About Planetary Defense
The asteroid Gaspra as seen by the Galileo spacecraft en route to Jupiter. Credit: NASA.
NASA scientists and engineers are studying asteroids and comets, and the effects of impacts on planets and moons. NASA’s Planetary Defense Coordination Office is researching technologies and techniques for deflecting asteroids away from Earth, and coordinating with other government agencies to develop a plan to address these issues.
Collisions are one of the most important processes throughout our solar system. Today, Earth encounters many bits of ice and rock—asteroids, comets, and meteoroids. Most meteoroids burn up in the atmosphere as meteors. Some chunks make it to Earth’s surface as meteorites. Larger rocks capable of damage to people or cities are rare. Currently, no known asteroids are predicted to collide with our world.
Early in the formation of the solar system, frequent and large impacts were common. These impacts left scars – gouges and craters. Many of the objects in the solar system bear those ancient scars to this day. Our Earth-Moon system formed after the impact of the early proto-Earth with another planet half of Earth’s width. The huge northern lowlands of Mars may be the result of impacts – or possibly one incredibly large impact!
This “heavy bombardment" period ended around 3.9 billion years ago. However, impacts still occur, at a reduced rate, and Earth continues to be a target. Scientists estimate that Earth and the other terrestrial planets are struck every million years by an average of five small asteroids (less than 2 kilometers wide). Larger impacts occur less frequently.
NASA is searching for and examining potentially hazardous asteroids and comets. Near-Earth Objects (NEOs) are those whose orbits bring them close to Earth. Large NEOs with orbits that cross Earth’s are considered potentially hazardous objects. The space near Earth is dominated by asteroids: scientists think millions of near-Earth asteroids may exist, but only about 20,000 have been discovered so far, and just a hundred near-Earth comets have been found.
Although many of the craters on Earth are no longer visible, it has actually been rocked by 13 to 20 times more impacts than our Moon!
Fifty thousand years ago, an iron asteroid struck what is today Arizona. The resulting impact crater, called Barringer (or Meteor) Crater, is still visible today. The relatively small 30 meter-wide (100 foot) asteroid that created this crater erased the vegetation over an area of the size of Los Angeles or Houston! Impacts this size occur once every few thousand years on Earth.
The environmental effects of the impact of the iron asteroid that formed Meteor Crater. Credit: University of Arizona/ LPL/David A. Kring
- A fireball would extend up to 6 miles (10 km) from the impact site (red circle)
- The impact shock wave would injure or kill animals up to 15 miles (24 km) (yellow circle)
- Hurricane-force winds would be felt as far away as 25 miles (40 km) (blue circle)
Asteroids and comets can also explode in the air. More common than impacts, airbursts can strike once every hundred years or so. The explosions generate shockwaves with energies greater than World War II-era atomic bombs. Such an airburst leveled 800 square miles of forest in Siberia in 1908. A smaller airburst exploded above Chelyabinsk, Russia on February 15, 2013, knocking people off their feet, crumbling walls, and shattering windows.
Scientists are examining asteroids’ characteristics and searching for those whose orbits may eventually intersect Earth. Radio observatories like the Arecibo Observatory in Puerto Rico use radio waves (radar) to determine an asteroid’s distance from Earth, its orbit, size, and how quickly it spins. Planetary scientists study impact craters on Earth and other bodies, and meteoritic samples of near-Earth asteroids that have fallen to Earth.
Planetary missions are analyzing the damage produced by impact cratering on asteroids themselves, and comparing the types and compositions asteroids. Impacts can alter the structure of an asteroid; some are solid throughout while others are loose collections of rubble. Understanding the structure of asteroids will help scientists and engineers create solutions to remove potential, future threats of impact.
Planetary defense research is critical to our assessment of future impact hazards and for developing spacecraft missions that will assist us in protecting our home.