SkyEye

Don't feel small. We all have
been demoted. Go on being
moon or rock or orb, buoyant
and distant, smallest craft ball…
— Maggie Deitz, "Pluto", 2016

What is the Definition?

The words asteroid, minor planet and planetoid are (unofficial and) interchangable. They each refer to the small rocky bodies that are mainly confined to the asteroid belt between the planets Mars and Jupiter. The radius of the asteroid belt varies from 2.0 au to 3.3 au from the Sun with holes (called Kirkwood gaps) in the distribution of objects. These gaps coincide with orbital periods which are exact fractions of the period of Jupiter. However, these objects can be found elsewhere in the solar system. Some fly alarmingly close to the Earth whilst others inhabit the shadowy realms of the Kuiper Belt out beyond the planet Neptune.

On 24 August 2006, the International Astronomical Union defined a dwarf planet as a celestial body that

• is in orbit around the Sun,
• has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
• has not cleared the neighbourhood around its orbit, and
• is not a satellite.

Furthermore, on 11 June 2008, the International Astronomical Union defined a plutoid as a celestial body that

• is in orbit around the Sun at a semi-major axis greater than that of the planet Neptune,
• has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and
• has not cleared the neighbourhood around its orbit.

Satellites of plutoids are not plutoids themselves.

Ceres, Pluto, Haumea, Eris and Makemake are now classified as dwarf planets and all but Ceres are also classified as plutoids.

A small solar-system body is anything that isn't a planet, a dwarf planet, or a satellite. This includes most asteroids and comets.

Some Interesting Bodies

The table below lists a few characteristics of some of the more interesting bodies. According to current policy, an official number is not given to a small solar-system body until a reliable orbit has been determined for it. Once the object has been numbered, the discoverer may name it.

Dwarf planets are marked with an asterisk *.

Notes

Ceres is the largest of all the objects in the asteroid belt and is classified as a dwarf planet. Like Pluto, it was originally identified as a planet but was assigned a different classification when it became clear that it was just one of many such objects inhabiting the region between Mars and Jupiter. It has its own artificial satellite, the Dawn spacecraft which arrived and went into orbit in 2015.

Pallas has an unusually high orbital inclination.

Like its neighbours Ceres, Pallas and Vesta, Juno was originally classified as a planet.

Vesta is the brightest of the asteroids and the only one that can reach the limit of naked-eye visibility, sixth magnitude. It is also the second-most massive object in the main asteroid belt. The Dawn spacecraft was in orbit about it during 2011 and 2012 before setting off to visit Ceres in 2015.

After a gap of 38 years, Astraea was discovered. Modern automated methods now mean thousands of such bodies are discovered every year.

Rosetta's ten-year mission provided it the opportunity to fly past two asteroids, Lutetia and the rare E-type asteroid Steins.

Ida was photographed by the Galileo spacecraft in 1994. This historic encounter provided the first conclusive evidence that asteroids could have natural satellites when Dactyl was discovered accompanying its parent body in orbit about the Sun.

In June 1997, NEAR Shoemaker passed by Mathilde on its way to a year 2000 Valentine's Day encounter with Eros, an Earth-grazing asteroid and the first one discovered to come inside the orbit of Mars. One year later, NEAR made history by becoming the first spacecraft to touch down on an asteroid when it made an amazing soft landing on Eros.

Numerical modelling suggests that members of the Baptistina asteroid group, named for Baptistina, may be responsible not only for the large lunar crater Tycho but also the terrestrial impact site at Chicxulub which seems to be implicated in the extinction of the dinosaurs.

The Trojan asteroids are two families of asteroids stationed at the Lagrangian points in Jupiter's orbit. The first one discovered was Achilles but the largest is Hektor.

The last of the 'lost' named and numbered asteroids, Albert was recovered in May 2000, almost 89 years after its original discovery. With its recovery, the position of every numbered asteroid is now known.

Hidalgo has a highly elliptical orbit which carries it from the main asteroid belt between Mars and Jupiter to out beyond Saturn. Some think that Hidalgo is actually an extinct comet.

In October 1991, the Galileo spacecraft took the first ever high-resolution photograph of an asteroid. The object concerned was Gaspra, a member of the Flora group of asteroids.

Amor is the prototype of the Amor group of asteroids. These objects come to perihelion between the orbits of Mars and Earth.

Icarus has an orbit that carries it closer to the Sun than Mercury, the innermost planet in the solar system.

Apollo is the prototype of the Apollo group of asteroids. These objects have perihelion distances of less than 1 au so they cross the Earth's orbit. Scientists have reconstructed the orbit of the meteroid that exploded in the region of Chelyabinsk, Russia, on 15 February 2013 and have concluded that it was an Apollo asteroid.

Chiron has an orbit entirely outside that of Jupiter. It has been observed to have a coma and may be a giant comet instead of an asteroid.

Aten is the prototype of the Aten group of asteroids. These are Earth-approaching objects with orbits lying mostly within that of the Earth.

Grissom is named in the memory of Apollo astronaut Virgil Ivan 'Gus' Grissom who was killed in the Apollo 1 launchpad fire on 27 January 1967.

Phaethon passes closer to the Sun than any other object in the solar system with the exception of a few comets. It was discovered by the IRAS satellite and is thought to be the parent body of the Geminid meteor shower.

The Astronomy Unit at Queen Mary University of London, is proud to have five of its faculty members represented in the far reaches of the solar system. McVittie is named for the late George McVittie who was appointed as the first Professor of Mathematics at Queen Mary College in 1948. Iwan is named for Prof. Iwan P. Williams, Carlmurray is named for Prof. Carl D. Murray, Donnison is named for Dr. John Richard Donnison and Richnelson is named for Prof. Richard P. Nelson.

In 2000, Masursky was examined by Cassini as the spacecraft headed out to its encounters with Jupiter and Saturn.

Toutatis had a close encounter with the Earth in December 1996 and again in September 2004.

Star Trek creator Gene Roddenberry is immortalised in the heavens as Roddenberry. Actors Leonard Nimoy (Mr Spock), George Takei (Hikaru Sulu) and Nichelle Nichols (Nyoto Uhura) from the series have been similarly honoured with Nimoy, Takei and Nichols. Wil Wheaton (Wesley Crusher from Star Trek: The Next Generation) has Wilwheaton named for him. Nanavisitor is named for Nana Visitor (Kira Nerys) and Terryfarrell honours Terry Farrell (Jadzia Dax) from Star Trek: Deep Space Nine. The starship Enterprise is also represented by Enterprise. Tenagra is the name of an island from the Star Trek: The Next Generation episode entitled 'Darmok'. The asteroid called Mr. Spock is actually named for a cat that accompanied the asteroid's discoverer but of course, the cat was named for the beloved Star Trek character.

Stardust was the first mission to return samples from a comet to the Earth. As a prelude to its cometary encounters, Stardust flew past Annefrank, discovering that it was much larger (and much dimmer) than anticipated.

Using stellar occultation data, scientists discovered that Chariklo is surrounded by two rings, the first asteroid to be identified as possessing a ring system. Chariklo is a Centaur, one of a class of small objects which orbit primarily between Jupiter and Neptune.

Deep Space 1, using an innovative ion propulsion system, made a very close fly by of near-Earth asteroid Braille in July 1999. The spacecraft went to on to fly past the coma of comet 19P/Borrelly two years later.

Itokawa is named for Hideo Itokawa, the father of Japanese rocketry. It's entirely appropriate that in 2005, the Japanese spacecraft Hayabusa landed not once but twice on the asteroid, making it the first spacecraft to successfully land and take off from the surface of an asteroid. Despite multiple problems and malfunctions, the spacecraft successfully collected a specimen from the asteroid and the sample capsule landed in Australia in June 2010 with its precious cargo.

Main belt asteroid Brianjones honours Brian Jones, author and founding member of the Bradford Astronomical Society. He revived the fortunes of the venerable amateur astronomy publication Yearbook of Astronomy in 2017 when its publisher dropped the title. The asteroid was first observed from Lowell Observatory in October 1998 and initially designated 1998 WK8. Reacquired by the Catalina Sky Survey in March 2000, it given the new designation 2000 EC144. The object varies in magnitude between +18 and +21 and never approaches closer than 1.7 au to Earth.

When Ixion was discovered it was originally thought to be larger than Ceres. Quaoar turned out to be even bigger although eventually it too was supplanted by other larger trans-Neptunian objects. Quaoar has one known moon called Weywot.

Lost just five days after its initial discovery in 1937 and only recovered in October 2003, Hermes turns out to be a binary object. Two asteroids for the price of one!

Bennu is a member of the Apollo group of asteroids and has an estimated 1-in-2700 chance of striking the Earth next century. The OSIRIS-REx spacecraft intends to visit this asteroid in 2020 and return a sample from it three years later.

Pluto was demoted from planet to dwarf planet status on 24 August 2006 and given the number 134340 a month later. It was the first trans-Neptunian object discovered and is one of the largest. It has five companions: Charon (discovered in 1978 by J. Christy), Hydra and Nix (discovered in 2005 by H.A. Weaver et al, Kerberos (discovered in 2011 by Showalter et al whilst searching for rings around Pluto) and finally Styx (discovered in 2012 by Showalter et al whilst looking for possible hazards to the wildly successful New Horizons flyby mission in 2015). It's surprising that such a small object has such a complicated satellite system.

Like Pluto, Haumea has satellites. They are named Hi'iaka and Namaka, both discovered in 2005 by Brown, Bouchez and the Keck Observatory Adaptive team. Haumea is the Hawaiian goddess of childbirth and fertility, and Hi'iaka and Namaka are two of her children. The minor planet Haumea is accompanied not only by her two satellites but lots of tiny fragments which resulted from an ancient collision.

Eris is the largest of the dwarf planets and the object which ultimately led to the demotion of Pluto! Informally known as 'Xena' since its discovery in 2003, it is appropriately named for the goddess of discord who caused Trojan War. It has a satellite named Dysnomia who, in Greek mythology, embodies the spirit of lawlessness and is the daughter of Eris.

Like other classical Kuiper belt objects, Makemake has the name of a creator deity, in this case, the creator of humanity according to the myths of Easter Islanders. It is pronounced ma kay ma kay.

Ryugu is a Near-Earth Object and is considered a potentially hazardous member of the Apollo group of asteroids. (This means it can come uncomfortably close to Earth and is big enough to do serious damage should it collide with our planet.) It is the destination of the sample return mission Hayabusa2 launched by the Japanese Aerospace Exploration Agency (JAXA).