7 Iris is a large

main-belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...

asteroid and possible remnant

planetesimal Planetesimals () are solid objects thought to exist in protoplanetary disks and debris disks. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...

orbiting the Sun between Mars and Jupiter. It is the fourth-brightest object in the

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...

. 7 Iris is classified as an

S-type asteroid S-type (stony-type or silicaceous-type) asteroids are asteroids with a spectral type that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are ...

, meaning that it has a stony composition.

Discovery and name

Iris was discovered on 13 August 1847, by J. R. Hind from

London London is the Capital city, capital and List of urban areas in the United Kingdom, largest city of both England and the United Kingdom, with a population of in . London metropolitan area, Its wider metropolitan area is the largest in Wester ...

, UK. It was Hind's first asteroid discovery and the seventh asteroid to be discovered overall. It was named after the

rainbow A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular ...

goddess

Iris Iris most often refers to: *Iris (anatomy), part of the eye * Iris (color), an ambiguous color term *Iris (mythology), a Greek goddess * ''Iris'' (plant), a genus of flowering plants * Iris (given name), a feminine given name, and a list of peopl ...

Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...

, who was a messenger to the gods, especially

Hera In ancient Greek religion, Hera (; ; in Ionic Greek, Ionic and Homeric Greek) is the goddess of marriage, women, and family, and the protector of women during childbirth. In Greek mythology, she is queen of the twelve Olympians and Mount Oly ...

. Her quality of attendant of Hera was particularly appropriate to the circumstances of discovery, as Iris was spotted following

3 Juno Juno (minor-planet designation: 3 Juno) is a large asteroid in the asteroid belt. Juno was the third asteroid discovered, in 1804, by German astronomer Karl Harding. It is tied with three other asteroids as the thirteenth largest asteroid, and ...

by less than an hour of

right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...

(

Juno Juno commonly refers to: *Juno (mythology), the Roman goddess of marriage and queen of the gods * ''Juno'' (film), the 2007 film Juno may also refer to: Arts, entertainment and media Fictional characters *Juno, a character in the book ''Juno of ...

is the

Roman Roman or Romans most often refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of Roman civilization *Epistle to the Romans, shortened to Romans, a letter w ...

equivalent of Hera). Iris's original symbol was a rainbow and a star: or more simply . It is in the pipeline for

Unicode Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...

17.0 as U+1CEC1 𜻁 ().

Characteristics

Geology

Iris is an S-type asteroid. The surface is bright and is probably a mixture of

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

metals and

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

- and iron-

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

s. Its spectrum is similar to that of L and

LL chondrite LL may refer to: * Ll or ll, a digraph that occurs in several natural languages Arts and entertainment *LL, the production code for the 1967 ''Doctor Who'' serial ''The Evil of the Daleks'' * ''Labyrinth Lord'', a fantasy role-playing game * ''L ...

s with corrections for

space weathering Space weathering is the type of weathering that occurs to any object exposed to the harsh environment of outer space. Bodies without atmospheres (including the Moon, Mercury, the asteroids, comets, and most of the moons of other planets) take ...

, so it may be an important contributor of these

meteorites A meteorite is a rock that originated in outer space and has fallen to the surface of a planet or moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical interactions with the atmospheri ...

. Planetary dynamics also indicates that it should be a significant source of meteorites. Among the

s, Iris ranks fifth in

mean diameter In applied sciences, the equivalent radius (or mean radius) is the radius of a circle or sphere with the same perimeter, area, or volume of a non-circular or non-spherical object. The equivalent diameter (or mean diameter) (D) is twice the equiva ...

after Eunomia,

Amphitrite In ancient Greek mythology, Amphitrite (; ) was the goddess of the sea, the queen of the sea, and her consort is Poseidon. She was a daughter of Nereus and Doris (or Oceanus and Tethys).Roman, L., & Roman, M. (2010). Under the influence ...

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.

Brightness

Iris's bright surface and small distance from the Sun make it the fourth-brightest object in the

after Vesta, Ceres, and

Pallas Pallas may refer to: Astronomy * 2 Pallas asteroid ** Pallas family, a group of asteroids that includes 2 Pallas * Pallas (crater), a crater on Earth's moon Mythology * Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena * Pa ...

. It has a ''

mean A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...

'' opposition

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

of +7.8, comparable to that of

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

, and can easily be seen with

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

at most oppositions. At typical

opposition Opposition may refer to: Arts and media * ''Opposition'' (Altars EP), 2011 EP by Christian metalcore band Altars * The Opposition (band), a London post-punk band * ''The Opposition with Jordan Klepper'', a late-night television series on Comedy ...

s it marginally outshines the larger though darker

. But at rare oppositions near perihelion Iris can reach a magnitude of +6.7 (last time on 31 October 2017, reaching a magnitude of +6.9), which is as bright as Ceres ever gets.

Surface features

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 International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

. The other 7 features are labeled A through G.

Rotation

Iris has a rotational period of 7.14 hours. Iris's north pole points towards the

ecliptic coordinates In astronomy, the ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small So ...

(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 In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbita ...

of 85°, 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.

Observations

Iris was observed

occulting An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks f ...

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

on 26 May 1995, and later on 25 July 1997. Both observations gave a

diameter In geometry, a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest Chord (geometry), chord of the circle. Both definitions a ...

of about 200 km. In February 2024, water molecules were discovered on 7 Iris, alongside

20 Massalia 20 Massalia is a stony asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital ...

, marking the first time water molecules were detected on asteroids.

Notes

References

External links

* *
2011-Feb-19 Occultation
(Durech Model)

JPL Ephemeris
* (displays Elong from Sun and V mag for 2011) * * {{DEFAULTSORT:000007 Background asteroids

S-type asteroids (Tholen) S-type asteroids (SMASS) 18470813