Rocks fly in space.
Some rocks come near our Earth. They can be big or small. They move around the Sun. We look at the sky to find them. It is cool to watch! Do you like looking at stars?
Rocks and ice move around the Sun.
Some of these move very close to Earth. We call these near-Earth objects. Most are rocky asteroids. Some are icy comets.
Some objects are quite large. If a big one hits Earth, it can make a hole. It can also make big waves in the sea.
Scientists watch the sky to stay safe. They track these objects to know where they go. It is good to be ready!
Space is full of small bodies that orbit the Sun.
We call these near-Earth objects, or NEOs. An NEO is any small body with an orbit that comes close to our home. This means it can get within 1.3 times the distance between Earth and the Sun. Most NEOs are rocky asteroids. Some are icy comets.
Some NEOs are a special kind. If their orbit crosses Earth's orbit and they are large, they are called potentially hazardous objects. We call these PHOs.
Impacts can change our world. Small objects might burn up in the air. Large ones can hit the ground and make craters. They can also hit the ocean and make tsunamis.
Scientists work hard to stay safe. They use the Torino scale to rate risks. This scale looks at how likely a hit is. It also looks at how bad the damage might be.
Since 1998, many nations have used a project called Spaceguard. They scan the sky to find NEOs. This helps us know if we need to move an object away.
Space is filled with many small bodies that orbit our Sun.
Scientists call these near-Earth objects, or NEOs. An NEO is any small body with an orbit that comes near our home. Specifically, its closest approach to the Sun must be less than 1.3 astronomical units. An astronomical unit is the distance between the Earth and the Sun. This definition depends on the object's path, not where it is right now. Even if an object is far away, it is still an NEO if its orbit stays close. Most NEOs are asteroids, but about one-third of one percent are comets.
Some of these objects are more important to watch closely. If an NEO's orbit crosses Earth's orbit and it is large, it is a potentially hazardous object, or PHO. To be a PHO, an object must be larger than 140 meters across.
These objects pose a danger because they could hit our planet. When an object hits Earth, the results depend on its size. Small objects might explode high in the atmosphere. Larger ones can hit land and create huge craters. If they hit the ocean, they can cause tsunamis.
Humans have been learning about these objects for a long time. The first near-Earth objects people saw were comets. In 1577, Tycho Brahe measured a comet to see how far away it was. Later, Edmond Halley calculated the orbit of a famous comet in 1705. This helped people predict when the comet would return. In 1898, humans discovered the first near-Earth asteroid, which was named 433 Eros.
Studying Eros helped scientists learn the exact distance from the Earth to the Sun.
Today, we use special tools to track these moving targets. Since 1998, many nations have joined the Spaceguard effort. This project scans the sky to find as many NEOs as possible. In 2011, NASA met a goal to find 90% of large NEOs.
We use the Torino scale to rate the risk of an object. This scale looks at the chance of an impact and the possible damage. There is also a more complex system called the Palermo scale. These scales help us understand if an object is a real threat.
We can even visit these objects with spacecraft. Many NEOs have low gravity and orbits like Earth's. This makes them easy targets for our robotic explorers. Spacecraft have visited five near-Earth comets and six near-Earth asteroids.
Some missions have even brought samples of these objects back to Earth. Scientists are also testing ways to move an object away from Earth. This is called deflection, and it is a way to keep our planet safe.
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.
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