1862 Apollo

FIELDS OF STUDY: Asteroid Impact Avoidance; Astronomy; Observational Astronomy

ABSTRACT: 1862 Apollo is the namesake object of the Apollo asteroid group. These are asteroids whose perihelion (minimum distance from the sun) lies within Earth’s orbit and whose aphelion (maximum distance from the sun) lies outside it, causing them to cross Earth’s orbit as they travel around the sun. 1862 Apollo is a near-Earth asteroid (NEA) that is also classified as a potentially hazardous object (PHO) because of its size and close orbit.

Near-Earth Asteroids (NEAs)

1862 Apollo is the original member of the Apollo asteroid group. Apollo asteroids are a particular type of asteroid whose perihelion, or minimum distance from the sun, lies within Earth’s orbit, while their aphelion, or maximum distance from the sun, lies outside it. This causes the asteroids to cross Earth’s orbit twice as it completes one full revolution around the sun. Apollo asteroids may cross the orbits of other planets as well. 1862 Apollo crosses the orbits of three planets in its travels: Earth, Mars, and Venus. Completing this orbit takes 1862 Apollo about two years.

Millions of asteroids orbit within the solar system. Astronomers believe that asteroids are debris left over from the formation of the solar system about 4.6 billion of years ago. Most of these are located in the asteroid belt, a doughnut-shaped ring of space between the orbits of Mars and Jupiter. However, some asteroids orbit in other parts of the solar system. Although scientists have different theories about where these asteroids came from, they know that many of them were ejected from the asteroid belt after colliding with one another or getting caught by Jupiter’s or Mars’s gravity. Additionally, scientists hypothesize that some asteroids may have originated as comets in the outer reaches of the solar system. After these comets burned off their volatile substances, they left behind rocky space bodies that were no longer able to form the distinctive comet’s tail.

Asteroids sometimes travel close to Earth and the sun. Apollo asteroids are a type of near-Earth asteroid (NEA), which is an asteroid that comes within 0.3 astronomical units (AU)—about 44.9 million kilometers, or 27.9 million miles—of Earth’s orbit, and thus 1.3 AU of the sun. (One AU is equal to about 149.6 million kilometers, or 92.9 million miles—approximately the distance from Earth to the sun.) NEAs are part of a larger group of space objects called near-Earth objects (NEOs). This group includes comets and asteroids that has been pulled close to the four inner planets—Mercury, Venus, Earth, and Mars—by gravitational forces.

Discovery of 1862 Apollo and Its Moon

German astronomer Karl Reinmuth (1892–1979) discovered 1862 Apollo in 1932. He noticed the asteroid tumbling past Earth at very high speed. However, since the asteroid appeared small in size, it did not seem important. Reinmuth noted the observation but did not carefully track the asteroid. It was lost shortly thereafter.

Astronomers at the Harvard Observatory rediscovered Reinmuth’s lost asteroid in 1973. They noted that its diameter was about 1.5 kilometers (about 0.9 mile) and it approached the sun at a distance of about 0.65 AU (97.2 kilometers, or 60.4 million miles). It was designated HA 1932. Later, it was officially named 1862 Apollo, after the mythological Greek god who is sometimes said to be the sun’s chariot driver. Astronomers determined that 1862 Apollo followed an orbit that brought it within 0.3 AU of Earth every two years.

In 2005, astronomers at the Arecibo Observatory in Puerto Rico announced the discovery of a satellite, or moon, orbiting 1862 Apollo. The satellite was detected via radar. It is believed to be about 80 meters (262.5 feet) in length. It travels a three-kilometer (about two miles) orbit around 1862 Apollo.

Apollo’s Composition and Rotation

1862 Apollo is classified as a Q-type asteroid. The composition of Q-type asteroids is similar to that of common meteorites. They are made up of rock, olivine, pyroxene, and several other metals. While Q-types are rarely observed in the asteroid belt, they are quite common among small NEAs. Apollo asteroids vary in size. The largest, 1866 Sisyphus, measures about 9 kilometers (5.6 miles) in diameter.

In 2007, astronomers confirmed that 1862 Apollo is subject to a phenomenon known as the YORP effect. The YORP (which stands for Yarkovsky-O’Keefe-Radzievskii-Paddack) effect states that exposure to sunlight affects asteroids’ rates of spin and orientation. When photons from sunlight reach an asteroid, it absorbs this energy and eventually reradiates it away from itself as heat. This process both increases the asteroid’s rate of spin and changes its orientation. This can greatly impact how the asteroid evolves over millions of years. The YORP effect is especially impactful on small asteroids, which can begin to spin so quickly that they can break into many pieces. Scientists believe that the YORP effect can explain why some asteroids have satellites: a large chunk spun off the asteroid’s body and then was caught in its orbit.

Potentially Hazardous Objects (PHOs)

The National Aeronautics and Space Administration (NASA) defines potentially hazardous objects (PHOs) as asteroids and comets that have the potential to collide with Earth. These are objects that pass within 0.05 AU (about 7.5 million kilometers, or 4.6 miles) of Earth’s orbit. PHOs are also large enough to survive the passage through Earth’s atmosphere. Objects of this size—about 50 meters (164 feet) or larger—could cause significant damage to Earth on impact.

Astronomers keep a close eye on known PHOs, including 1862 Apollo, because of the potential for collisions with Earth. Earth has experienced major collisions with space objects in the past. A huge crater near Chicxulub, Mexico, is likely evidence of a collision between Earth and a large asteroid about sixty-five million years ago. This collision may have played a major role in the Cretaceous-Tertiary mass extinction of dinosaurs and other large life-forms. More recently, in 2013, a relatively small NEA entered Earth’s atmosphere and exploded over Russia, releasing about the same amount of energy as a large atomic bomb.

NASA created the Near-Earth Object Program (NEOP) to detect and track PHOs and other NEOs. This program studies these objects and is exploring technologies that will someday allow scientists to divert NEOs from Earth.

Additionally, NASA's Double Asteroid Redirection Test (DART) is the world's first full-scale mission to test techology that will be used to defend Earth against collisions with asteroids and comets. NASA launched a high-speed spacecraft at an asteroid in an attempt to redirect its path. The impact is predicted to occur on September 26, 2022.

PRINCIPAL TERMS

• Apollo asteroid group: the group of asteroids whose perihelion lies within Earth’s orbit and whose aphelion lies outside it, causing them to cross Earth’s orbit as they travel around the sun.
• asteroid: a small, irregularly shaped space rock that orbits the sun.
• potentially hazardous object: a space object greater than 140 meters (about 460 feet) in diameter with an orbit that passes within 0.05 astronomical units (about 7.5 million kilometers, or 4.6 million miles) of Earth’s orbit.
• Q-type asteroid: an asteroid that is made up of olivine (a rare iron silicate), pyroxene, and metals.
• YORP effect: a phenomenon in which objects in space change their rate of spin when exposed to rays from the sun.

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