Interplanetary Radar: How NASA’s Planetary Radar Helps Protect Earth From Asteroids

A space rock does not need to be large enough to destroy the world to cause a catastrophe. An asteroid measuring tens or hundreds of metres across could cause devastating regional damage if it struck a populated area. The challenge for planetary defence is finding potentially dangerous objects, determining their orbits and working out whether they could threaten Earth.

NASA has a powerful tool for part of that job: planetary radar.

Using the Goldstone Solar System Radar in California, scientists can transmit radio waves toward certain near-Earth asteroids and analyse the faint echoes that return. Those measurements can reveal an asteroid’s distance, motion, rotation, shape and surface features, helping researchers improve predictions of where it will travel.

But there is an important distinction. Planetary radar is not an invisible shield around Earth, nor does it independently scan the entire solar system for incoming asteroids. It is one part of a broader planetary-defence effort that also depends on optical telescopes, infrared surveys, orbit calculations and, potentially, spacecraft designed to deflect dangerous objects.

NASA’s radar observations have already helped scientists investigate asteroids passing relatively close to Earth. Meanwhile, the agency is developing a dedicated infrared space telescope designed to find many of the difficult-to-detect objects that ground-based surveys can miss.

Together, these technologies are helping answer two essential questions: What is approaching Earth, and would we have enough warning to respond?

What is NASA’s planetary radar system?

NASA’s planetary radar capability is centred on the Goldstone Solar System Radar, which operates as part of the agency’s Deep Space Network near Barstow, California.

The system uses a powerful antenna to transmit radio signals toward selected objects in the solar system. When those signals strike an asteroid, a small fraction of the energy scatters back toward Earth. Sensitive receiving equipment and sophisticated processing techniques allow scientists to extract useful information from the returning signal.

Radar observations are particularly valuable because they provide measurements that ordinary images cannot always deliver as precisely.

For example, an optical telescope may reveal a moving point of light against the background stars. Repeated observations can establish its orbit, but the object’s apparent brightness alone does not reliably determine its size because its reflectivity is usually unknown.

Radar can provide an independent measurement of distance and line-of-sight motion. Under favourable observing conditions, radar imaging can also reveal an asteroid’s shape, rotation and surface structures.

NASA’s Goldstone system has been used to observe planets and other solar-system bodies as well as near-Earth asteroids. For planetary defence, its greatest value is improving the physical and orbital information available for objects that are close enough and suitably positioned for observations.

NASA explains the technique in its overview of planetary radar astronomy.

How radar tracks an asteroid

The process begins with an asteroid that has already been identified or whose position is sufficiently well known for the radar antenna to point toward it.

A powerful radio transmission travels through space at the speed of light. If the signal reaches the target, a tiny fraction reflects back toward Earth. The receiving system records the echo, which may be extremely weak by the time it arrives.

Scientists analyse several properties of that echo.

1. Measuring distance

Radio waves travel at a known speed. By measuring the time between transmission and reception, astronomers can calculate the target’s distance.

The basic relationship is:

Distance = speed of light × round-trip time ÷ 2

The division by two is necessary because the signal travels to the asteroid and then back to Earth.

This measurement can sharply improve an asteroid’s estimated position, although the actual precision depends on the strength of the echo, the observing geometry and the measurement technique.

2. Calculating motion

The frequency of the returning signal can shift because of the relative motion between Earth and the asteroid. This is known as the Doppler effect.

Analysing that shift helps scientists measure the asteroid’s motion along the line of sight. Combined with optical observations and previous orbit calculations, the result can improve estimates of its trajectory.

More accurate orbital measurements help astronomers predict where an asteroid will be in the future and assess whether its path could bring it dangerously close to Earth.

3. Revealing shape and rotation

A radar echo does not necessarily return from the asteroid as a single, uniform signal. Different parts of its surface may reflect the radio waves at different times and frequencies.

By analysing these variations, scientists can construct radar images and models of the object’s shape and rotation.

Depending on the target and observing conditions, the results may reveal an elongated body, surface depressions, ridges, boulders or even a smaller companion orbiting the main asteroid.

These details matter because asteroids are not uniform balls of rock. Some are relatively solid, while others may be loose collections of material held together by gravity.

Understanding an asteroid’s physical properties would be useful if scientists ever needed to plan a deflection mission.

A real example: NASA’s radar observations of asteroid 2025 OW

Planetary radar is already contributing to the study of real near-Earth objects.

In July 2025, NASA’s Goldstone Solar System Radar observed asteroid 2025 OW as it passed Earth at a distance of approximately 640,000 kilometres, or about 1.6 times the average distance between Earth and the Moon.

The asteroid had been discovered on July 4, 2025, by the Pan-STARRS2 survey telescope in Hawaii. Goldstone’s observations on July 28 produced a series of radar images suggesting that the asteroid was approximately 60 metres across and had an irregular shape.

The observations also indicated that it was spinning unusually rapidly, completing a rotation in roughly 1.5 to three minutes.

Scientists were able to resolve surface features approximately 3.75 metres across.

The asteroid passed safely. The observations were not a last-minute interception of an object on a collision course with Earth. Instead, they demonstrate how radar can add valuable information after a near-Earth asteroid has been discovered.

NASA’s report on the 2025 OW observations explains how the measurements improved scientists’ understanding of the object’s size, shape and future motion.

The distinction is important: radar helped characterise the asteroid, while the discovery itself came from an optical survey.

Why radar cannot find every dangerous asteroid

Radar is powerful, but it has significant limitations.

First, the signal becomes weaker as it travels across vast distances and spreads through space. The returning echo is weaker still. As a result, planetary radar observations are generally most useful for objects that are relatively close to Earth and have favourable observing geometry.

Second, astronomers must know where to point the antenna. Radar does not function like a giant radio net that continuously detects every asteroid crossing the solar system. It is normally used to follow up targets discovered through other observations.

Third, the number of available observing opportunities is limited. Scientists must choose which objects to study and how to allocate telescope time, equipment and personnel.

These limitations mean that planetary radar cannot replace the wide-field surveys responsible for finding new near-Earth objects.

Optical telescopes remain essential for discovering asteroids by detecting reflected sunlight. Infrared telescopes can help identify objects that are difficult to spot in visible light, including dark asteroids that absorb sunlight and emit it as heat.

The most effective planetary-defence strategy combines these different methods rather than relying on a single technology.

NASA’s next major step: NEO Surveyor

One of the most important future additions to planetary defence is NASA’s Near-Earth Object Surveyor, usually called NEO Surveyor.

Unlike Goldstone radar, NEO Surveyor is designed specifically to discover potentially hazardous asteroids and comets. It will use a space-based infrared telescope to search for objects that can be difficult to detect using visible light.

Dark asteroids are a particular challenge. An object that reflects relatively little sunlight can appear faint even when it is physically large. But sunlight also heats an asteroid, causing it to emit infrared radiation. A sensitive infrared telescope can use that thermal emission to help find and characterise such objects.

NEO Surveyor is also designed to search for asteroids approaching from the direction of the Sun, where glare can make them difficult for ground-based optical surveys to observe.

NASA’s current mission information lists a launch no earlier than September 2027. That means NEO Surveyor is a planned mission, not an operational asteroid-detection network already protecting Earth.

After launch, the spacecraft is intended to operate near the Sun-Earth L1 region, about 1.5 million kilometres from Earth toward the Sun. Its survey is designed to make substantial progress toward finding at least two-thirds of near-Earth objects measuring 140 metres or larger during its five-year baseline mission.

That size category matters because an impact from an object roughly 140 metres across could cause major regional damage.

Read NASA’s official NEO Surveyor mission overview for its objectives and planned capabilities.

How many dangerous asteroids remain undiscovered?

Astronomers have discovered most near-Earth objects larger than one kilometre, but the smaller population is much more difficult to catalogue completely.

NASA’s Center for Near-Earth Object Studies reports that more than 90% of near-Earth objects larger than one kilometre have been discovered. The agency has also highlighted the continuing need to find objects at least 140 metres across, a category for which the known population remains incomplete.

These smaller objects are still capable of causing serious destruction. However, an asteroid’s size alone does not establish that it is dangerous: its orbit, impact probability, speed, composition and potential impact location all matter.

There is an important difference between an asteroid being classified as potentially hazardous and an asteroid being predicted to hit Earth. Potentially hazardous objects merit monitoring because their orbits bring them relatively close to Earth’s orbit and they are sufficiently large. Most are not on a collision course with our planet.

NASA’s Near-Earth Object Observations Program explains how surveys discover objects, refine their orbits and assess potential hazards.

The goal is not to eliminate every uncertainty overnight. It is to build a progressively more complete catalogue and identify any genuine threat early enough for scientists and decision-makers to respond.

What happens if an asteroid really is on a collision course?

Finding a potentially hazardous asteroid is only the first stage. If observations showed that an object had a meaningful chance of striking Earth, scientists would need to refine its orbit, estimate its size and determine how much warning time remained.

The appropriate response would depend on those details.

For a threat identified years in advance, a spacecraft could potentially be sent to alter the asteroid’s trajectory. A very small change in velocity, applied early enough, can lead to a substantial difference in where an object eventually travels.

NASA has already tested one approach through the Double Asteroid Redirection Test, or DART.

On September 26, 2022, DART deliberately struck Dimorphos, a small asteroid moonlet orbiting the larger asteroid Didymos. The impact shortened Dimorphos’s orbital period around Didymos by approximately 33 minutes, demonstrating that a spacecraft collision could measurably alter an asteroid’s motion.

Neither object threatened Earth; the system was selected as a safe target for the experiment.

In March 2026, NASA reported further research showing that the impact also changed the orbital motion of the Didymos–Dimorphos system around the Sun by a small amount. This provided additional evidence that a kinetic impact can affect the motion of a celestial body.

DART was a technology demonstration, not proof that every threatening asteroid could be deflected. The effectiveness of a future mission would depend on the object’s size, structure, composition, orbit and available warning time.

Nevertheless, it established an important principle: humanity can deliberately change the motion of an asteroid.

NASA’s DART mission page provides details about the experiment and its results.

Why Arecibo is no longer part of the planetary-radar effort

Some descriptions of future planetary defence mention upgrading the Arecibo Observatory in Puerto Rico. That requires an important correction.

The observatory’s 305-metre radio telescope suffered structural failures and was decommissioned in 2020 before collapsing in December of that year. Its former planetary-radar capability is no longer available.

Arecibo made valuable contributions to radar astronomy, including the study of near-Earth objects. But it should not be described as an operating radar telescope being upgraded for current asteroid tracking.

NASA’s 2020 statement on Arecibo’s decommissioning explains the decision and the observatory’s former role.

Future planetary-defence capabilities will depend on operational facilities, new missions and continued investment in observing systems—not on assuming that retired infrastructure is available.

Could artificial intelligence improve asteroid defence?

Artificial intelligence could support planetary defence by helping researchers process large quantities of telescope observations, identify candidate objects and prioritise follow-up measurements.

Automated systems can assist with matching observations across different nights, flagging unusual trajectories and identifying patterns in data that merit human review.

However, AI does not remove the need for reliable observations or independent verification. An algorithm can misclassify an object, inherit biases from its training data or produce an incorrect result when faced with unusual circumstances.

A credible asteroid-warning system must rely on accurate measurements, well-tested orbital calculations and transparent uncertainty estimates. AI can make parts of that process faster, but it is not a substitute for physical evidence.

The same principle applies to public asteroid alerts: information should be based on verified orbit calculations and communicated with clear explanations of what the risk estimates actually mean.

Frequently asked questions

1. Does NASA have an interplanetary radar shield around Earth?

No. NASA operates planetary radar equipment, including Goldstone Solar System Radar, which can observe selected asteroids and other solar-system objects. It is part of a wider planetary-defence programme, not a shield that continuously scans all of space.

2. Can radar detect an asteroid before optical telescopes do?

Not generally as a first-discovery method. Radar usually follows up on an object whose location is already known. Wide-field optical surveys and infrared observatories are central to discovering new near-Earth asteroids.

3. Can NASA’s radar predict exactly where an asteroid will hit?

Radar measurements can improve an asteroid’s orbit and help scientists calculate future positions. Whether an impact can be predicted depends on the quality of all available observations and the uncertainties in the object’s orbit. Radar does not automatically provide a precise impact forecast for every object.

4. What was asteroid 2025 OW?

Asteroid 2025 OW was a near-Earth asteroid discovered by the Pan-STARRS2 survey in July 2025. Goldstone radar observations later helped scientists estimate its size, shape and rapid rotation. It passed Earth safely.

5. What is NEO Surveyor?

NEO Surveyor is a planned NASA space telescope designed to discover and characterise potentially hazardous asteroids and comets using infrared observations. NASA lists its launch as no earlier than September 2027.

6. Has NASA successfully tested asteroid deflection?

Yes. NASA’s DART spacecraft struck Dimorphos in 2022 and measurably changed its orbit around Didymos. The mission demonstrated the kinetic-impact technique, although the response to a real threat would depend on the target and the available warning time.

7. Are large asteroids still a threat to Earth?

Asteroid impacts are a real natural hazard, but no known asteroid larger than 140 metres is currently identified by NASA as having a significant chance of hitting Earth over the next 100 years. Continued observations remain important because the catalogue of near-Earth objects is incomplete, especially for smaller objects.

The bottom line: finding an asteroid early is the first line of defence

Planetary radar cannot stop an asteroid from approaching Earth, and it cannot discover every object before it comes close. What it can do is provide precise measurements that help astronomers understand an asteroid’s motion and physical properties.

Those measurements become more valuable when combined with optical surveys, infrared observations, orbit modelling and technology demonstrations such as DART.

NASA’s developing NEO Surveyor mission could help close important gaps in the search for dark and difficult-to-detect asteroids. Meanwhile, radar observations continue to turn passing space rocks into objects that scientists can measure and study rather than merely speculate about.

The most effective planetary defence is not a dramatic last-minute explosion. It is a sustained effort to discover potential threats, understand their trajectories and prepare a response while there is still time to act.

Earth does not have a perfect shield against asteroids. But improved detection, better measurements and tested deflection techniques are helping humanity build something more useful: a growing ability to recognise a genuine threat before it becomes a catastrophe.

Editorial disclaimer: This article explains established NASA planetary-defence capabilities and planned missions. It does not claim that an asteroid is currently on a collision course with Earth. Asteroid risk assessments can change as new observations refine an object’s orbit.

Sources and further reading

• NASA — Goldstone planetary radar tracks asteroid close approaches
• NASA — Goldstone radar observations of asteroid 2025 OW
• NASA — Near-Earth Object Observations Program
• NASA — NEO Surveyor mission
• NASA — DART mission
• NASA — Planetary radar astronomy explained
• NASA — Arecibo Observatory decommissioning statement