Near-Earth object

A near-Earth object (NEO) is a small object in space that comes close to Earth. These objects move around the Sun and come within 1.3 times the distance between Earth and the Sun. Most NEOs are asteroids, but some are comets. Many NEOs have been found, and some might come close to Earth.

Some NEOs might cross Earth's path. If they are bigger than 140 meters across, they are called potentially hazardous objects. In the past, collisions with these objects have changed Earth's history. Even smaller asteroids can cause damage where they land, and larger ones can create craters or big waves in the ocean.

Because of this risk, many countries have been watching for NEOs since the 1980s. Space agencies work on ways to stop a dangerous NEO from hitting Earth. Scientists also study these objects with spacecraft, and some have brought samples back to Earth for study. There are also ideas about using asteroids for resources in the future.

Definitions

Near-Earth objects (NEOs) are small objects in space that go around the Sun and come close to Earth. They are defined by the International Astronomical Union as any small object that goes around the Sun and comes within 1.3 times the distance from the Sun to Earth. These objects can be either asteroids or comets.

Some of these objects have paths that cross Earth’s orbit and could come very close to our planet. If these objects are larger than 140 meters across, they are called potentially hazardous objects. This means they might pose a risk if they ever hit Earth. Scientists study these objects to learn about their paths and sizes to help keep our planet safe.

History of human awareness of NEOs

The first near-Earth objects humans saw were comets. We learned they came from space after Tycho Brahe measured one in 1577 and found it far from Earth. In 1705, Edmond Halley worked out the orbit of what we now call Halley's Comet. His guesses were right when it returned in 1758–1759, the first time we predicted a comet’s return.

We also found out that meteors (shooting stars) come from space after watching the 1833 Leonid meteor shower. This led us to think these meteors likely come from a comet, which we later proved when we found 55P/Tempel–Tuttle had the same path as the Leonids.

The first near-Earth asteroid found was 433 Eros in 1898. Studying its path helped us learn more about how far Earth is from the Sun.

Encounters with Earth

When a near-Earth object shares the same path as Earth, it can come very close — or even hit our planet.

Close approaches

Only 23 comets have come close to Earth. Two, Halley's Comet and 73P/Schwassmann–Wachmann, have made many close passes. The closest was Lexell's Comet in 1770. More recently, in 2025, asteroid 2025 UC11 passed very close — just 6,600 km above Earth’s surface. Other asteroids like (308635) 2005 YU55 and 367943 Duende (2012 DA₁₄) also made close, safe passes.

Earth-grazers

Some small asteroids enter Earth’s upper atmosphere at a shallow angle, glow from the heat, and then leave without hitting us. On August 10, 1972, many people saw such an object, called the 1972 Great Daylight Fireball, pass within 58 km of Earth’s surface. Another was seen over Czechoslovakia and Poland in 1990.

Impacts

When a near-Earth object hits Earth, smaller ones usually break apart in the atmosphere. Larger ones can cause big waves in water or create craters on land.

We’ve learned about these impacts by studying craters on Earth and the Moon, and by simulating orbits. For example, the Cretaceous–Paleogene extinction event about 65 million years ago is thought to have been caused by a large asteroid impact.

Risk

Throughout history, people have seen these objects as signs of change, spectacular sights, or even threats to the world. With better technology, we now understand the real risks better.

Scientists have developed ways to rate these risks, helping us share clear information with everyone. Projects like the Palomar Planet-Crossing Asteroid Survey and others have worked to find these objects early, giving us time to plan if needed.

Main article: Asteroid impact avoidance

Number and classification

Near-Earth objects (NEOs) are small bodies in our solar system that come close to Earth. When an NEO is found, its details are sent to special groups for tracking. These groups keep lists of known NEOs.

NEOs can be meteoroids, asteroids, or comets, depending on their size and what they’re made of. As of late 2024, over 37,000 NEOs have been found, with most being asteroids.

Finding NEOs can be tricky because some are harder to see than others. Bigger objects reflect more light and are easier to spot. Space telescopes help by looking at heat instead of reflected light.

Exploratory missions

Further information: List of minor planets and comets visited by spacecraft

Some near-Earth objects are interesting because spacecraft need less fuel to reach them. This is because these objects move slowly and have very little gravity. They let us study space rocks up close and might give us materials for future space projects.

Missions to NEAs

In the early days of space exploration, scientists thought visiting asteroids was too hard. But later, they found that some asteroids could be reached with less energy than it takes to get to Mars. The first asteroid visited was 433 Eros by NASA's NEAR Shoemaker probe in 2000 and 2001. Another was 25143 Itokawa, explored by Japan's Hayabusa mission from 2005 to 2007, which brought samples back to Earth. More recent missions have also explored asteroids and brought back samples.

Asteroid deflection tests

In 2022, NASA tested a way to protect Earth by crashing a spacecraft into an asteroid's moon. This changed the moon's orbit a little, showing that this method could work. In 2024, a European spacecraft was sent to study the effects of this test. China also plans to send its own probes to test this method.

Space mining

From the 2000s, some companies have wanted to mine asteroids for materials. One company tested small satellites, but closed down. Another company launched a probe in 2025 to check if an asteroid contains metal.

Missions to NECs

The first near-Earth comet visited by a spacecraft was 21P/Giacobini–Zinner in 1985. Later, several probes visited Halley's Comet. In 2014, a European probe studied another near-Earth comet and landed on its surface.