Frequency of impacts
- Related Topics:
- small body
- planetary defense
- near-Earth object
Because there are far fewer large NEOs and long-period comets in space than smaller ones, the chances of a collision decrease rapidly with increasing size. The impact-hazard community—primarily scientists with an interest in the issue—has defined a global catastrophe to be an impact that leads to the death of one-fourth or more of the world’s population. An impact by a 1-km- (0.6-mile-) diameter NEO, the smallest believed capable of causing such a catastrophe, is estimated to occur about once per 100,000 years on average, based on the assumed population in space of such objects. On the other hand, an impact by a 100-metre (328-foot) NEO, the smallest believed capable of causing regional devastation, is estimated to occur about once every 1,000 years on average. (An impact from a body the size of the Chelyabinsk meteorite of 2013 [17 metres (56 feet)] is expected to occur once per century.) The hazard posed by long-period comets is less certain because fairly few such objects are known, but it is thought to be perhaps as high as 25 percent of that for NEOs.
The major difference between the threat posed by the impact of an asteroid or comet and that posed by other natural disasters is the extent of the damage that could be done. In some parts of the world at high risk for floods or earthquakes, the chances of dying in such an event are 100–200 times greater than the risk of dying from a cosmic impact. What distinguishes the impact hazard, however, is that it is the only known natural disaster, with the possible exception of an exceedingly large volcanic eruption, that could result in the death of a significant fraction of Earth’s population and, in the most extreme case, the extinction of the human species.
NEO search programs
The outlook for detecting long-period comets that are specifically on a collision course with Earth is poor. Long-period comets, by definition, would likely be discovered on their way into the inner solar system only a few months—or, at best, a few years—before impact. (It is possible that an NEO destined for a collision with Earth also might not be discovered until the “last minute,” but the probability of finding it many years before impact is much higher because NEOs make frequent passages near Earth before colliding with it.) Although it might be feasible to detect long-period comets as early as about six months before impact, this knowledge would be useful only if a practical technology were already in place for preventing the collision. And if the comet is not on a collision course, then it is of little immediate concern, because it will not return for at least 200 years and perhaps not for millennia.
In principle, the outlook for identifying the larger NEOs is more promising. Using current technology, it is possible to find virtually all NEOs with diameters greater than 1 km (0.6 mile) and most of those half as large.
A number of loosely coordinated programs to search for NEOs have been instituted. Among them are several sponsored by the National Aeronautics and Space Administration (NASA) in the United States and others in China, Japan, and Italy (as a joint Italian-German project). Their objective is to find objects capable of causing global catastrophe were they to hit Earth. In 1998 NASA stated its official program goal to be the “detection of 90 percent of the NEO population larger than 1 km within a decade.” As of 2020, about 900 NEOs at least a kilometre in size were known.
These search programs use charge-coupled-diode (CCD) sensors, similar to those in digital cameras, on reflecting telescopes with primary mirrors in the 1-metre- (40-inch-) diameter range to obtain three or more images of the same region of the sky over a short period of time, generally some tens of minutes. The images are then compared with one another to find objects that have moved rapidly. The distance that the object has moved between images and its brightness provide clues to its distance and size. For example, fast-traveling, bright objects are almost certainly very close to Earth. A definitive orbit, however, is required before an accurate prediction can be made of the object’s true distance and future path through space. This generally requires several days to acquire an “arc” of sufficient length to allow computation of an accurate orbit.
For every NEO a kilometre or larger in size, there are thousands more as small as about 100 metres (328 feet). An impact by a 100-metre object has the explosive power of about 100 megatons of TNT, roughly equivalent to the largest man-made nuclear explosions. (If the blast of the Tunguska event had an energy of 15 megatons, as some damage-based estimates have placed it, then the colliding object likely had a diameter of about 30–50 metres [100–164 feet].) Search programs in the 1990s discovered several NEOs in this smaller size range passing close enough to Earth to attract attention in the popular press. In fact, for each one detected, hundreds of unobserved objects passed just as close or closer. Dozens in this range were larger than 100 metres and thus large enough to cause a devastating tsunami or, if one were to hit land, to destroy an area the size of a small country. The chance of an impact by a 100-metre NEO is about 1 in 10 per century. For each actual impact, however, there will be numerous near misses. Many of these NEOs are certain to be detected as a by-product of searches for the larger objects capable of causing global catastrophes.
Determining the hazard potential of an NEO
When an NEO is first discovered, its orbit and size are uncertain. If sufficient observations are made during its discovery apparition, a fairly good orbit can be computed. In practice, however, few orbits are reliably determined during the first apparition, and later observations of the object are required to learn how its position has changed in the interim. Observations to determine its size are rarely made (perhaps several in 100 are so observed), because they require specialized techniques such as radar or thermal infrared radiometry; rather, the size of an NEO is estimated from its brightness. Sizes estimated this way are uncertain by about a factor of 2—that is, an object reported as being 1 km (0.6 mile) in diameter could have a diameter between 0.5 and 2 km (0.3 and 1.2 miles).
In most cases, sufficient observation of an object will establish that the chances of its colliding with Earth are negligible. In some cases, however, there is no opportunity for additional observation. This happens, for example, when the object is small and discovered while passing very close to Earth; it quickly becomes too faint to observe further. Even a larger and more distant object can be lost because of poor weather (a factor taken into account in choosing observing sites for search programs). Without the observations needed to compute a reliable orbit, prediction of the object’s future close approaches to Earth is highly uncertain.
When computations indicate that a NEO estimated to be larger than about 200 metres (656 feet) could strike Earth during the next century or two, the object is called a potentially hazardous asteroid (PHA). As of 2019 there were about 2,000 identified PHAs. Observations of PHAs are continued until their orbits are refined to the point where their future positions can be reliably predicted.
While an object remains on the PHA list, its hazard potential is described by the Torino Impact Hazard Scale, an indicator named after the city of Turin (Italian: Torino), Italy, where it was presented at an international NEO conference in 1999. The purpose of the scale is to quantify the level of public concern warranted. The scale’s values, which are integers between 0 and 10, are based on both an object’s collision probability and its estimated kinetic energy. The value for a given object can change as probability and energy estimates are refined by additional observations.
On the Torino scale, a value of 0 indicates that the likelihood of a collision is zero or well below the chance that a random object of the same size will strike Earth within the next few decades. This designation also applies to any small object that, should it collide, is unlikely to reach Earth’s surface intact. A value of 10 indicates that a collision is certain to occur and is capable of causing a global climatic catastrophe; such events occur on timescales of 100,000 years or longer (the mass extinction event at the end of the Cretaceous Period falls here). Intermediate values categorize impacts according to various levels of probability and destructiveness. A Torino scale value is always reported together with the predicted date of the close encounter to convey further the level of urgency that is warranted. Since the implementation of the Torino scale, the highest level reached was 4 for the asteroid Apophis, which, shortly after its discovery in 2004, had a 1.6 percent impact probability on April 13, 2029, but subsequent observations reduced the uncertainty in Apophis’s orbit, and the Torino level fell to 0. Other objects often have received initial Torino values of 1 or greater, but these values proved fictitious once the needed additional observations were made and more accurate orbits calculated.

Defending Earth from a colliding object
Even with the best of search programs, whether anything can be done about an object found to be on a collision course with Earth depends on many factors. The most important are the amount of lead time and the physical properties of the object—its size, shape, spin rate, density, strength, and other characteristics. Scientists believe that kinetic energy interception is adequate for the majority of objects, including those of intermediate size and most likely to cause destructive tsunamis. Such a strategy would involve the use of a nonexplosive projectile sent to strike the object in a particular location at high speed to change its orbit and possibly to fragment it. For the remainder, more aggressive measures, likely involving the use of powerful thermonuclear devices, are thought to be necessary to achieve the same results. Because the physical properties of NEOs are so poorly known, however, it is possible that such measures could do more harm than good—e.g., by breaking a large object into numerous smaller, but still potentially destructive, pieces without deflecting them enough to miss Earth. Validating these options requires additional theory, laboratory experiments, and safe experiments involving actual NEOs in space.
The Double Asteroid Redirection Test (DART) mission was the first such experiment in planetary defense. On September 26, 2022, the DART spacecraft collided with the asteroid Dimorphos, which orbits the larger asteroid Didymos. Dimorphos orbited Didymos every 11 hours and 55 minutes. Mission scientists considered success to be the alteration of Dimorphos’s orbit by at least 73 seconds. DART shortened Dimorphos’s orbital period to 11 hours and 23 minutes, a much larger change, and even changed Dimorphos’s shape.
Edward F. Tedesco The Editors of Encyclopaedia Britannica