Near Earth Asteroids: How many are there, how are they classified, and what risk do they represent?
Last Updated: October 13, 2023
When you hear the word asteroid, you may think of a video/ arcade game, fantastical spaceships dodging around in the asteroid belt, or the damage that a lone object hurtling through space can cause when it crashes into something else.
But, when does one get close enough to be called a Near-Earth Asteroid? How big do they get? When does an asteroid become a concern for a collision with Earth? What are the risks? What plans do we have in place for those that are of concern?
As we approach the UN-designated Asteroid Day on June 30th, let’s explore more about these small bodies, let’s dive into Near Earth Asteroids!
Introduction and Background
We’ve talked before about the smaller parts of our solar system including asteroids, comets, and meteoroids, and the different types of asteroids, but what about the ones that particularly impact Earth’s neighborhood in space? Near Earth Objects (NEOs) are asteroids that due to the gravitational impacts of the planets are within Earth’s neighborhood of space, specifically ones with closest approaches within 1.3 AU of the Sun (1 Astronomical Unit or AU is equal to the average distance of the Sun to the Earth which is equal to approximately 93 million miles or 150 million kilometers).
NEOs range in size from meters to tens of kilometers in length. The vast majority of NEOs are asteroids (currently over 31,800 of the 600,000 known asteroids in the solar system) and Near Earth Comets (NECs) are short-period comets with an orbital period of less than 200 years (currently totaling over a hundred).
NEOs that are specifically asteroids are identified as Near Earth Asteroids or NEAs. Since these asteroids either intersect or come close to intersecting our orbit, there is a chance of collision and therefore detecting, cataloging, and tracking them has become an important aspect of planetary defense for today and into the future.
Classifications of NEAs
NEAs are classified or grouped by orbit with 3-4 main groups (Aten, Apollo, Amor, and Atira though the last isn’t always included) based on their closest and furthest distance from the Sun (known perihelion and aphelion respectively) as well as their semi-major axes which are the long axis of the orbit since many are more oval shaped than circular.
Keeping in mind that NEAs are asteroids that have a perihelion distance of 1.3 AU or less, the following metrics are used to classify them further.
The semi-major axis is less than 1 AU and the aphelion distance is greater than 0.983 AU
NEAs whose orbit crosses the Earth’s orbit and with semi-major axes smaller than Earth’s
The semi-major axis is greater than 1 AU and the perihelion distance is less than 1.017 AU
NEAs whose orbit crosses the Earth’s orbit and with semi-major axes larger than Earth’s
The semi-major axis is greater than 1 AU and their perihelion distance is between 1.017 and 1.3 AU
NEAs whose orbit is Earth approaching but between Earth’s orbit and Mars’
The semi-major axis is less than 1 AU and their aphelion distance is less than 0.983 AU
NEAs whose orbits are entirely within the Earth’s orbit around the Sun
Observing and tracking
Since asteroids typically move faster with respect to the background sky than more distant objects, many asteroids are discovered thanks to amateur astronomers and citizen scientists who spy a body moving faster relative to the background and submit the position in the sky and observation time to the International Astronomical Union (IAU) Minor Planet Center.
These are later added to the catalog of known asteroids if confirmed through sufficient subsequent observation. Professional astronomers also discover asteroids thanks to their regular observations of the night sky. As of May 1, 2023, over 393,700,000 observations have been submitted to the Minor Planet Center.
NEAs are also discovered using all-sky surveys from various telescopes including upcoming projects such as the Vera C. Rubin Observatory and the ESA’s Flyeye network of observatories. The NEO Surveyor will be the first space telescope dedicated to discovering asteroids and comets, with a particular goal of discovering more than 90% of all PHAs.
We also use a variety of radar systems to find and track NEOs as part of the Planetary Defense Project. As of May 1, 2023, over 10,400 discovered NEAs are over 140 meters and over 850 of these are larger than a kilometer. There are an estimated 15,000 NEAs over 140 meters yet to be discovered with an estimated 50 of these being larger than a kilometer. What are currently some of the biggest most well-known NEAs?
You can check out the Next Five Asteroid Approaches at NASA JPL’s Asteroid Watch website as well as other tracking and resources related to asteroids such as NASA’s interactive 3D visualization Eyes on Asteroids program including live countdowns and visualizations of upcoming asteroid close approaches.
Diameter: 23.4 miles (37.68 km) in diameter
Comparable in size to the city of Houston
Perihelion distance: 1.25 AU
Aphelion distance: 4.09 AU
0.34 AU from Earth’s orbit at the closest point
Orbital Period: 1,590 days (4.35 years)
Listed as the largest NEA
Diameter: 10.4 miles (16.84 km)
Comparable in size to the city of Philadelphia
Perihelion Distance: 1.13 AU
Aphelion Distance: 1.79 AU
Orbit Period: 643 days (1.76 years)
Orbit approaches Earth’s but does not cross it
Close approach within 0.40 AU on Nov 30th, 2025
1st NEO ever found in August 1898
1st asteroid ever orbited by a spacecraft (NEAR-Shoemaker)
Diameter: 3.36 miles (5.4 km)
Comparable in size to the San Francisco Bay
Perihelion Distance: 0.95 AU
Aphelion Distance: 4.13 AU
Orbit Period: 1,480 days (4.06 years)
Closest Approach to Earth at less than 0.05 AU (1,845,076 miles/ 2,969,518 km) on Nov. 5, 2069
Diameter: 0.21 miles (0.33 km)
Comparable in size to a football field
Larger than 90% of asteroids, but relatively small compared to large asteroids
Perihelion Distance: 0.95 AU
Aphelion Distance: 1.69 AU
Orbit Period: 557 days (1.52 years)
Crosses the orbit of Earth
Next closest approach will be March 30, 2030, at a distance of 0.376 AU and the next closest approach at less than 0.05 AU will be June 11, 2036, with the nearest distance to Earth at 3,774,006 miles (6,073,674 km)
1st asteroid to be the target of a sample return mission (Hayabusa)
Which of these classes is most likely to be a potential concern to Earth? It could be any of them. To determine that, we need to narrow down the parameters a little more, particularly in regard to variables that would create threatening close approaches to Earth including distance from our orbit and size.
Potentially Hazardous Objects (PHOs) are ones that get within 4,650,000 miles (7,480,000 km) of our orbit and are bigger than 500 feet or 140 meters in diameter. Most PHOs are asteroids, labeled as Potentially Hazardous Asteroids (PHAs).
Being labeled as a PHO does not mean that the object will impact the Earth in the foreseeable future, but it does mean we pay extra attention to them since they are closer to our orbit than most and in the unlikely event of a collision, the results would likely be disastrous at least in the local area of impact. In the month of April 2023, 10 NEAs passed closer to Earth than the Moon in the past month and 103 did so in the last year, meaning that these are fairly regular occurrences and usually nothing to be concerned about.
Recent and upcoming “noteworthy” closest approaches include the following:
Size and Class
Diameter: 166.67 ft (50.18 meters)
About the size of an airplane
March 25, 2023
Over 100,000 miles (160,934 km) away, about half the distance between the Earth and the Moon
Perihelion distance: 1 AU
Aphelion distance: 3.32 AU
Orbital Period: 3.17 years
Next closest approach less than 0.05 AU is April 3, 2026
Not a PHA
size of a 40 to 80-story building (393 to 853 ft or 120 to 260 meters)
April 6, 2023
1.8 million miles or 2.9 million km (about 7.5 times the average distance between the Earth and the Moon)
Perihelion Distance: 0.85 AU
Aphelion Distance: 3.47 AU
Orbital Period: 3.17 years
Diameter: 232.5 feet (70.88 meters)
July 4, 2023
Now calculated at a distance of about 6.2 million miles or 10 million kilometers (more than 20x the distance of the Moon)
Perihelion Distance: 0.67 AU
Aphelion: 2.27 AU
Orbital Period: 1.78 years
On January 7, 2023 (1 day after its discovery), it was flagged as a potential impact for July 4th with a Palermo Scale rating of -1.5; the highest ranking on the Palermo Scale seen in a decade (-0.66) recorded a few days later; full moon hid it for a few days, causing increased worry; immediately after the moon was dim enough again, observations immediately reduced the risk level to no impact risk
Again, for the most part, all of this is completely normal and nothing to be concerned about. It doesn’t mean that we shouldn’t keep an eye on them, but it also doesn’t mean that an asteroid passing between the Moon and the Earth should immediately be a cause for concern over an impact.
In fact, the majority of PHAs contain no threat of impact (without significant changes to their orbit) for the next 100 years.
So if even PHOs/ PHAs aren’t necessarily on a collision course with Earth for the foreseeable future, what are the risks and how do we classify them? Which are the most concerning? The early solar system was a chaotic period, full of collisions as matter swirled around the Sun, attempting to collate into bigger bodies.
Looking at the cratered surface of the moon (devoid of atmosphere, tectonic activity, and environmental erosion factors) details a past of frequent impacts. Eventually, things calmed down and generally settled into where they are now.
The gravitational impacts of planets, moons, and even smaller bodies make slight changes to the orbits of objects around them, but in general, impacts are much less common than they used to be. The Earth is still frequently bombarded by tiny micrometeoroids (often smaller than a grain of sand) that appear as meteor showers (shooting stars) in the night sky as they tear through our atmosphere and usually burn up from the immense heat. 100 tons of dust and sand-sized particles bombard Earth daily. Rarely are they big enough to get through and fall to the ground as meteorites.
However, all we need to do is look to the past to see evidence of asteroid impacts that have severely altered the landscape of the surrounding area and even the globe. The main theory for the Cretaceous Period extinction event (including the dinosaurs) was an impact from an asteroid estimated to have been 6 miles (10 km) in diameter. An asteroid measuring 330 feet (100 meters) in diameter exploded over Siberia in 1908, destroying over half a million acres of forest.
So, how do we determine the risk of the various PHOs, primarily PHAs, in our neighborhood of space? We primarily utilize two scales for NEO risk determination.
The Torino Scale focuses on a broad picture of risk based on impact probability (how likely that particular NEO is to hit the Earth or its atmosphere) and kinetic energy (the amount of energy that would be released upon impact, relating to how fast the object is moving and how massive it is).
Ranging from 0 to 10 where 0 is no hazard and 10 is a certain impact with likely catastrophic global effects, several NEAs are initially categorized as a 1 with the chance of collision extremely unlikely, further data usually downgrades the asteroid to a level 0 where the collision likelihood is zero or effectively zero, even including those rare meteorite falls that rarely cause damage.
The scale ranks as follows:
No Hazard: chance of collision is zero or effectively zero
Normal: chance of collision is extremely unlikely; subsequent observations very likely to downgrade to Level 0
2: Object makes a somewhat close but not highly unusual pass near the Earth, perhaps regularly; merits the attention of astronomers, but not public attention or concern; subsequent observations usually downgrade the level over time
3: A close encounter with a 1% or greater chance of collision capable of localized destruction; public attention merited if the encounter is less than 10 years away; subsequent observations usually downgrade the level over time
4: A close encounter with a 1% or greater chance of collision capable of regional devastation; public attention warranted if the encounter is less than 10 years away; subsequent observations usually downgrade the level over time
5: A close encounter posing a serious, but uncertain threat of regional devastation; critical attention by astronomers to determine the risk; governmental contingency planning may be warranted if the encounter is less than 10 years away
6: A close encounter with a large object poses a serious but uncertain threat of global catastrophe; critical attention by astronomers is needed and if the encounter is less than 30 years away, governmental contingency planning may be warranted
7: A very close encounter by a large object which poses an unprecedented but uncertain threat of a global catastrophe; if the encounter is within 100 years, international contingency planning is warranted
8: A collision capable of causing localized destruction is certain; these events occur on average between once per 50 years and once per several thousand years
9: A collision causing unprecedented regional devastation is certain; such events occur once per 10,000 years and once per 100,000 years
10: A collision capable of causing global climatic catastrophe; such events occur on average once per 100,000 years or less
According to the IAU, as of this writing, only 1 NEO rates higher than level 0 on the Torino Scale.
The Palermo Technical Impact Hazard Scale compares the probability of an impact and its kinetic energy to the “status quo” frequency of similarly sized objects colliding with Earth. It is mainly used by scientists to understand how populations of objects of a particular size behave over long periods.
Due to the wide range of sizes and velocities among NEOs, this creates a wide range range of results, meaning the Palermo Scale is often discussed on a logarithmic scale to discover patterns. A NEO with a scale of -2 is 1% as likely to occur as a random “status quo” background event and one with a scale of 0 is just as likely to occur as a random background event. Most NEOS are between -2 and 0. Positive scores produce concern on the part of astronomers. None currently rank above 0 as of this writing.
Sentry (from NASA JPL) is a highly automated collision monitoring system that continually scans the most up-to-date asteroid catalog for possibilities of future impact with Earth over the next century. When a potential impact is detected, it is analyzed and added to the Sentry Table. When subsequent observations downgrade the possibility out of concern for the next 100 years, they are removed from the table and documented as such.
As of May 2023, there are 25 asteroids on the Sentry table. However, it is worth noting a few points. None of these have a positive score on the Palermo Scale and all except 2 are classified as 0 on the Torino Scale (with the exceptions being over 100 years in the future and therefore not applicable). All of them have significantly tiny probabilities for impact. The majority are predicted for at least 20 years from now. Again, there are currently no Near Earth Asteroids that have a statistically significant possibility of impacting Earth in the next 100 years.
The ESA keeps a Risk List of NEOs that have a non-zero impact probability based on the Palermo Scale. There are currently just under 1,500 on the list and 2 on the Special Risk List (Bennu and 29075 1950DA) both of which have a potential impact date in over a century (over 1,000 and 8,000 years to be exact). Again, it is important to remember that this does not mean we have to worry about 1,500 asteroids currently. Just that there is a non-zero chance of impact.
Let’s go into a few details on NEAs that are currently expected to have a collision course with Earth in the future and have achieved some attention recently.
.73 miles (1.173 km) in diameter
Comparable in size to the U.S. Pentagon
Perihelion distance: 0.83 AU
Aphelion distance: 5.01 AU
0.05 AU from Earth’s orbit at the closest point
Orbital period: 1,830 days (5.01 years)
Nov 2022 announcement of the discovery of three asteroids hiding in the Sun’s glare; extremely low probability of an impact in the foreseeable future, but the probability increases in the next few thousand years and if it did collide, it would be a mass extinction level event
1,100 ft (340m) across, 0.21 miles (0.34) km in diameter
Comparable in size to the U.S. Capitol Building
Perihelion Distance: 0.75 AU
Aphelion Distance: 1.10 AU
Orbital Period: 324 days
Its closest point to Earth’s orbit is under .01 AU, which is of concern
March 6, 2021: nearly 10.6 miles (16,852,647 km) from Earth and radar observations ruled out any impact risk in 2029, 2068, or long after
April 13, 2029: less than 20,00 miles (32,000) km from Earth
Used to be considered one of the asteroids of most concern with a possible impact as early as 2029 (reaching the highest rating ever on the Torino Scale of 4 in 2004), but subsequent observations have reduced concern for at least the next 100 years and was even been removed from the Sentry Risk Table in 2021; the close approach in 2029 means it will be visible to the naked eye for observers in the Eastern Hemisphere and provide an unparalleled opportunity to study a relic of the early solar system
probably between 0.003 to 0.006 kilometers in diameter
Comparable in size to a school bus or smaller
Perihelion Distance: 0.80 AU
Aphelion Distance: 2.38 AU
Orbit Period: 731 days (2.00 years)
Dec. 17, 2022: 27,627 km from Earth
Dec. 17, 2024: 758,091 km from Earth
Not a PHA, but Sentry lists it as having a 0.23% chance of impact in 2024 (the fifth highest chance of impact currently on Sentry with the higher ones all being a minimum of 40 years away)
Diameter: 1,614 feet (492 meters)
Larger than the Empire State Building
Perihelion Distance: 0.9 AU
Aphelion Distance: 1.36 AU
Orbit Period: 437 days (1.2 years)
Close Approach to Earth every 6 years
Next closest approach at less than 1.05 AU will be on September 29, 2054; 2135 is the current date for possible concern, but still not technically an expected impact until 2182
Found in 1999 and named by a 3rd grade student who won a contest to name it
OSIRIS-REx mission orbited Bennu, studying its surface in unprecedented detail and collected a sample in 2020 that will be returned to Earth in September 2023; -1.41 Palermo Scale rating, the highest currently listed on Sentry
Diameter: 2 km
Comparable in size to Mt Everest
Perihelion Distance: 0.83 AU
Aphelion Distance: 2.56 AU
Orbit Period: 808 days (2.21 years)
Will pass within 11,332,561 km of Earth March 2, 2032 2032
It once had the highest known probability of impacting Earth with a 0.17 Palermo rating for a possible collision in 2880; more recent observations have reduced its Palermo rating to -2.05
Current Risk Mitigation Organizations include:
- The IAU NEO Working Group
- Center for Near Earth Object Studies (CNEOS) at NASA JPL (24 years for the Near Earth Objects Observation Program as of 2023!)
- Sentry Impact Detection System: performs long-term analyses of possible future orbits of hazardous asteroids
- Scout System which monitors new NEO observations, even before they are officially confirmed and cataloged
- ESA/ Space Safety NEO Segment
- ESA Near-Earth Object Coordination Centre (NEOCC)
- UN Office of Space Affairs (UNOOSA) and UN Committee on the Peaceful Uses of Outer Space (COPUOS) have worked together to ensure international cooperation in discovering, monitoring, and physically characterizing PHOs to help all nations be better prepared
- International Asteroid Warning Network (IAWN) to distribute information and devise communication plans in case of an emergency in addition to assisting governments in policy recommendations and impact mitigation responses; currently includes representatives from North and South America, Europe, and Asia
- Space Mission Planning Advisory Group: provides the same research and communication as IAWN, but for space missions
Mitigation Measures/ Projects currently include:
Double Asteroid Redirection Test (DART) and Hera: In November 2021, NASA launched DART to collide with Dimorphos, the moon of the NEA Didymos on September 26, 2022. Astronomers were able to measure the change in the orbital period of the small moon, confirming that DART had a consequent effect on its trajectory. In October 2024, ESA will launch Hera to Didymos with an arrival in early 2027 to provide more data on the outcome of the DART impact.
Hayabusa2: Building on its predecessor mission which retrieved an asteroid sample and provided unique insights into the early solar system, Hayabusa2 both collected a surface sample and tested the surface material’s response to an explosive event. After returning the sample to Earth, the mission has been extended to perform a flyby of another asteroid in 2026 and even rendezvous with a 60-meter NEA which is similar to the estimated NEO that exploded over Siberia in 1908 to better understand it and the complex navigation and control that would facilitate future mission to deflect PHOs.
OSIRIS-APEX and other Apophis missions: After returning the samples of NEA Bennu to Earth in September 2023, the OSIRIS-REx mission will start its new OSIRIS-APEX (APophis EXplorer) mission to perform a rendezvous with the famous NEA Apophis a few days after its close approach to Earth (36,000 km) on April 13, 2029, which will allow us to study the tidal effects from the Earth on the asteroid. Other missions are planned to study it before and during the encounter as well.
CNEOS and other organizations’ hypothetical impact exercises inform scientists and decision-makers of the warning systems and impact mitigation strategies that could be employed in the event a threatening object is identified.
On April 3, 2023, the White House Office of Science and Technology Policy (WHOSTP) released the National Preparedness Strategy and Action Plan for Near-Earth Object Hazards and Planetary Defense with six key goals:
- Enhance NEO detection, tracking, and characterization capabilities.
- Improve NEO modeling, prediction, and information integration.
- Develop technologies for NEO reconnaissance, deflection, and disruption missions.
- Increase international cooperation on NEO preparedness.
- Strengthen and routinely exercise NEO impact emergency procedures and action protocols.
- Improve U.S. management of planetary defense through enhanced interagency collaboration.
While many if not all of these have been priorities for both the U.S. and international planetary defense initiatives, the official commitment to these goals will ensure the continued importance of each of these points and their annual budgets to further protect us.
While the majority of asteroids are located in the asteroid belt and Kuiper Belt, there are over 30,000 known asteroids within 1.3 AU of Earth. Space agencies around the world have made identifying, tracking, and cataloging these Near Earth Objects, including Near Earth Asteroids, a priority.
This research has helped us better chart our neighborhood in space and actually confirmed that most of these pose little threat to Earth now and in the foreseeable future. Many cross between us and the Moon on a regular basis with no reason to worry.
While it is important to track these bodies and continue our efforts to find as many as possible to reduce the chance of unexpected collisions, it is important to remember that there are currently no known asteroids that have a significant chance of impact with Earth (or its atmosphere) in the next century.
The chance of finding out about an NEA on a collision course with Earth in the very near future is extremely low. Even the scare of 2022 AE1 in January was quickly downgraded to level 0. Therefore, if this were to change, this would be a very big deal and it would be very big news that everyone would be talking about. Approach any near/ possible collision stories with caution and skepticism as they likely aren’t accurate.
Our risk mitigation focuses primarily on information gathering through identification, cataloging, and tracking. Knowing the location of as many NEOs as possible and their orbits currently and in the future helps us to know the risks of each. In addition, missions like OSIRIS-REx and its next mission OSIRIS-APEX will help us better understand asteroid close approaches with Earth, the concerns, and our options in mitigating risk.
Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at https://medium.com/@sarah-marie and authors self-help and children’s books.