By Sabine Stanley, Ph.D., Johns Hopkins University
There are various types of asteroids, some linked through their composition to meteorites found on Earth. Explore the near-Earth asteroids, the asteroid belt, and many asteroids in the outer solar system.
Kirkwood Gaps
Kirkwood gaps occur in the asteroid belt where the missing asteroids have orbital periods integer multiples of Jupiter’s orbital period. The gaps were named after Daniel Kirkwood, who first noticed them around 1860. The gravitational perturbations of Jupiter are in the same direction and become amplified, causing larger perturbations of an asteroid’s orbit until it gets flung out of the orbital location.
Jupiter never stops pushing and replacements never stop arriving. There are many forces across the asteroid belt that perturb additional asteroids into one of the Kirkwood gaps. But once there, resonances with Jupiter begin pushing, and the asteroid doesn’t stay there for long.
Forces of Perturbation
The forces that perturb asteroids into different orbits also include collisions between asteroids that affect their orbits. But there is also a subtle force with a big effect on the orbits of smaller asteroids due to light. Like planets and moons, asteroids absorb solar radiation on their Sun-facing side. But photons of sunlight carry momentum, they push on an asteroid when absorbed, and are reradiated from the surface of the rotating asteroid. When sunlight is absorbed by one side of an asteroid, there is a time lag before the photons are reradiated back to space. During that time lag, the asteroid rotates, so the hot side of the asteroid shifts. The reradiated photons point in a different direction from when they were absorbed. As the photons radiate from the surface, they push the asteroid, just like a skateboarder jumping off a skateboard pushes the skateboard in the opposite direction.
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The Yarkovsky Effect
The pushing force is known as the Yarkovsky effect, but if an asteroid changes its orbital speed, then the orbital distance changes. That is how light changes an asteroid’s orbit. The Yarkovsky effect influences all objects in the solar system that absorb light, from dust to planets, having an important influence on objects between 10 centimeters and 10 kilometers in size. Most near-Earth objects have their orbits changed by light.
Types of Asteroids
The asteroid belt has a pattern in the composition of the asteroids. Approaching the asteroid belt from its inner edge, a predominance of silicate-rich asteroids is seen, known as stony, or S type, asteroids. Moving outward through the asteroid belt, the asteroids have more carbon, and the outer belt is predominantly C type, or carbonaceous, asteroids. In the outer belt, where it’s colder, carbon and other icy volatiles were sometimes more likely to be captured and included in asteroids.
Throughout the belt, there is another class of asteroid known as an M type asteroid. M stands for metallic, and these objects’ surfaces are very iron-metal rich. These asteroids are fractured pieces from a larger asteroid’s metallic core. NASA plans a mission to the metal-rich asteroid Psyche in order to study what is believed to be the core of a larger object.
Asteroids-Meteorites Connection
Some of the asteroids are linked through their composition to meteorites found on Earth. The asteroid Vesta is the source of HED meteorites found on Earth. Vesta is the largest and most intriguing metal-rich asteroid. It is the second-largest asteroid, with a diameter of around 530 kilometers. It is spherical, but there are large impact craters in the southern hemisphere which make it look less spherical and these large impacts eject material from Vesta, arriving as the HED meteorites found on Earth. Vesta is differentiated into a rocky mantle and iron core, the only known asteroid to have a metal core and a rocky mantle. Vesta was heated up enough in its past to have melted, allowing the separation of the denser iron from the lighter rocky components in Vesta. Its metal core is about 200 kilometers in diameter, almost Vesta’s total diameter.
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Magnetized Meteorites
Some of the meteorites from Vesta are magnetized, formed in the presence of an ancient global magnetic field that died long ago when the iron core cooled so much that it froze. Vesta’s less weathered surface is explained by leftover magnetic fields in crustal rocks, shielding the surface from the Sun’s high-energy radiation, just like Earth’s magnetic field shields the Earth from solar radiation. From 2011 to 2012, the Dawn mission visited the asteroid Vesta, orbiting between 2.1 and 2.6 astronomical units, a chance to map the connections between HED meteorites on Earth and the source material on Vesta.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Size of Ceres Asteroid
After escaping Vesta’s orbit, the Dawn mission journeyed to Ceres in 2015. Ceres is by far the biggest asteroid with a diameter of almost 1,000 kilometers, containing about of the mass of the asteroid belt and considered a planet for about 50 years after its discovery. Ceres is about 31⁄2 times smaller than Earth’s Moon, but still large enough that its self-gravity makes it a spherical body. This qualifies Ceres as a dwarf planet, large enough to be spherical and orbiting the Sun. Ceres’s orbit is fairly elliptical, changing its distance from the Sun between 2.5 and 3 astronomical units during its orbit. The inclination of Ceres’s orbit is about 10°.
Hidden Treasures of Ceres
The Dawn mission revealed Ceres to be a crater-covered world with a surface rich in water-ice and hydrated minerals. Underneath is a mantle layer that is about 100 kilometers thick, also rich with water-ice. The interior is rocky, without significant metals. In addition to craters, Ceres also has one large mountain named Ahuna Mons, a cryovolcano. Ceres has bright spots across the surface.
The brightest spot is located in a large crater named Occator. These spots are made of different salts, suggesting that the water on Ceres is more like a brine. There are also hints that Ceres has a subsurface ocean at the base of the icy layer. It rotates with a period of 9 hours and a small axial tilt of about 4°, due to which some of the craters in Ceres’s polar regions are permanently shadowed and may be places for water-ice to collect, like on Mercury and the Moon.
Learn more about exploring the Earth-moon system.
Asteroids Spread Far and Wide
Asteroids near Earth and in the asteroid belt are closer and get more attention, but there are as many asteroids in the outer solar system too. The most important location where asteroids are concentrated in the outer solar system is along Jupiter’s own orbit, 60° ahead of Jupiter and 60° behind. These are the L4 and L5 Lagrange points, stable locations near the planet’s orbit where gravitational forces from the Sun and the planet balance each other. Over 7,000 of Trojan asteroids were discovered by 2018. These two locations have about a million asteroids over one kilometer, all about 5.2 astronomical units from the Sun. That is similar in number to the asteroids in the entire main asteroid belt. A primary reason they can all fit is that the orbital angles of inclination range more widely than the main belt, up to 40°.
Trojans
Trojans have small tadpole orbits around their respective Lagrange points. Other planets revealed only a handful of Trojans. In 2011, the first Earth Trojan was found, a 300-meter object traveling ahead of Earth at the L4 point and orbiting at an inclination angle of 20° to the plane of the solar system. Asteroids are the most dynamic and changeable phenomena anywhere in the solar system. Efforts to catalog every asteroid, even below the size of the Chelyabinsk meteor, continues worldwide under the Asteroid Grand Challenge. Asteroids, though small, have large impacts on the solar system. Understanding asteroids is important, not only for future space travel throughout the solar system but also for the protection of planet Earth.
Common Questions About Asteroids
The missing asteroids have orbital periods integer multiples of Jupiter’s orbital period which causes the Kirkwood gaps. These are places where the gravitational perturbations of Jupiter are in the same direction and become amplified.
The Yarkovsky effect influences all objects in the solar system that absorb light, from dust to planets, having an important influence on objects between 10 centimeters and 10 kilometers in size. Most near-Earth objects have their orbits changed by light.
There are three types of asteroids namely the stony or S type, carbonaceous or C type, and metallic or M type asteroid.
Trojans have small tadpole orbits around their respective Lagrange points. Other planets have revealed only a handful of Trojans. In 2011, the first Earth Trojan was found, a 300-meter object traveling ahead of the Earth.
