Near-Earth object

A near-Earth object (NEO) is any small object in space that comes close to Earth. These objects orbit the Sun and come within 1.3 times the distance between Earth and the Sun. Most NEOs are asteroids, but some are comets. There are over 37,000 known NEOs, and many of them could potentially come close to Earth.

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

Because of this risk, many countries have been looking for NEOs since the 1980s. Space agencies work on ways to prevent a dangerous NEO from hitting Earth. Scientists also study these objects with spacecraft, and some have even 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 orbit the Sun and come close to Earth. They are defined by the International Astronomical Union as any small object that orbits 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 potentially 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 collide with Earth. These objects are studied to understand their paths and sizes to keep our planet safe.

History of human awareness of NEOs

The first near-Earth objects humans observed were comets. We learned they were from outer space after Tycho Brahe measured one in 1577 and found it much farther away than Earth. In 1705, Edmond Halley calculated the orbit of what we now call Halley's Comet. His predictions came true when it returned in 1758–1759, the first time we predicted a comet’s return.

We also discovered that meteors (shooting stars) come from space after studying the 1833 Leonid meteor shower. This led us to realize these meteors likely come from a comet, which we later confirmed 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 within a certain distance of 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 scales 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 and those that might become hazards.

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. A small number are considered potentially hazardous because they might come close to Earth and are large enough to cause damage.

Finding NEOs can be tricky because some are harder to see than others. Bigger objects reflect more light and are easier to spot. Also, where an object is in its orbit affects how bright it looks from Earth. Space telescopes help by looking at heat instead of reflected light, which makes it easier to find hidden objects.

Exploratory missions

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

Some near-Earth objects are interesting because they need less fuel for spacecraft to reach them compared to even missions to the Moon. This is because these objects move slowly and have very little gravity. They offer chances to study space rocks up close and might one day provide 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 realized 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 explored other asteroids and returned samples as well.

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.