Near-Earth object

A near-Earth object, or NEO, is a small body in our Solar System that orbits the Sun.

To be classified as an NEO, an object's perihelion, or its closest point to the Sun, must be less than 1.3 astronomical units (AU). An astronomical unit is the distance between the Earth and the Sun. This classification is based on the object's orbital path rather than its current location. An object is still an NEO even when it is far from Earth.

Most NEOs are asteroids, but about 0.3 percent are comets.

Some objects are more significant because their orbits intersect with Earth's orbit. These are called potentially hazardous objects, or PHOs. To be a PHO, an object must be larger than 140 meters in diameter.

Scientists also use specific parameters to identify potentially hazardous asteroids (PHAs). A PHA must have an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less. It must also have an absolute magnitude of 22.0 or brighter. This magnitude serves as a rough indicator of the object's size.

The impact of an NEO depends heavily on its size and where it hits. Small objects, up to a few tens of meters across, often explode in the upper atmosphere. These explosions can vaporize most of the solid material. Larger asteroids can penetrate the atmosphere and strike the surface of the Earth. If they hit a continent, they produce impact craters. If they strike the sea, they can create tsunamis.

Researchers use impact models to estimate how often these events occur. For example, an asteroid 140 meters across might hit Earth every 5 years. An asteroid 1 kilometer across might hit every 440,000 years. A 10-kilometer asteroid might hit only once every 18 million years.

Humans have studied these objects for centuries, though our understanding has changed. The first near-Earth objects observed were comets. In 1577, Tycho Brahe used parallax to prove comets were far beyond the Moon. In 1705, Edmond Halley calculated the orbit of a returning comet. This allowed for the first predicted appearance of a comet in 1758.

Later, in 1833, Denison Olmsted analyzed a meteor shower to show meteors come from space. In 1898, the first near-Earth asteroid, 433 Eros, was discovered.

Studying Eros helped scientists determine the distance between the Earth and the Sun.

Since the 1980s, awareness of the risks posed by NEOs has increased. This led to the Spaceguard effort, which began in 1998. The United States, the European Union, and other nations scan the sky to catalog these objects. NASA received a mandate from the U.S. Congress to catalog 90% of NEOs at least 1 kilometer in diameter. This goal was met by 2011.

Surveys have since expanded to include smaller objects that could cause large-scale damage. To manage risk, scientists use the Torino scale and the Palermo scale. These scales rate the probability of an impact and the severity of the consequences.

Modern space exploration has allowed us to visit these objects directly. NEOs have low surface gravity and orbits similar to Earth's. This makes them accessible targets for spacecraft. We have visited five near-Earth comets and six near-Earth asteroids.

Some missions have even returned samples of these objects to Earth. Scientists are also researching mitigation, or ways to avoid impacts. One successful deflection test has already been conducted.

This shows that it is possible, in principle, to change an object's path.

Beyond safety, NEOs are important for scientific study and future industry. Because they are easy to reach, some private companies have drafted plans for commercial asteroid mining. While few of these plans are currently being pursued, the potential remains. Studying NEOs helps us understand the geological and biological history of Earth. Collisions in the past have played a significant role in shaping our planet. By watching the sky, we learn more about our place in the Solar System.