An asteroid is a small planet in the inner solar system. Asteroids vary significantly in size and shape, from 1 meter rocks to a dwarf planet almost 1000 km in diameter; They are rocky, metallic or icy bodies without an atmosphere.
Of the approximately one million known asteroids, most lie between the orbits of Mars and Jupiter, about 2 to 4 AU from the Sun, in the main asteroid belt. Asteroids are generally classified into three types: C-type, M-type, and S-type. These are named after and generally identified with carbonaceous, metallic, and siliceous compositions, respectively. Asteroids vary greatly in size; the largest, Ceres, is almost 1,000 km (600 miles) across and is considered a dwarf planet. The total mass of all asteroids together is only 3% of the mass of Earth's moon. Most main belt asteroids follow stable, slightly elliptical orbits, rotate in the same direction as Earth, and take three to six years to complete a full orbit around the Sun.
Historically, asteroids have been observed from Earth; The Galileo spacecraft provided the first close-up observation of an asteroid. Several special asteroid missions have since been launched by NASA and JAXA, with plans for more missions in the works. NASA's NEAR Shoemaker studied Eros and Dawn observed Vesta and Ceres. The JAXA Hayabusa and Hayabusa2 missions examined and returned samples from Itokawa and Ryugu, respectively. OSIRIS-REx probed Bennu and collected a sample in 2020 to return to Earth in 2023. NASA's Lucy, launched in 2021, will study eight different asteroids, one main belt asteroid and seven Jupiterian Trojans. Psyche, scheduled for launch in 2023, will study a metallic asteroid of the same name.
Near-Earth asteroids could threaten all life on the planet; An asteroid impact triggered the Cretaceous-Paleogene extinction. Several strategies have been proposed to deflect asteroids. The Double Asteroid Redirection Test (DART) spacecraft launched in 2021 and intentionally impacted Dimorphos in September 2022, successfully changing its orbit upon colliding with it.
Also known as a minor planet or planetoid, an asteroid is one of many small bodies approximately 1,000 km (600 mi) in diameter or less that orbit the Sun, primarily between the orbits of Mars and Jupiter, in a known nearly flat ring. like asteroid. . . It's called the asteroid belt. Due to their small size and large number compared to the large planets, asteroids are also known as minor planets. The two designations have been used interchangeably, although the term asteroid is generally accepted by the general public. Among scientists, those who study individual objects with dynamically interesting orbits or groups of objects with similar orbital properties often use the term minor planets, while those who study the physical properties of such objects often refer to them as asteroids. The distinction between asteroids and meteoroids of the same origin is imposed culturally and is essentially one of size. Asteroids that are the size of a house (a few tens of meters in diameter) and smaller are generally called meteoroids, although the choice may depend somewhat on context, for example, if one is considering objects orbiting in space (asteroids). ) or objects with the potential to collide with a planet, natural satellite or other comparatively large body, or with a spacecraft (meteoroids).
Important Milestones in Asteroid Research
The first asteroid was discovered by astronomer Giuseppe Piazzi on January 1, 1801 in Palermo, Italy. At first Piazzi thought he saw a comet; However, after calculating the object's orbital elements, it became clear that the object was traveling in a planet-like orbit between the orbits of Mars and Jupiter. Due to illness, Piazzi was only able to observe the object until February 11. Although the discovery was reported in the press, Piazzi only shared details of the observations of it with a few astronomers and did not publish a full set of observations of it until months later. With the mathematics available at the time, the short observation arc did not allow an orbit to be calculated accurately enough to predict where the object would reappear if it returned to the night sky, leading some astronomers to disbelieve the discovery at all.
It might have remained so if this object had not been at the heliocentric distance predicted by Bode's law of planetary distances, proposed by the German astronomer Johann D. Titius in 1766 and popularized by his compatriot Johann E. Bode. who used the scheme to promote the notion of a "missing" planet between Mars and Jupiter. The discovery of the planet Uranus in 1781 by the British astronomer William Herschel at a distance exactly as predicted by Bode's law was considered strong evidence of its accuracy. Some astronomers were so convinced that, during an astronomical conference in the 1800s, they agreed to conduct a systematic search. Ironically, Piazzi was not involved in this attempt to locate the missing planet. Despite this, based on the preliminary orbit, Bode and others believed that Piazzi had found it and then lost it. This led the German mathematician Carl Friedrich Gauss, in 1801, to develop a method for calculating the orbits of minor planets from a few observations, a technique that has not improved much since. Orbital elements calculated by Gauss showed that the object was moving in a planet-like orbit between the orbits of Mars and Jupiter. Using Gauss's predictions, the German-Hungarian astronomer Franz von Zach (ironically, the one who proposed a systematic search for the "lost" planet) rediscovered the Piazzi object on December 7, 1801. (It was also independently rediscovered by the astronomer German Wilhelm Olbers on December 7, 1801). January 2, 1802.) Piazzi named this object Ceres after the ancient Roman goddess of grain and tutelary goddess of Sicily, beginning a tradition that continues today: asteroids are named after their discoverers (unlike comets, named after their discoverers).
The discovery of three more dim objects in similar orbits over the next six years (Pallas, Juno, and Vesta) complicated this elegant solution to the missing-planet problem and led to the surprisingly enduring, though no longer accepted, idea that asteroids were remnants. . exploded planet.
After this burst of activity, the search for the planet seems to have been abandoned in 1830, when Karl L. Hencke renewed it. In 1845 he discovered a fifth asteroid which he named Astraea.
Classical scholar Charles Burney, Jr. suggested to Herschel the name Asteroid (Greek for "star-like"). through his father, music historian Charles Burney, Sr., who was a close friend of Herschel's. Herschel proposed the term in 1802 at a meeting of the Royal Society. However, it was not accepted until the mid-19th century, when it became clear that Ceres and the other asteroids were not planets.
In 1866, 88 asteroids were known when the next great discovery was made: Daniel Kirkwood, an American astronomer, discovered that there were gaps (now known as Kirkwood gaps) in the distribution of asteroids' distances from the Sun (see Distribution of asteroids). Kirkwood and Breccias) . The introduction of photography to the search for new asteroids in 1891, when 322 asteroids had already been identified, accelerated the pace of discovery. The asteroid discovered in 1891, designated (323) Brucia, was the first to be discovered by photography. By the end of the 19th century, 464 had been found, and that number had risen to 108,066 by the end of the 20th century and reached nearly 1,000,000 in the third decade of the 21st century. The explosive growth was a byproduct of research aimed at find 90 percent of asteroids larger than a kilometer in diameter that could cross Earth's orbit and thus have the potential to collide with the planet (see Near-Earth Asteroids below).
In 1918, the Japanese astronomer Hirayama Kiyotsugu recognized a group of three orbital elements (semi-major axis, eccentricity, and inclination) of different asteroids. He speculated that the objects sharing these elements were formed by explosions of larger parent asteroids and called these groups of asteroids "families."
In the mid-20th century, astronomers began to entertain the idea that, during the formation of the solar system, Jupiter was responsible for stopping the accretion of a planet from a swarm of planetesimals extending about 2.8 units apart. astronomical (AU) distance from the sun; for an elaboration of this idea, see Origin and evolution of asteroids below. (An astronomical unit is the average distance from Earth to the Sun, about 150 million km [93 million miles].) Around the same time, calculations of the lifetimes of asteroids with orbits close to those of the major planets showed that most of these asteroids were destined to collide with a planet or be ejected from the solar system on time scales that they range from a few hundred thousand to a few million years. Since the age of the solar system is approximately 4.6 billion years, this means that asteroids seen in such orbits today must have entered them recently and implies that there was a source for these asteroids. At first, this source was believed to be comets that had been captured by the planets and lost their volatile material through repeated passages within the orbit of Mars. It is now known that most of these objects originate from regions of the main asteroid belt near Kirkwood gaps and other orbital resonances.
For much of the 19th century, most discoveries about asteroids were based on studies of their orbits. Most of the knowledge about the physical properties of asteroids, for example their size, shape, period of rotation, composition, mass, and density, has been acquired since the 20th century, particularly since the 1970s. These objects went from from being mere "little" planets to little worlds in their own right. The following discussion follows this advance in knowledge, focusing first on asteroids as orbiting bodies and then on their physical nature.
Geography of the asteroid belt
Geography in its truest sense is a description of the features of the surface of the Earth or any other planet. Three coordinates (latitude, longitude, and altitude) are enough to locate all these features. Similarly, the position of any object in the solar system can be specified by three parameters: heliocentric ecliptic longitude, heliocentric ecliptic latitude, and heliocentric distance. However, such positions are only momentary, since all objects in the solar system are in constant motion. Therefore, a better description of the "location" of an object in the Solar System is the path, called an orbit, that it follows around the Sun (or, in the case of a planetary satellite [Moon], the path around its orbit). planet). dad) . .
All asteroids revolve around the Sun in elliptical orbits and move in the same direction as the major planets. Some elliptical orbits are nearly circular, while others are highly elongated (eccentric). An orbit is fully described by six geometric parameters called its elements. The orbital elements, and therefore the shape and orientation of the orbit, also change over time as each object gravitationally impacts and is impacted by all other bodies in the solar system. In most cases, these gravitational effects can be taken into account, allowing accurate predictions of past and future locations and a defined mean orbit. These mean orbits can be used to describe the geography of the asteroid belt.
Asteroid names and orbits
Due to their wide occurrence, asteroids are assigned numbers and names. Numbers are assigned sequentially after determining the precise orbital elements. Ceres is officially known as (1) Ceres, Pallas as (2) Pallas, and so on. Of the 990,933 asteroids discovered up to 2020, 55% were numbered. Asteroid discoverers have the right to choose names for their discoveries after numbering them. The selected names will be submitted to the International Astronomical Union (IAU) for approval. (In 2006, the IAU decided that Ceres, the largest known asteroid, also qualifies as a member of a new category of objects in the Solar System called dwarf planets.)
Prior to the mid-20th century, asteroids were sometimes given numbers before the precise orbital elements had been determined, so some numbered asteroids could not be located later. These objects have been called "lost" asteroids. The last numbered missing asteroid, (719) Albert, was recovered in 2000 after 89 years. Many newly discovered asteroids are still "lost" due to insufficient observation periods, but new asteroids do not receive numbers until their orbits are known reliably.
The Center for Minor Planets at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, maintains computer files for all asteroid position measurements. In 2020, there were more than 268 million of these jobs in its database.
Distribution and Kirkwood gaps
The vast majority of known asteroids lie in orbits between those of Mars and Jupiter. Most of these orbits, in turn, have semi-major axes, or average distances from the Sun between 2.06 and 3.28 AU, a region called the main belt. The mean distances are not evenly distributed, showing population declines or "gaps". These so-called Kirkwood spaces are due to resonances of the mean motion with Jupiter's orbital period. For example, an asteroid with a mean distance from the Sun of 2.50 AU makes three orbits around the Sun in the time it takes Jupiter, which has a mean distance of 5.20 AU, to complete one orbit. The asteroid is said to be in a three to one resonance orbit (written 3:1) with Jupiter. Consequently, Jupiter and an asteroid in such an orbit would be in the same relative positions in all three orbits, and the asteroid would be subject to a gravitational force in a fixed direction. Repeated application of this force would eventually change the average distance of this asteroid, and others in similar orbits, creating a gap of 2.50 AU. Large spaces are produced at distances from the Sun corresponding to resonances with Jupiter of 4:1, 3:1, 5:2, 7:3 and 2:1, the respective mean distances being 2.06, 2.50, 2.82 , 2.96. and 3.28. The main gap at the 4:1 resonance defines the next extension of the main belt; the gap in the 2:1 resonance is the widest.
Some motion mean resonances are observed rather than scattered asteroids to gather them. Outside the main belt boundaries, asteroids cluster near resonances of 5:1 (at 1.78 AU, called the Hungaria Group), 7:4 (at 3.58 AU, Cybele Group), 3:2 (at AU3.97, Hilda) . group), 4:3 (at 4.29 AU, the Thule group) and 1:1 (at 5.20 AU, the Trojan groups). The presence of other resonances, so-called secular resonances, complicates the situation, especially at the sunward edge of the belt. Secular resonances, in which two orbits interact through the motions of their ascending nodes, perihelion, or both, act on time scales of millions of years to alter the eccentricity and inclination of asteroids. Combinations of mean motion and secular resonances can lead to long-term stabilization of asteroid orbits at certain mean motion resonances, as demonstrated by the Hungaria, Cybele, Hilda, and Trojan asteroid groups, or cause the orbits to evolve away from each other. of the resonances, as shown by the Kirkwood gaps.
Asteroids that can come close to Earth are called near-Earth asteroids (NEAs), although not all NEAs cross Earth's orbit. NEAs are divided into several classes of orbitals. Asteroids that belong to the class farthest from Earth (those asteroids that can cross the orbit of Mars but are more than 1.3 AU from perihelion) are known as Martian cruisers. This class is further divided into two: shallow Martian cruisers (perihelion distances not less than 1.58 AU but less than 1.67 AU) and deep Martian cruisers (perihelion distances greater than 1.3 AU but less than 1. 58 AU).
The next class farthest from the NEAs are the Cupids. Members of this group have perihelion distances greater than 1.017 AU, which is equal to the aphelion distance from Earth but not greater than 1.3 AU. Therefore, the Amor asteroids do not currently cross Earth's orbit. However, due to the strong gravitational perturbations generated by their approach to Earth, the orbital elements of all approaching asteroids except flat-bottomed Martian cruisers change considerably on time scales as short as years or decades. Because of this, about half of the known Cupids, including (1221) Cupid, the group's namesake, are part-time Earth Cruisers. Only asteroids that cross the orbits of the planets - i. H. Earth-approaching asteroids and idiosyncratic objects such as (944) Hidalgo and Chiron (see Asteroids in Unusual Orbits below) experience significant changes in their orbital elements on time scales of less than many million years.
There are two classes of NEAs that traverse low-Earth orbit almost continuously. The first to be discovered were the Apollo asteroids, named for (1862) Apollo, which was discovered in 1932 but lost shortly thereafter and not rediscovered until 1978. The average distances of Apollo asteroids from the Sun are greater than or equal to to 1 AU, and their perihelion distances are less than or equal to Earth's aphelion distance of 1.017 AU; Therefore, they intersect Earth's orbit when they approach the closest points to the Sun in their own orbit. The other class of Earth-crossing asteroids is called Atens de (2062) Aten, discovered in 1976. Aton asteroids have mean distances from the Sun of less than 1 AU and aphelion distances greater than or equal to 0.983 AU, the perihelion distance. from the earth. Land; They cross Earth's orbit when they are near the farthest points of their orbits from the Sun.
The most recently recognized class of NEAs consists of asteroids whose orbits lie entirely within Earth's orbit. Known as the Atira asteroids after (163693) Atira, they have mean distances from the Sun of less than 1 AU and aphelion distances of less than 0.983 AU; They do not cross Earth's orbit.
In 2020, the known Atira, Aten, Apollo, and Amor asteroids of all sizes numbered 42, 1,771, 11,851, and 9,837 respectively, though these numbers are steadily increasing as asteroid exploration programs progress. Most of them have been discovered since the 1970s, when the targeted search for this type of asteroid began. Astronomers have estimated that there are about 15 Atiras, 45 Atens, 570 Apollos, and 270 Amors larger than 1 km (0.6 mi) in diameter.
Because they can come so close to Earth, some of the best information available on asteroids comes from ground-based radar surveys conducted by NEA. In 1968, the asteroid Apollo (1566) Icarus was the first NEA observed with radar. As of 2020, close to 1000 NEAs have been observed in this way. Due to continuous improvements in the radar systems themselves and in the computers used to process the data, the information provided by this technique has increased dramatically from the last decade of the 20th century onwards. For example, the first images of an asteroid, (4769) Castalia, were taken using radar data taken in 1989, more than two years before the first flyby of an asteroid (951) Gaspra by the Galileo spacecraft in 1991 (see Exploration then) . . of the spaceship). The Castalia observations provided the first evidence in the Solar System for a bilobed object, interpreted as two roughly equal bodies in contact. Radar observations of (4179) Toutatis in 1992 showed it to be several kilometers long and shaped like a peanut shell; Like Castalia, Toutatis seems to mainly consist of two components in Kontakt, one about twice the size of the other. Higher-resolution images show craters with diameters of 100 to 600 meters (about 300 to 2,000 feet). The radar images of (1620) geographers obtained in 1994 were numerous enough and of sufficient quality to create an animation when rotated.
The orbital characteristics of the NEAs cause some of these objects to come close to the Earth and occasionally collide with it. For example, in January 1991, an Apollo asteroid (or, alternatively, a large meteoroid) estimated to be 10 meters (33 ft) in diameter passed the Earth less than halfway across the Moon. Such passages are not particularly rare. On October 6, 2008, the approximately 5-meter-wide asteroid 2008 TC3 was discovered. It crashed the next day in the Nubian desert in Sudan. However, due to the small size of NEAs and the short time they spend close enough to Earth to be seen, it is unusual for such narrow passages to be observed. An example of NEA where the lead time for the observation is long is (99942) Apophis. This asteroid Aten, approximately 375 meters (1,230 feet) in diameter, is expected to pass within 32,000 km (20,000 miles) of Earth on April 13, 2029 - i. H. closer than communications satellites in geostationary orbits; During this pass, its probability of hitting Earth is believed to be close to zero. It is widely accepted that the collision of a large enough NEA with Earth represents a great potential danger to humans and potentially all life on the planet. For an in-depth discussion of this topic, see Land Impact Risk.
Main Belt Asteroid Families
Within the main belt there are groups of asteroids clustered with respect to certain mean orbital elements (semi-major axis, eccentricity, and inclination). These groups are called families and are named after the asteroid with the lowest number in the family. Asteroid families form when an asteroid is destroyed in a catastrophic collision, with the family members being parts of the original asteroid. Theoretical studies suggest that catastrophic collisions between asteroids are common enough to explain the number of observed families. About 40 percent of the largest asteroids belong to these families, but up to 90 percent of small asteroids (ie those around 1 km in diameter) could be members of the family, since each catastrophic collision it produces many more smaller fragments than large and smaller ones. asteroids are associated with them and they are more likely to be completely destroyed.
The three largest families in the main asteroid belt are called Eos, Koronis, and Themis. Each family was determined to be homogeneous in terms of composition; that is, all members of a family appear to have the same basic chemical composition. If the asteroids belonging to each family are seen as fragments of a single main body, then their main bodies must be 200, 90, and 300 km (124, 56, and 186 mi) in diameter, respectively. The smaller families in the main belt have also not been studied because their numbered members are getting smaller (and therefore fainter when viewed telescopically). Some of the Earth-crossing asteroids and the vast majority of meteorites that strike the Earth's surface are thought to be fragments formed in collisions similar to those that created families of asteroids. For example, the asteroid Vesta, whose surface appears to be basaltic rock, is the parent body of the meteorites known as basaltic achondrite HEDs, a grouping of related meteorite types, howardite, eucrite, and diogenite.
Hungary and outer belt asteroids
Only one known concentration of asteroids, the Hungaria Group, occupies the region between Mars and the inner edge of the main belt. The orbits of all Hungarians are outside the orbit of Mars, whose aphelion distance is 1.67 AU. The Hungarian asteroids have nearly circular orbits (with little eccentricity) but large inclinations relative to Earth's orbit and the general plane of the Solar System.
Four known asteroid groups lie outside the main belt but within or near the orbit of Jupiter, at mean distances from the Sun between about 3.28 and 5.3 AU, as indicated in the Kirkwood distribution and gaps section above. . Collectively known as outer belt asteroids, they have orbital periods ranging from more than half the Jovian period to about the Jovian period. Three of the outer belt clusters, Cybeles, Hildas, and Thule, are named after the lowest numbered asteroid in each cluster. Members of the fourth group are called Trojan asteroids. As of 2020, there are approximately 2,034 Cybeles, 4,493 Hildas, 3 Thules, and 8,721 Trojans. These groups should not be confused with asteroid families, since they all share a common parent asteroid. Some of these groups, for example, the Hildas and the Trojans, contain families.
In 1772, the French mathematician and astronomer Joseph-Louis Lagrange predicted the existence and positions of two groups of small bodies located near two gravitationally stable points along Jupiter's orbit. These are positions (now called Lagrange points and denoted L4 and L5) where gravitational forces can hold a small body at one vertex of an equilateral triangle whose other vertices are occupied by the massive bodies of Jupiter and the Sun. These positions, which they are ahead (L4) and behind (L5) of Jupiter by 60° in the plane of its orbit, they are two of the five theoretical Lagrange points in the solution of the three-body problem restricted to the circle of celestial mechanics. (see Celestial Mechanics: The Restricted Three-Body Problem). The other three points lie along a line through the Sun and Jupiter. However, the presence of other planets, notably Saturn, perturbs the Sun-Jupiter-Troy asteroid system so much that these points become destabilized and, in fact, no asteroids have been found at them. Due to this destabilization, most of Jupiter's Trojan asteroids move in orbits inclined up to 40° from Jupiter's orbit and offset up to 70° from the positions before and after the true Lagrangian points.
In 1906, the first of the predicted objects, (588) Achilles, was discovered near the Lagrange point, which precedes Jupiter in its orbit. Within a year, two more were found: (617) Patroclus, near the back Lagrange point, and (624) Hector, near the main Lagrange point. Later, it was decided to continue naming these asteroids after the participants in the Trojan War, as recounted in Homer's epic The Iliad, as well as naming those near the main point after the warriors. Greeks, and those who were near the end point in homage to the Trojan warriors. With the exception of the two "out of place" names already adopted (Hector, the lone Trojan in the Greek camp, and Patroclus, the lone Greek in the Trojan camp), this tradition has been maintained.
As of 2020, of the 8,721 Jupiterian Trojan asteroids discovered, roughly two-thirds were near the Lagrange hotspot, L4, and the remainder near the vanishing point, L5. Astronomers estimate that between 1,800 and 2,200 of the total population of Trojans on Jupiter are larger than 10 km (6 miles) in diameter.
Since the discovery of Jupiter's orbital companions, the term Trojan has been applied to any small object that occupies the equilateral Lagrange points of other pairs of relatively massive bodies. The astronomers searched for Trojan objects on Earth, Mars, Saturn, Uranus and Neptune and the Earth-Moon system. Due to the gravitational perturbations of the large planets, it was long considered doubtful that stable orbits could exist near these Lagrange points. However, in 1990, an asteroid named (5261) Eureka was discovered oscillating around the trailing Lagrange point of Mars, and three others have since been found, two trailing and one trailing. Twenty-nine Neptune Trojans have been discovered since 2001, all but five associated with Lagrange's principal point, Neptune's Sun. This asteroid passes through L3 on its way from L4 to L5. Earth's first Trojan asteroid, 2010 TK7, which orbits L4, was discovered in 2010. The first Uranus Trojan, 2011 QF99, which is one pound around L4, was discovered in 2011. Floating objects have been found around the Lagrange points of the formed systems. by Saturn and its moon Thetis and Saturn and its moon Dione are known.