What are the shapes of asteroids?

Question

I want a specific one

Answer 1

Shape--

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Asteroids have usually irregular shapes due partly to the process of collisional fragmentation they have passed through and partly to the fact that they are too small to "pull" themselves in spherical shape by gravitational attraction.

While the asteroid follows its trajectory, it usually rotates around an axis and the area of the face which is turned towards the observer changes in time. As a first result, the apparent brightness will change in time, with a periodical curve that follows the rotation (also periodical). This curve is called light curve

(http://spaceguard.esa.int/NScience/neo/n...

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A rocky or metallic object, generally smaller than a planet but bigger than a meteoroid, that orbits the Sun or another star. Asteroids are also called minor planets. The largest asteroid, Ceres, however, has been classified as a dwarf planet.

More than 20,000 asteroids have been given official designations, most of them in the asteroid belt between the orbits of Mars and Jupiter. Asteroids outside this belt include the Trojans, which share Jupiter’s orbit, and the near-Earth asteroids. A number of different asteroid groups have been distinguished on the basis of their similar orbital characteristics. Other sub-planetary objects, considered distinct from asteroids, are Centaurs, Kuiper Belt objects, and comets, though classification is ambiguous in some cases. Some of the smaller moons in the solar system appear to be captured asteroids, including the two moons of Mars, and a number of the outer moons of the four gas giants.

Asteroids range in size from a few meters to over 900 km across, and vary greatly in composition. Although none is visible to the naked eye, many can be seen at times with binoculars or small telescopes, including the four largest: (1) Ceres, (2) Pallas, (4) Vesta, and (10) Hygiea. Thirty known asteroids exceed 200 km in diameter and the census of asteroids larger than 100 km in diameter is believed to be virtually complete. In the 10 to 100 km range, probably about half await discovery. However, of the estimated one million asteroids bigger than 1 km across, only a tiny percentage is known. The total mass of all the asteroids, most of which is concentrated in the main belt, is about one-twentieth that of the Moon and about three times that of Ceres. Some asteroids, such as Ceres, Pallas, and Vesta, are nearly spherical; others, like (15) Eunomia (see Eunomia family), (107) Camilla, and (511) Davida, are quite elongated; still others, such as (4769) Castalia, (216) Kleopatra, and (4179) Toutatis, have bizarre shapes. Several asteroids, including (243) Ida, (45) Eugenia, and (762) Pulcova, are known to have small moons of their own. The discovery of these moons is important because it enables an accurate determination of the parent asteroid’s mass and average density. The density then gives a clue to the asteroid's makeup – either in terms of composition or of structure. There are also binary asteroids, such as (90) Antiope and, possibly, (1620) Geographos, in which two components of roughly equal size orbit each other at very close range. Several asteroids have been studied by passing space probes, including Ida and (951) Gaspra (by Galileo), and (253) Mathilde and (433) Eros (by NEAR-Shoemaker).

Most asteroids move in orbits that are somewhat more inclined and eccentric than those of the major planets (with the exception of Pluto) – the orbit of an average main-belt asteroid being inclined at about 10° to the plane of the ecliptic with an eccentricity of about 0.15. But some asteroids, such (3200) Phaethon and (944) Hidalgo, have highly inclined and/or elliptical paths, suggesting they may be defunct cometary nuclei. Rotational periods of asteroids range from 2.3 hours to 48 days, but in more than 80% of cases are 4 to 20 hours. Albedos vary from just under 0.02 to over 0.5, with the majority of asteroids tending toward the lower (dark) end of this range. Low-albedo asteroids are generally found in the outer half of the asteroid belt, while higher-albedo objects tend to occupy the inner half. This fact stems from compositional differences, which in turn are related to how far from the Sun asteroids of different types formed.



Asteroid Eros from NEAR-Shoemaker in orbit

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Asteroids are thought to be the remnants of a stillborn planet. According to this idea, the newborn Jupiter gravitationally scattered nearby large planetesimals – accreting lumps of matter in the embryonic stage of planet-formation – some of which may have been as massive as Earth is today. Some of these big planetesimals strongly perturbed the orbits of the planetesimals in the region of the asteroid belt, raising their mutual velocities to the average 5 km/s seen today. As a result, what had been mild accretionary collisions in the future belt region became catastrophic disruptions. Only objects larger than about 500 km in diameter could have survived 5 km/s collisions with objects of comparable size. Ever since, the asteroids have been collisionally evolving so that, with the exception of the largest, most present-day asteroids are either remnants or fragments of past impacts.

While breaking down larger asteroids into smaller ones, collisions expose deeper layers of asteroidal material. If asteroids were compositionally homogeneous, this would have no noticeable result. Some of them, however, became differentiated; in other words, after they formed from primitive material in the solar nebula, they were heated (by radioactive decay or other means) to the point where their interiors melted and geochemical processes occurred. In some cases, temperatures became high enough for iron to form. Being denser than other materials, the iron sank to the center, forming an iron core and forcing basaltic lavas to the surface. At least one asteroid with a basaltic surface, Vesta, survives to this day. Other differentiated asteroids were disrupted by collisions that stripped away their crusts and mantles and exposed their iron cores. Still others may have had only their crusts partially stripped away, which exposed surfaces such as those visible today on the A-, E-, and R-class asteroids.

Collisions gave rise to the Hirayama families and at least some of the planet-crossing asteroids. Tiny fragments from the latter enter Earth’s atmosphere to become sporadic meteors, while larger pieces survive passage through the atmosphere to end up as meteorites. The very largest produce craters such as the Barringer Crater, and one may have been responsible for the extinction of the dinosaurs. Luckily, collisions of this sort are rare. According to current estimates, a few asteroids of 1-km diameter collide with Earth every 1 million years. Past collisions between asteroids and the Earth appear to have played a crucial role in the evolution of life on this planet (see cosmic collisions, biological effects). In particular, the impact of an asteroid about 65 million years ago caused a mass extinction in which the last of the dinosaurs were wiped out (see Cretaceous-Tertiary Boundary). The detection and tracking of near-Earth objects that might collide with the Earth in the future is receiving increasing attention. In addition, a number of space missions to investigate asteroids more closely, including the return of samples, have taken place or are underway. These include NEAR-Shoemaker, MUSES-C, and Deep Space 1.

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Answer 2

impossible to explain specifically but can be moon-shaped