Asteroids- small space bodies of irregular shape, bending around the sun in different orbits. These bodies are more than 30 meters in diameter and do not have their own atmosphere.
It is believed that the solar system may contain from 1.1 to 1.9 million asteroids, more than 1 km in size.
Most of them are located in the belt, which stretches between the orbits of Jupiter and Mars . The belt is torus-shaped, and its density decreases as the distance from the Sun is 3.2 AU.
Until August 24, 2006, Ceres was considered the largest asteroid (975×909 km), but they decided to change its status, assigning it the title of a dwarf planet. And the total mass of all objects in the main belt is small – 3.0 – 3.6.1021 kg, which is 25 times less than the mass of the Moon .
Photo of the dwarf planet Ceres
Sensitive photometers make it possible to study changes in the brightness of cosmic bodies. The result is a light curve, by the shape of which you can find out the period of rotation of the asteroid and the location of the axis of its rotation. The frequency ranges from several hours to several hundred hours. Also, the light curve can help in identifying asteroid shapes. Only the largest objects approach the shape of a ball, the rest have an irregular shape.
By the nature of the change in brightness, it can be assumed that some asteroids have satellites, while others are binary systems or bodies that roll over each other’s surfaces.
The orbits of asteroids change under the powerful influence of planets, especially Jupiter affects their orbits. It led to the fact that there are entire zones where small planets are absent, and if they manage to get there, then for a very short time. Such zones, called Kirkwood hatches or gaps, alternate with regions filled with cosmic bodies that form families. Most of the asteroids are divided into families, which are most likely formed from the crushing of larger bodies. These clusters are named after their largest member.
At a distance after 3.2 AU. in Jupiter’s orbit, two flocks of asteroids – the Trojans and the Greeks – are circling. One flock (Greeks) overtakes the gas giant, while the other (Trojans) lags behind. These groups move quite steadily, because they are at the “Lagrange points”, where the gravitational forces acting on them are equal. Their divergence angle is the same – 60 °. Trojans have been able to accumulate long after the evolution of collisions of various asteroids. But there are other families with very close orbits, formed by the recent decays of their parent bodies. Such an object is the Flora family, which has about 60 members.
Interaction with the Earth
Not far from the inner edge of the main belt are groups of bodies whose orbits can intersect with the orbits of the Earth and the terrestrial planets. The main objects include the groups of Apollo, Amur, Aton. Their orbits are not stable, depending on the influence of Jupiter and other planets. The division into groups of such asteroids is rather arbitrary, because they can move from group to group. Such objects cross the Earth’s orbit, which creates a potential threat. The earth’s orbit is periodically crossed by about 2000 objects, the size of which is more than 1 km.
They are either fragments of larger asteroids, or cometary nuclei, from which all the ice has evaporated. In 10 – 100 million years, these bodies will surely fall on the planet that attracts them, or on the Sun.
Asteroids in the past of the Earth
The most famous event of this kind was the fall of an asteroid 65 million years ago, when half of everything living on the planet died. It is believed that the size of the fallen body was about 10 km, and the epicenter was the Gulf of Mexico. Traces of a hundred-kilometer crater were also found on Taimyr (in the bend of the Popigai River). On the planet’s surface, there are about 230 astroblems – large impact ring formations.
Composition
Asteroids can be classified according to their chemical composition and morphology. Determining the size of such a small body as an asteroid in the huge solar system, which, moreover, does not emit light, is extremely difficult. This helps to carry out the photometric method – measuring the brightness of a celestial body. The properties and nature of the reflected light are used to judge the properties of asteroids. So, using this method, all asteroids were divided into three groups:
- Carbonaceous– type C. There are most of them – 75%. They reflect light poorly, and are located on the outside of the belt.
- Sandy– type S. These bodies reflect light more strongly and are located in the inner zone.
- Metallic– type M. Their reflectivity is similar to the bodies of group S, and they are located in the central zone of the belt.
The composition of the asteroids is similar to that of the meteorite , because the latter are actually their fragments. Their mineralogical composition does not differ in variety. Only about 150 minerals have been identified, while there are more than 1000 of them on Earth.
Other asteroid belts
Similar space objects exist outside the orbit of Jupiter . There are a lot of them in the peripheral parts of the solar system. Beyond the orbit of Neptune is the Kuiper belt, which contains hundreds of objects ranging in size from 100 to 800 km.
Between the Kuiper belt and the main asteroid belt is another collection of similar objects belonging to the “Centaur class”. Their main representative was the asteroid Chiron, which sometimes pretends to be a comet, covering itself with a coma and spreading its tail. This two-faced type has a size of about 200 km and is proof that there are many similarities between comets and asteroids.
Origin hypotheses
What is an asteroid – a fragment of another planet or prototype? This is still a mystery, which they have been trying to solve for a long time. There are two main hypotheses:
Explosion of the planet. The most romantic version is the exploded mythical planet Phaethon. It was allegedly inhabited by intelligent beings who have reached a high standard of living. But a nuclear war broke out, eventually destroying the planet. But the study of the structure and composition of meteorites revealed that the substance of only one planet is not enough for such a variety. And the age of meteorites – from a million to hundreds of millions of years – shows that the crushing of asteroids was prolonged. And the planet Phaethon is just a beautiful fairy tale.
Collisions of protoplanetary bodies. This hypothesis prevails. She quite reliably explains the origin of asteroids. The planets were formed from a cloud of gas and dust. But in the regions between Jupiter and Mars, the process ended with the creation of protoplanetary bodies, from the collision of which the asteroids were born. There is a version that the largest of the minor planets are the embryos of the planet that failed to form. These objects include Ceres, Vesta, Pallas.
Largest asteroids
Ceres. It is the largest object in the asteroid belt, with a diameter of 950 km. Its mass is almost a third of the total mass of all bodies in the belt. Ceres consists of a stone core surrounded by an ice mantle. It is assumed that liquid water is present under the ice. The dwarf planet revolves around the Sun in 4.6 years at a speed of 18 km / sec. The period of its rotation is 9.15 hours, and the average density is 2 g / cm 3 .
Pallas. The second largest object in the asteroid belt, but with the transfer of Ceres to the status of a dwarf planet, became the largest asteroid. Its parameters are 582x556x500 km. The flight of the star takes 4 years at a speed of 17 km / sec. The day on Pallas is 8 hours, and the surface temperature is 164 ° K.
Vesta. This asteroid has become the brightest and only one that can be seen without the use of optics. The dimensions of the body are 578x560x458 km, and only the asymmetric shape does not allow Vesta to be attributed to dwarf planets. Inside it is an iron-nickel core, and around it is a stone mantle.
There are many large craters on Vesta, the largest of which is 460 km across and is located in the region of the South Pole. The depth of this formation reaches 13 km, and its edges rose above the surrounding plain by 4 – 12 km.
Eugene. This is a fairly large asteroid with a diameter of 215 km. It is interesting in that it has two satellites. They are the Little Prince (13 km) and S / 2004 (6 km). They are 1200 and 700 km away from Evgenia, respectively.
The study
A detailed study of asteroids was initiated by the Pioneer spacecraft. But the first to take pictures of objects Gaspra and Ida apparatus “Galileo” in 1991. A detailed examination was also carried out by NEAR Shoemaker and Hayabusa devices. Eros, Matilda and Itokawa became their target. From the latter, soil particles were even delivered. In 2007, the Dawn station went to Vesta and Ceres, reaching Vesta on July 16, 2011. This year the station should arrive at Ceres, and then it will try to reach Pallas.
It is unlikely that any life will be found on asteroids, but there are certainly many interesting things. You can expect a lot from these objects, but you don’t want only one thing: their unexpected arrival to visit us.