Our solar system is a very busy place. Earth is not alone as it orbits the Sun. We share our neighborhood with other planets, moons, and countless smaller objects like comets and asteroids. These are ancient pieces of rock and ice left over from when our solar system was formed. Most of them are very far away and keep to themselves. However, some of these objects have paths, or orbits, that bring them close to Earth.
These objects are called Near-Earth Asteroids, or NEAs. NASA, the U.S. space agency, has a very important job: to find and keep track of these cosmic neighbors. This job is called “Planetary Defense.” It is not science fiction; it is a real and active program. Scientists are constantly scanning the skies to protect our planet. They need to know where these asteroids are, where they are going, and if any of them pose a threat to Earth.
This task is incredibly difficult. Most asteroids are not bright and shiny. Many are as dark as a lump of coal, making them extremely hard to see against the blackness of space. They are also often very small and moving at high speeds. So, how does NASA manage to find a small, dark rock moving thousands of miles per hour in the vastness of space?
What Exactly Is a Near-Earth Asteroid?
First, it helps to know what scientists are looking for. The term “Near-Earth Asteroid” can sound a little scary, but the word “near” means something different in space. An NEA is an asteroid (or comet) whose orbit brings it within 1.3 astronomical units (AU) of the Sun. One AU is the distance from the Earth to the Sun, which is about 93 million miles (or 150 million kilometers). This definition just means the asteroid’s path crosses into Earth’s general neighborhood. As of 2025, scientists have found over 37,000 NEAs of all sizes.
Scientists have another, more specific name for the asteroids they watch most closely: “Potentially Hazardous Asteroids,” or PHAs. An asteroid gets this label if it meets two conditions. First, its orbit must bring it very close to Earth’s orbit, within about 4.65 million miles (7.5 million kilometers). This is about 20 times the distance to the Moon. Second, the asteroid must be larger than about 460 feet (140 meters) across. An object this size is large enough to pass through our atmosphere and cause serious regional damage if it were to hit. The main goal of NASA’s asteroid-hunting program is to find at least 90 percent of all PHAs.
Who Is in Charge of Protecting Earth from Asteroids?
The official group at NASA responsible for this work is the Planetary Defense Coordination Office (PDCO). This office was created in 2016 to manage all of NASA’s efforts in planetary defense. Its mission is clear and has several parts. First, it must find the asteroids and comets that get close to Earth. Second, it must track them and calculate their paths for the next 100 years or more. Third, it works to “characterize” them, which means learning their size, shape, and what they are made of.
Finally, if a credible threat is ever found, the PDCO is responsible for coordinating the response. This includes warning the public and working with other government agencies in the U.S. and around the world. Planetary defense is a global team effort. NASA works very closely with other space agencies, like the European Space Agency (ESA), and with a worldwide group of scientists called the International Asteroid Warning Network (IAWN). Everyone shares their data to create the most complete picture possible of what is happening in our solar system.
How Do Telescopes First Discover a New Asteroid?
The search for new asteroids begins on the ground, with powerful telescopes scanning the night sky. These telescopes act like giant, robotic searchlights. They do not just stare at one spot. Instead, they take many pictures of large patches of the sky, one after another, all night long. A few hours later, they will come back and take pictures of the same patch of sky again. Special computer software then compares these images.
The stars in the background will not move. They are fixed in place. But an asteroid, which is much closer to us, will have moved a small amount between the pictures. The computer flags this tiny, moving dot of light. This is how most new asteroids are discovered. NASA funds several major survey telescopes that do this work. The most productive ones include the Catalina Sky Survey in Arizona and the Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) in Hawaii. Pan-STARRS has an incredibly large digital camera that can photograph a huge area of the sky at once, making it one of the world’s best asteroid hunters.
What Happens Immediately After an Asteroid Is Found?
Finding a new moving dot is just the first step. This single detection is not nearly enough information. To know where that asteroid is going, scientists need more data, and they need it fast. As soon as a survey telescope like Pan-STARRS flags a potential new object, it sends the information to one central place: the Minor Planet Center (MPC). The MPC is the world’s official clearinghouse for all asteroid and comet observations. It is funded by NASA and operated by a team at the Smithsonian Astrophysical Observatory.
The MPC gives the new object a temporary name, which looks something like 2025 AB. This name is a code that tells other astronomers it is a new, unconfirmed discovery. The MPC then posts this object on a public web page for other telescopes around the world to see. This is a call to action. Astronomers, both professional and skilled amateurs, will point their own telescopes at that spot in the sky to get “follow-up” observations. They must find the object and measure its position again. This global teamwork is essential. Without these extra observations, the new asteroid could be quickly “lost” and its path could not be calculated.
How Does NASA Calculate an Asteroid’s Exact Path?
Once the Minor Planet Center has several observations of the new asteroid from different telescopes, the data is sent to another key group: the Center for Near Earth Object Studies (CNEOS). This team is based at NASA’s Jet Propulsion Laboratory (JPL) in California. The scientists at CNEOS are the master navigators of the solar system. Their job is to take all these data points and use powerful computers to calculate the asteroid’s precise orbit around the Sun.
This calculation is very complex. It must account for the powerful gravity of the Sun, but also the smaller gravitational pulls from every planet, including Earth, Jupiter, and Mars. Over many decades, these small tugs can slowly change an asteroid’s path. CNEOS runs a highly advanced, automated system called Sentry. This system is always working. It takes the orbit of every known Near-Earth Asteroid and projects its path forward in time for the next 100 years or more. It constantly checks to see if any of these paths will cross Earth’s position at the same time. This system is what allows NASA to say with great confidence whether a known asteroid is a threat or not.
Do Space Telescopes Help Find Asteroids Too?
Yes, absolutely. Ground-based telescopes are amazing, but they have some big limitations. They can only work at night, and they can be stopped by clouds or bad weather. Also, Earth’s atmosphere blurs their view. A telescope in space flies above all these problems and can hunt for asteroids 24 hours a day. More importantly, a space telescope can see in different kinds of light that our atmosphere blocks. This is a huge advantage for finding asteroids.
NASA’s most valuable asteroid-hunting space telescope was NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer). This mission, which ended its operations in 2024, did not look for visible light like our eyes do. Instead, it looked for heat, also known as infrared light. This was its superpower. Most asteroids are very dark and reflect very little sunlight, making them nearly invisible to normal telescopes. However, these dark asteroids are warmed up by the Sun, and they glow with their own heat in infrared light. NEOWISE was able to spot these dark, hidden asteroids. It also gave scientists a much better way to measure an asteroid’s size. A small, shiny asteroid and a large, dark asteroid might look equally bright in a normal telescope, but in infrared, the large asteroid will glow much more brightly.
What Is the Next Generation of Asteroid Hunting?
The current system has found tens of thousands of NEAs, but it is not perfect. Many asteroids are still hiding, especially the ones that orbit closer to the Sun than Earth does. From our viewpoint on Earth, these asteroids are lost in the Sun’s bright glare. They are very hard to see with ground-based telescopes, which can only look for them in the brief twilight hours. The famous Chelyabinsk meteor that exploded over Russia in 2013 came from this direction, which is why no telescope saw it coming.
To solve this problem, NASA is building a brand-new, dedicated asteroid-hunting space telescope called the NEO Surveyor. This mission is the next generation of planetary defense. It will be a powerful infrared telescope, just like NEOWISE, but far more advanced. It will not orbit Earth. Instead, it will be sent to a special spot in space called the L1 Lagrange point, which is located between the Earth and the Sun. From this viewpoint, it will be able to look “outward” and “sideways” to easily spot the asteroids that are currently hiding in the Sun’s glare. NEO Surveyor is designed to find the majority of the dangerous PHAs that are still undiscovered. Its launch, planned for the late 2020s, will completely change the game for planetary defense.
How Does NASA Know If an Asteroid Is Actually Dangerous?
Once CNEOS calculates an asteroid’s orbit, the Sentry system checks for any future impact possibilities. If it finds one, that asteroid is placed on a public “Risk List.” This may sound alarming, but it is actually a sign of the system working. For a newly discovered asteroid, the orbit is often a bit uncertain. Sentry might show a 1-in-10,000 chance of an impact in the year 2085. This does not mean it will hit. It means scientists need more data. Astronomers will keep observing that asteroid, and as they add new data, the orbit calculation becomes more precise. In almost every case, the new data rules out the impact completely, and the asteroid is removed from the list.
To communicate this risk to the public in a simple way, scientists use the Torino Scale. This is a color-coded chart that rates the danger from 0 to 10.
- Level 0 (White): No hazard. This means the asteroid will miss Earth, or it is so small it will safely burn up in the atmosphere.
- Level 1 (Green): Normal. This is for a new discovery that needs more observation. The chance of impact is extremely unlikely.
- Levels 2–4 (Yellow): Merits attention. These objects are rare. Astronomers will make them a high priority for observation.
- Levels 5–7 (Orange): Threatening. A close call with a serious chance of collision. This level would trigger major planning.
- Levels 8–10 (Red): Certain collision. This means a definite impact is coming, with Level 10 representing a global catastrophe.
As of 2025, every single known asteroid in the sky has a rating of 0 (White) on the Torino Scale. There are no known credible impact threats to Earth for the next 100 years.
What Can We Do If a Dangerous Asteroid Is Found?
Finding a threat is only half the battle. NASA and its partners are also testing ways to stop a dangerous asteroid. The most important thing we need is time. If we find a dangerous asteroid only a few weeks before impact, there is very little we can do. But if we find it 10 or 20 years in advance, the solution is much easier. We would not need to destroy it with a massive bomb. Instead, we would just need to give it a tiny nudge. A very small change in its speed, years in advance, would cause it to miss Earth by thousands of miles when it finally arrives.
In 2022, NASA proved this is possible with the DART (Double Asteroid Redirection Test) mission. DART was a spacecraft, about the size of a small car, that was sent on a one-way trip. Its target was a small asteroid moonlet named Dimorphos, which was orbiting a larger asteroid named Didymos. Neither of these asteroids was a threat to Earth; they were just a practice target. The DART spacecraft slammed into Dimorphos at over 14,000 miles per hour. The mission was a huge success. The impact changed Dimorphos’s orbit around Didymos by 33 minutes, which was far more than scientists had expected. DART proved that the “kinetic impactor” technique works. We now know that we have the technology to defend ourselves, as long as we find the threat early enough.
Conclusion
Finding and tracking Near-Earth Asteroids is a massive and ongoing effort. It is a 24/7 job that connects telescopes on mountains all over the world with advanced computer systems at NASA. It starts with survey telescopes like Pan-STARRS finding the “dots that move.” It continues with the Minor Planet Center, which acts as a global traffic controller for new data. Then, NASA’s CNEOS team calculates the object’s precise path and uses the Sentry system to check for any long-term risk, which is then communicated simply using the Torino Scale.
Space telescopes like NEOWISE and the upcoming NEO Surveyor are critical for finding the dark, hidden asteroids that ground-based telescopes might miss. And finally, missions like DART have successfully proven that we are not helpless. We have the ability to protect our planet. This quiet, constant, and collaborative work is the real story of planetary defense. It is a system built not on fear, but on careful science, international teamwork, and preparation.
Now that you know how this global neighborhood watch works, does the active search for asteroids change how you view our place in the solar system?
FAQs – People Also Ask
What is the difference between an asteroid and a meteor?
An asteroid is a large rocky body that orbits the Sun. A meteoroid is a much smaller piece of rock or debris from an asteroid or comet. When a meteoroid enters Earth’s atmosphere and burns up, it creates a streak of light we call a meteor, or a “shooting star.” If any piece of it survives the fall and lands on the ground, it is called a meteorite.
Has NASA ever found an asteroid that will definitely hit Earth?
No. As of 2025, NASA has not found any asteroid that poses a credible threat of impacting Earth within the next 100 years. The Sentry risk list is constantly updated, but every object that has ever been on it has been downgraded to a 0 on the Torino Scale (meaning no hazard) after more observations were made.
How many Near-Earth Asteroids have been found?
As of 2025, scientists have discovered more than 37,000 Near-Earth Asteroids. This number grows every single day as survey telescopes continue to scan the sky. However, this is still only a fraction of the total number that are believed to be out there, especially the smaller ones.
What was the DART mission’s main goal?
The main goal of the DART (Double Asteroid Redirection Test) mission was to prove that we can change an asteroid’s motion in space. By crashing a spacecraft into the asteroid moonlet Dimorphos in 2022, NASA successfully altered its orbit. This test confirmed that the “kinetic impactor” method is a viable technique for planetary defense.
Can a small asteroid still cause damage?
Yes. An asteroid does not need to be miles wide to be dangerous. The meteor that exploded over Chelyabinsk, Russia, in 2013 was only about 60 feet (20 meters) wide. It exploded in the air with the force of a small nuclear weapon, and its shockwave injured over 1,500 people and damaged thousands of buildings.
What is the Torino Scale used for?
The Torino Scale is a simple, color-coded system from 0 to 10 that scientists use to communicate the potential impact risk of a Near-Earth Object. A score of 0 means there is no hazard. A score of 1 is “normal,” and higher numbers (which have never been assigned long-term) indicate a more serious threat, with 10 being a certain, global catastrophe.
Why is it so hard to find asteroids in the Sun’s glare?
Telescopes on Earth can only look at the night sky, away from the Sun. Asteroids that orbit between Earth and the Sun are lost in the daylight or the bright twilight glare. To see them, you must look toward the Sun, which can damage telescope optics and is only possible for brief moments, making detection very difficult from the ground.
What is a gravitational keyhole?
A gravitational keyhole is a very small, precise region of space near a planet, like Earth. If a passing asteroid flies through this exact “keyhole,” the planet’s gravity will pull on it in just the right way to put it on a definite collision course for a future orbit. This is why scientists work so hard to precisely measure an asteroid’s path, to rule out any future keyhole passages.
Does NASA work with other countries on asteroid defense?
Yes, planetary defense is a global effort. NASA’s Planetary Defense Coordination Office works very closely with the European Space Agency (ESA) and other space-faring nations. They also share all of their data through the International Asteroid Warning Network (IAWN) and the Minor Planet Center to ensure all scientists are working with the same information.
What is the NEO Surveyor telescope?
NEO Surveyor is NASA’s next-generation asteroid-hunting space telescope, planned for launch in the late 2020s. It is an infrared telescope that will be placed at a special point between the Earth and Sun. From this location, it will be able to find thousands of asteroids, especially the dark ones and those hiding in the Sun’s glare that are currently invisible to us.