Size Reference

How big are
near-Earth asteroids?

Asteroid size spans an enormous range - from pebbles to objects larger than cities. The size of an asteroid determines whether it burns up harmlessly in the atmosphere or poses a genuine risk to the surface below.

See the largest known NEOs →

Size classes and what they mean

Each size class has a characteristic effect on Earth - determined by how much kinetic energy survives to the surface.

< 25 m ≈ Double-decker bus (11 m long)

Burns up in atmosphere. Fragments may reach ground. No significant surface impact.

Examples: 2023 BU (4–8 m), Chelyabinsk object (~20 m)

25–140 m ≈ Football pitch (105 m long)

Can reach the surface or cause major airburst. Local to regional damage.

Examples: Tunguska object (~50–80 m)

140 m – 1 km ≈ The Shard, London (310 m tall)

Potentially hazardous. Regional to continental damage on impact.

Examples: Apophis (~370 m), Bennu (~490 m)

1–5 km ≈ Ben Nevis, Scotland (1,345 m)

Continental to global effects. Potential climate disruption.

Examples: 4179 Toutatis (~4.5 km)

> 5 km ≈ Mount Everest (8,849 m)

Global catastrophe. Mass extinction risk.

Examples: Chicxulub impactor (~10–15 km, 66 Ma ago)

Relative scale (logarithmic)

Car

5 m

Double-decker bus

11 m

Football pitch

105 m

Chelyabinsk (2013)

20 m

Tunguska object

65 m

The Shard

310 m

Apophis

370 m

Ben Nevis

1.3 km

Mount Everest

8.8 km

Chicxulub

12.0 km

Scale is logarithmic. Grey bars = landmarks. Blue bars = asteroids/events.

Well-known near-Earth asteroids by size

A selection of named objects, from the largest to the most closely watched.

Asteroid	Diameter	Class	PHA	Note
1036 Ganymed	~37.7 km	Amor	No	Largest known NEA
433 Eros	~34.4 km	Amor	No	Visited by NEAR Shoemaker (2001)
3552 Don Quixote	~19 km	Apollo	No	Likely extinct comet nucleus
4179 Toutatis	~4.5 km	Apollo	No	Slow rotator, irregular shape
101955 Bennu	~490 m	Apollo	Yes	Sample returned by OSIRIS-REx (2023)
99942 Apophis	~370 m	Apollo	Yes	2029 flyby at ~38,000 km from Earth
25143 Itokawa	~535 m	Apollo	No	Visited by Hayabusa (2010)
162173 Ryugu	~900 m	Apollo	No	Sample returned by Hayabusa2 (2020)

Why size matters

Impact energy scales with the cube of diameter - double the diameter, and the released energy increases eightfold. A 50-metre asteroid carries roughly 500 times more kinetic energy than a 10-metre one, even at the same speed.

This is why NASA's planetary defence threshold sits at 140 metres. Below that size, the atmosphere absorbs enough energy during entry that ground-level effects are limited. Above it, surface impacts produce enough energy to cause widespread regional destruction. At 1 kilometre and above, the effects become global.

Largest near-Earth asteroids

The biggest known NEOs ranked by size.

Impact probability

How size and frequency are linked.

How close do asteroids pass?

Miss distances - from lunar distances to kilometres.

Asteroids passing Earth in 2026

Live table of upcoming close approaches.

Common questions

How big are near-Earth asteroids?

Near-Earth asteroids range across many orders of magnitude. The smallest tracked objects are a few metres across - roughly the size of a car. The largest known near-Earth asteroid, 1036 Ganymed, is about 37 km in diameter, larger than Greater London. Most of the asteroids in NASA's close-approach catalogue are small - under 50 metres - because survey telescopes are now sensitive enough to detect them.

What is the smallest asteroid that can cause damage on the ground?

Objects smaller than about 25 metres typically disintegrate in the atmosphere before reaching the surface. The 2013 Chelyabinsk event involved an object roughly 20 metres across that exploded at altitude and never reached the ground intact - yet its shockwave still broke windows and injured around 1,500 people. Objects in the 25-50 metre range can produce partial penetration to the ground in denser material. Above 50 metres, the risk of surface damage rises significantly.

How big was the asteroid that killed the dinosaurs?

The Chicxulub impactor that caused the Cretaceous-Paleogene extinction event 66 million years ago is estimated at between 10 and 15 kilometres across. For comparison, that is roughly the height of Mount Everest (8.8 km) plus a few kilometres. The impact released energy equivalent to billions of nuclear weapons and triggered global climate effects that lasted years. NASA has catalogued more than 95% of near-Earth objects at this scale. None are on a collision trajectory.

How big is the largest known near-Earth asteroid?

1036 Ganymed is the largest known near-Earth asteroid, with a diameter of approximately 37.7 km. It is an Amor-class asteroid, meaning its orbit does not currently cross Earth's orbit. The second largest is 433 Eros at about 34.4 km - a peanut-shaped object visited by NASA's NEAR Shoemaker spacecraft in 2001. Neither poses an impact risk.

How do scientists estimate asteroid size?

Several methods are used. The simplest is apparent brightness combined with an assumed reflectivity (albedo). Darker asteroids reflect less light so must be larger to appear equally bright. Thermal infrared measurements - which detect the heat an asteroid radiates - give more reliable size estimates regardless of surface colour. Radar observations provide the most precise measurements, resolving shape and rotation as well as size. For asteroids visited by spacecraft, direct imaging gives exact dimensions.