (5 km × 4 km × 6 km) [1] 225 km × 190 km × 190 km [6] |
199±10 km [7] [8] 214±5 km [1] 199.83±10 km (IRAS) [4] | |
Flattening | 0.42 [lower-alpha 1] |
538460 km2 [lower-alpha 2] | |
Volume | 37153500 km3 [lower-alpha 2] |
Mass | (13.5±2.3)×1018 kg [8] (13.75±1.3)×1018 kg [1] |
Mean density | 3.26±0.74 g/cm3 [8] 2.7±0.3 g/cm3 [1] |
Equatorial surface gravity | 0.08 m/s² |
Equatorial escape velocity | 0.131 km/s |
7.138843 h (0.2974518 d) [1] | |
Equatorial rotation velocity | 25.4 m/s [lower-alpha 2] |
0.279 [8] 0.2766±0.030 [4] | |
Temperature | ~171 K max: 275 K (+2°C) |
S | |
6.7 [9] [10] to 11.4 | |
5.64 [4] | |
0.32" to 0.07" | |
7 Iris is a large main-belt asteroid and possible remnant planetesimal orbiting the Sun between Mars and Jupiter. It is the fourth-brightest object in the asteroid belt. 7 Iris is classified as an S-type asteroid, meaning that it has a stony composition.
Iris was discovered on August 13, 1847, by J. R. Hind from London, UK. It was Hind's first asteroid discovery and the seventh asteroid to be discovered overall. It was named after the rainbow goddess Iris in Greek mythology, who was a messenger to the gods, especially Hera. Her quality of attendant of Hera was particularly appropriate to the circumstances of discovery, as Iris was spotted following 3 Juno by less than an hour of right ascension (Juno is the Roman equivalent of Hera).
Iris's original symbol was a rainbow and a star: or more simply . It is in the pipeline for Unicode 17.0 as U+1CEC1 ( ). [11] [12]
Iris is an S-type asteroid. The surface is bright and is probably a mixture of nickel-iron metals and magnesium- and iron-silicates. Its spectrum is similar to that of L and LL chondrites with corrections for space weathering, [13] so it may be an important contributor of these meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites. [14]
Among the S-type asteroids, Iris ranks fifth in mean diameter after Eunomia, Juno, Amphitrite and Herculina. Its shape is consistent with an oblate spheroid with a large equatorial excavation, suggesting it is a remnant planetesimal. No collisional family can be associated with Iris, likely because the excavating impact occurred early in the history of the Solar System, and the debris has since dispersed. [1]
Iris's bright surface and small distance from the Sun make it the fourth-brightest object in the asteroid belt after Vesta, Ceres, and Pallas. It has a mean opposition magnitude of +7.8, comparable to that of Neptune, and can easily be seen with binoculars at most oppositions. At typical oppositions it marginally outshines the larger though darker Pallas. [15] But at rare oppositions near perihelion Iris can reach a magnitude of +6.7 (last time on October 31, 2017, reaching a magnitude of +6.9), [9] which is as bright as Ceres ever gets.
A study by Hanus et al. using data from the VLT's SPHERE instrument names eight craters 20 to 40 km in diameter, and seven recurring features of unknown nature that remain nameless due to a lack of consistency and their occurrence on the edge of Iris. The names are Greek names of colors, corresponding to the rainbow as the sign of Iris. It is unknown whether these names are under consideration by the IAU. The other 7 features are labeled A through G. [1]
Feature | Pronunciation | Greek | Meaning |
---|---|---|---|
Chloros | /ˈkloʊrɒs/ | χλωρός | 'green' |
Chrysos | /ˈkraɪsɒs/ | χρῡσός | 'gold' |
Cirrhos | /ˈsɪrɒs/ | κιρρός | 'orange' [lower-alpha 3] |
Cyanos | /ˈsaɪənɒs/ | κύανος | 'blue' |
Erythros | /ˈɛrɪθrɒs/ | ἐρυθρός | 'red' |
Glaucos | /ˈɡlɔːkɒs/ | γλαυκός | 'grey' [lower-alpha 4] |
Porphyra | /ˈpɔːrfɪrə/ | πορφύρα | 'purple' |
Xanthos | /ˈzænθɒs/ | ξανθός | 'yellow' |
Iris has a rotational period of 7.14 hours. Iris’ north pole points towards the ecliptic coordinates (λ, β) estimated to be (18°, +19°) with a 4° uncertainty (Viikinkoski et al. 2017) or (19°, +26°) with a 3° uncertainty (Hanuš et al. 2019). This gives an axial tilt of 85°, [17] so that on much of each hemisphere, the sun does not set during summer, and does not rise during winter. On an airless body this gives rise to very large temperature differences.
Iris was observed occulting a star on May 26, 1995, and later on July 25, 1997. Both observations gave a diameter of about 200 km.
In February 2024, water molecules were discovered on 7 Iris, alongside 20 Massalia, marking the first time water molecules were detected on asteroids. [18] [19]
16 Psyche is a large M-type asteroid, which was discovered by the Italian astronomer Annibale de Gasparis, on 17 March 1852 and named after the Greek goddess Psyche. The prefix "16" signifies that it was the sixteenth minor planet in order of discovery. It is the largest and most massive of the M-type asteroids, and one of the dozen most massive asteroids. It has a mean diameter of approximately 220 kilometers (140 mi) and contains about one percent of the mass of the asteroid belt. It was thought to be the exposed core of a protoplanet, but recent observations cast doubt on that hypothesis. Psyche will be explored by NASA, with a spacecraft of the same name, marking the first time a manmade object will journey to a metallic asteroid, launched on 13 October 2023, with an expected arrival in 2029.
Juno is a large asteroid in the asteroid belt. Juno was the third asteroid discovered, in 1804, by German astronomer Karl Harding. It is one of the twenty largest asteroids and one of the two largest stony (S-type) asteroids, along with 15 Eunomia. It is estimated to contain 1% of the total mass of the asteroid belt.
5 Astraea is an asteroid in the asteroid belt. This object is orbiting the Sun at a distance of 385 million kilometres (2.5735 AU) with a period of 4.13 yr and an orbital eccentricity of 0.19. The orbital plane is inclined at an angle of 5.37° to the plane of the ecliptic. It is spinning with a period of 16.8 h. The surface of Astraea is highly reflective and its composition is probably a mixture of nickel–iron with silicates of magnesium and iron. It is an S-type asteroid in the Tholen classification system.
6 Hebe is a large main-belt asteroid, containing around 0.5% of the mass of the belt. However, due to its apparently high bulk density, Hebe does not rank among the top twenty asteroids by volume. This high bulk density suggests an extremely solid body that has not been impacted by collisions, which is not typical of asteroids of its size – they tend to be loosely-bound rubble piles.
8 Flora is a large, bright main-belt asteroid. It is the innermost large asteroid: no asteroid closer to the Sun has a diameter above 25 kilometres, and not until 20-km 149 Medusa was discovered was an asteroid known to orbit at a closer mean distance. It is the seventh-brightest asteroid with a mean opposition magnitude of +8.7. Flora can reach a magnitude of +8.1 at a favorable opposition near perihelion, such as occurred in November 2020 when it was 0.88 AU from Earth.
9 Metis is one of the larger main-belt asteroids. It is composed of silicates and metallic nickel-iron, and may be the core remnant of a large asteroid that was destroyed by an ancient collision. Metis is estimated to contain just under half a percent of the total mass of the asteroid belt.
10 Hygiea is a major asteroid located in the main asteroid belt. With a mean diameter of between 425 and 440 km and a mass estimated to be 3% of the total mass of the belt, it is the fourth-largest asteroid in the Solar System by both volume and mass, and is the largest of the C-type asteroids in classifications that use G type for 1 Ceres. It is very close to spherical, apparently because it had re-accreted after the disruptive impact that produced the large Hygiean family of asteroids.
11 Parthenope is a large, bright main-belt asteroid.
13 Egeria is a large main-belt G-type asteroid. It was discovered by Annibale de Gasparis on November 2, 1850. Egeria was named by Urbain Le Verrier, whose computations led to the discovery of Neptune, after the mythological nymph Egeria of Aricia, Italy, the wife of Numa Pompilius, second king of Rome.
15 Eunomia is a very large asteroid in the middle asteroid belt. It is the largest of the stony (S-type) asteroids, with 3 Juno as a close second. It is quite a massive asteroid, in 6th to 8th place. It is the largest Eunomian asteroid, and is estimated to contain 1% of the mass of the asteroid belt.
29 Amphitrite is one of the largest S-type asteroids, approximately 200 kilometers in diameter, and probably fifth largest after Eunomia, Juno, Iris and Herculina.
44 Nysa is a large and very bright main-belt asteroid, and the brightest member of the Nysian asteroid family. It is classified as a rare class E asteroid and is probably the largest of this type.
48 Doris is one of the largest main belt asteroids. It was discovered on 19 September 1857 by Hermann Goldschmidt from his balcony in Paris.
88 Thisbe is the 13th largest main-belt asteroid. It was discovered by C. H. F. Peters on June 15, 1866, and named after Thisbe, heroine of a Roman fable. This asteroid is orbiting the Sun at a distance of 2.768 AU with a period of 4.60 years and an orbital eccentricity (ovalness) of 0.165. The orbital plane is inclined at an angle of 5.219° to the ecliptic.
130 Elektra is a large outer main-belt asteroid and quadruple system with three minor-planet moons. It was discovered on 17 February 1873, by astronomer Christian Peters at Litchfield Observatory, New York, and named after Electra, an avenger in Greek mythology.
704 Interamnia is a large F-type asteroid. With a mean diameter of around 330 kilometres, it is the fifth-largest asteroid, after Ceres, Vesta, Pallas and Hygiea. Its mean distance from the Sun is 3.067 AU. It was discovered on 2 October 1910 by Vincenzo Cerulli, and named after the Latin name for Teramo, Italy, where Cerulli worked. Its mass is probably between fifth and tenth highest in the asteroid belt, with a mass estimated to be 1.2% of the mass of the entire asteroid belt. Observations by the Very Large Telescope's SPHERE imager in 2017–2019, combined with occultation results, indicate that the shape of Interamnia may be consistent with hydrostatic equilibrium for a body of its density with a rotational period of 7.6 hours. This suggests that Interamnia may have formed as an equilibrium body, and that impacts changed its rotational period after it fully solidified.
324 Bamberga is one of the largest asteroids in the asteroid belt. It was discovered by Johann Palisa on 25 February 1892 in Vienna. It is one of the top-20 largest asteroids in the asteroid belt. Apart from the near-Earth asteroid Eros, it was the last asteroid which is ever easily visible with binoculars to be discovered.
354 Eleonora is a large, stony main-belt asteroid that was discovered by the French astronomer Auguste Charlois on January 17, 1893, in Nice.
423 Diotima is one of the larger main-belt asteroids. It is classified as a C-type asteroid and is probably composed of primitive carbonaceous material.