June 30 is International Asteroid Day, which marks the anniversary of the 1908 Tunguska impact in Siberia. Earth has had its share of cosmic collisions, but these extraterrestrial impacts pose increasing threats as human settlements have stretched across most of Earth. Now, planetary scientists and engineers are designing plans, tools and experiments to defend the planet and humanity.
In December 2024, NASA discovered a 100-meter-wide asteroid named 2024 YR4 and issued a warning that there was a one- to three-percent chance that this “city killer” might hit Earth in 2032. Later monitoring ruled out a collision; nevertheless, as the book “How to Kill an Asteroid” by Robin George Andrews emphasizes, an asteroid hitting Earth is not uncommon. For example, 12 years ago, a 180-meter-wide asteroid that weighed 9,100 tons broke up in the sky above Chelyabinsk in Russia. Within minutes, the shock waves injured 1,491 people and damaged 7,200 buildings.
When discussing meteorite impacts, the Chicxulub catastrophe that wiped out dinosaurs and many other species 66 million years ago is often what comes to mind. Meteorites of Chicxulub size – about ten-kilometers wide – can hit Earth once every 100 to 200 million years, but current maps of large asteroids and their movements indicate that such impacts are unlikely in the next 100 years or so.
The real danger is from midsize bodies, 100–200-meter-wide “city killers” or 1–2 kilometer-wide “country crushers” that might cross Earth’s path. Their population and risk are considerable. Space rocks smaller than 25 meters entering Earth’s atmosphere are unlikely to reach the ground as meteorites: They typically burn up in the atmosphere.
Asteroid Defense Measures
The popular image of planetary defense against an incoming asteroid is to blast it directly with a nuclear bomb as dramatized in sci-fi movies like “Armageddon” and “Deep Impact.” But this might not be a real solution if the nuked asteroid splits into several fragments and some of them still fall to Earth.
Creating a catalog of asteroids and their trajectories involves repeated photography of the night sky, then tracing the moving spots. So far, researchers have mapped more than 1.3 million asteroids of all sizes. The vast majority of them reside in the Main Asteroid Belt between Mars and Jupiter. Some 37,000 are grouped as the Near-Earth Objects (which also includes 120 known short-period comets) circling the sun at distance of 1.3 astronomical units. These numbers will rise with better monitoring. This is the first critical step in our planetary defense, but it is not without challenges.
For one, in dark space, we can only see objects coming toward the sun – what’s called the “opposition effect.” Objects coming from the direction of the Sun will not be illuminated, and those near the Sun will be outshone. Moreover, at best, our celestial catalog captures the regular dynamism of asteroids and comets, which is good for 100 years or so. In what is called “chaotic dynamism,” the path and pace of asteroids and comets change under the influence of planetary or lunar gravity. The more and the bigger telescopes we have on Earth and in space, the more precise our mapping and monitoring will become.
Deflection may be another possible line of defense. In September 2022, NASA conducted the first-ever mission to deliberately crash a spacecraft into a 160-meter-wide asteroid, named Dimorphos, to slightly alter its path. The mission was called Double Asteroid Redirection Test (DART) because Dimorphos is a minor satellite circling the bigger asteroid, Didymos, every 12 hours. By choosing a binary asteroid, NASA wanted to measure the orbital change of Dimorphos without sending a second spacecraft for that task.
A detonation near an incoming asteroid could also send it to a new trajectory. In 2023, a team of scientists at Sandia National Laboratories blasted tiny chips of silica with X-rays in a vacuum. As reported in Nature Physics, the rays imparted some force to the chips, but the real push came from the heat that vaporized the surface of silica. The expanding gas pushed the material in the opposite direction at a high speed. The researchers believe that this procedure can be upscaled to deflect a real asteroid several kilometers by detonating a nuclear bomb in its proximity. Laser beams might also produce a similar effect.
Creating an Asteroid Armor
For the first time in Earth’s history, humans are developing a planetary defense system. Our defense mechanisms, whether averting the impactor or evacuating the target area, will crucially depend on how much advance warning, time and information we have about the impactor’s position and motion.
Asteroid hunting is just beginning, and space can surprise us. Planetary defense has a long way to go, but as Andrews writes in “How to Kill an Asteroid”: “Science and technology, with the right people in power, can provide forms of armor to make sure an asteroid does not strike Earth.”