Gravity and colisions: what can hit us?

Carme Jordi, ICCUB-IEEC
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In the Solar System where we find ourselves immersed as inhabitants of the planet Earth, we do not find ourselves alone. Not only are there other planets and satellites, but we also find asteroids, comets and millions of dust particles, each following their path around the sun. The earth in its trajectory around the sun, year after year, performs a constant sweeping of cosmic material and collides with these bodies and materials. The comets are quite spectacular with bright heads and long tails that extend into space being visible for several weeks. Especially in ancient days they were considered signs of bad omen, based on the belief that the future could be read in the sky. Those stars of sporadic appearance were considered evil.

Both asteroids and dust particles, are often burned as visible meteors in the outer layer of the atmosphere, and only the larger or more compact material can survive atmospheric friction and reach the ground as meteorites. The meteors have a short duration and not being as spectacular as the comets, have not been popular attraction nor have caused panic or fear to anyone in the same degree. Usually they fall one by one, and sometimes they do so in the form of a meteor shower. We will describe these lesser-known components of the Solar system and we will focus mainly on those that can cause impacts. We will present some historical evidence and the chances of success.



Comets are composed of icy gases and rocky particles (dust). They are relatively small, typically a few kilometers in diameter, and their mass is less than ten- thousand times the mass of the moon. Their orbits are very elliptical and spend most of the time far away from the sun, and therefore not visible; Periodically they near the sun, rapidly complete the innermost part of their orbits, and then re-submerge themselves into the depths of the Solar system. In this fleeting visit, the heat of the sun begins to melt the ice and the flow of gas and dust forms a wrapper around the nucleus, making them visible. The solar wind, caused by the atoms escaping from the sun, expel gases and cause the typical tail of the comet (of millions of kilometers), which always points in the direction of the sun. Most of the orbits of these comets do not cross the orbits of the planets. 

Given their small mass, they have no noticeable effect on the trajectories of planets and satellites, even if they pass very closely. On the other hand, Jupiter and Saturn can modify the comets' orbits, even ejecting them from the Solar System. The average comet is so fragile that the Sun, Jupiter and Saturn could dissolve it into pieces if the comet passes too nearby, because the tidal forces caused by these large masses are much stronger than the mutual gravitational pull of the comet's material and this breaks the comet into fragments. This happened to comets Biela and West which were both broken up by the Sun in 1826 and 1975, respectively. We have also observed comet fragmentation by Jupiter: the Brooks 2 in 1886, and, more recently, in 1992 the Shoemaker Leavy 9 that broke into about 28 pieces.

Bright and spectacular comets are quite rare: one appears every 10 years on average. Currently, one is unlikely to suddenly appear. There are observers who watch for the appearance of new comets and follow them to calculate their trajectory in order to predict when they will approach the Sun and Earth, so that we will be prepared. A number of periodic comets are well known, the most famous being comet Halley which has a period of 76 years. The probability of impact of a comet with Earth is not zero, and the first idea was launched by Edmund Halley in 1705. In fact, it is believed that what razed the forest in an area of more than 1000 km2 near the Tunguska River (Siberia) in 1908, was the tip of a small comet. In 1994, we saw the collision of comet Shoemaker Leavy 9 with another planet in our system, Jupiter.

Fragmentation and the wear and tear a comet edures on each visit to the Sun, make them quite ephemeral bodies. When the ice in the comet has completely evaporated, only a cloud of dust and loose rocks survive, which are not compact enough to stay together by mutual gravitational pull and gradually disperse along the comet's orbit. The interplanetary space is full of these trails, which can be more or less dense. Invisible in space, this cosmic matter becomes apparent only when the Earth in its orbital motion casually stumbles upon it. Then, the fragment heated by friction with the atmosphere is vaporized. This is a meteor.

Meteors and meteorites

On a clear moonless night you can see bright streaks crossing the sky. We call these luminous stripes meteors or falling stars because of their star-like appearance. In general, however, they are rather weak and usually last less than a second, and at the most 5 seconds.

The Solar System contains a large number of particles, most no larger than a grain of sand, moving around the Sun. The Earth's orbital velocity is about 30km/s, so the relative particle speed is between 35 and 95 km/s. If one of these particles intercepts the Earth and enters the atmosphere, friction with air quickly heats it up to incandescence and it becomes visible as a streak of light. At the same time, the air along the path of the meteor is ionized. Meteor light can only be seen at night, but the path of ionized gas reflects radio waves and can be detected by radar both during the day and at night.

The word meteor is sometimes used confusingly to describe both the visual effect, the ionized gas and the solid particle itself responsible for this effect. It is customary to use the word meteoroid when we talk about the particle before reaching the Earth's atmosphere and the word meteor for the observable streak produced in the atmosphere. Although most meteors are completely destroyed during their journey through this atmosphere, some penetrate to the surface. They are then known as meteorites.

Meteors typically begin to be visible about 100-150 km above the surface of the earth, since it is too dim to produce incandescence above the atmosphere. The brightest meteors remain visible up to a height of 55 km or less, while the weakest (the smallest) are consumed entirely before reaching about 80 km from the ground. Although an observer can see between 6 and 8 meteors per hour, in fact they are only seeing the brightest meteors on a small part of the earth's surface. The total number of particles entering the atmosphere every day is enormous; In terms of mass, a total of approximately 1000 tons are estimated to fall to Earth every day, although this number is very uncertain. Most of this mass is in the form of micrometeorites.

The brightest meteors are called bolides, they are like fireballs and can leave a trail of hundreds of kilometers that last for minutes. They can be brighter than Venus and can even be seen by day, illuminating the landscape around them. Occasionally they are accompanied by a thunderous noise that is surely a sonic bomb like the one associated with a supersonic plane. When a bolide burns, it releases hot gases that while escaping sometimes cause the bolide to explode into pieces that fall to the ground. They are rare. One, in 1896, was observed for 5.5 hours. Most meteoroids that cause bolides are made of very fragile material, possibly originally comets that are destroyed in the Earth's atmosphere. The few that survive and give meteorites are probably of a different nature and are almost certainly small asteroids.

The number of meteors we can see is not constant and varies overnight and throughout the year. It is always bigger after midnight than before, and at our latitudes it is larger in autumn and winter. Some nights you can see a much larger number of meteors, which is known as meteor shower. Many of the meteor showers are associated with comets. As we have said, cometary material is dispersed throughout its orbit and when the Earth intercepts it, usually once a year (exceptionally two), meteor showers occur. In many cases, the material is distributed unevenly throughout the orbit and therefore the intensity of the meteor showers will vary from one year to the next. Several are known and are named after the constellation where the radiant point is located. Due to our perspective, all meteors coming from the same direction seem to radiate from the same point. Most last a few days, but others, such as the well-known Perseids lasted almost the entire month of August.

The collisions with larger meteorites cause impact craters, which are larger than the meteorite's own size due to the associated hot gases. One of the most famous craters known is Barringer in Arizona. Smaller meteorites, on the other hand, slow down and fall like simple stones or dust or are consumed entirely in the upper atmosphere and do not reach the surface. Approximately 500 meteorites fall to Earth each year. Since only 30% of the surface is earth, only about 150 of them can be collected, but usually only about 10 are actually collected. Thousands of meteorites have been found. One of the best places to find them is in Antarctica as they are dragged by ice towards the edge of the continent. The largest ever collected Hoba (Namibia) and Tucaman (South America) have masses of 60 and 15 tons, respectively.



Between the orbits of Mars and Jupiter there is a belt of small planets that we call asteroids. It is not known for certain how this belt formed. One of the existing hypotheses is that they are the remains of a planet that broke up. What we are most interested in highlighting here is that there are millions of pieces and many different sizes, the largest of which can become visible to the naked eye as bright spots in the sky, and the smallest of which are the size of a grain of sand. In fact, establishing the difference between an asteroid and a meteoroid is a difficult question. 

The composition, shape, density and surface characteristics are quite varied, from one to another. Most have irregular shapes (very few are known to be practically spherical forms). The composition is more similar to any planet in the Solar System, very different from the agglomeration of dust and ice of comets. The first asteroid found in an orbit that intersects Earth's orbit (and therefore, which can potentially produce a collision) is asteroid 1862 Apollo, in 1932. Since then more have been discovered, most with diameters less than 1 km, but the largest are about 10 km in diameter. The effect that one of these bodies can produce in a collision with our planet is like that of a meteoroid. The smallest will be invisible to the naked eye, larger ones can give rise to meteors, and only larger or more compact ones will reach the surface in the form of meteorites. In 1947, a meteorite crash created approximately 100 1m craters in SikhoteAlin( Siberia) and revived studies on the Tunguska phenomenon, coming to the conclusion that it could have been caused by the impact of a comet or an asteroid.



It is thought that the large rain of cosmic projectiles during the formation of the Solar System (which are probably responsible for the many craters we observe on the Moon, Mercury, some on Mars, Calisto, Ganímedes, etc., and perhaps responsible for the Earth's breakup in the Earth-Moon system) ended about 3.8 to 109 years ago. However, the impacts have continued regularly, albeit with less intensity, ever since.

Despite the rapid erosion and techtonism of The Earth, some 140 impact craters of asteroids and comets have been identified. The largest are Vredefort in South Africa with a 300 km diameter, Sudbury (Ontario, Canada) with a 250 km diameter, and Chickulub in Mexico at 170 km in diameter. The first to be recognized as an impact crater is Barringer in Arizona (USA). UU.) 1.2 km in diameter. It is estimated that the impact occurred 20000-50000 years ago and that the meteorite had to be about 150 m diameter and fall at a speed between 15 and 20 km/s. In written history there is no record of a meteorite falling as huge as those that formed these craters.

As we have said, the meteorite collision of 1947, in Sikhote-Alin (Siberia) is thought to have come from fragments due to the impact of a small asteroid. A little earlier, in 1908, what is thought to be the tip of a comet fell in the Tunguska region, also in Siberia, causing a devastated area of more than 1000 km2.

The effect of meteorite impacts on Earth's geological history and the evolution of life has been the subject of interdisciplinary study since the idea emerged that extinction in the Cretaceous/Tertiary may have been caused by the impact of a comet or asteroid about 10 km in diameter. This hypothesis has been accepted from the identification of a probable place of impact in the Yucatan (Chicxulub) and an intense concentration of noble elements in the sediment layer corresponding to the Cretaceous-Tertiary border. Currently, approximately 140 craters are recognized. 


Probability and consequences of an impact

There is an extremely low risk associated with falling meteorites and there is no real fatality associated with meteorite impact (although some cars have been impacted), given the low effective section of a human being. However, large projectiles, such as comets or asteroids, can cause damage. Obviously, the consequences and odds depend on the size and speed of the impact, as well as the composition of the body that collides with Earth. We will take a look different risks and their possible consequences.

  • Individual effects 

Behaviours that affect personal health and safety, such as smoking or driving cars are vastly more important for life expectancy than disasters of any kind affecting a group of people at once. Most of the risks we face in our lives actually happen to someone we know or to someone we read in newspapers. In contrast, very few people have died in modern times due to the impact of an alien object, and the likelihood of this happening to someone in the next century is actually very small. The Earth's atmosphere acts as a barrier to meteoroids. Even for megaton energies (1MT=106 tons of TNT= 4.2 1015 J) most meteoroids are broken up and consumed before reaching the inner part of the atmosphere. 

According to this calculation, the impact of a bolide with hiroshima bomb energy (0.015 MT) occurs every year, and every century there are impacts of a megaton. In general, however, we do not worry about these events because most take place at high altitudes and the shock wave does not reach the Earth's surface. On the other hand, aerodynamic forces also crush, fragment and disperse the larger meteoroids. The level of fragmentation depends on the constitution of the meteoroid. Only iron can reach the floor in one piece. For those meteoriods that are not iron, the minimum equivalent energy to penetrate to the surface is about 10 MT or, equivalently, about 50 m in diameter.


  • Local effects 

If the meteor is able to penetrate up to 25 km from the surface at speeds of tens of km/s, the resulting explosion can cause damage similar to that of a nuclear bomb of similar energy, but without gamma radiation or neutrons.

An example of local devastation would be a rocky or metal projectile about 250 m in diameter (1000 MT of energy) that could easily penetrate to the surface; if it crashed, it would produce a crater about 5 km in diameter. A comet of similar dimensions would fragment before reaching the surface and having devastating effects on the ground. A collision of this kind occurs every 10,000 years, which means that the probability of it happening during a person's life is 1%. The affected area would be about 10,000 km2 , that is 0.002% of the earth's area. The effect would therefore be essentially local (or along the coast in case of an ocean impact). In the case of the Tunguska impact, which had a shock wave large enough to devastate an area of more than 1000 km2, it is estimated to be equivalent to about 10-20 MT.


  • Global effects

The higher energy impacts would have global consequences in addition to those produced at the site of the impact itself. An obvious but extreme example would be the impact on the Cretaceous/ Tertiary about 65 million years ago, which broke the ecosystem and is widely believed to have caused the extinction of more than half of the species on Earth. The impact of a 10 km (approximately 108 MT) object excavated a crater 180 km (110 mi) in Chicxulub, caused chemical changes in the atmosphere and ocean and climate changes produced by a few million kg of submicrometric dust injected into the stratosphere.

There are many uncertainties when calculating the minimum energy necessary to produce a global catastrophe: Is it an asteroid or a comet; is the impact in the northern or southern hemisphere; what is the angle of incidence; etc. There are uncertainties when evaluating the consequences on the environment and moreover on humanity. A projectile hundreds of meters in diameter that falls into the ocean will indirectly affect coastal areas. Climate effects can become important.

The dominant cause of catastrophic deaths in recent history have been due to natural disasters (earthquakes, floods, droughts, cyclones, ...), wars and especially hunger and epidemics, not forgetting some technological accidents such as the Bopal disaster or plane accidents. Tunguska- type impacts in urban areas are at least 100 times less frequent. The statistical risk of an impact can be compared with that of other current risks. 


Surveillance projects

Aware of the non-zero probability of an Earth collision with an extraterrestrial body, since the 1862 asteroid Apollo was discovered, a great effort has been made to discover and catalog all potential asteroids impacts with Earth. These asteroids, once discovered, are regularly observed to update elements of their orbits and predict their future positions. Both the European space agency (E.S.A.) and the American (N.A.S.A.) in collaboration with the International Astronomical Union maintain research teams in this field in order to predict possible impacts. Databases are maintained with objects discovered. For queries:,, among others.

Of all the objects listed so far, none represent a danger to Earth ( lists/CloseApp.html). In the next 33 years, the body that will pass the closest is 2000 SB45 and it will do so in October 2037 at a distance of about 210,000 km, about two-thirds of the distance from Earth to the moon.


About the author

Carme Jordi is an astronomer, professor in the Department of Quantum Physics and Astrophysics and the Institute of Cosmos Sciences at the University of Barcelona, and a member of the Institute of Space Studies of Catalonia. His research focuses on the space mission of the European Space Agency (ESA), Gaia, which aims to create the largest and most accurate three-dimensional map of our Galaxy by conducting a survey of one billion stars with unprecedented accuracy in position and movement. Carme Jordi is also an active science disseminator, she participates in the "Toc-Toc" program of La UBDivulga, and gives talks and conferences in several centres and entities. She is also a professor at the "Universitat de l'Experiència de la UB", where she teaches astronomy for people older than 55 years old. 

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