Near-Earth object (NEO)
A Near-Earth Object (NEO) is defined as an asteroid or comet that orbits the Sun within 195 million kilometers (121 million miles) and comes within 48 million kilometers (30 million miles) of Earth's orbit. NEOs can range in size from small particles that disintegrate upon entering the Earth's atmosphere to large bodies that could potentially cause significant damage. As of June 2024, astronomers have identified over 35,000 near-Earth asteroids and about 100 near-Earth comets.
NEOs are classified based on their orbits, with the majority being asteroids. Some asteroids can be categorized as potentially hazardous objects (PHOs) if they are larger than 140 meters and are projected to approach closely to Earth. While the possibility of a significant impact from a NEO is considered low, scientists continuously monitor these objects for any potential risks.
Most NEOs burn up upon atmospheric entry, with only larger ones capable of reaching the surface. Historical events, like the 2013 Chelyabinsk incident, highlight the potential effects of NEOs, demonstrating how even relatively small objects can cause substantial damage. The monitoring of NEOs is crucial for understanding and mitigating any future impact risks to Earth.
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Near-Earth object (NEO)
A near-Earth object (NEO) is an asteroid or comet that orbits the Sun within a distance of 195 million kilometers (121 million miles) and passes within about 48 million kilometers (30 million miles) of Earth's orbit. They range from small objects that burn up if they enter Earth's atmosphere to miles-wide bodies that could cause catastrophic damage to the planet. The first NEO was discovered in the late nineteenth century, and by 2017, astronomers had cataloged more than seventeen thousand. While NEOs can potentially pose a danger to Earth, it is highly unlikely a significant impact will occur in the foreseeable future.
Background
The solar system was formed about 4.6 billion years ago, when gravitational forces caused a large interstellar cloud of gas and dust to begin collapsing in on itself. As the collapsing cloud began to rotate, gas and dust started to clump together at its center. The cloud's center accumulated more and more matter until it became so dense it began fusing together atoms at its core, giving birth to the Sun. In the region around the cloud's center, smaller groups of gas and dust were also clumping together to form the planets.
When the Sun "turned on," charged particles from its atmosphere, called the solar wind, pushed away much of the lighter gas and dust from the inner solar system, leaving behind smaller, rocky worlds. In the outer solar system, the solar wind was not as strong, allowing the large gas giants such as Jupiter to form. The pieces of rock that did not clump together to form the planets became asteroids. The gravitational influence of Jupiter kept many of these rocks from becoming larger objects and relegated them to a region between Mars and Jupiter called the Asteroid Belt. Comets are giant frozen balls of gas, dust, and rock that are also left over from the birth of the solar system. Comets originate in the outer regions of the solar system, occasionally approaching the Sun in a long orbit that can take hundreds of millions of years to complete.
Overview
Occasionally, the gravity of Jupiter, Mars, or other space bodies can nudge an asteroid out of the Asteroid Belt and send it on an orbit closer to Earth. As comets approach the Sun, they can also be affected by planetary gravity and pushed into smaller orbits. To be classified a near-Earth object, an asteroid or comet must make its closest approach to the Sun within 1.3 astronomical units (au). An astronomical unit is the average distance between Earth and the Sun, or about 150 billion meters. A distance of 1.3 au would be about 194 million kilometers (121 million miles) and would bring the object to within 48 million kilometers (30 million miles) of Earth's orbit. For a comet to be classified as an NEO, it must also be a short-period comet—a comet with an orbital period of two hundred years or less.
Asteroids make up the great majority of NEOs. Near-Earth asteroids (NEAs) are classified into four groups based on their orbits and their closest approach to Earth's orbit. Amor asteroids cross the orbit of Mars, approaching Earth but do not cross its orbital path. Apollo asteroids occasionally cross Earth's path, but primarily travel outside the planet's orbit. Aten asteroids also intersect with Earth's path but spend most of their time inside its orbit. Atira asteroids have orbits contained entirely within Earth's path.
The first NEO was discovered independently by German astronomer Carl Gustav Witt and French astronomer Auguste Charlois in 1898. Named 433 Eros, the object is a potato-shaped Amor-class asteroid about 33 kilometers (20.5 miles) long by 13 kilometers (8.1 miles) wide. At its closest approach, the orbit of Eros brought it within 22.5 million kilometers (14 million miles) of Earth in 1975. The largest known NEO is the asteroid 1036 Ganymed, a 32 to 34 kilometer (20 to 21 mile) wide asteroid with an orbit that never brings it within 56 million kilometers (34.8 million miles) of Earth.
By June 2024, astronomers had detected more than thirty-five thousand near-Earth asteroids and more than one hundred near-Earth comets. More than eight hundred of the asteroids are 1 kilometer (0.62 miles) or larger, and more than seven thousand are 140 meters (460 feet) or larger. If an NEO larger than 140 meters is projected to come within 8 million kilometers (5 million miles) of Earth's orbit, it is classified as a potentially hazardous object (PHO). A PHO is not necessarily on a possible collision course with Earth; the classification only means the object will be monitored by the Center for NEO Studies (CNEOS) at the National Aeronautics and Space Administration's Jet Propulsion Laboratory in California.
The CNEOS observes near-Earth objects to determine an impact risk factor. Because astronomical calculations are constantly being reanalyzed and updated, NEOs that are placed on a watch list are often removed when new statistics are examined. In a 2024 article in the Independent, researchers, including some from NASA’s Jet Propulsion Laboratory, found only one NEO, asteroid 7482, that has a strong possibility of encountering Earth during the next one thousand years. This discovery did not mean that a collision was imminent; 7482 was predicted to spend a significant amount of time near Earth during these years and had a greater chance of colliding than other NEOs.
Earth is constantly colliding with space objects; about 100 tons of dust and small asteroid fragments enter the planet's atmosphere every day. Most of this space debris burns up and disintegrates before reaching the surface. Since the 1990s, astronomers have detected about six hundred NEOs a few meters wide that have entered the atmosphere and caused spectacular fireballs across the sky as they burned up. NEOs larger than 30 to 50 meters (98 to 160 feet) can survive the atmosphere and reach the surface. In 2013, an NEO about 16.7 meters (55 feet) wide exploded over Chelyabinsk, Russia, generating a shock wave that damaged buildings and injured about 1,200 people. Astronomers estimate a similar-size NEO can impact the planet several times a century. In 2024, a bus-sized space rock known as 2024 VM1 came within 193,000 miles of Earth.
Larger objects striking Earth are rarer and more deadly. An NEO about 50 meters (160 feet) wide is predicted to impact Earth every few hundred years. A 100-meter (328-foot) NEO is believed to strike the planet every ten thousand years. Objects 1 kilometer (.62 miles) or larger strike about every one million years and can have a devastating global impact. Objects larger than 10 kilometers (6.2 miles) can cause mass extinctions, but they occur only about every one hundred million years.
Bibliography
"433 Eros: In Depth." National Aeronautics and Space Administration, solarsystem.nasa.gov/planets/433eros/indepth. Accessed 7 Nov. 2024.
Binzel, Richard P., and Cristina Thomas. "Sizing Up the Threat from Near-Earth Objects (NEOs)." Planetary Society, www.planetary.org/explore/space-topics/asteroids-and-comets/sizing-up-the-threat.html. Accessed 7 Nov. 2024.
Dobrijevic, Daisy. "How Did the Solar System Form?" Space.com, 27 Aug. 2024, www.space.com/35526-solar-system-formation.html. Accessed 7 Nov. 2024.
Irizarry, Eddie. "Asteroid Florence Found to Have 2 Moons." Earth Sky, 2 Sept. 2017, earthsky.org/astronomy-essentials/asteroid-1981-et3-3122-florence-pass-earth-2017-binoculars. Accessed 7 Nov. 2024.
"NASA Tracking Bus-Sized Asteroid Nearing Earth Today." Newsweek, 6 Nov. 2024, www.newsweek.com/nasa-tracking-bus-asteroid-earth-today-1981379/. Accessed 7 Nov. 2024.
Nelson, Stephen A. "Meteorites, Impacts, and Mass Extinction." Tulane University, 1 Dec. 2014, www.tulane.edu/~sanelson/Natural‗Disasters/impacts.htm. Accessed 7 Nov. 2024.
"NEO Basics." Center for Near Earth Object Studies, cneos.jpl.nasa.gov/about/basics.html. Accessed 7 Nov. 2024.
Sankaran, Vishwam. "Astronomers Puzzled by Spinning Asteroid That Is Flinging Rocks into Space." Independent, 31 Mar. 2023, www.the-independent.com/space/nasa-asteroid-spinning-flinging-rocks-b2311717.html. Accessed 10 Nov. 2024.
Yeomans, Donald K. Near-Earth Objects: Finding Them Before They Find Us. Princeton UP, 2016.