Planetary defense
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What is planetary defense?
Planetary defense involves detecting asteroids and comets that could significantly affect Earth and preventing or mitigating any impacts.
NASA consolidated its planetary defense programs into the Planetary Defense Coordination Office (PDCO) in 2016. Besides searching for dangerous asteroids, the PDCO notifies other federal agencies about the approach or potential impact of such objects and collaborates with FEMA if an asteroid impact is unavoidable.
The first experimental test of deflecting an asteroid was the Double Asteroid Redirection Test (DART) mission.
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Why do we need planetary defense?
Planetary defense is needed to detect and prevent impacts from asteroids and comets that could significantly affect Earth. History shows that impacts from space have caused mass extinctions, such as the event that killed the dinosaurs 66 million years ago. Even smaller objects can cause significant damage, as demonstrated by the Tunguska event in 1908, where an asteroid or comet flattened a vast area of Siberia. Programs like NASA's Planetary Defense Coordination Office (PDCO) work to identify threatening objects and develop strategies to deflect them, as seen in the Double Asteroid Redirection Test (DART) mission.
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What kinds of objects does planetary defense protect Earth from?
Planetary defense aims to protect Earth from potentially hazardous asteroids and comets that could significantly affect the planet upon impact. These objects range in size, with the greatest potential danger coming from the largest projectiles, as their collisions with Earth are infrequent but pose a substantial threat to human life and the environment.
NASA classifies near-Earth objects (NEOs) as asteroids and comets with orbits that approach Earth's orbit around the Sun within 45 million km. A subset of NEOs, those that are 140 meters or larger and come within 7.5 million km of Earth's orbit, are considered potentially hazardous objects (PHOs).
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How do scientists detect asteroids and comets that could hit Earth?
Scientists identify potentially hazardous asteroids and comets through Near-Earth Object (NEO) search programs. These programs use reflecting telescopes with digital cameras to take multiple images of the same sky region over a short time, then compare the images to find fast-moving objects. The object's motion and brightness offer clues to its distance and size.
NASA launched the NEO Surveyor space telescope in 2026 to detect more asteroids. The Planetary Defense Coordination Office (PDCO) consolidates NASA’s planetary defense programs and is in charge of strategies for deflecting asteroids that could hit Earth.
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What are some methods scientists might use to prevent an asteroid impact?
To prevent asteroid impacts, scientists are exploring methods of planetary defense that involve detecting and mitigating the effects of asteroids and comets that could significantly affect Earth.
One strategy involves kinetic energy interception, where a nonexplosive projectile hits the object at high speed to change its orbit or fragment it. For larger objects, more aggressive measures using thermonuclear devices might be needed.
The Double Asteroid Redirection Test (DART) was the first experiment testing asteroid deflection. In 2022, the DART spacecraft collided with Dimorphos, altering its orbit and shape, proving the feasibility of asteroid deflection.
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How do international organizations work together on planetary defense missions?
International cooperation in planetary defense involves various organizations. For example, the United Nations Committee on the Peaceful Uses of Outer Space addresses scientific, technical, and legal issues related to international space activities and has developed treaties governing space use. NASA's Planetary Defense Coordination Office (PDCO) consolidates planetary defense programs, searches for dangerous asteroids, alerts federal agencies, and collaborates with FEMA. The PDCO also manages strategies for deflecting asteroids that could impact Earth, demonstrated by the Double Asteroid Redirection Test (DART) mission, which successfully altered the orbit of the asteroid Dimorphos.
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Planetary defense, the detection of asteroids and comets whose impact with Earth could significantly affect the planet and the prevention or mitigation of any such impacts.
Meteorite falls have been scientifically accepted since the early 19th century, but recognition of the awesome power of meteorite impacts did not follow until the 20th century with the realization of the true nature of the Tunguska event, in which an asteroid or a comet flattened 2,000 square km (500,000 acres) in Siberia on June 30, 1908, and the discovery that a 1,200-metre (4,000-foot) diameter crater in Arizona had been caused by a meteorite impact 50,000 years ago. That Earth impacts could pose a danger to humanity was made more evident with the proposal in 1980 that an asteroid impact had likely caused the extinction of 80 percent of Earth’s animal life, including the dinosaurs, 66 million years ago.
In 1990 Congress asked the National Aeronautics and Space Administration (NASA) to study how to discover more asteroids that would cross Earth’s orbit (and thus would be likely to collide with Earth) and how to alter the orbits of or to destroy such dangerous asteroids. It was determined that the most dangerous asteroids were those with a diameter larger than 1 km (0.6 miles), the so-called “planet-killer” asteroids. Congress then asked NASA to find at least 90 percent of such asteroids by 2010. NASA did so, but, in 2005, during the course of the search for the planet-killers, Congress asked it to complete a similar search for the “city-killer” asteroids, those with a diameter greater than 140 metres (460 feet), by 2020. As of 2022, NASA has found about 40 percent of such asteroids and plans to launch a space telescope, Near-Earth Object (NEO) Surveyor, in 2026 to detect many more such asteroids.
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In 2016 NASA consolidated its planetary defense programs into the Planetary Defense Coordination Office (PDCO). In addition to searching for dangerous asteroids, the PDCO would also notify other federal agencies about the approach or possible impact of such objects and work with the Federal Emergency Management Agency (FEMA) in case an asteroid impact is unavoidable.
The PDCO is also in charge of any mitigation strategies for deflecting an asteroid that could impact Earth. The first experimental test of deflecting an asteroid was the Double Asteroid Redirection Test (DART) mission. On September 26, 2022, the DART spacecraft collided with the asteroid Dimorphos, which orbits the larger asteroid Didymos. Dimorphos orbited Didymos every 11 hours and 55 minutes. Mission scientists considered success to be the alteration of Dimorphos’s orbit by at least 73 seconds. DART changed Dimorphos’s orbital period to 11 hours and 23 minutes, a much larger change. The large change in orbital period was much more than that which would have been caused solely by DART’s collision with Dimorphos, and the momentum transfer from the spacecraft to the asteroid was enhanced by the spacecraft’s impact causing an ejection of tons of material from the asteroid. About eight percent of Dimorphos’s mass was restributed, significantly changing the asteroid’s shape.