Planetary defense involves finding and tracking asteroids or comets that come close to Earth. These space rocks, like the asteroid Bennu shown above, can vary in size from small boulders to objects hundreds of meters across. Scientists call these near-Earth objects (NEOs). A NEO is any comet or asteroid whose orbit brings it near our planet. By studying their orbits and physical properties, researchers work to predict and avoid possible impacts. As of 2025, surveys have catalogued tens of thousands of near-Earth asteroids, and new ones are found every week. For context, an object about 20 meters wide exploded over Chelyabinsk, Russia in 2013, injuring over a thousand people. A larger asteroid hit Siberia in 1908 (the Tunguska event), flattening forests over hundreds of square kilometers. While big impacts are rare, even smaller collisions can be dangerous. This is why nations around the world invest in planetary defense.

Finding Near-Earth Objects

Ground observatories are the backbone of asteroid detection. Several major surveys scan the night sky each clear evening for moving objects:

• Pan-STARRS in Hawaii uses wide-field telescopes to survey large regions of the sky rapidly.
• The Catalina Sky Survey in Arizona searches for asteroids and comets across wide swaths of sky each night.
• ATLAS in Hawaii operates multiple telescopes to spot smaller asteroids on short notice, often weeks or days before a possible impact.
• The Vera C. Rubin Observatory in Chile (under construction) will expand searches with an 8.4-meter telescope and a powerful digital camera, greatly increasing the rate of discovery.

Telescopes like the one shown above then perform follow-up observations of newly discovered asteroids to refine their orbits. Data from these surveys are reported to the Minor Planet Center (MPC) in Massachusetts. The MPC compiles observations from astronomers worldwide and calculates the orbits of all known asteroids and comets. This shared database allows researchers everywhere to work from the same information and identify any objects that might warrant a closer look. By observing how an asteroid moves against the background of stars, scientists can compute its trajectory and determine whether it might come close to Earth.

Space-Based Observatories

Space telescopes watch for asteroids from above Earth’s atmosphere, using infrared or visible-light sensors. For example, NASA’s Wide-field Infrared Survey Explorer (WISE) satellite mapped the sky in infrared, and after its main mission it was repurposed as NEOWISE to hunt near-Earth asteroids by their heat signatures. In the future, a new mission called NEO Surveyor will carry a sensitive infrared telescope dedicated to detecting and tracking asteroids that ground telescopes might miss. Because infrared sensors can see the warmth of an asteroid, space telescopes can pick out dark or distant objects that are too faint to spot in visible light. Observing from space also allows detection of asteroids that approach from the direction of the Sun (which are invisible to Earth-based observatories during the day). These space-based systems complement ground surveys by covering parts of the sky and wavelengths that ground telescopes cannot.

Radar and Tracking

After an asteroid is discovered, radar observatories can be used to refine its orbit and learn more about its shape. Powerful facilities like the Goldstone Solar System Radar in California or the Green Bank Observatory in West Virginia can bounce radio waves off a passing asteroid. The reflected signals allow scientists to measure the asteroid’s distance, speed, and size with very high precision. (Until 2020, the Arecibo Observatory in Puerto Rico also performed this role with one of the most powerful radar dishes on Earth.) Radar measurements dramatically improve orbit predictions, because they reduce the uncertainty in an asteroid’s future path. In some cases, radar can even resolve surface details or shape information for larger asteroids. By knowing the orbit very accurately, space agencies can judge whether an asteroid poses any realistic risk to Earth in the coming years.

Deflection Technologies

If a potentially hazardous asteroid is found on a collision course with Earth, scientists have proposed several ways to avert disaster. These methods fall into two broad categories: kinetic impactors (hitting the asteroid to change its motion) and various non-impact approaches (using gravity, explosives, or other forces to shift the asteroid). No single method is perfect, and many are still under study, but they offer multiple options depending on the asteroid’s size, composition, and the warning time available.

Kinetic Impactors

The image above, taken by the Hubble Space Telescope, shows the debris cloud produced by NASA’s DART spacecraft when it deliberately crashed into an asteroid in 2022. This illustrates a kinetic impactor approach, in which a spacecraft collides with an asteroid to change its course. In NASA’s Double Asteroid Redirection Test (DART), a 610‑kg spacecraft was aimed at the 160‑meter moonlet Dimorphos of the asteroid Didymos. The impact was a success: follow-up observations showed that Dimorphos’s orbit around Didymos was shortened by about 32 minutes. This confirmed that a collision can transfer momentum to an asteroid and alter its trajectory. In practice, if a threatening asteroid were discovered, one or more impactor spacecraft could be launched years in advance to strike the asteroid at high speed. Each collision would impart a small change in velocity; over time and over multiple hits, this could be enough to nudge the asteroid away from Earth. The effectiveness of a kinetic impactor depends on factors like the asteroid’s mass and how much material is ejected by the collision, but it is a relatively straightforward technique that has now been demonstrated in space.

Nuclear Explosive Devices

Using nuclear explosives is another proposed deflection strategy, especially for very large or late-discovered threats. A nuclear device could be detonated near or on the surface of an asteroid to push it off course. The blast could vaporize or eject material, creating a recoil force on the asteroid. While no nuclear test has ever been done for planetary defense, studies suggest this could be a powerful method if there were little time to act. However, this approach has complications. International treaties currently ban nuclear explosions in space, and setting off a bomb near an asteroid carries the risk of breaking it into many fragments. That could create multiple hazardous objects instead of one, making the situation worse. For these reasons, nuclear options are generally considered a last resort, to be used only if an impact is imminent and no other methods are feasible. Research continues on whether a standoff nuclear detonation (a blast that does not touch the asteroid) could deliver enough push without fragmentation. But many planners prefer non-nuclear methods when possible.

Gravity Tractor and Other Concepts

Several other deflection ideas have been proposed, though they remain largely theoretical. One is the gravity tractor. In this concept, a heavy spacecraft would hover near the asteroid for months or years, using its own gravity to slowly tug the asteroid. The spacecraft’s thrusters would keep it in position, and over long periods this gentle pull could shift the asteroid’s orbit. Because the force is extremely small, a gravity tractor would require very early detection (decades in advance) and a very accurate approach, but it has the advantage of not risking fragmentation of the asteroid.

Other ideas involve exerting continuous forces on the asteroid. For example, a spacecraft could use an ion-beam shepherd (IBS) concept. In this case, the spacecraft hovers near the asteroid and fires a stream of charged particles (an ion beam) at the surface. The recoil from the ions hitting the asteroid would nudge it off course without direct contact. Similarly, laser ablation has been studied: powerful lasers (potentially space-based) could heat a spot on the asteroid’s surface until material vaporizes. The outgassing would act like a tiny thruster jet, gradually pushing the asteroid in the opposite direction. Another method would be surface alteration: by changing part of the asteroid’s color or reflectivity (for example, by painting it white or covering it with a reflective material), the way sunlight heats the asteroid would change. This would modify the Yarkovsky effect, a small force caused by asymmetric thermal emission, and over time could alter the orbit. Each of these approaches — gravity tractor, ion beams, lasers, painting — would work very slowly and require long lead times, but they offer alternative strategies if an asteroid is found many years before a predicted impact. None of these methods have been tested in space yet, but studies and small-scale experiments continue to evaluate their feasibility.

International Cooperation

Planetary defense is a global endeavor. In the United States, NASA’s Planetary Defense Coordination Office (PDCO) directs government efforts to find and track asteroids and coordinate responses. Europe’s European Space Agency (ESA) has its own planetary defense initiatives under its Space Safety program, and it has partnered internationally on missions. For example, ESA is building the Hera mission to follow up on NASA’s DART test and study the impacted asteroid system in detail. Other space agencies, like Japan’s JAXA and agencies in Russia, India, and elsewhere, also contribute telescopes, data analysis, or mission concepts. In 2013 the United Nations endorsed two collaborative groups: the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG). These networks bring together space agencies, astronomers, and planetary scientists to share observations and plan what to do if a threatening asteroid is found. Even private and non-profit organizations get involved. The B612 Foundation (a non-profit of scientists and former astronauts) has advocated for improved asteroid surveys and even proposed its own space telescope mission. Thanks to this international cooperation, the world benefits from shared data and expertise. By pooling resources, countries can scan the skies more thoroughly and prepare a coordinated response, increasing the chances of deflecting any dangerous asteroid well before it arrives.

Summary

Protecting Earth from asteroids relies on a combination of technology and teamwork. Modern surveys use powerful telescopes on the ground and in space to detect and track near-Earth objects. Radar observatories refine those trajectories to high precision. If a true impact threat is identified, scientists have several methods in theory to change an asteroid’s path. So far, the most mature technique — smashing a spacecraft into an asteroid — has been proven by NASA’s DART mission. Other methods like nuclear blasts, gravity tugs, lasers, or ion beams remain concepts under study. Agencies around the world continue to plan and test these ideas, and international coordination ensures that knowledge and plans are shared. In short, the current strategy is to find it early and then move it early. With vigilance and advanced technology, humanity plans to spot dangerous asteroids well in advance and apply the appropriate defense before they come too close to Earth. The ongoing work of global space agencies, observatories, and researchers increases the odds that a potential asteroid impact can be prevented, keeping our planet safe from cosmic hazards.