Asteroid facts for kids

Images of visited asteroids illustrating their differences: (top row) 433 Eros and 243 Ida with its moon Dactyl, (bottom row) Ceres and 101955 Bennu. Sizes are not to scale.

An asteroid is a minor planet—an object that is neither a true planet nor comet— that orbits within the inner Solar System. They are rocky, metallic, or icy bodies with no atmosphere. Asteroids can be quite small (a few meters) to very large (up to about 940 kilometers in diameter for Ceres, which is classified as a dwarf planet).

They orbit the Sun in elliptical paths and can have irregular shapes, often resembling a "rubble pile" rather than a solid body.

Many asteroids are the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. A body is classified as a comet, not an asteroid, if it shows a coma (tail) when warmed by solar radiation. Also, asteroids are sometimes confused with meteoroids; however, meteoroids are typically smaller fragments that can enter a planet's atmosphere and become meteors or meteorites if they reach the surface.

Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. In 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA's Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice.

Asteroids are believed to contain clues about the early solar system and the processes that led to planet formation. Studying them can provide insights into the history of our solar system.

Composition

Asteroids are primarily composed of rock and metal. They can be classified into three main types based on their composition:

• C-type (carbonaceous): Rich in carbon and organic materials.
• S-type (silicaceous): Composed mainly of silicate minerals and nickel-iron.
• M-type (metallic): Made up mostly of metallic iron and nickel.

History of observations

Despite their large numbers, asteroids are a relatively recent discovery. The first one—Ceres— was only identified in 1801. Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye in dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be briefly visible to the naked eye.

Asteroids have historically been observed from Earth. The first close-up observation of an asteroid was made by the Galileo spacecraft. Several dedicated missions to asteroids were subsequently launched by NASA and JAXA.

Discovery of Ceres

In 1772, German astronomer Johann Elert Bode, citing Johann Daniel Titius, published a numerical procession known as the Titius–Bode law (now discredited). Except for an unexplained gap between Mars and Jupiter, Bode's formula seemed to predict the orbits of the known planets. Bode's formula predicted another planet would be found with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun.

In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal Monatliche Correspondenz (Monthly Correspondence), sent requests to 24 experienced astronomers (whom he dubbed the "celestial police"), asking that they combine their efforts and begin a methodical search for the expected planet.

One of the astronomers selected for the search was Giuseppe Piazzi, a Catholic priest at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. He was searching for "the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another". Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet.

Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin. He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet". In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the Monatliche Correspondenz.

Piazzi named the newly discovered object Ceres Ferdinandea, "in honor of the patron goddess of Sicily and of King Ferdinand of Bourbon".

Discovery process and naming

Cumulative discoveries of just the near-Earth asteroids known by size, 1980–2024

In the past, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. A body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.

The last step is sending the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.

By 1851, the Royal Astronomical Society decided that asteroids were being discovered at such a rapid rate that a different system was needed to categorize or name asteroids. In 1852, when de Gasparis discovered the twentieth asteroid, Benjamin Valz gave it a name and a number designating its rank among asteroid discoveries, 20 Massalia. Sometimes asteroids were discovered and not seen again. So, starting in 1892, new asteroids were listed by the year and a capital letter indicating the order in which the asteroid's orbit was calculated and registered within that specific year. For example, the first two asteroids discovered in 1892 were labeled 1892A and 1892B. However, there were not enough letters in the alphabet for all of the asteroids discovered in 1893, so 1893Z was followed by 1893AA. A number of variations of these methods were tried, including designations that included year plus a Greek letter in 1914. A simple chronological numbering system was established in 1925.

Currently all newly discovered asteroids receive a provisional designation (such as 2002 AT₄) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros).

Observation and exploration

Until the age of space travel, objects in the asteroid belt could only be observed with large telescopes, their shapes and terrain remaining a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope can only resolve a small amount of detail on the surfaces of the largest asteroids. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (variation in brightness during rotation) and their spectral properties. Sizes can be estimated by timing the lengths of star occultations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. Spacecraft flybys can provide much more data than any ground or space-based observations; sample-return missions gives insights about regolith composition.

Largest asteriods

Three largest objects in the asteroid belt, Ceres, Vesta, and Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourth-largest asteroid, Hygiea, appears nearly spherical although it may have an undifferentiated interior, like the majority of asteroids. The four largest asteroids constitute half the mass of the asteroid belt.

Asteroid mining

Artist's concept of a crewed mission to an asteroid

The concept of asteroid mining was proposed in 1970s. Matt Anderson defines successful asteroid mining as "the development of a mining program that is both financially self-sustaining and profitable to its investors". It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on Earth, or materials for constructing space habitats. Materials that are heavy and expensive to launch from Earth may someday be mined from asteroids and used for space manufacturing and construction.

As resource depletion on Earth becomes more real, the idea of extracting valuable elements from asteroids and returning these to Earth for profit, or using space-based resources to build solar-power satellites and space habitats, becomes more attractive. Hypothetically, water processed from ice could refuel orbiting propellant depots.

From the astrobiological perspective, asteroid prospecting could provide scientific data for the search for extraterrestrial intelligence (SETI). Some astrophysicists have suggested that if advanced extraterrestrial civilizations employed asteroid mining long ago, the hallmarks of these activities might be detectable.

Potential threats to Earth

Near-Earth asteroids can threaten all life on the planet, as in the impact which may have inflicted the Cretaceous–Paleogene extinction. As an experiment to meet this danger, in September 2022 the Double Asteroid Redirection Test spacecraft successfully altered the orbit of the non-threatening asteroid Dimorphos by crashing into it.

Fiction

Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spacecraft traveling between two other points, and as a threat to life on Earth or other inhabited planets, dwarf planets, and natural satellites by potential impact.

Interesting facts about asteriods

• Of the roughly one million known asteroids, the greatest number are located between the orbits of Mars and Jupiter, in a region known as the main asteroid belt.
• The total mass of all the asteroids combined is only 3% that of Earth's Moon.
• The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.
• As of April 2022^[update], the Minor Planet Center had data on 1,199,224 asteroids in the inner and outer Solar System, of which about 614,690 had enough information to be given numbered designations.
• Asteroids become darker and redder with age due to space weathering.

Images for kids

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  2013 EC, shown here in radar images, has a provisional designation

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  Artist's impression shows how an asteroid is torn apart by the strong gravity of a white dwarf.

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  A top view of asteroid group location in the inner solar system.

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  A map of planets and asteroid groups of the inner solar system. Distances from sun are to scale, object sizes are not.

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  Cratered terrain on 4 Vesta

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  Mineral map of Vesta, showing its spectra

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  A complex horseshoe orbit (the vertical looping is due to inclination of the smaller body's orbit to that of the Earth, and would be absent if both orbited in the same plane)      Sun ·       Earth ·       (419624) 2010 SO16

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  The 70m antenna at Goldstone Observatory

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  Radar observations of near-Earth asteroid (505657) 2014 SR₃₃₉ as seen by Arecibo

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  WISE infrared space telescope

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  Asteroid 6481 Tenzing, center, is seen moving against a background of stars in this series of images taken by the James Webb Space Telescope's instrument NIRCam

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  Asteroids and comets visited by spacecraft as of 2019 (except Ceres and Vesta), to scale

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  Artist's concept of a crewed mission to an asteroid

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  Frequency of bolides, small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere

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  Artist's impression of an asteroid impact on Earth

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  Artist's concept of spacious structure of near-Earth asteroid 2011 MD

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  Hayabusa2

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  Dawn

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  Lucy

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  Psyche

See also

In Spanish: Asteroide para niños

• Exoasteroid
• List of minor planets
• List of exceptional asteroids
• List of asteroid close approaches to Earth
• Lost minor planet
• Meanings of minor-planet names