Asteroids are rocky, airless remnants formed during the early stages of our solar system, approximately 4.6 billion years ago. These celestial bodies provide scientists with a unique window into the conditions that existed when planets were first forming. Their diverse compositions and locations offer valuable insights into the solar system’s evolution.
Notable Asteroids and Their Unique Features
Ceres, the largest object in the main asteroid belt between Mars and Jupiter, is classified as a dwarf planet. It measures approximately 940 kilometers (584 miles) in diameter and is composed of rock and ice. Research suggests subsurface liquid water, with evidence of a briny slush reservoir beneath its Occator crater. Ceres holds more water than any other inner solar system body except Earth.
Vesta is the second most massive body in the asteroid belt and is considered a protoplanet. It has a differentiated interior with a metallic iron-nickel core, an olivine mantle, and a basaltic crust. Vesta’s surface is bright, and fragments from ancient impacts have fallen to Earth as howardite-eucrite-diogenite (HED) meteorites, providing direct samples for study.
Pallas, the third-largest asteroid, has a highly inclined orbit of about 34.8 degrees to the plane of the asteroid belt, and a moderate eccentricity similar to Pluto’s. This makes it a challenging target for spacecraft missions. Pallas is roughly 510 kilometers (317 miles) in diameter, and its surface is likely composed of silicate material, resembling carbonaceous chondrite meteorites.
Eros, an S-type stony asteroid, is an elongated, near-Earth object. It was the first asteroid orbited by a spacecraft and the first to have a spacecraft land on its surface. Eros measures approximately 34.4 by 11.2 by 11.2 kilometers (21.4 by 7.0 by 7.0 miles) in size.
Bennu and Ryugu are loose, rubble-pile asteroids and are carbonaceous, rich in carbon. They are significant targets for sample return missions due to their primitive compositions, holding clues to the early solar system’s history and the origins of organic compounds.
Classifying Asteroids
Asteroids are categorized into three main compositional types: C-type, S-type, and M-type. C-type, or carbonaceous asteroids, are the most common, comprising about 75% of known asteroids. They are dark, composed primarily of clay and silicate rocks, and found in the outer regions of the main asteroid belt. Ceres is an example of a C-type asteroid.
S-type, or stony asteroids, are the second most common type, accounting for approximately 17% of known asteroids. These are brighter than C-types and consist mainly of silicate materials and nickel-iron. S-type asteroids dominate the inner asteroid belt, and Eros is a notable example.
M-type, or metallic asteroids, comprise many of the remaining known asteroids and are found in the middle region of the main belt. They are bright and primarily composed of metallic iron and nickel. Some M-type asteroids are thought to be the exposed metallic cores of larger, differentiated bodies that lost their outer layers.
Asteroids are also grouped by their orbital locations.
Main Belt Asteroids
Main Belt Asteroids reside in the primary region between the orbits of Mars and Jupiter, where the greatest concentration of these objects is found. This belt is estimated to contain millions of objects, though its total mass is less than that of Earth’s Moon.
Near-Earth Asteroids (NEAs)
Near-Earth Asteroids (NEAs) are those with orbits that bring them within approximately 48 million kilometers (30 million miles) of Earth’s orbit. The majority of NEAs originate from the main asteroid belt and are perturbed into inner solar system orbits by gravitational influences. This group includes Amor asteroids, which cross Mars’ orbit but do not cross Earth’s orbit, and Apollo and Aten asteroids, which do cross Earth’s orbit.
Trojan Asteroids
Trojan asteroids share an orbit with a larger planet, clustering around stable gravitational points, known as Lagrangian points, located 60 degrees ahead (L4) or 60 degrees behind (L5) the planet in its orbit. While Trojans exist for other planets like Mars and Neptune, the term “Trojan asteroid” most commonly refers to the large group of asteroids co-orbital with Jupiter. Jupiter’s Trojans are distributed in two elongated, curved regions around these points.
Exploring Asteroids: Robotic Missions and Discoveries
The NEAR Shoemaker mission was the first spacecraft to orbit an asteroid and subsequently land on its surface. Launched in 1996, it studied the S-type asteroid 433 Eros, entering orbit in February 2000 and landing in February 2001. The mission provided detailed images and data, revealing Eros’s physical properties, morphology, and mineral components, and confirmed that asteroids can be solid bodies rather than loose rubble piles.
NASA’s Dawn mission, launched in 2007, explored two of the largest objects in the main asteroid belt: Vesta and Ceres. It orbited Vesta from 2011 to 2012, confirming its status as a protoplanet with a differentiated interior and linking it to HED meteorites found on Earth. Dawn then proceeded to Ceres, orbiting it from 2015 to 2018, where it discovered bright spots, identified as salty deposits from subsurface briny water that had seeped to the surface.
The Japanese Aerospace Exploration Agency’s (JAXA) Hayabusa2 mission targeted the C-type asteroid Ryugu. Launched in 2014, it arrived at Ryugu in 2018 and successfully collected both surface and subsurface samples, returning them to Earth in December 2020. Analysis of these pristine samples confirmed Ryugu as a carbonaceous chondrite and provided insights into the early solar system’s formation, including organic molecules and water-formed minerals.
NASA’s OSIRIS-REx mission, launched in 2016, visited the carbonaceous near-Earth asteroid Bennu, collecting samples in October 2020 and returning them to Earth in September 2023. The mission revealed Bennu’s surface to be rich in boulders and showed evidence of past water activity. Preliminary analyses of the returned samples have confirmed abundant organic compounds, nitrogen-rich material, and past liquid water, suggesting that the building blocks of life were widespread in the early solar system.