What Are Near-Earth Asteroids?
Near-Earth Asteroids (NEAs) are rocky objects orbiting the Sun that come within 1.3 astronomical units (AU) of Earth—about 120 million miles or 195 million kilometers. To put this in perspective, Earth orbits at 1 AU from the Sun, so NEAs are those asteroids whose paths bring them relatively close to our planet's orbital neighborhood.
Most NEAs are remnants from the early solar system, 4.6 billion years old. They're leftover building blocks that never coalesced into planets. Their orbits can be influenced by the gravitational pull of planets, particularly Jupiter, which occasionally nudges asteroids from the main asteroid belt (between Mars and Jupiter) into orbits that cross Earth's path.
Currently, we've discovered over 34,000 NEAs, with new ones found regularly. They range in size from small boulders just a few meters across to massive objects over 30 kilometers (18 miles) in diameter.
Categories of Near-Earth Objects
NEAs are classified into groups based on their orbital characteristics:
Atens
Asteroids with orbits that lie mostly inside Earth's orbit. Their average distance from the Sun is less than 1 AU, but their elliptical paths cross Earth's orbit. About 2,500 known Atens exist.
Apollos
The largest NEA group with over 17,000 known members. Apollos have orbits larger than Earth's (more than 1 AU from the Sun) but cross Earth's orbital path during part of their journey around the Sun. These pose the most common potential impact threat.
Amors
Asteroids with orbits entirely outside Earth's orbit but inside Mars' orbit. While they don't currently cross Earth's path, gravitational perturbations could eventually change their orbits to become Earth-crossers. About 10,000 Amors are known.
Atiras
Rare asteroids with orbits entirely inside Earth's orbit. Only about 40 are known because they're difficult to observe—they're always close to the Sun from our perspective, making detection challenging.
Potentially Hazardous Asteroids (PHAs)
Not all NEAs are considered potentially hazardous. An asteroid earns the PHA designation if it meets two criteria:
- Size: Larger than approximately 140 meters (460 feet) in diameter
- Proximity: Orbit comes within 0.05 AU (4.6 million miles / 7.5 million km) of Earth's orbit
The size threshold represents the point where an impact would cause regional devastation rather than just local damage. The proximity criterion identifies objects that pass close enough to potentially collide if orbital perturbations occur.
Currently, about 2,300 PHAs are known. The "potentially hazardous" label doesn't mean they will hit Earth—it means they're large enough to cause significant damage if they did, and they pass close enough that careful tracking is warranted. For most PHAs, we can predict their orbits decades or even centuries ahead with high precision.
How We Detect and Track Asteroids
Discovering and monitoring asteroids requires systematic observation of the night sky:
Survey Programs
- Catalina Sky Survey (CSS): Uses telescopes in Arizona to scan the sky
- Pan-STARRS: Two telescopes in Hawaii with wide-field cameras
- ATLAS: Four telescopes designed for detecting fast-moving objects
- NEOWISE: Space-based infrared telescope detecting asteroids by their heat signature
Detection Process
Telescopes photograph the same region of sky multiple times per night. Software compares images to identify moving objects—asteroids appear as points of light that shift position between frames while stars remain stationary. Once detected, the object is tracked across multiple nights to determine its orbit precisely.
NASA's Center for Near-Earth Object Studies (CNEOS) at JPL maintains a database of all known NEAs, calculating their orbits and predicting close approaches for decades into the future. This data is publicly available and used by observatories worldwide.
The Real Risk: Should We Worry?
Hollywood loves asteroid disaster movies, but what's the actual threat level?
Historical Context
Earth has been struck by asteroids throughout its history. The most famous impact occurred 66 million years ago when a 10-kilometer (6-mile) asteroid struck near Mexico's Yucatan Peninsula. The impact created the Chicxulub crater and contributed to the extinction of the dinosaurs, along with about 75% of all species.
More recently, the 1908 Tunguska event in Siberia saw a 50-100 meter asteroid or comet fragment explode in the atmosphere, flattening 2,000 square kilometers of forest. In 2013, a 20-meter asteroid exploded over Chelyabinsk, Russia, with the energy of 500 kilotons of TNT, injuring over 1,500 people (mostly from broken glass).
Current Threat Assessment
Thanks to detection programs, we've found over 95% of NEAs larger than 1 kilometer—the "civilization-ending" size. None of these pose any threat for at least the next 100 years. For smaller but still dangerous objects (140+ meters), we've found about 40% of the estimated population.
Impact probability is measured on the Torino Scale (0-10) and Palermo Technical Impact Hazard Scale. Currently, no known asteroid rates above 0 (no hazard) on the Torino Scale. When new asteroids are discovered, they sometimes briefly show a low impact probability that disappears as we refine their orbit with additional observations.
Statistically:
- Asteroids capable of global catastrophe (~1 km) strike roughly every 500,000 years
- Regional devastation-size impacts (~140 m) occur every 10,000-20,000 years
- Tunguska-sized events (~50 m) happen every few centuries
- Chelyabinsk-sized events (~20 m) occur every few decades
Planetary Defense: The DART Mission
In September 2022, NASA successfully demonstrated humanity's ability to defend against asteroid threats. The Double Asteroid Redirection Test (DART) deliberately crashed a spacecraft into Dimorphos, a small moon orbiting the asteroid Didymos.
DART's kinetic impact changed Dimorphos' orbital period by about 33 minutes—far exceeding the mission's success criteria of just 73 seconds. This proved we can alter an asteroid's trajectory if given enough warning time.
Key lessons from DART:
- Kinetic impact works: Hitting an asteroid can meaningfully change its orbit
- Time is crucial: The earlier we detect a threat, the smaller the deflection needed
- Ejecta matters: Material ejected from impact contributed significantly to the momentum change
- Follow-up needed: ESA's Hera mission will visit in 2026 to study the impact crater in detail
Asteroids as Resources
While asteroids can pose threats, they also represent extraordinary opportunities. These space rocks contain valuable materials:
Metal-Rich Asteroids
M-type asteroids consist primarily of metallic iron and nickel, often with platinum-group metals. A single metallic asteroid 500 meters across could contain more platinum than has ever been mined on Earth. The asteroid 16 Psyche (target of NASA's Psyche mission) might contain metal worth $10,000 quadrillion at current market prices—though flooding the market would crash prices.
Water-Rich Asteroids
C-type (carbonaceous) asteroids contain water ice and organic compounds. Water can be split into hydrogen and oxygen—rocket fuel. Establishing "gas stations" in space using asteroid water could enable much cheaper deep space exploration since we wouldn't need to launch all fuel from Earth.
Scientific Value
Asteroids are primitive, unchanged remnants from the solar system's formation. Studying them helps us understand:
- How planets formed
- The composition of the early solar system
- How organic molecules might have arrived on Earth
- The potential for life's building blocks elsewhere
Recent and Upcoming Asteroid Missions
Several spacecraft have visited or are en route to asteroids:
- OSIRIS-REx (NASA): Returned samples from asteroid Bennu in 2023
- Hayabusa2 (JAXA): Returned samples from asteroid Ryugu in 2020
- DART (NASA): Successfully impacted Dimorphos in 2022
- Hera (ESA): Will arrive at Didymos system in 2026 to study DART's impact
- Psyche (NASA): En route to metallic asteroid 16 Psyche, arriving 2029
- Lucy (NASA): Touring Jupiter's Trojan asteroids through 2033
What You Can Do
You don't need to be a professional astronomer to contribute to asteroid science:
- Track asteroids: Use our asteroid tracker to monitor upcoming close approaches
- Citizen science: Projects like Asteroid Zoo let volunteers help classify asteroids
- Stay informed: Follow NASA CNEOS for official impact assessments
- Amateur astronomy: Backyard telescopes can contribute observations to global tracking networks