By Sabine Stanley, Ph.D., Johns Hopkins University
Near-Earth asteroids are not just objects to be feared. Studying asteroids can help scientists learn about the composition and formation of the solar system, and near-Earth asteroids are the easiest asteroids to visit. Learn more about the origin of these asteroids, the asteroid belt, and the asteroid family.
The Merit Side of a Near-Earth Asteroid
Near-Earth asteroids hold mineral and water resources that could be used in the future. The first mission to orbit a near-Earth asteroid was the NEAR Shoemaker mission to the asteroid Eros in the year 2000. Eros is a potato-shaped asteroid with dimensions of around 33×13×13 kilometers. It is the second-largest known near-Earth asteroid and was the first NEA to be discovered, back in 1898. It is currently in a Mars-crossing orbit in the Amor subgroup, but models suggest it may eventually be perturbed into an Earth-crossing orbit.
Learn more about near-Earth asteroids and the asteroid belt.
The NEAR Shoemaker Mission
The NEAR Shoemaker mission orbited Eros for about a year before lowering the orbit, touching down in 2001, on a smooth, flat deposit of regolith, called a pond. Eros has a density similar to the rocks in Earth’s crust, around 2,700 kilograms per cubic meter. It has craters, grooves, ridges, and boulders covering the surface. Even though gravity is really low on Eros, there are signs of downslope movement of material and ponding of material in craters, suggesting gravity effects on the motion at the surface. Eros is a stony fragment of a previously larger body, with three percent metals, some of which are precious. If the mass of Eros is a million megatons, and if precious metals account for, say, 10 to 100 parts per million, that’s 10 to 100 megatons of precious metals.
Learn more about human futures in the solar system.
Itokawa: A Rubble Pile Asteroid
Itokawa is the smallest asteroid, called a rubble pile asteroid, covered in rocks and boulders with only a few craters. It has a low average density of about 1,900 kilograms per cubic meter, due to a lot of porosity, or empty space. Itokawa is made up of fragments, or previous asteroids, loosely held together by gravity, making it more of a rubble pile than an intact, cohesive body.
In 2005, the Japanese mission Hayabusa not only rendezvoused with Itokawa, but it also took the bold step of touching down twice on the surface, collecting a sample of dust and grains. Those samples were found to be stony, similar in composition to the most common meteorite found on Earth. Itokawa’s dimensions are less than a kilometer each, but the asteroid is big enough for a Torino scale nine event, having an Earth-crossing orbit.
Learn more about how the solar system family is organized.
The Story of Ryugu Asteroid
Ryugu is a larger one-kilometer asteroid, crossing the orbits of both Mars and Earth. Diamond-shaped, or a blocky spinning top, it has an unusual G class composition similar to Ceres. A mission named Hayabusa 2 arrived at asteroid Ryugu in June 2018, marking the first time that rovers had landed on an asteroid, and included four rovers that jumped around the surface. In September 2018, the first two rovers were released from the spacecraft. Another with a different design landed in October 2018 and the fourth in July 2019.
The Similarities of Bennu and Ryugu
The C class asteroid Bennu is the same size as Ryugu and very similar in shape. In fact, when the first close-up images of Bennu came in from the OSIRIS-REx mission during December 2018, astronomers joked that they had arrived at the wrong asteroid. In 2019, the OSIRIS-REx mission imaged pebbles and rocks sporadically ejected from Bennu’s surface at specific locations. These rocks either began orbiting Bennu or escaped the system altogether.
The Origin of Near-Earth Asteroids
Most near-Earth asteroids are destroyed or ejected from the vicinity of Earth’s orbit after a few million years. But there are still tens of thousands of NEAs, replenishing from somewhere. Studies of the composition and physical characteristics of NEAs indicate that most of these objects originated in the asteroid belt.
The asteroid belt is a ring-shaped disk that lies between the orbits of Mars and Jupiter, home to millions of asteroids. The total mass in the asteroid belt is about four percent of the mass of Earth’s Moon, and about half of that mass is from the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea. About 200 asteroids in the belt have diameters larger than 100 kilometers.
The densest part of the asteroid belt is known as the main belt, extending from 2.2 to 3.5 astronomical units. There are roughly one to two million asteroids with diameters greater than one kilometer spread across the belt. Most have orbital inclinations less than 30° above or below the plane of the solar system, and the belt is more of a donut. The main belt is a vast region, where the average distance between any two asteroids of one kilometer or larger is the distance from Earth to the Moon. Someone on a spaceship in the asteroid belt would have difficulty spotting a single asteroid anywhere around them. Any one-kilometer object is thousands of kilometers away.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Explaining the Asteroid Belt
The asteroid belt is not a planet, but in the early 19th century, when multiple asteroids were first discovered, one theory was that the asteroids used to be part of a planet that suffered some catastrophic impact, breaking it into pieces. But there were problems with that theory, because asteroids in different parts of the asteroid belt have different compositions, and it would require an extreme amount of energy to break up a planet.
Jupiter’s gravity makes it impossible to form a planet in the asteroid belt because its gravity strongly perturbs objects in that region of space, giving too much energy to small objects, preventing them from clinging together in large quantities to produce a planet. Jupiter has a lot of influence in shaping the asteroid belt. First, gravitational perturbations from Jupiter tend to keep the orbits of asteroids more eccentric and inclined than seen with planets. The orbit of the belt’s third-largest asteroid, Pallas, is a perfect example with its orbital inclination at about 35°, highly elliptical, orbiting between 2.1 to 3.4 astronomical units. This asteroid covers almost the entirety of the asteroid belt in height and width.
The Result of an Asteroid Collision
As the orbits get more eccentric, there is a larger chance of collision between the asteroids. These collisions have resulted in the breakup of many objects in the asteroid belt. Some asteroids become moons of other asteroids, like Ida, a small, unassuming asteroid having a tiny moon named Dactyl. Some of the fragments from the collisions recoalesce through gravitational attraction into loosely held rubble piles, like Itokawa. Other fragments are ejected from the asteroid belt, some ending up as near-Earth asteroids. Another possibility for fragments staying in the asteroid belt is for them to orbit the Sun without recoalescing. These co-orbiting groups of fragments are known as asteroid families—these are objects having similar orbital properties and compositions. For example, the Chelyabinsk meteor may have come from a rubble pile asteroid in the Flora family from the inside of the main belt, possibly breaking up during an earlier gravitational encounter with Earth 1.2 million years ago.
Identified Asteroid Families
Dozens of asteroid families have been identified, becoming most evident when the asteroids are plotted on graphs of their orbital properties. For example, by plotting their orbital inclination versus eccentricity, clumps of higher density objects at certain inclinations and eccentricities are seen. If orbital inclination versus their orbital distance is plotted, clumps are seen again. Spectral characterization of the asteroids in the clumps is used to confirm that these asteroids are the fragments of a once larger asteroid rather than an interloping asteroid.
The Reason for Asteroid Belt Gaps
Jupiter is responsible for gaps in the asteroid belt. If the number of asteroids as a function of orbital distance are plotted, certain locations lack asteroids. These gaps happen to occur where the missing asteroids would have orbital periods that are integer multiples of Jupiter’s orbital period. For example, the gap near 2.5 astronomical units turns out to be a 3:1 orbital resonance, where an asteroid would orbit three times the Sun every one orbit of Jupiter. Another big gap results at the 5:2 resonance and the main belt ends at the 2:1 resonance.
Common Questions About Near-Earth Asteroids
The first mission to orbit a near-Earth asteroid was the NEAR Shoemaker mission to the asteroid Eros launched in the year 2000.
Itokawa, the smallest asteroid where a spacecraft has landed, is made up of fragments, or previous asteroids, loosely held together by gravity, making it more of a rubble pile than an intact, cohesive body. It is covered in rocks and boulders with only a few craters.
In 2005, the Japanese mission Hayabusa not only rendezvoused with Itokawa, but it also took the bold step of touching down twice on the surface, collecting a sample of dust and grains, and returning that sample to Earth.
Ryugu is a large, one-kilometer asteroid that crosses the orbits of both Mars and Earth. Diamond-shaped, it has an unusual G class composition similar to Ceres. A mission named Hayabusa 2 arrived at asteroid Ryugu in June of 2018 marking the first time that rovers had landed on an asteroid.
