By Sabine Stanley, Ph.D., Johns Hopkins University
In the solar system planets and moons orbit around the Sun. Also orbiting throughout the solar system are millions of smaller planetoids, known as asteroids. The largest concentration of asteroids is located between Mars and Jupiter. But how many are there in near-Earth orbit?
What Are Asteroids?
Asteroids are small rocky bodies that orbit the Sun. The very biggest asteroid, Ceres, is round, but the rest have irregular shapes, including shapes like peanuts, potatoes, dog bones, and even diamonds. Besides having a wide variety of shapes, these small bodies are interesting for three reasons. First, their orbits get perturbed a lot, putting them on collision courses with planets, including Earth. Second, they are like leftovers from solar system formation, holding clues to planet formation. Third, these have resources, like precious metals and water, that humans may find useful in the future.
Learn more about the planets, moons, asteroids,and long-period comets.
Near-Earth Asteroids and the Subset PHA
The closest asteroids to Earth are called near-Earth asteroids, or NEAs, having orbits that travel partly or fully inside a distance of 1.3 astronomical units from the Sun. However, there is a subset of near-Earth asteroids known as potentially hazardous asteroids, or PHAs, that are even closer.
To qualify as potentially hazardous, an asteroid’s orbit needs to be within plus or minus 0.05 astronomical units from Earth—that’s about 71⁄2 million kilometers from Earth or about 20 times the Earth-Moon distance—and the asteroid must be larger than 120 meters. An asteroid smaller than this size is likely to explode or burn up in the Earth’s atmosphere, for an indirect ground impact.
The Effects of Smaller Asteroids
However, asteroids smaller than the potentially hazardous asteroid limit can still cause significant destruction on Earth, even if they burn up before hitting the surface. In February 2013, the city of Chelyabinsk in Russia experienced the biggest such event in over a century when an asteroid of about 20 meters in extent entered Earth’s atmosphere. It exploded 30 kilometers high in the atmosphere with the energy of 40 Hiroshima nuclear bombs. Less than 0.05% of the meteor landed in pieces on Earth, but the flash of light was brighter than the Sun, causing some severe cases of sunburn, damaging buildings, especially windows, and injuring 1,500 people. Footage of the meteor in the atmosphere captured by car dashcams in Chelyabinsk allowed scientists to determine its trajectory and entry speed. It originated in the asteroid belt, ejected through gravitational resonances with Jupiter.
Learn more about how the solar system family is organized.
The Tunguska Event
Almost 1,600 kilometers from Chelyabinsk, and 105 years earlier, the Tunguska event in Eastern Siberia flattened over 2,000 square kilometers of forest in 1908, knocking down around 80 million trees. It happened due to an asteroid around 36 meters across that disintegrated around five to 10 kilometers high in the atmosphere.
The energy released from the disintegration of the asteroid was equivalent to 2.8 million tons of TNT, about 185 times greater than the energy of the atomic bomb dropped on Hiroshima, Japan. An event like that would have caused serious destruction if it had occurred over a large city. There are serious potential hazards from objects much smaller than 120 meters.
The Impact of Craters
The famous asteroid or comet which had a bigger impact, likely killing the dinosaurs, was 10 to 15 kilometers in size. It made the Chicxulub impact crater in the Yucatán Peninsula of Mexico, which is 150 kilometers wide. Even though Earth’s surface is constantly changing, traces of some 200 impact craters have been found. Chicxulub near Mexico is the second-largest found, but almost as large is the Sudbury crater in Ontario, Canada, which is 1.85 billion years old. Larger than both is a 2-billion-year-old crater in South Africa at Vredefort with a rim-to-rim diameter of 160 kilometers and a damage diameter of 300 kilometers.
Arthur C. Clarke’s novel Rendezvous with Rama proposed a ‘SpaceGuard’ as an early warning system for hazardous asteroids. In 2005, the US Congress mandated the NASA Near-Earth Objects Program, including comets and asteroids, finding targets bigger than one kilometer which can cause global destruction.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Asteroids and Comets
The main difference between an asteroid and a comet is that the asteroids are composed of rock and include iron-rich metal cores, and sometimes have some ice, too. Comets are ice-rich and rarely have intact rocky interiors. Another difference is that asteroids are found and formed in the inner solar system. Icy comets have formed and survived by being mostly in the outer solar system, with occasional trips near the Sun. It’s only when a comet is closer to the Sun that the ices begin vaporizing from the higher temperatures and the glorious tail of a comet is seen. A third difference is that almost all known near-Earth objects are asteroids. Comets, being made of ice, don’t survive long if they spend a lot of time as close to the Sun as Earth is.
The Search for Near-Earth Asteroids
In 1980, about 50 near-Earth asteroids were known, slowly increasing to 500 by 1998. Within two years, by the year 2000, the number of known near-Earth objects suddenly doubled to 1,000 and then increased to around 20,000 by 2019. Around 1,000 of known near-Earth objects are larger than one kilometer. Global efforts continue to search for even smaller objects.
Near-Earth Asteroid Groups
Asteroids near Earth are subdivided into four groups based on their current orbits. The Amors have orbits outside of Earth’s orbit but interior to Mars’s orbit. The Apollos’ orbits cross Earth’s orbit but are mostly outside it. In contrast, the Atens’ orbits cross Earth’s orbit but are mostly inside it. The Atiras have orbits entirely inside Earth’s orbit. It is the Apollos and Atens that have the potential to collide with Earth. Asteroid orbits get perturbed by gravitational and thermal forces all the time, so an asteroid can go from not currently crossing Earth’s orbit to a more high-risk Apollo or Atens classification.
Learn more about near-Earth asteroids and the asteroid belt.
Sensing Danger Through the Torino Scale
The Torino scale reports the danger of a near-Earth asteroid over 20 meters. The scale combines a number for the size or expected energy of an asteroid with a second number for the probability that the asteroid will hit Earth. The Torino scale goes from zero, unlikely to collide or cause damage, and then jumps around until arriving at eight, nine, and ten, meaning a collision is certain, with increasing levels of damage. The impact that killed the dinosaurs is rated ten on the Torino scale. The Tunguska event is rated eight. But no monitored object has ever been rated above a level four, which means only one percent chance or more of a collision of regional devastation. Asteroids move around on the Torino scale, usually downwards. One asteroid initially rated level four is a 340-meter wide object called Apophis. In 2029, Apophis is expected to come within the orbit of our geosynchronous satellites. The risk of collision with Earth has been downgraded to level zero since 2004 and all the known near-Earth asteroids in 2018 had a zero rating on the Torino scale.
The Double Asteroid Redirection Test
NASA plans to test some ideas. In 2022, the Double Asteroid Redirection Test, known as DART, will intentionally impact the small moon of the asteroid Didymos. The moon is about 160 meters in diameter, without a formal name. But some scientists call it Didy-moon. The DART mission will test our current ability to deflect an asteroid of this size.
Common Questions About Asteroids
The difference between an asteroid and a comet is that the asteroids are composed of rock and include iron-rich metal cores, sometimes with some ice, too. Comets, on the other hand, are ice-rich and rarely have intact rocky interiors. Asteroids are found and formed in the inner solar system. Icy comets are formed by being in the outer solar system, with occasional trips near the Sun.
The asteroid with the big impact that probably killed the dinosaurs was 10 to 15 kilometers in size. It made the Chicxulub impact crater in the Yucatán Peninsula of Mexico, which is 150 kilometers wide.
The Torino scale is used to read the danger of a near-Earth asteroid over 20 meters. The scale combines a number for the size or expected energy of an asteroid with a second number for the probability that the asteroid will hit Earth. The Torino scale goes from zero, unlikely to collide, or too small to cause damage, and then jumping around until arriving at eight, nine, and ten, meaning a collision is certain, with increasing levels of damage.
In 2022, NASA plans the Double Asteroid Redirection Test, known as DART, which will impact the small moon of the asteroid Didymos.
