Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago.

The current known asteroid count is: 795,148.

Most of this ancient space rubble can be found orbiting the sun between Mars and Jupiter within the main asteroid belt. Asteroids range in size from Vesta—the largest at about 329 miles (530 kilometers) in diameter – to bodies that are less than 33 feet (10 meters) across. The total mass of all the asteroids combined is less than that of Earth’s Moon.

Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered. As they revolve around the sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go. More than 150 asteroids are known to have a small companion moon (some have two moons). There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.

The three broad composition classes of asteroids are C-, S-, and M-types.

• The C-type (chondrite) asteroids are most common, probably consist of clay and silicate rocks, and are dark in appearance. They are among the most ancient objects in the solar system.
• The S-types (“stony”) are made up of silicate materials and nickel-iron.
• The M-types are metallic (nickel-iron). The asteroids’ compositional differences are related to how far from the sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface.

Jupiter’s massive gravity and occasional close encounters with Mars or another object change the asteroids’ orbits, knocking them out of the main belt and hurling them into space in all directions across the orbits of the other planets. Stray asteroids and asteroid fragments slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth.

Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth’s orbit, and near-Earth asteroids that approach Earth’s orbital distance to within about 45 million kilometers (28 million miles) and may pose an impact danger. Radar is a valuable tool in detecting and monitoring potential impact hazards. By reflecting transmitted signals off objects, images and other information can be derived from the echoes. Scientists can learn a great deal about an asteroid’s orbit, rotation, size, shape, and metal concentration.

Main Asteroid Belt: The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.

Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points). There, the gravitational pull from the sun and the planet are balanced by a trojan’s tendency to otherwise fly out of the orbit. The Jupiter trojans form the most significant population of trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune trojans, and NASA announced the discovery of an Earth trojan in 2011.

Near-Earth Asteroids: These objects have orbits that pass close by that of Earth. Asteroids that actually cross Earth’s orbital path are known as Earth-crossers. As of June 19, 2013, 10,003 near-Earth asteroids are known and the number over 1 kilometer in diameter is thought to be 861, with 1,409 classified as potentially hazardous asteroids – those that could pose a threat to Earth.

In space, a large rocky body in orbit about the Sun is referred to as an asteroid or minor planet whereas much smaller particles in orbit about the Sun are referred to as meteoroids. Once a meteoroid enters the Earth’s atmosphere and vaporizes, it becomes a meteor (i.e., shooting star). If a small asteroid or large meteoroid survives its fiery passage through the Earth’s atmosphere and lands upon the Earth’s surface, it is then called a meteorite. Cometary debris is the source of most small meteoroid particles. Many comets generate meteoroid streams when their icy cometary nuclei pass near the Sun and release the dust particles that were once embedded in the cometary ices. These meteoroid particles then follow in the wake of the parent comet. Collisions between asteroids in space create smaller asteroidal fragments and these fragments are the sources of most meteorites that have struck the Earth’s surface.

Because they are readily available for study, many meteorites have already been subjected to detailed chemical and physical analyses in laboratories. If particular asteroids can be identified as the sources for some of the well-studied meteorites, a detailed knowledge of the meteorite’s composition and structure will provide important information on the chemical mixture and conditions from which the parent asteroid formed 4.6 billion years ago.

Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid’s potential to make threatening close approaches to the Earth. Specifically, all asteroids with a minimum orbit intersection distance (MOID) of 0.05 au or less and an absolute magnitude (H) of 22.0 or less are considered PHAs. In other words, asteroids that can’t get any closer to the Earth (i.e. MOID) than 0.05 au (roughly 7,480,000 km or 4,650,000 mi) or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not considered PHAs.

This “potential” to make close Earth approaches does not mean a PHA will impact the Earth. It only means there is a possibility for such a threat. By monitoring these PHAs and updating their orbits as new observations become available, we can better predict the close-approach statistics and thus their Earth-impact threat.

Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth’s neighborhood. Composed mostly of water ice with embedded dust particles, comets originally formed in the cold outer planetary system while most of the rocky asteroids formed in the warmer inner solar system between the orbits of Mars and Jupiter.

In technical terms, NEOs are NEAs (near-Earth asteroids) and NECs (near-Earth comets). NEAs are asteroids whose perihelion distance is less than 1.3 au. NECs are comets whose perihelion distance is less than 1.3 au and whose orbital period is less than 200 years.

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.

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.

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 240,00 miles, or 384,000 kilometers).

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.

An asteroid’s orbit is computed by finding the elliptical path about the sun that best fits the available observations of the object. That is, the object’s computed path about the sun is adjusted until the predictions of where the asteroid should have appeared in the sky at several observed times match the positions where the object was actually observed to be at those same times. As more and more observations are used to further improve an object’s orbit, we become more and more confident in our knowledge of where the object will be in the future.

An asteroid on a trajectory to impact Earth could not be shot down in the last few minutes or even hours before impact. No known weapon system could stop the mass because of the velocity at which it travels – an average of 12 miles per second.

Research indicates that the best technique to use to divert an asteroid from its impact course with Earth is scenario-dependent. That is, the choice of method for impact mitigation depends on the orbit of the object and its composition, bulk properties, and relative velocity, as well as the probability of impact and the predicted impact location. Some NEOs could be in orbits that are especially hard to reach unless we find them many years to decades in advance of impact. Some asteroids are essentially rubble piles, making them difficult to “push on” without breaking them up, while others could be coherent monolithic bodies. Some are too small or fragile to reach the surface of Earth (for example, the asteroid that disintegrated over Chelyabinsk, Russia in 2013) and would not warrant a mitigation mission but rather emergency response planning. Prior to mounting a mitigation mission, it is especially important to adequately characterize the asteroid.

Asteroid impacts are a continuously occurring natural process. Every day, 80 to 100 tons of material falls upon Earth from space in the form of dust and small meteorites (fragments of asteroids that disintegrate in Earth’s atmosphere). Over the past 20 years, U.S. government sensors have detected nearly 600 very small asteroids a few meters in size that have entered Earth’s atmosphere and created spectacular bolides (fireballs). Experts estimate that an impact of an object the size of the one that exploded over Chelyabinsk, Russia, in 2013 – approximately 55 feet (17 meters) in size – takes place once or twice a century. Impacts of larger objects are expected to be far less frequent (on the scale of centuries to millennia). However, given the current incompleteness of the NEO catalogue, an unpredicted impact – such as the Chelyabinsk event – could occur at any time.