7 Iris ( , or as in ) is a large main belt asteroid. Among
S-type asteroidS-type asteroids are of a siliceous composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
s it ranks fifth in
geometric meanThe geometric mean, in mathematics, is a type of mean or average, which indicates the central tendency or typical value of a set of numbers. It is similar to the arithmetic mean, which is what most people think of with the word "average," except that instead of adding the set of numbers and then...
diameter after
Eunomia15 Eunomia is a very large asteroid in the inner main asteroid belt. It is the largest of the stony asteroids, and somewhere between the 8th to 12th largest Main Belt asteroid overall...
,
JunoJuno , formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main belt asteroids, being one of the two largest stony asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid...
,
Amphitrite29 Amphitrite is one of the largest S-type asteroids, probably third in diameter after Eunomia and Juno, although Iris and Herculina are similar in size....
and
Herculina532 Herculina is a very large asteroid, with a diameter of around 225 km.-Discovery:It was discovered on April 20, 1904, by Max Wolf in Heidelberg, and initially catalogued as 1904 NY. The origin of its name is not known; it may be named after the mythical Hercules, or after an unknown woman...
.
Its bright surface and small distance from the Sun make Iris the fourth brightest object in the
asteroid beltThe asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
after Vesta, Ceres, and Pallas. But at typical
oppositionOpposition is a term used in positional astronomy to indicate when one celestial body is on the opposite side of the sky when viewed from a particular place . In particular, two planets are in opposition to each other when their ecliptic longitudes differ by 180°.The symbol of opposition is...
s it marginally outshines the larger though darker
Pallas2 Pallas is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802...
. It has a
meanIn statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....
opposition
magnitudeThe apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere...
of +7.8, comparable to that of Neptune, and can easily be seen with
binocularsBinoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes with binocular vision when viewing distant objects. Most are sized to be held...
at most oppositions. At rare oppositions near perihelion Iris can reach a magnitude of +6.7, which is as bright as Ceres ever gets, and reports of it being seen without optical aid are unverified.
Discovery and name
It was the seventh
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
discovered, on August 13, 1847 by
J. R. HindJohn Russell Hind FRS was an English astronomer. Some sources give his name as John Russel Hind with only one "L". However, 19th century British astronomical magazines consistently spell his name with two "L"s....
from
London[]London is the capital of England and the United Kingdom. It has been a major settlement for two millennia, and the history of London goes back to its founding by the Romans, when it was named Londinium. London's core, the ancient City of London, the 'square mile', retains its medieval boundaries...
,
UKThe United Kingdom of Great Britain and Northern Ireland is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of Ireland, and many small islands...
. It was Hind's first asteroid discovery.
Iris was named after the
rainbowA rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere...
goddess
IrisIn Greek mythology, Iris is the personification of the rainbow and messenger of the gods. As the sun unites Earth and heaven, Iris links the gods to humanity...
of
Greek mythologyGreek mythology is the body of myths and legends belonging to the ancient Greeks concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
, sister of the Harpies and messenger of the gods, especially
HeraIn the Olympian pantheon of classical Greek Mythology, Hera or Here was the wife and older sister of Zeus. Her chief function was as goddess of women and marriage. In Roman mythology, Juno was the equivalent mythical character. The cow, and later, the peacock were sacred to her...
. Her quality of attendant of Hera was particularly appropriate to the circumstances of discovery, as she was spotted following
3 JunoJuno , formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main belt asteroids, being one of the two largest stony asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid...
(
JunoJuno was an ancient Roman goddess, the protector and special counselor of the state. She is a daughter of Saturn and sister of the chief god Jupiter and the mother of Juventas, Mars, and Vulcan...
is the
RomanRoman mythology, or Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its main city, Ancient Rome. It can be considered as having two parts; One part, largely later and literary, consists of borrowings from Greek mythology...
equivalent of Hera) by less than an hour of
right ascensionRight ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
.
According to the
OEDThe Oxford English Dictionary , published by the Oxford University Press , is a comprehensive dictionary of the English language...
, the correct
adjectival formIn grammar, an adjective is a word whose main syntactic role is to modify a noun or pronoun, giving more information about the noun or pronoun's referent...
of the name is
Iridian.
Characteristics
Lightcurve analysis indicates a somewhat angular shape and that Iris' pole points towards
ecliptic coordinatesThe ecliptic coordinate system is a celestial coordinate system that uses the ecliptic for its fundamental plane. The ecliptic is the path that the sun appears to follow across the sky over the course of a year. It is also the projection of the Earth's orbital plane onto the celestial sphere...
(β, λ) = (10°, 20°) with a 10° uncertainty. This gives an
axial tiltIn astronomy, axial tilt is the angle between an object's rotational axis and a line perpendicular to its orbital plane. The angle is measured between the line perpendicular to object's orbital plane and object's rotational axis passing through north pole at which the planet appears to rotate...
of 85°, so that on almost a whole hemisphere of Iris, 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' surface likely exhibits
albedoThe albedo of an object is the extent to which it diffusely reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure...
differences, with possibly a large bright area in the northern hemisphere.
The surface of Iris is overall very bright and is probably a mixture
nickelNickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
-
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
metalA metal is a chemical element that is a good conductor of both electricity and heat, forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions...
s and
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
- and
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
-
silicateA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
s. Its spectrum is similar to that of
LThe L type ordinary chondrites are the second most common type of meteorite, accounting for approximately 35% of all those catalogued, and 40% of the ordinary chondrites....
and
LL chondriteThe LL chondrites are the least abundant group of the ordinary chondrites, accounting for about 10-11% of observed ordinary-chondrite falls and 8-9% of all meteorite falls ....
s with corrections for
space weatheringSpace weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...
, so it may be an important contributor of these meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites.
Iris was observed
occultingAn occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and can also be used in a general sense to describe when an object in the foreground occults objects in the background...
a
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
on May 26, 1995 and later on July 25, 1997. Both observations gave a
diameterIn geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
of about 200 km.
Iris regularly comes within 0.4AU of
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
and will next do so on November 2, 2054.
External links