What Are Near-Earth Objects?

Near-Earth objects (NEOs) are asteroids and comets that travel through space on paths that bring them close to Earth’s orbit around the Sun. Scientists define a near-Earth object as any small body that comes within about 30 million miles (50 million kilometers) of Earth’s path. Most NEOs are asteroids made of rock and metal, while a smaller number are comets made of ice, dust, and rock. As of 2024, scientists have discovered more than 34,000 near-Earth asteroids. The vast majority of these objects will never actually hit our planet, but scientists track them carefully just in case.

Asteroids vs. Comets

Asteroids and comets are both leftovers from when our solar system formed about 4.6 billion years ago. Asteroids are mostly made of rock and metal, and most of them orbit the Sun in the asteroid belt between Mars and Jupiter. Comets are made of ice, dust, and rock, and they usually come from the outer edges of the solar system. When a comet gets close to the Sun, the heat causes its ice to turn into gas, creating a glowing tail that can stretch for millions of miles. Both asteroids and comets can become near-Earth objects if gravity from a planet nudges them onto a new path that brings them closer to Earth.

How Big Are They?

Near-Earth objects come in many different sizes, from tiny pebbles to enormous space rocks several miles across. Most NEOs are quite small, less than 100 feet (30 meters) wide, and would burn up in Earth’s atmosphere before reaching the ground. Scientists pay the most attention to NEOs larger than 460 feet (140 meters) because these could cause serious damage if they hit Earth. The largest known near-Earth asteroid is called 1036 Ganymed, and it is about 22 miles (35 kilometers) across. Fortunately, Ganymed’s orbit does not put it on a collision course with our planet.

Famous Impacts in History

About 66 million years ago, a massive asteroid roughly 6 miles (10 kilometers) wide struck what is now the Yucatan Peninsula in Mexico. This impact created the Chicxulub crater, which is more than 90 miles (150 kilometers) across, and led to the extinction of the dinosaurs along with about 75% of all species on Earth. In 1908, a smaller object exploded in the air above Tunguska, Russia, flattening about 800 square miles of forest even though it never touched the ground. More recently, in 2013, a small asteroid about 60 feet (18 meters) wide exploded over the city of Chelyabinsk, Russia, shattering windows and injuring about 1,500 people. These events remind us why it is important to watch the skies.

How Scientists Track NEOs

Scientists around the world use powerful telescopes to search for and track near-Earth objects. NASA runs a program called the Planetary Defense Coordination Office (PDCO), which keeps track of all known NEOs and calculates whether any could hit Earth. Ground-based telescopes like the Catalina Sky Survey in Arizona scan the sky every clear night looking for new asteroids. NASA also launched a space telescope called NEOWISE that helped discover and study thousands of asteroids from orbit. When scientists find a new NEO, they carefully measure its position over time to figure out exactly where it will travel in the future.

The DART Mission

In September 2022, NASA made history by crashing a spacecraft into an asteroid on purpose. The mission was called DART, which stands for Double Asteroid Redirection Test. DART slammed into a small asteroid moon called Dimorphos at about 14,000 miles per hour (22,500 kilometers per hour). The impact successfully changed the asteroid’s orbit, shortening the time it took to go around its larger partner asteroid by 33 minutes. This was the first time humans ever changed the motion of a natural object in space, proving that we might be able to deflect a dangerous asteroid if one were ever headed toward Earth.

Planetary Defense Plans

If scientists discovered a large asteroid heading toward Earth, they would have several options to try to stop it. The best strategy depends on how much warning time we have and how big the asteroid is. If we spot the threat years or decades in advance, a spacecraft could slowly push or pull the asteroid off course using gravity or a gentle nudge. For larger or closer threats, a kinetic impactor like DART could slam into the asteroid at high speed to change its path. As a last resort, scientists have studied the idea of using a nuclear explosion near an asteroid to vaporize part of its surface and push it away, though this has never been tested.

Why Planetary Defense Matters

Studying near-Earth objects is not just about preventing disasters — it also teaches us about the history of our solar system. The asteroids and comets near Earth are like time capsules from billions of years ago, holding clues about how the planets formed. Scientists believe that comets and asteroids may have delivered water and important building blocks of life to early Earth. By tracking NEOs and developing ways to protect our planet, space agencies around the world are working together to keep Earth safe. With better telescopes and spacecraft technology, scientists are more prepared than ever to find and respond to any potential threats from space.