Asteroid Monitoring: How Scientists Track Objects That Pass Near Earth

Earth is constantly moving through a busy region of space. Millions of objects orbit the Sun, including planets, comets, asteroids and smaller fragments left over from the formation of the solar system. Most of these objects never come close to Earth, but some follow orbits that bring them into our planet’s wider neighborhood. Scientists continuously monitor these objects to understand where they are going, how large they are and whether any could pose a future impact risk.

The process may sound simple, but tracking an asteroid across space is a complex scientific task. Astronomers cannot simply point a telescope at an asteroid once and know exactly where it will be years later. They need repeated observations from different times and locations, mathematical models of orbital motion and increasingly precise measurements to reduce uncertainty.

NASA’s Center for Near-Earth Object Studies, or CNEOS, is one of the organizations responsible for calculating high-precision orbits of known near-Earth objects and assessing possible future close approaches and impact risks. The Minor Planet Center collects observations of small bodies from observatories around the world, providing an important international foundation for this work.

Asteroid monitoring is therefore not a single telescope or a single observation. It is an ongoing global process that combines astronomy, mathematics, computing, radar and international cooperation.

What Counts as a Near-Earth Object?

Before understanding how scientists track asteroids, it is important to understand what astronomers mean by a near-Earth object, or NEO. A near-Earth object is an asteroid or comet whose orbit brings it into Earth’s broader orbital neighborhood.

NASA currently defines NEOs as objects whose orbits bring them within about 120 million miles, or 195 million kilometers, of the Sun. This includes objects whose paths can approach Earth’s orbit even if they never come particularly close to Earth itself. Most NEOs are not dangerous. Their orbits may bring them into the general neighborhood of Earth without creating a realistic possibility of collision.

Scientists pay particular attention to a smaller category known as potentially hazardous asteroids. NASA describes these as asteroids roughly 140 meters or larger whose orbits can bring them within about 7.5 million kilometers of Earth’s orbital path. The classification does not mean that an asteroid is going to hit Earth. It means that its size and orbital characteristics justify closer monitoring.

This distinction is important because headlines about an asteroid “passing near Earth” can sometimes create unnecessary concern. A close approach in astronomical terms can still involve millions of kilometers of separation.

How Scientists First Find an Asteroid

The first step in monitoring an asteroid is discovering it. Astronomers use wide-field telescopes to repeatedly photograph large areas of the night sky. Most objects visible in those images are stars and distant galaxies, which appear essentially fixed relative to one another. An asteroid within the solar system moves noticeably against that background.

By comparing images taken several minutes apart, astronomers can identify points of light that have shifted position. A moving object may then be investigated to determine whether it is a previously known asteroid, a new discovery, a comet or another type of object.

NASA-supported surveys have included systems such as the Catalina Sky Survey, Pan-STARRS and ATLAS. These surveys are designed to search large portions of the sky and identify moving objects that could potentially be near-Earth objects. The discovery process is therefore based not simply on seeing an object but on recognizing movement against the much larger background of stars.

Why One Observation Is Not Enough

Seeing an asteroid once does not provide enough information to predict its future accurately. Imagine seeing a car from a distance at one moment and trying to determine exactly where it will be ten hours later. Without knowing its speed, direction, acceleration and road conditions, there would be considerable uncertainty.

The same principle applies to objects in space, although the mathematics is far more complex. Astronomers need observations collected at different times. Each observation provides another position in the sky. When these positions are combined, scientists can calculate an orbital path that best matches the object’s observed movement.

CNEOS explains that an asteroid’s orbit is calculated by finding the path around the Sun that best fits the available observations. As additional observations are collected, scientists can refine the orbital solution and become more confident about the object’s future position.

This is why an asteroid’s predicted risk can sometimes change significantly after its discovery. Early calculations may be based on only a small number of observations, leaving considerable uncertainty.

The Role of the Minor Planet Center

Asteroid monitoring is an international effort, and the Minor Planet Center plays a central role in collecting observations. Observatories around the world submit measurements of small bodies to the Minor Planet Center. These observations provide information about where an asteroid appeared in the sky at a particular time.

The data can then be used by organizations such as NASA’s CNEOS and other international orbit-determination systems to refine the object’s trajectory. This global approach is essential because an asteroid does not remain visible from one location indefinitely. Earth’s rotation, daylight, weather and the asteroid’s position can all limit when a particular telescope can observe it.

Observations from multiple locations can extend the amount of time during which the object can be followed. NASA describes the Minor Planet Center as the internationally recognized clearinghouse for small-body position measurements, with observations contributed by observatories around the world, including amateur observers.

Follow-Up Observations Make Predictions Better

After a potential near-Earth asteroid is discovered, astronomers often conduct follow-up observations. The purpose is to obtain additional measurements and improve the orbital calculation. A newly detected object may initially have a wide range of possible future positions. As more observations are added, that range can become considerably narrower.

Follow-up observations can also reveal physical characteristics. Telescopes can help scientists estimate an asteroid’s brightness, rotation and other properties, while observations at different wavelengths can provide additional information.

The process is particularly important for newly discovered objects that may initially appear to have a possible future encounter with Earth. An apparent impact possibility does not necessarily mean the impact will happen. It can simply reflect the uncertainty of the initial orbit.

Why an Asteroid’s Risk Estimate Can Change

One of the most important concepts in asteroid monitoring is uncertainty. When scientists first detect an object, they do not know its orbit with perfect precision. The available observations contain measurement uncertainties, and there may not yet be enough observations to determine the object’s future trajectory precisely. As additional measurements become available, scientists update the orbital solution.

NASA’s experience with asteroid 2024 YR4 illustrates this process. In early 2025, observations initially produced an elevated calculated probability of an Earth impact in 2032. As more observations were collected, the calculated probability dropped substantially, and NASA ultimately determined that the asteroid was not expected to impact Earth.

This example demonstrates why early asteroid headlines should be interpreted carefully. A preliminary probability is not necessarily a final prediction. Scientific monitoring is specifically designed to reduce uncertainty over time.

How Computers Calculate an Asteroid’s Future Path

The motion of an asteroid is governed primarily by gravity, but calculating its future position can become complicated. The Sun’s gravity is the dominant influence on most asteroid orbits, but planets also exert gravitational forces.

During relatively close encounters with planets, these gravitational interactions can alter an asteroid’s trajectory. Scientists therefore use detailed orbital models that incorporate available observations and relevant gravitational effects. CNEOS operates automated systems that continually update orbit calculations as new observations become available.

Its Sentry system performs long-term analyses of possible asteroid trajectories and searches for potential future impacts. CNEOS also operates Scout, which assesses possible trajectories of newly detected but not yet confirmed objects. The goal is not simply to predict where an asteroid will be tomorrow. Scientists can use these calculations to examine possible trajectories years or even decades into the future.

The Importance of Radar

Optical telescopes are essential for discovering and tracking asteroids, but radar can provide a different type of information. Planetary radar works by transmitting radio signals toward an asteroid and measuring the echoes that return. When conditions are suitable, radar observations can provide highly precise information about an object’s distance and motion.

Radar can also help scientists study an asteroid’s shape, rotation and surface characteristics. Because radar observations require an object to be relatively close and detectable, they are not available for every asteroid.

Nevertheless, when an asteroid approaches Earth closely enough, radar can significantly improve scientists’ understanding of its trajectory. NASA identifies planetary radar as an important component of its near-Earth object observations, particularly for precise tracking and characterization during close approaches.

Telescopes in Space Add Another Perspective

Ground-based observatories are extremely important, but observing from space can provide advantages. Earth’s atmosphere can interfere with observations, and some wavelengths of light are difficult or impossible to observe effectively from the ground. Space-based telescopes can therefore complement terrestrial surveys.

NASA’s NEOWISE mission demonstrated the value of infrared observations for studying asteroids. The spacecraft made millions of infrared measurements during its mission and contributed observations of thousands of near-Earth objects. The mission ended in 2024, while NASA’s planned NEO Surveyor mission is intended to strengthen future infrared searches for potentially hazardous objects.

Infrared observations are particularly valuable because an asteroid’s thermal emission can provide information that is difficult to obtain from visible light alone. This can help scientists estimate an object’s size more effectively.

Why Infrared Observation Matters

An asteroid can appear bright or faint for reasons that do not directly reveal its physical size. Surface reflectivity, distance and viewing geometry all influence how bright an object appears. Infrared measurements can provide another way to estimate an asteroid’s physical characteristics because objects absorb sunlight and emit thermal radiation.

This is important for planetary defense because knowing an object’s size is a major part of understanding its potential impact consequences. A relatively small object and a large object can appear similar under certain observing conditions, so combining different types of observations improves scientific estimates.

NASA’s planned NEO Surveyor is designed specifically to search for near-Earth objects using infrared observations from space.

How Scientists Determine Whether an Asteroid Is a Risk

Once scientists have an orbital solution, they can calculate whether the asteroid’s future path could intersect Earth’s position.

This involves considering the uncertainty surrounding the orbital solution. Scientists do not simply draw one perfectly precise line through space. Instead, they analyze a range of possible trajectories based on measurement uncertainties and the limits of the available data.

If some of those possible trajectories intersect Earth in a future calculation, an impact probability can initially appear.

As new observations reduce uncertainty, many possible trajectories can be eliminated.

CNEOS’s Sentry system continuously evaluates known near-Earth objects and searches for possible impacts over a century-long period. Its calculations are updated as new observations become available.

This long-term monitoring allows scientists to identify objects that may require closer observation well before a potential encounter.

What Makes a Close Approach Different From an Impact Threat?

The words “close approach” and “impact risk” describe different things.

A close approach simply means that an asteroid’s orbit brings it relatively near Earth. The distance can still be millions of kilometers.

An impact threat requires something much more specific: the object’s future trajectory must have a meaningful possibility of intersecting Earth.

NASA notes that the majority of known near-Earth objects have orbits that do not bring them dangerously close to Earth. Only a smaller fraction require heightened attention because of their size and orbital characteristics.

This distinction is particularly important when interpreting news about asteroids. An asteroid can pass close to Earth without posing a significant danger.

Why Scientists Keep Monitoring Objects That Are Not Dangerous Today

An asteroid may be harmless during one particular encounter but still worth monitoring over longer periods.

Small changes in an orbit can accumulate over time. Gravitational interactions with planets can alter an object’s path, while additional observations can reveal that the original orbital estimate was less precise than initially believed.

This is why planetary defense is an ongoing activity rather than a one-time check.

The purpose of long-term monitoring is to identify potentially concerning objects early enough that scientists can understand their trajectories and, if necessary, consider possible responses.

NASA’s planetary-defense program focuses on finding, tracking and characterizing near-Earth objects, while also studying possible mitigation techniques for objects that could present a genuine future impact threat.

The Role of International Cooperation

No single country can observe every part of the sky continuously. Weather, daylight and geographic location create natural limitations.

International cooperation therefore forms an essential part of asteroid monitoring.

Observatories in different countries can observe the same object at different times. Researchers can share measurements, compare orbital calculations and coordinate follow-up observations.

Organizations such as NASA’s Planetary Defense Coordination Office work with the International Asteroid Warning Network and other international partners to coordinate planetary-defense activities.

This collaboration also creates a system in which independent observations can help verify calculations.

The more complete the observation record becomes, the better scientists can understand an asteroid’s future motion.

What Happens If Scientists Find a Genuine Threat?

Finding a potentially hazardous asteroid does not automatically mean that an emergency is imminent.

The first response would be continued observation and analysis. Scientists would need to determine the object’s orbit, size, composition and impact probability as accurately as possible.

If a genuine impact threat were confirmed, planetary-defense organizations could evaluate possible mitigation strategies. NASA has already demonstrated one possible technique through the Double Asteroid Redirection Test, or DART, which deliberately impacted the asteroid moonlet Dimorphos to test kinetic impact technology.

However, planetary defense is not based on one universal solution. The appropriate response would depend on the asteroid’s size, composition, trajectory and the amount of warning time available.

The most valuable resource in planetary defense is therefore often time. Finding an object early gives scientists more opportunity to understand it and evaluate possible responses.

The Future of Asteroid Monitoring

Asteroid monitoring is becoming increasingly sophisticated. Improvements in telescope technology, automated image processing, computing power and data-sharing systems are allowing astronomers to identify and characterize more objects.

Artificial intelligence and machine-learning techniques may also assist with processing enormous quantities of astronomical imagery, although scientific verification remains essential.

Space-based infrared observatories could improve the ability to detect objects that are difficult to see from the ground. Meanwhile, increasingly powerful computational models can analyze orbital uncertainties and identify possible future encounters.

The future of planetary defense will therefore involve an interconnected system of telescopes, radar, satellites, databases and computational models rather than one single detection instrument.

The objective is straightforward even if the science is complex: find potentially dangerous objects as early as possible, understand their trajectories accurately and provide enough information for informed decisions.

Conclusion: Tracking Is the First Line of Planetary Defense

Asteroid monitoring may appear to be about watching small points of light move across the night sky, but the science behind those observations is considerably more sophisticated.

Astronomers first detect moving objects using wide-field surveys. Additional observatories then follow them and provide more measurements. The Minor Planet Center collects positional observations, while organizations such as NASA’s CNEOS use those observations to calculate precise orbits and assess possible future close approaches.

Radar can improve measurements when an asteroid comes sufficiently close, while space-based infrared observations can help scientists identify and characterize objects that may be difficult to detect using visible light alone.

Most near-Earth objects do not pose an immediate threat to our planet. Even when an asteroid initially appears concerning, additional observations can dramatically reduce uncertainty and change the calculated risk.

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