Asteroids: Remnants of the Early Solar System

Asteroids are small, rocky objects that orbit the Sun . These remnants from the early formation of our solar system about 4.6 billion years ago provide a valuable window into how the planets, including Earth, formed and evolved . While they orbit the Sun like planets, asteroids are much smaller, ranging in size from Vesta – the largest at about 329 miles (530 kilometers) in diameter – to bodies less than 33 feet (10 meters) across . Most asteroids are irregularly shaped, though a few are nearly spherical, and they often have pitted or cratered surfaces . In fact, asteroids aren't all round like planets; they have jagged and irregular shapes .   

Asteroids are significant because they offer a glimpse into the conditions and processes that prevailed during the early solar system. Unlike planets, which have undergone significant geological changes over time, asteroids have remained relatively unchanged, preserving valuable information about the solar system's history . They can even contain water and organic molecules, and by impacting the early Earth, some may have helped deliver the ingredients for life to our planet .   

The Asteroid Belt

Most asteroids reside in the main asteroid belt, a region between the orbits of Mars and Jupiter . This belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 miles) in diameter, and millions of smaller ones . Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today .   

Types of Asteroids in the Asteroid Belt

Asteroids are classified into three main types based on their composition:

• C-type (chondrite) asteroids: These are the most common type, making up about 75% of known asteroids . They are dark in appearance due to their composition of clay and silicate rocks . C-type asteroids are considered among the most ancient objects in the solar system and may contain organic carbon, water ice, and other primordial materials . They are primarily found in the outer regions of the asteroid belt .   
• S-type ("stony") asteroids: These asteroids are composed of silicate materials and nickel-iron . They are found mostly in the inner asteroid belt . Eros, the first discovered and second-largest near-Earth asteroid, is an example of an S-type asteroid .   
• M-type (metallic) asteroids: These asteroids are made up of metallic iron and nickel and are thought to be the source of metallic meteorites . Some M-type asteroids have iron cores, suggesting they experienced high temperatures after formation, causing iron to sink to the center . Others may be the remnants of planetesimal cores that lost their outer layers through collisions . Psyche, a large M-type asteroid, is the target of NASA's Psyche mission to study its composition and history .   

Studying the composition of asteroids is crucial because it can teach us about the early Solar System. All of the planets and moons we know of have changed since their initial formation, with their surfaces dramatically reshaped by processes like volcanism, erosion, and tectonics. Asteroids, by contrast, can offer pristine records of the early Solar System . By studying what asteroids are made of, we can learn about the building blocks of planets, how the Solar System has changed over time, and even how Earth itself evolved over time .   

Trojan Asteroids

Not all asteroids in the asteroid belt follow a standard orbit around the Sun. Trojan asteroids share an orbit with a larger planet but do not collide with it because they gather around two special places in the orbit called the L4 and L5 Lagrangian points . At these points, the gravitational pull from the Sun and the planet are balanced by a Trojan asteroid's tendency to otherwise fly out of orbit . The Jupiter Trojans form the most significant population of Trojan asteroids and are thought to be as numerous as the asteroids in the asteroid belt . There are also Mars and Neptune Trojans, and NASA announced the discovery of an Earth Trojan in 2011 .   

The Kuiper Belt

While the asteroid belt is the most prominent region of asteroids, another significant area lies beyond the orbit of Neptune: the Kuiper Belt. This belt is a disk-shaped region extending from about 30 AU from the Sun . (One AU is the average distance between Earth and the Sun.) The Kuiper Belt is thought to be home to hundreds of thousands of icy bodies larger than 100 kilometers (62 miles) in diameter and trillions of smaller objects . It is composed of leftover material from the solar system's formation .   

Objects in the Kuiper Belt

The Kuiper Belt objects (KBOs) are composed mainly of frozen volatiles, such as water, methane, and ammonia . Pluto, once considered the ninth planet, is now classified as a dwarf planet and the largest known KBO. Other notable KBOs include Eris, Makemake, and Haumea.   

Due to their distance from the Sun, KBOs have experienced minimal heating and radiation, making them relatively pristine remnants of the early solar system . They provide valuable insights into the conditions and processes that existed during the solar system's formation. Further, their great distance means solar heat and radiation have minimal effects on their surface. KBOs are also spared of the frequent impacts faced by objects in the denser, inner solar system. These facts make KBOs among the most pristine targets of interest in the solar system .   

Interstellar Asteroids

While most asteroids originate within our solar system, some come from interstellar space – the space between stars. These interstellar asteroids are objects that have been ejected from their own star systems and have traveled through interstellar space before encountering our solar system.

The first known interstellar asteroid, ʻOumuamua, was discovered in 2017 . It had a highly unusual elongated shape and exhibited non-gravitational acceleration, suggesting it was not a typical asteroid . Another interstellar object, 2I/Borisov, was discovered in 2019 and was confirmed to be a comet .   

These interstellar visitors provide a unique opportunity to study the composition and characteristics of objects that formed in other star systems, offering clues about the diversity of planetary systems in the galaxy. Studying these objects helps us understand how unique or common our solar system is compared to other planetary systems in the Milky Way galaxy .   

Asteroid Threat to Earth

While most asteroids pose no threat to Earth, some asteroids have orbits that bring them close to our planet. These Near-Earth Objects (NEOs) are closely monitored by scientists to assess their potential for impact . NEOs are objects that have orbits that pass close by that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers .   

The impact of a large asteroid could have devastating consequences for Earth, with impact debris spreading throughout the Earth's atmosphere, potentially causing acid rain, blocking sunlight, and starting firestorms . Therefore, efforts are underway to detect and track NEOs and to develop strategies for deflecting any potentially hazardous asteroids . These efforts include dedicated surveys like the Catalina Sky Survey and Pan-STARRS, which scan the skies for moving objects, and space missions like NEOWISE, which uses an infrared telescope to detect and characterize asteroids .   

Economic Potential of Asteroids

Asteroids also offer a source of volatiles and an extraordinarily rich supply of minerals that can be exploited for the exploration and colonization of our solar system . It has been estimated that the mineral wealth resident in the belt of asteroids between the orbits of Mars and Jupiter would be equivalent to about 100 billion dollars for every person on Earth today . These raw materials could be used in developing space structures and in generating the rocket fuel that will be required to explore and colonize our solar system . By closely investigating the compositions of asteroids, intelligent choices can be made as to which ones offer the richest supplies of raw materials .   

Famous Asteroids

• Ceres: The largest object in the asteroid belt, Ceres is classified as both an asteroid and a dwarf planet . It was the first asteroid to be discovered, in 1801. Ceres is unique because it is thought to contain a significant amount of water ice beneath its surface . This subsurface ocean makes Ceres a potential target for future exploration to search for signs of past or present life.   
• Vesta: The second-largest asteroid in the asteroid belt, Vesta is a bright object that can be seen with the naked eye under favorable conditions. It is an S-type asteroid with a differentiated interior, meaning it has a distinct core, mantle, and crust. Vesta's surface shows evidence of past volcanic activity, making it a fascinating object for studying the geological processes that occurred in the early solar system.
• Bennu: A near-Earth asteroid, Bennu is the target of NASA's OSIRIS-REx mission . The mission successfully collected a sample from Bennu's surface in 2020, which is expected to be returned to Earth in 2023 . Bennu is a C-type asteroid and is considered a potentially hazardous object due to its small chance of impacting Earth in the future. Analysis of the Bennu sample will provide valuable insights into the composition and history of this asteroid and the early solar system.  

Asteroids are fascinating remnants of the early solar system, providing valuable insights into its formation and evolution. They are diverse in size, shape, and composition, ranging from small pebbles to objects hundreds of miles across. Asteroids hold clues to the processes that shaped our planetary system, including the formation of planets and the delivery of water and organic molecules to Earth. While some asteroids pose a potential threat to Earth, ongoing efforts to detect, track, and characterize them are crucial for planetary defense. The study of asteroids continues to reveal new information about the history of our solar system, the diversity of objects that exist within it, and the potential resources they hold for future space exploration. As we continue to explore and study these remnants of the early solar system, we gain a deeper understanding of our place in the cosmos and the dynamic processes that have shaped our world.