What is a near-Earth object?
A near-Earth object (NEO) is an asteroid or comet whose orbit brings it within a zone approximately 121 million miles (195 million kilometers) from the Sun, meaning that it can pass within about 30 million miles (50 million kilometers) of Earth’s orbit. Like the planets, asteroids and comets orbit the Sun. Some of the smaller moons of other planets may be captured asteroids. Most asteroids are in what is called “the main belt” between Mars and Jupiter. The vast majority of near-Earth asteroids have come from inner part of the main belt where, over tens of millions of years, their orbits were altered by the gravitational influence of Jupiter and Mars, and some by mutual collisions. Although the vast majority of NEOs that enter Earth’s atmosphere disintegrate before reaching the surface (and more than 100 tons of dust particles disintegrate in Earth’s atmosphere daily), those NEOs that are larger than around 98 to 164 feet (30 to 50 meters) in size and could cause widespread damage in and around their impact sites. A potentially hazardous object (PHO) is a near-Earth object whose orbit brings it within 4.7 million miles (7.5 million km) of Earth’s orbit, and is greater than 500 feet (140 meters) in size.
How often do near-Earth objects come close to Earth?
Small asteroids a few meters in size are detected passing between Earth and the Moon’s orbit several times a month. Meteoroids – very small fragments of asteroids and comets less than 3 feet (1 meter) in size – hit Earth’s atmosphere and explode virtually every day, causing the bright meteor events that people see at night and sometimes leave remnants – meteorites – on the ground. The Jet Propulsion Laboratory’s Center for NEO Studies maintains close approach tables that are updated daily.
Do we know of any asteroids that pose a threat to Earth?
No known asteroid poses a significant risk of impact with Earth over the next 100 years. The highest risk of impact for a known asteroid is a 1 in 714 chance of impact by an asteroid designated 2009 FD in 2185, meaning that the possibility that it could impact then is less than 0.2 percent. The Sentry Impact Risk Table, which is maintained by the Jet Propulsion Laboratory’s Center for NEO Studies, is updated continuously as new asteroids are discovered and known asteroids are further observed.
One asteroid that NASA is studying up close, called Bennu, has a 1/2700 chance of impacting Earth between 2175 and 2195. The OSIRIS-REx spacecraft will complete a 2-year investigation of Bennu before plucking a sample of asteroid material off its surface and delivering it back to Earth. Along with collecting a sample, OSIRIS-REx will also be studying how light absorbed from the Sun and re-radiated by Bennu affects its orbit—and consequently, how that orbit could become more dangerous for Earth. Read more about the OSIRIS-REx mission’s contribution to planetary defense here.
How are NEOs found and tracked?
Observers find and track NEOs using ground-based telescopes around the world, and, currently, NASA’s space-based NEOWISE infrared telescope. The basic method of finding NEOs is to look for small objects moving across the background of relatively fixed stars. Observers track NEOs by using their predicted orbits, based on initial observations, to look for the objects at the time and in the place where they have been predicted to be visible to telescopes again. It takes a week to a month of observations for scientists to establish a good orbit determination. Observers provide their data to a global database maintained by the Minor Planet Center, which is sanctioned by the International Astronomical Union and funded by NASA’s NEO Observations Program.
Watch these two videos for a deeper look into asteroid-hunting:
How do we spot NEOs?
The international asteroid hunt, how the hunt is coordinated by NASA.
What is a NEO close approach?
A NEO close approach occurs when an object passes by Earth, but it is of particular interest when it passes within the distance from the Earth to the Moon, or a “lunar distance”. NEO close approaches are often measured in lunar distances (1 LD=approximately 240,000 miles, or 384,000 kilometers). The Jet Propulsion Laboratory’s Center for NEO Studies maintains close approach tables that are updated daily.
How can we prevent an asteroid from hitting Earth?
Currently, an asteroid impact is the only natural disaster we might be able to prevent. There are a few methods that NASA is studying to deflect an asteroid on a course to impact Earth. One of these techniques is called a gravity tractor—it involves a spacecraft that would rendezvous with an asteroid (but not land on its surface) and maintain its relative, optimal position to use the mutual gravity attraction between the satellite and the asteroid to slowly alter the course of the asteroid. A gravity tractor spacecraft could even enhance its own gravitational attraction by first plucking a boulder off the surface of the asteroid to add to its own mass.
A kinetic impactor is currently the simplest and most technologically mature method available to defend against asteroids. In this technique, a spacecraft is launched that simply slams itself into the asteroid at several km per second speed. Scientists will test the kinetic impact technique by the Double-Asteroid Redirect Test mission (DART) on an asteroid system called Didymos in 2022. DART’s target is a binary asteroid system where one football-stadium-sized asteroid (Didymos B) is orbiting a half-mile-wide asteroid (Didymos A). NASA’s goal is to send the car-sized DART spacecraft slamming into Didymos B at 25,000 kilometers per hour (16,000 miles per hour) to determine by how much the impact can shift the orbit of Didymos B around Didymos A. After all, we’d only need to nudge an asteroid’s orbit enough to make it either seven minutes early or seven minutes late in its intersection with Earth’s orbit. It takes seven minutes for the Earth to travel the distance of its diameter, so if an asteroid arrives seven minutes early or late—it’ll miss us completely.
Nuclear explosive device methods are considered the last resort when it comes to NEO deflection, though they may be the most effective for preventing a cataclysmic event. When warning time is short or the asteroid is large, deploying a nuclear device is the most effective option. A standoff detonation is the method with the most controllability and predictability for using a nuclear device to deflect an asteroid. This method works by detonating a nuclear device at a few hundred meters above the surface of the asteroid. The energy from the device is primarily in the form of X-rays, which near instantly strike the surface of the asteroid. The material in the top layers of the asteroid is super-heated and vaporized by this radiation, causing a blow-off of material from the surface. The momentum push from the vaporized and blown off surface material imparts momentum to the rest of the asteroid and pushes it onto a new trajectory. Therefore, it is not the force from the explosion itself that moves the asteroid but rather the force of the radiated energy onto the surface of the asteroid.
For more, see Frequently Asked Questions