Frequently Asked Questions

What are asteroids?

What are comets?

What are the differences between an asteroid, comet, meteoroid, meteor and meteorite?

What is a Near-Earth Object (NEO)?

What size of NEOs are dangerous?

How many Near-Earth Objects have been discovered so far?

Are any NEOs predicted to hit the Earth?

What is the risk of impacts?

How much warning will we have?

What are Atens, Apollos and Amors?

What is a Potentially Hazardous Asteroids (PHA)?

How is an orbit calculated?

Why do your first calculations of an orbit often look more threatening than later ones?

How soon after the discovery is a search for potential collisions initiated?

How often do these results change?

Why are the results published by NEODyS not the same as those published by Sentry?

How can we protect ourselves?

Why study asteroids?

Why study comets?


Most asteroids are believed to be the remnants of the material from the circumstellar disk that coalesced to form the planets. Former theories involving the break-up of a planet between Mars and Jupiter have now been discounted. They therefore represent samples of the primordial solar system, largely unchanged since its formation 4500 million years ago. In addition, there is evidence that some asteroids are related to comets, being either debris ejected from the parent body, or inert comet nuclei.

The sizes of asteroids range from dust particles to significant bodies up to the largest, Ceres, which is 913 kilometres in diameter. Their composition can be determined by the spectrum of the sunlight that they reflect.

The majority of asteroids are confined to the main asteroid belt, orbiting the sun between Mars and Jupiter. The orbits of these main belt asteroids are generally stable, but mutual interaction or the gravitational influences of Mars or Jupiter can perturb them.

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Comets come in two main types, Short Period Comets and Long Period Comets.

All comets are believed to share roughly the same composition, having formed from essentially the same pool of material. They have been described as “dirty snowballs”, a description reinforced, but complicated by the detailed studies of Comet Halley in 1986. “Icy mudball” may be a more accurate description. The main constituents of nuclei are volatile ices, mainly water, mixed with dust and hydrocarbons. The one nucleus that has been closely studied (Comet Halley) is blacker than coal, due to a coating of dark hydrocarbons. Until the Giotto mission in 1986 that photographed Halley, it was assumed that cometary nuclei would have high albedos, due to their icy composition. The very low albedo of Halley has caused a reassessment, and a consequent upward revision of size and mass estimates of cometary nuclei. The increased temperatures encountered as the nucleus approaches the Sun cause volatile ices to sublime, releasing gas and dust which form a cloud, or coma around the nucleus, and the dust element of the “tail” popularly associated with comets. The other element, the plasma tail, is caused by the interaction of the solar wind with ions released from the nucleus during outgassing.

There is increasing evidence (Bailey and Emel-Yanenko, 1997) that there might be a significant population of “dead” comets occupying Halley type orbits. Once a comet has outgassed all of the available volatiles, its coma and tail will disappear, and the remaining, inert nucleus will take on the appearance of a low albedo asteroid. Finding such bodies could present new challenges to search programmes, requiring the use of infrared technology.

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Asteroid A relatively small, inactive, rocky body orbiting the Sun.
Comet A relatively small, at times active, object whose ices can vaporize in sunlight forming an atmosphere (coma) of dust and gas and, sometimes, a tail of dust and/or gas.
Meteoroid A small particle from a comet or asteroid orbiting the Sun.
Meteor The light phenomenon which results when a meteoroid enters the Earth’s atmosphere and vaporizes; a shooting star.
Meteorite A meteoroid that survives its passage through the Earth’s atmosphere and lands upon the Earth’s surface.

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Near-Earth-Objects (NEOs) are asteroids and comets with orbits that regularly bring them close to the Earth and which, therefore, are capable someday of striking our planet.

You can also read our comprehensive guide to Near Earth Objects.

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The Earth’s atmosphere protects us from most NEOs smaller than a modest office building (40 m diameter, or impact energy of about 3 megatons). From this size up to about 1 km diameter, an impacting NEO can do tremendous damage on a local scale. Above an energy of a million megatons (diameter about 2 km), an impact will produce severe environmental damage on a global scale. The probable consequence would be an “impact winter” with loss of crops worldwide and subsequent starvation and disease. Still larger impacts can cause mass extinctions, like the one that ended the age of the dinosaurs 65 million years ago (15 km diameter and about 100 million megatons).

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As of June 24, 2012, 9064 Near-Earth objects have been discovered. Some 847 of these NEOs are asteroids with a diameter of approximately 1 kilometer or larger. Also, 1318 of these NEOs have been classified as Potentially Hazardous Asteroids (PHAs).

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As of the end of 2011, astronomers had discovered more than 90% of the larger Near Earth Asteroids (diameter greater than 1 km). None of the known asteroids is a threat, but we have no way of predicting the next impact from an unknown object. The count of known NEAs can be obtained daily from the NASA Program Office website at

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We don’t know when the next NEO impact will take place, but we can calculate the odds. Statistically, the greatest danger is from an NEO with about 1 million megatons energy (roughly 2 km in diameter). On average, one of these collides with the Earth once or twice per million years, producing a global catastrophe that would kill a substantial (but unknown) fraction of the Earth’s human population. Reduced to personal terms, this means that you have about one chance in 40,000 of dying as a result of a collision. Such statistics are interesting, but they don’t tell you, of course, when the next catastrophic impact will take place – next year or a million years from now. The purpose of the Spaceguard Survey is not to improve these statistical estimates, but to find any individual rock that may be on a collision course.

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With so many of even the larger NEOs remaining undiscovered, the most likely warning today would be zero — the first indication of a collision would be the flash of light and the shaking of the ground as it hit. In contrast, if the current surveys actually discover a NEO on a collision course, we would expect many decades of warning. Any NEO that is going to hit the Earth will swing near our planet many times before it hits, and it should be discovered by comprehensive sky searches like Spaceguard. In almost all cases, we will either have a long lead time or none at all.

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Atens, Apollos and Amors are subgroups of Near-Earth asteroids, and are categorized by their orbits. NEOs are asteroids and comets with perihelion distance q less than 1.3 AU. The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). NEAs are further divided into the following groups according to their perihelion distance (q), aphelion distance (Q) and their semi-major axes (a):

Group Description Definition
NEAs Near-Earth Asteroids q<1.3 AU
Atens Earth-crossing NEAs with semi-major axes smaller than Earth’s (named after asteroid 2062 Aten). a<1.0 AU, Q>0.983 AU
Apollos Earth-crossing NEAs with semi-major axes larger than Earth’s (named after asteroid 1862 Apollo). a>1.0 AU, q<1.017 AU
Amors Earth-approaching NEAs with orbits exterior to Earth’s but interior to Mars’ (named after asteroid 1221 Amor). a>1.0 AU, 1.017<q<1.3 AU

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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.

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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.

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Because orbits stemming from very limited observation sets are more uncertain it is more likely that such orbits will “permit” future impacts. However, such early predictions can often be ruled out as we incorporate more observations and reduce the uncertainties in the object’s orbit. Most often, the threat associated with a specific object will decrease as additional observations become available, and so objects will be posted to, and later removed from, our Impact Risk Page. The Palermo Scale values will typically start out at less negative values when the object’s orbit is most uncertain and evolve to more negative values (and eventually off the list) as more and more observations allow the object’s orbit to be continually improved.

On the other hand, in the unlikely case where a particular potential impact event persists until the orbit is relatively well constrained, the impact probability and associated risk will tend to increase as observations are added. This is not too paradoxical: if an asteroid is indeed going to come very near the Earth then a collision cannot be ruled out early on. The impact probability will tend to grow as the orbit is refined and alternative and safer trajectories are eliminated. Eventually, the impact probability will drop (usually quite abruptly) to zero or, if the asteroid is really on a collision trajectory, it will continue to grow until it reaches 100%.

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When the discovery of a new NEA is announced by the Minor Planet Center (MPC), Sentry automatically (usually within an hour or two) prioritizes the object for an impact risk analysis. If the prioritization analysis indicates that the asteroid cannot pass near the Earth or that its orbit is very well determined then the computationally intensive nonlinear search for potential impacts is not pursued. If, on the other hand, a search is deemed necessary then the object is added to a queue of objects awaiting analysis. Its position in the queue is determined by the estimated likelihood that potential impacts may be found.

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NEA orbits and close approach tables are continuously and automatically updated whenever new observations are made available, generally within a couple of hours of the release of the information. Whenever an NEA orbit is updated the object is re-prioritized and, if appropriate, it is re-queued for a new potential impact search. This process is ongoing – taking place anytime, day and night, seven days a week.

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The differences between the two systems are generally not substantial, and in some sense they are reassuring. Independent systems using different software and theoretical approaches are not expected to produce the same results from statistical searches. Experience has shown that there is excellent agreement between the two systems for the more serious potential collision detections.

One of the differences between the two systems stems from different approaches to computing the impact probability. This computation is rough by its very nature, and different techniques may be used; impact probabilities different by a factor of ten or so are not extraordinary.

Another important variation is that Sentry uses a different sampling strategy, one that should detect nearly all potential impacts with probability greater than 10-8 (1 in 100 million), and does not expend much effort pursuing less likely cases, although it may find some anyway. In any case, nothing with impact probability below 10-10 (1 in 10 billion) is published by Sentry. In contrast, NEODyS may not detect as many potential impacts at probabilities below 10-6 (1 in 1 million), but in certain cases it can detect very low probability events that Sentry does not.

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NEO impacts are the only major natural hazard that we can effectively protect ourselves against, by deflecting (or destroying) the NEO before it hits the Earth. The first step in any program of planetary defense is to find the NEOs; we can’t protect against something we don’t know exists. We also need a long warning time, at least a decade, to send spacecraft to intercept the object and deflect it. Many defensive schemes have been studied in a preliminary way, but none in detail. In the absence of active defense, warning of the time and place of an impact would at least allow us to store food and supplies and to evacuate regions near ground zero where damage would be the greatest.

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The scientific interest in asteroids is due largely to their status as the remnant debris from the inner solar system formation process. Because some of these objects can collide with the Earth, asteroids are also important for having significantly modified the Earth’s biosphere in the past. They will continue to do so in the future. In addition, asteroids offer a source of volatiles and an extraordinarily rich supply of minerals that can be exploited for the exploration and colonization of our solar system in the twenty-first century.

Asteroids represent the bits and pieces left over from the process that formed the inner planets, including Earth. Asteroids are also the sources of most meteorites that have struck the Earth’s surface and many of these meteorites have already been subjected to detailed chemical and physical analyses. If certain asteroids can be identified as the sources for some of the well-studied meteorites, the detailed knowledge of the meteorite’s composition and structure will provide important information on the chemical mixture, and conditions from which the Earth formed 4.6 billion years ago. During the early solar system, the carbon-based molecules and volatile materials that served as the building blocks of life may have been brought to the Earth via asteroid and comet impacts. Thus the study of asteroids is not only important for studying the primordial chemical mixture from which the Earth formed, these objects may hold the key as to how the building blocks of life were delivered to the early Earth.

Every day, the Earth is bombarded with more than 100 tons of dust and sand-size particles. Many of the incoming particles are so small that they are destroyed in the Earth’s atmosphere before they reach the ground. These particles are often seen as meteors or shooting stars. The vast majority of all interplanetary material that reaches the Earth’s surface originates as the collision fragments of asteroids that have run into one another some eons ago. With an average interval of about 100 years, rocky or iron asteroids larger than about 50 meters would be expected to reach the Earth’s surface and cause local disasters or produce the tidal waves that can inundate low lying coastal areas. On an average of every few hundred thousand years or so, asteroids larger than a mile could cause global disasters. In this case, the impact debris would spread throughout the Earth’s atmosphere so that plant life would suffer from acid rain, partial blocking of sunlight, and from the firestorms resulting from heated impact debris raining back down upon the Earth’s surface. The probability of an asteroid striking the Earth and causing serious damage is very remote but the devastating consequences of such an impact suggests we should closely study different types of asteroids to understand their compositions, structures, sizes, and future trajectories.

The asteroids that are potentially the most hazardous because they can closely approach the Earth are also the objects that could be most easily exploited for raw materials. These raw materials could be used in developing the space structures and in generating the rocket fuel that will be required to explore and colonize our solar system in the twenty-first century. By closely investigating the compositions of asteroids, intelligent choices can be made as to which ones offer the richest supplies of raw materials. It has been estimated that the mineral wealth resident in the belt of asteroids between the orbits of Mars and Jupiter would be equivalent to about 100 billion dollars for every person on Earth today.

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Life on Earth began at the end of a period called the late heavy bombardment, some 3.8 billion years ago. Before this time, the influx of interplanetary debris that formed the Earth was so strong that the proto-Earth was far too hot for life to have formed.

Under this heavy bombardment of asteroids and comets, the early Earth’s oceans vaporized and the fragile carbon-based molecules, upon which life is based, could not have survived. The earliest known fossils on Earth date from 3.5 billion years ago and there is evidence that biological activity took place even earlier – just at the end of the period of late heavy bombardment. So the window when life began was very short. As soon as life could have formed on our planet, it did. But if life formed so quickly on Earth and there was little in the way of water and carbon-based molecules on the Earth’s surface, then how were these building blocks of life delivered to the Earth’s surface so quickly? The answer may involve the collision of comets with the Earth, since comets contain abundant supplies of both water and carbon-based molecules.

As the primitive, leftover building blocks of the outer solar system formation process, comets offer clues to the chemical mixture from which the giant planets formed some 4.6 billion years ago. If we wish to know the composition of the primordial mixture from which the major planets formed, then we must determine the chemical constituents of the leftover debris from this formation process – the comets. Comets are composed of significant fractions of water ice, dust, and carbon-based compounds.

Since their orbital paths often cross that of the Earth, cometary collisions with the Earth have occurred in the past and additional collisions are forthcoming. It is not a question of whether a comet will strike the Earth, it is a question of when the next one will hit. It now seems likely that a comet or asteroid struck near the Yucatan peninsula in Mexico some 65 million years ago and caused a massive extinction of more than 75% of the Earth’s living organisms, including the dinosaurs.

Comets have this strange duality whereby they first brought the building blocks of life to Earth some 3.8 billion years ago and subsequent cometary collisions may have wiped out many of the developing life forms, allowing only the most adaptable species to evolve further. Indeed, we may owe our preeminence at the top of Earth’s food chain to cometary collisions. A catastrophic cometary collision with the Earth is only likely to happen at several million year intervals on average, so we need not be overly concerned with a threat of this type. However, it is prudent to mount efforts to discover and study these objects, to characterize their sizes, compositions and structures and to keep an eye upon their future trajectories.

As with asteroids, comets are both a potential threat and a potential resource for the colonization of the solar system in the twenty first century. Whereas asteroids are rich in the mineral raw materials required to build structures in space, the comets are rich resources for the water and carbon-based molecules necessary to sustain life. In addition, an abundant supply of cometary water ice can provide copious quantities of liquid hydrogen and oxygen, the two primary ingredients in rocket fuel. One day soon, comets may serve as fueling stations for interplanetary spacecraft.

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