Minor planets: asteroids
General RemarksThe word "asteroid" means "star-like". In all but the largest telescopes, asteroids appear to be bright points of light indistinguishable from stars. Asteroids move with respect to the background stars, which is how they were discovered. Asteroids are named with (i) a number indicating the order of discovery, starting from 1, and (ii) a proper name sometimes suggested by the discoverer. The name must be approved by the naming committees of the International Astronomical Union (IAU). Asteroids are composed of rock, which distinguishes them from comets; the latter have a high proportion of low-density, icy material that is emitted when they are warmed by the Sun. Chunks of rock with diameters greater than a few 100 km will be pulled into spherical shapes by their own gravity. Only a few asteroids of the estimated total number of about 100,000 are large enough to be spheres. The vast majority of asteroids will have irregular shapes, like potatoes. Asteroids are too small for radioactive heating to have made them molten, unlike the terrestrial planets. Asteroids are therefore "undifferentiated", meaning that their composition is uniform throughout. Asteroids therefore represent the unprocessed composition of the rocky content of our solar system from the time of its formation. Mass and numberThere are probably a few 100,000 asteroids in our solar system. Ceres is the largest, with a diameter of about 1,000 km. The next largest, Pallas, is only half as large. The sizes of the asteroids drop off quickly. In fact, the number of asteroids at a given diameter decreases as the inverse square of the diameter. # of asteroids with diameter D = K D-2 In the above relation, K is a constant of proportion. This inverse square relationship is what would be expected from the asteroid system when the distribution of sizes is produced from collisions. A consequence of this distribution is that there are very few large asteroids and a great many small asteroids. However, the largest asteroid, Ceres, contains nearly half the total mass of all the asteroids. So most of the mass of the asteroids is contained in the largest few objects. We can therefore confidently guess the total mass of the asteroids in our solar system, without having to discover all of them. The total mass of the asteroids is estimated to be about 0.2% that of the Earth. Modern asteroid catalogs list many tens of thousands of objects. We have discovered probably all of the larger asteroids over about 25 km in diameter, and a significant proportion over the size of a few km. Asteroid orbits
Main belt asteroids: Most asteroids are found in the main belt between Mars and Jupiter. These asteroids are not evenly distributed in the belt. Rather, there are narrow lanes that seem to be devoid of asteroids. These avoided areas are called "resonance gaps" or "Kirkwood gaps". They are named for astronomer Daniel Kirkwood (1814-1895), who discovered them in 1886. In the context of orbits, "resonance" refers to a situation when the orbital period of one object is related to another by a ratio that is a simple fraction, like 1/2, 2/3, 3/5, and so on. When such period relation exists, the objects can have a gravitational effect on each other whose effects accumulate over time. In the case of the main asteroid belt, the asteroids react most strongly to Jupiter's presence. The gaps in the asteroid belt occur at those orbits whose period is a simple fraction of Jupiter's orbit. The most obvious gaps are those at ratios asteroid:Jupiter of 4:1, 3:1, 5:2, and 2:1. For example, suppose an asteroid is in the 4:1 resonance orbit. For every 4 orbits traversed by the asteroid, Jupiter traverses exactly 1 orbit. Therefore, the asteroid and Jupiter will always meet at the same places in their orbits. Jupiter's gravitational pull on the asteroid at this location will always be in the same direction. The asteroid will be systematically displaced from its location. Other asteroids whose orbits do not relate simply to Jupiter's orbit will not feel a repeated gravitational pull, so will not be systematically displaced from their orbits. Note that the asteroids also exert their gravitational tug on Jupiter. Jupiter is so much more massive than an asteroid that Jupiter's orbit does not change appreciably. Trojan asteoids: Orbit resonances exist all over the solar system. A special orbital resonance is the 1:1 resonance of the Trojan group. These asteroids share an orbit with Jupiter. They are located near the apex of an equilateral triangle whose other two apexes are the Sun and Jupiter. These two points belong to a group of five locations called the Lagrangian points. These points are named after Joseph-Louis Lagrange (1736-1813), a French mathematician. The two stable locations lead and lag behind Jupiter by a 60° angle. Near-Earth asteroids: Asteroids whose paths cross the Earth's are collectively called near-Earth asteroids (NEAs). These paths do not necessarily intersect the Earth's, but do stray across the 1 AU average Earth-Sun distance. These asteroids pose both a unique opportunity and unique threat to life on Earth. Regularly coming close to Earth, they are potential targets for exploration, mining, and colonization. However, should one strike the Earth, the consequences could be severe. There are estimated to be 1,000 NEAs with diameters above 1 km, and perhaps a few hundred thousand with diameters above 100 m. There are countless numbers of smaller asteroids down to microscopic in size, the smallest of which are called dust grains. We have discovered about 2,000 NEAs. Every month, an asteroid of size 1 m or greater burns up in our atmosphere. Our atmosphere protects us from asteroids smaller than about 40 m in diameter. Changes in climate are thought to be associated with mass extinctions of species on Earth. The extinction of the dinosaurs is thought to be associated with the climate change brought about by the after-effects of an impact with an asteroid a few 10s of km in diameter. Other climate changes are thought to be due to other effects, such as long-term cycles of either the geologic activity of the Earth, or the power output of the Sun. Most of these NEAs probably result from collisions between main belt asteroids. After the collision, some of the rocks fall into elongated orbits with smaller average distances from the Sun. Compositions of asteroids
Asteroids fall into about 3 different classes based upon their elemental make-up: carbonaceous, silicate, and metallic. Carbonaceous asteroids (C-type) contain more carbon and other volatile materials than other asteroids. A volatile substance is one that evaporates at low temperatures. Examples are water and ammonia. Water can incorporated into the crystalline structure of rock and remain as a low-density solid. Silicate asteroids (S-type) are sometimes called "stony" because their composition is dominated by silicon-oxygen compounds, similar to the Earth's surface. Metallic asteroids (M-type) have a high proportion of iron, and possibly nickel. The third largest asteroid, Vesta (V-type), appears to be in a class of its own. Its surface is made of a solidified volcanic type of rock, called basalt, which is rich in iron and magnesium compounds. If so, than Vesta is the only asteroid to have experienced a (brief) molten phase. A few NEAs have compositions so similar to Vesta that we guess that they separated from it somehow, probably in collisions with other asteroids. The above table shows that asteroids vary in composition from region to region within the solar system. The variation is not random. Carbonaceous asteroids tend to be found in the outer solar system, far from the Sun, where the temperature is cooler. At these lower temperatures, volatile materials would be less likely to boil away to space. Volatile material collected here to form the outer asteroids. Material that becomes a gas at high temperatures could survive in the inner solar system, and so coalesced to form the inner belt asteroids. Collisions then injected the inner solar system with some of these bodies. How asteroids are investigatedThe nature of asteroids has been revealed through occultations of background stars, the nature of the light they reflect, remote sensing using Earth-based radar, and visitation by remote probe. Web references
|