Apollo Asteroids
Apollo asteroids are a specific group of near-Earth asteroids (NEAs) characterized by their orbits that intersect the paths of both Earth and Mars. These small, irregularly shaped celestial bodies, generally no larger than 6 miles (10 kilometers) in diameter, are part of a broader category of near-Earth objects (NEOs) that includes comets and other asteroids influenced by gravitational forces. Apollo asteroids represent over 60 percent of known NEAs, with more than 1,600 identified so far. Their proximity to Earth raises concerns about potential collisions, as demonstrated by significant historical impacts, such as the event believed to have contributed to the extinction of the dinosaurs.
Scientific interest in Apollo asteroids stems not only from their collision potential but also from their role as remnants of the solar system's formation around 4.6 billion years ago. NASA has established initiatives to monitor these objects, including the Center for Near-Earth Object Studies (CNEOS), which employs advanced tracking technologies and space missions to gather vital information about them. Ongoing research and missions aim to enhance our understanding of these asteroids and develop strategies for planetary defense against possible threats.
Apollo Asteroids
FIELDS OF STUDY: Astronomy; Observational Astronomy
ABSTRACT: Apollo asteroids are a type of near-Earth asteroid (NEA). These space objects orbit around the sun close enough to cross the orbits of both Earth and Mars. Planetary scientists study NEAs because they have the potential to collide with Earth. Additionally, NEAs can provide information about the early solar system.
Near-Earth Asteroids
Apollo asteroids are small, irregularly shaped space objects that travel around the sun in the solar system. Like other objects in the solar system, they orbit the sun in a roughly elliptical (oval) shape. However, Apollo asteroids are of particular interest because their paths of travel cross the orbits of both Earth and Mars.
The solar system contains millions of asteroids, most of which can be found within the asteroid belt, an oval-shaped ring located between the orbits of Mars and Jupiter, about 2 to 4 AU (about 186 to 370 million miles) from the sun. However, some asteroids are ejected from the asteroid belt into the larger solar system. This can occur when asteroids collide or become trapped in the massive gravitational force surrounding Jupiter.
Asteroids that are pushed into the solar system sometimes end up orbiting close to Earth and the sun. near-Earth asteroids (NEAs) are asteroids that come within 0.3 AU (about 44.8 million kilometers, or about 31 million miles) of Earth. NEAs are categorized by distinguishing characteristics, such as their average distance from the sun (both near and far) and the size and shape of their orbital paths.
NEAs are part of a larger grouping of space objects called near-Earth objects (NEOs). This group includes comets and asteroids that have been pulled near the four inner planets (Mars, Mercury, Earth, and Venus) by gravitational forces.
Asteroid Characteristics
Asteroids are made up mainly of rock. Their surfaces are covered with visible craters. Asteroids vary greatly in size. The largest known asteroid, Ceres, measures about 966 kilometers (600 miles) in diameter. However, most asteroids are considerably smaller. Apollo asteroids are on the smaller side, measuring no more than about 6 miles (10 kilometers) in diameter. At 8.5 kilometers wide, 1866 Sisyphus is the largest known Apollo asteroid.
Astronomers have identified more than 1,600 Apollo asteroids in the solar system. They are the most common type of NEA, comprising more than 60 percent of known NEAs. They are distinguished by their unique orbital paths, which cross the orbits of both Earth and Mars.
Astronomers keep a close eye on known Apollo asteroids, because they come close enough to Earth to collide with the planet.
What NEAs Tell Us
Scientists are interested in asteroids and other NEOs because they are thought to be remaining debris from the formation of the solar system about 4.6 billion years ago. They can provide valuable information about the early solar system. Additionally, about 20 percent of NEAs come close enough to Earth to raise concerns about potential collisions with Earth. Although smaller space objects such as meteors collide with Earth frequently, these objects are usually so small that they burn up in the atmosphere before they can reach the planet’s surface.
Major collisions between Earth and NEAs have occurred in the past. For example, a huge crater near Chicxulub, Mexico, is believed to be evidence of a past collision between Earth and a large NEA. Many experts believe this collision, which probably had the force of a nuclear explosion, played a major role in the Cretaceous-Tertiary mass extinction 65 million years ago. During this extinction, dinosaurs and other large life forms on the planet died out. Researchers believe that this NEA was about the same size as Apollo asteroid 1866 Sisyphus.
Other NEAs have collided with Earth more recently. For example, in February 2013, a relatively small NEA exploded over Russia, releasing the energy of a large atomic bomb. Another small NEA hit Earth's atmosphere over the Norwegian Sea before disintegrating on March 11, 2022. Scientists predict that on April 13, 2029, asteroid 99942 Apophis will pass within 19,000 miles (31,000 kilometers) of Earth. This is a much shorter distance than most asteroids.
NASA’s Near-Earth Object Program
The National Aeronautic and Space Administration (NASA) created the Center for Near-Earth Object Studies (CNEOS) to detect and track NEOs. This program uses teams around the world to learn more about these objects and to develop technologies that will someday allow scientists to divert NEOs away from Earth. These teams use state-of-the art technologies such as massive radars that bounce radio waves off asteroids in order to identify, track, and learn about NEOs.
Additionally, NASA has successfully launched many robotic spacecraft missions to study NEOs. For example, the NEAR-Shoemaker mission was launched in 1996 to study the asteroid Eros, while the Dawn mission was launched in 2007 to study asteroids Vesta and Ceres. NASA's OSIRIS-REx was launched in 2018 to study the asteroid Bennu.
Finally, NASA is conducting a Double Asteroid Redirection Test (DART) in which it will launch a spacecraft at a high speed to crash into an asteroid and alter its path. The impact should occur in September 2022. NASA believes that this is an important step in preparing Earth's planetary defenses.
PRINCIPAL TERM:
- near-Earth asteroid (NEA): a small, irregularly shaped celestial body with an orbital path that brings it close to Earth. NEAs travel within 0.3 astronomical units (AUs) of Earth’s orbit and within 1.3 AUs of the sun. An AU is equal to about 149.6 million kilometers (about 93 million miles).
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