What Is a Near-Earth Object (NEO)? Asteroid Threats Explained

📘 Definition

Near-Earth objects are small solar system bodies — primarily asteroids, but also some comets — whose orbits bring them into the vicinity of Earth. NEOs are defined as having a perihelion (closest approach to the Sun) of less than 1.3 AU. A subset called potentially hazardous asteroids (PHAs) are those larger than 140 metres with an orbit that brings them within 0.05 AU (7.5 million km) of Earth's orbit — large enough to cause regional devastation on impact. NASA's Planetary Defense Coordination Office (PDCO) coordinates NEO detection and response. The DART mission (2022) was the first test of asteroid deflection technology — successfully altering the orbit of asteroid moon Dimorphos.

35,000+ (as of 2026)

Known NEOs

2,400 PHAs

Potentially Hazardous

> 1 km diameter

Extinction-Class Threshold

First asteroid deflection (2022)

DART Mission

Understanding NEO

NEO Size and Risk

Size	Impact Frequency	Effect	Detection Status
< 25 m	Yearly	Burns up in atmosphere; airburst possible	Largely undetected
25–140 m	Centuries	City-scale destruction (Tunguska-class)	Partially catalogued
140 m – 1 km	Tens of thousands of years	Regional devastation	40% detected
> 1 km	Millions of years	Global climate disruption	95% detected
> 10 km	100 million years	Mass extinction (Chicxulub-class)	All known; none threatening

Planetary Defence

NASA's DART mission (Double Asteroid Redirection Test) successfully demonstrated kinetic impactor deflection in September 2022 by crashing a spacecraft into Dimorphos, a 160-metre asteroid moon, and measurably changing its orbital period. This was the first real-world test of planetary defence technology. ESA's Hera mission (launched 2024) will survey the impact site in detail. Future deflection methods under study include gravity tractors (a spacecraft hovering near an asteroid, using its own gravity to slowly tug it), ion beam deflection, and nuclear standoff detonation for larger objects requiring rapid response.

Detection and Tracking

NEOs are primarily discovered by ground-based survey telescopes including the Catalina Sky Survey, Pan-STARRS, and ATLAS. NASA's NEO Surveyor space telescope (planned launch 2028) will use infrared observations to dramatically accelerate detection of smaller, darker asteroids that ground-based optical surveys miss. The goal mandated by the US Congress is to catalogue 90% of NEOs larger than 140 metres — a target that current ground-based surveys alone cannot meet.

Frequently Asked Questions

Are any asteroids going to hit Earth? ▼

No known asteroid poses a significant impact threat for at least the next 100 years. NASA's Planetary Defense Coordination Office continuously monitors all known NEOs and regularly updates risk assessments. The asteroid that briefly topped risk charts — Apophis (370 m) — was cleared of any impact threat for at least the next century after refined orbit calculations in 2021.

What was the DART mission? ▼

DART (Double Asteroid Redirection Test) was NASA's first planetary defence test mission. In September 2022, a spacecraft deliberately crashed into Dimorphos — a 160-metre asteroid moonlet orbiting the larger asteroid Didymos — at 6.6 km/s. The impact shortened Dimorphos's orbital period by 33 minutes, demonstrating that kinetic impactor technology can meaningfully alter an asteroid's trajectory.