How close, how big, how often

Asteroids pass closer to Earth than the Moon about twice a week, and most of them are small. In NASA's close-approach records for 2016 to 2025, asteroids under 15 m across made about 100 such passes a year, and larger ones only about 12.^[1][2] The largest asteroids pass that close least often of all, and they are also the best known. This guide explains the distances, the sizes and the odds.

How close: the lunar distance

Flyby distances are usually given in lunar distances, written LD. One LD is the average distance between Earth and the Moon, 384,400 km.^[3] A pass at 2 LD is twice as far away as the Moon, and a pass under 1 LD is closer than the Moon.^[3]

For the very closest passes, satellites make a better yardstick. Satellites in geosynchronous orbit circle about 36,000 km above Earth, and on 13 April 2029 the asteroid Apophis will pass about 32,000 km from Earth's surface, inside that ring.^[4] CNEOS computes the close approaches of every known near-Earth object from the year 1900 to 2200, so passes like this are known decades in advance.^[1]

How big: why sizes are ranges

Very few asteroids have measured sizes.^[5] Most are seen only as points of light, so their size is estimated from brightness: the absolute magnitude, written H, and an assumed albedo, the share of sunlight the surface reflects.^[5] A dark asteroid has to be larger than a shiny one to look equally bright. An asteroid with H of 22 is about 140 m across if it reflects 15% of the light that falls on it, and about 240 m across if it reflects only 5%.^[5] H itself is usually uncertain by about half a magnitude.^[5]

That is why a new asteroid is often described as, for example, 40 to 90 m across, as 2024 YR4 was after its discovery.^[6] A firmer size needs radar, infrared measurements or a spacecraft visit.^[6]

How often: size by size

The list below groups asteroids by size and gives, for each group, how often one hits Earth, what an impact would do, and how often one passes closer than the Moon. The pass counts come from NASA/JPL's close-approach data: for the three smaller groups, passes actually spotted from 2016 to 2025; for the larger ones, past and predicted passes of known asteroids from 1926 to 2125.^[1][2] Both counts are floors, because the estimated population of small near-Earth asteroids far exceeds the number found so far.^[7]

• Under 15 m, up to the length of a bus. Objects this size enter Earth's atmosphere every year, several times a year for the smallest.^[8] They burn up as bright fireballs high in the atmosphere, and at most small meteorites reach the ground, as when the asteroid 2008 TC3, about 4 m across, exploded over Sudan in 2008, the first asteroid discovered before it hit.^[9][10] From 2016 to 2025, 1,019 passes this close were recorded, roughly 100 a year.^[1][2]
• 15 to 50 m, up to the length of an Olympic pool. One hits roughly once every 50 to 100 years.^[11] The result is an airburst: the Chelyabinsk asteroid, about 19 m across, exploded over Russia in 2013 and injured more than 1,500 people, mostly through broken glass.^[12] From 2016 to 2025, 117 passes closer than the Moon were recorded, about 12 a year.^[1][2]
• 50 to 150 m, up to the height of the Great Pyramid. An asteroid of about 50 m hits roughly once every 1,000 years.^[13] It can flatten a forest or a city-sized area, as the 1908 Tunguska airburst did to about 830 square miles of Siberian forest.^[14] From 2016 to 2025, 8 passes closer than the Moon were recorded, among them 2019 OK, estimated at 60 to 130 m, which passed about 65,000 km above Earth with very little warning.^[1][2][15]
• 150 to 300 m, up to the height of the Eiffel Tower. Impacts this size happen on the order of once every 10,000 to 20,000 years.^[16] An asteroid of about 140 m would leave a crater 1 to 2 km wide and could cause deaths across a metropolitan area.^[13] Known asteroids this size make 9 passes closer than the Moon between 1926 and 2125, about one every 22 years.^[1][2]
• 300 m to 1 km. Impacts this size happen on the order of once every 70,000 to 500,000 years and could cause damage on a national to continental scale.^[16][17] Known asteroids this size make 5 passes closer than the Moon between 1926 and 2125, about one every 40 years; Apophis in 2029 is one of them.^[1][2]
• 1 km and larger. Impacts this size happen on the order of once every 500,000 years or more, and an asteroid of about 1 km would cause devastation around the world.^[16][13] The Chicxulub impactor that ended the age of the dinosaurs 66 million years ago was about 10 km across.^[18] Between 1926 and 2125, known asteroids this size make 9 passes within 5 LD of Earth and fewer than 3 within 2 LD.^[1][2]

What this means

The rule across every size is the same: the larger the asteroid, the rarer both the impacts and the close passes, and the more completely the population has been found. NASA's search goal is at least 90% of near-Earth objects 140 m and larger; fewer than half of the estimated 25,000 are known so far, and none of the known ones has a significant chance of hitting Earth in the next 100 years.^[19] For how a possible impact is assessed, see Impact risk.