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There is simple evidence that, even in recent times, the Earth has suffered major impacts from asteroids and comets. What dangers do they hold for our civilisation? Major efforts now being made to detect those that might harm us and strategies are being developed to prevent their impacts. But we do have a problem with comets...

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Professor Ian Morison




An image of the Moon is a salutary reminder that bodies in the solar system, including the Earth, have suffered millions of impacts in the past - and will continue to do so, but happily at a far reduced rate.  If anything, due to its greater mass, the Earth will have suffered more impacts that the Moon but erosion has remove the evidence of all but a few from its surface.  The impacts of solar system debris, such as asteroids and comets, give rise to what are termed Impact Craters, a name that can be applied to any depression resulting from the impact at very high velocity of an object into a larger body.  Impact craters are approximately circular depressions that usually have raised rims, and range from small, smooth, bowl-shaped depressions to large craters having terraced rims often with a central peak or peaks.  Massive impacts give rise to giant impact basins such as the Mare on the Moon where the lunar crust was breached and so lava was able to well up from below and fill the depression caused by the impact.  These 'mare' depressions, also seen on Mercury, were the result of a period of intense bombardment in the inner Solar System that ended about 3.8 billion years ago.  Since then, though still appreciable, the rate of crater production within the inner solar system has been considerably lower but about every million years the Earth experiences a few impacts large enough to produce a 20 km diameter crater.  Erosion on Earth quickly destroys them, but about 170 terrestrial impact craters have been identified - the largest of which are listed in the table at the end of this section.  Their size ranges from a few tens of metres up to about 300 km in diameter.   Most are less than 200 million years old. 

Until the 1930's it was widely believed that the craters found on the Earth were volcanic in origin rather than the result of impacts (Patrick Moore was quite convinced until relatively recently that the Lunar craters were volcanic in origin!) and it was not until the 1960's that researchers, notably Eugene M. Shoemaker, found clear evidence that they had been created by impacts, identifying, for example, shocked quartz which could only be formed in an impact event.  By 1970, more that 50 impact craters had been found on the Earth.

Crater Formation

The speed at which an object hits the Earth can range from about 11km/s up to 70 km/s with a typical impact speed of ~ 25 km/s - these speeds are derived from the orbital speeds of objects within the solar system.  It is now possible to simulate such events and it appears that the impacting object will normally penetrate the ground and then cause a below ground explosion that melts and vaporises material.   In this case, it does not normally matter at what angle the object has hit the Earth and the resulting craters are nearly always circular with only very low angle impacts giving rise to elliptical craters.   In large impacts, much material will be ejected and mostly fall within a few crater radii, but some may travel significant distances and may form 'rays' as seen centred on the lunar crater, Tycho.  Some ejected material may even exceed the planet or satellite's escape velocity and then travel within the solar system to perhaps fall as meteorites on the Earth.  This is how we have some Martian rock samples to investigate for signs of life.  [The team who have investigated the Martian meteorite ALH 84001 believe that new investigations show evidence of simple life forms within it.]

Craters on the Earth

Perhaps the best known Impact crater on Earth is Meteor Crater, some 43 miles east of Flagstaff in northern Arizona, USA, which was formed about 50,000 years ago when the area was open grassland inhabited by woolly mammoths!   It is often referred to as the 'Barringer Crater' in honour of Daniel Barringer who was first suggested that it was produced by meteorite impact.  It is about 1.2 km in diameter and 170 m deep, surrounded by a rim that rises 45 m above the surrounding plains.  At its centre is a ~220 m pile of rubble.  The remains of the meteorite are believed to be embedded under the rim at the south side of the crater.  It was a nickel-iron meteorite about 50 meters across, which, it is thought, impacted the plain at a speed of 12.8 km/s (28,600 mph).  The meteorite would have initially weighed ~600,000 tons but it is suspected that half may have vaporised as it passed through the Earth's atmosphere.  The meteorite that formed the crater is officially called the Canyon Diablo Meteorite - it is named after the town of Canyon Diablo, Arizona, which, now a ghost town, was 12 miles to the north.

In 1960, research by Eugene M. Shoemaker confirmed Barringer's hypothesis with the discovery of the presence in the crater of the mineral stishovite.  This is a rare form of silica found only where quartz-bearing rocks have been severely shocked by through an impact event or nuclear explosion.  It cannot be created by volcanic action.   Since Shoemaker produced the first definitive proof of an extraterrestrial impact on the Earth's surface many impact craters have been identified around the world but Meteor Crater is still the most visually impressive.  It was used by Shoemaker to train the Astronauts in crater geology prior to the Apollo missions in the 1960's.

The Nördlinger-Ries and Steinheim Craters in Germany

Two large impact craters in the south of Germany are the nearest significant impact craters to the United Kingdom and are thought to result from the impact of a binary asteroid (two co-rotating asteroids) about 14.4 million years ago.  The larger, the Ries crater, is 24 km across and its floor is about 120 m below the eroded remains of the rim.  On a very clear day it is possible to see from one side across to the far rim (as has the author), but the size is such that the crater does not appear too obvious.  As with the Barringer crater, Eugene Shoemaker showed that it was caused by meteorite impact as, with Edward Chao, they found shocked quartz (coesite) in the stone that had been used to build Nördlingen town church which lies within the crater.  Interestingly, stone buildings in Nördlingen contain millions of tiny diamonds, all less than 0.2 millimeter across - formed when part of the asteroid impacted a local graphite deposit from which stone has been quarried over the centuries.

The smaller,Steinheim, crater is 42 km west-southwest from the centre of the Ries crater and is 3.8 km across.  Viewing from the crater wall at one side, its crater form is quite obvious and there is a low outcrop of material on the crater floor.   It is thought that the Reis crater was formed by an asteroid of 1.5 km diameter and the Steinheim crater one of 150 m diameter.  The pair impacted the region at an angle around 40 degrees from the surface from a west-south-westerly direction.  They impacted at about 20 km/s and had an explosive power of 1.8 million Hiroshima bombs.  Ejecta from the Reis crater has been found up to 450 km to the north-east and is believed to be the source of moldavite tektites found in Bohemia and Moravia.

The largest known impact craters on Earth:

The Chixilub Event

The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico with its centre located near the town of Chicxulub.  The crater is more than 180 km in diameter, making it the third largest confirmed impact structures in the world - formed by the impact of an asteroid at least 10 km across.  The crater was discovered by a geophysicist, Glen Penfield, who had been prospecting for oil during the late 1970s.  Within his data, Penfield found a huge underwater arc 70 km in length which agreed with a gravitational anomaly shown on map of the Yucatán made in the 1960s.  This also suggested a giant crater but was not publicised due to commercial considerations.  Penfield found another arc on land which, together with offshore arc, formed a circle, 180 km wide.  As at the sites of other impact craters, shocked quartz was found confirming the structural and gravitational evidence of a massive impact.

The age of the rocks and isotope analysis show that this impact structure dates from the end of the Cretaceous Period (called the K-T boundary), roughly 65 million years ago and so it is thus implicated in causing the extinction of the dinosaurs.  However this may not have been the sole reason for their demise.  The impact released the equivalent of 100,000,000 megatons of TNT - 2 million times greater than the most powerful man-made explosion!  In 2007, a paper published in 'Nature' proposed that the 'Chicxulub asteroid' resulted from a collision in the asteroid belt 160 million years ago that gave rise to the creation of the Baptistina family of asteroids.  There is evidence that the impactor was a member of a rare class of asteroids called carbonaceous chondrites, like the Baptistina family of which the largest surviving member of is 298 Baptistina.  The impact would have caused one of the largest tsunamis in the Earth's history, reaching several thousand feet high whilst a cloud of super-heated dust, ash and steam would have spread from the crater.  Debris, both from the impactor and the impact area, would have been thrown out of the atmosphere by the blast, heated to incandescence upon re-entry and so igniting global wildfires.  Shockwaves from the blast may well have triggered earthquakes and volcanic eruptions across the globe.  The emission of dust and particles could have covered the entire surface of the Earth for several years greatly reducing solar radiation and so interrupting plant photosynthesis and thus affecting the entire food chain. 

The main evidence of a giant impact, besides the crater itself, is contained in a thin layer of clay present in the geological record from across the world dating from the K/T boundary.  It contains an abnormally high concentration of iridium, reaching 6 parts per billion by weight or more as compared to just to 0.4 for the Earth's crust as a whole.  In contrast to the Earth, meteorites can contain around 470 parts per billion of iridium.  It seemed a reasonable hypothesis that the iridium was spread into the atmosphere when the impactor was vaporized, mixed with other material thrown up by the impact and settled across the Earth's surface so producing the layer of iridium-enriched clay.

The discovery of the Chicxulub Crater supported the theory that the extinction of numerous animal and plant groups, including the dinosaurs, may have resulted from a giant impact.  The time of this 'extinction' event approximately agrees with the Chicxulub event but recent core samples indicate that the impact occurred about 300,000 years before the mass extinction, and thus it could not have been the direct cause of the dinosaur's demise.  It may have been just one of a number of large impacts spaced over several hundred thousand years.  In addition, at around this time the Earth's temperature will have risen due to carbon dioxide released by a massive eruption of lava that formed the Deccan traps of India and this would have been a real problem for the dinosaurs.   A further impact, 300,000 years after Chicxulub, possibly in the sea bed beneath the Indian Ocean, may have been the final straw

Tungusta - June 30th,1908.

The Tunguska Event was a powerful explosion that occurred near the Tunguska River in Siberia, at around 7:14 a.m. on June 30, 1908.  The explosion is believed to have been caused by the air burst of a large meteoroid or comet fragment a few ten's of metres across at an altitude of 5-10 km above the Earth's surface.  Though the meteor or comet burst in the air rather than directly hitting the surface, this event is still referred to as an impact which released around 12 megatons of TNT - about 1,000 times as powerful as the atomic bomb dropped on Hiroshima.  The Tunguska event is the largest impact event over land in the Earth's recent history.  The blast from the explosion knocked over an estimated 80 million trees spread over 2,150 square kilometres.  Had the explosion happened a few hours later it would have destroyed St Petersburg. 

At around 7:14 a.m. local time, Tungus natives and Russian settlers in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the Sun, moving across the sky followed by a flash and a sound similar to artillery fire. The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of miles away and the explosion equivalent to 5.0 on the Richter scale was registered on seismic stations across Eurasia.  Ice particles formed at extremely cold temperatures in the high atmosphere increased light levels around dawn and dusk allowing Londoners to read newspapers from their light!

An Eyewitness Report

Testimony of S. Semenov, as recorded by Leonid Kulik's expedition in 1930.

 "At breakfast time I was sitting by the house at Vanavara Trading Post (65 kilometres south of the explosion), facing north. I suddenly saw that directly to the north, over Onkoul's Tunguska Road, the sky split in two and fire appeared high and wide over the forest.  The split in the sky grew larger, and the entire northern side was covered with fire.  At that moment I became so hot that I couldn't bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few yards.  I lost my senses for a moment, but then my wife ran out and led me to the house.  After that such noise came, as if rocks were falling or cannons were firing, the earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it.  When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops.  Later we saw that many windows were shattered, and in the barn a part of the iron lock snapped."

There was little scientific curiosity about the impact at the time and, possibly due to the isolation of the Tunguska region, and any records of early expeditions to the site were likely to have been lost during World War 1, the Russian Revolution of 1917 and the Russian Civil War.  In 1921, the Russian mineralogist Leonid Kulik, visited the Tunguska River basin as part of a survey for the Soviet Academy of Sciences and deduced from local accounts that the explosion had been caused by a giant meteorite impact.  He returned in 1927 and finally reached the impact site where, to their surprise, they could not find a crater but instead a region 8 km across of scorched tree trunks.  At greater distances the trees were knocked down pointing away from the centre.  In the 1960's it was found that a butterfly-shaped area 70 km across and 55 km in length had been levelled but still no crater was found.  Tiny spheres of silicate and magnetite contained high proportions of nickel relative to iron leading to the conclusion they were of extraterrestrial origin, this being found in meteorites.  In addition, the bogs that cover much of the region contain an unusually high proportion of iridium, similar to the iridium layer found in the K-T boundary. This is believed to result from debris from the impacting body that had been deposited in the bogs.

Using model forests (made of matches on wire stakes) and small explosive charges slid downward on wires, experiments produced butterfly-shaped blast patterns strikingly similar to the pattern found at the Tunguska site and suggested that the object had approached at an angle of about 30 degrees from the ground and had exploded in mid-air.

Controversy remains as to whether the Tunguska body was a meteorite or small cometary body - which would leave no obvious traces.   A cometary origin could explain the glowing skies observed across Europe for several evenings after the impact, the result of dust and ice that had been dispersed from the comet's tail across the upper atmosphere.  It might have been a fragment of the short-period Comet Encke, which is responsible for the Beta Taurid meteor shower, its timing and direction of approach would have been consistent with this hypothesis.  Some argue that a cometary body could not reach so close to the ground, but others suggest that the object was an extinct comet covered with a stony mantle that allowed it to penetrate the atmosphere.  To support the opposing view, resin extracted from the core of the trees in the area of impact, showed high levels of material commonly found in rocky asteroids and rarely found in comets.  In June 2007 it was announced that scientists from the University of Bologna had identified a lake in the Tunguska region as a possible impact crater from the event.  They do not dispute that the Tunguska body exploded mid-air, but believe that a one meter fragment survived the explosion and impacted the ground.  The arguments continue!

The Earth is continuously being bombarded by meteoroids usually travelling at a speed of more than 10 km/s.  The majority are small but occasionally a larger one enters.  Due to the heat generated as they travel through the atmosphere, most burn up or explode before they reach the ground.  It appears that stony meteoroids of about 10 m in diameter produce an explosion of around 20 kilotons, similar to the Nagasaki A-Bomb, in the upper atmosphere more than once a year.  However, megaton-range events like Tunguska are much rarer only occurring about once every 300 years.

Some recent events

On August 10th, 1972 a meteor that became known as The 1972 Great Daylight Fireball was witnessed moving north over the Rocky Mountains from the U.S. into Canada.  It was an Earth-grazing meteoroid that passed within 57 kilometres of the Earth's surface.  Many saw its passage through the atmosphere and a tourist at the Grand Teton National Park in Wyoming was able to film it with an 8-millimeter colour movie camera. 

On March 23rd, 1989 the 300 meter diameter Apollo asteroid, 4581 Asclepius, missed the Earth by 700,000 km (400,000 miles) passing through the exact position where the Earth was only 6 hours before.  Had the asteroid impacted the Earth, it would have created the largest explosion in recorded history, thousands of times more powerful the most powerful nuclear bomb ever exploded by man.

On June 6th, 2002 an object with an estimated diameter of 10 meters entered the Earth's atmosphere over the Mediterranean Sea, between Greece and Libya, and exploded in mid-air.  The energy released was estimated to be equivalent to 26 kilotons of TNT, comparable to a small nuclear weapon.

On March 18th, 2004, a 30 meter asteroid, 2004 FH, passed Earth at a distance of only 42,600 km, about one-tenth the distance to the moon, and the closest miss ever observed.  Similar sized asteroids are thought to come this close about every two years.

On the 5th of October 2008, scientists calculated that a just discovered small Near-Earth asteroid, 2008 TC3, would impact the Earth on the 6thOctober over Sudan, at 05:46 local time.  The asteroid entered the atmosphere just as predicted - the first time that an asteroid impact on Earth has been accurately predicted.  The object was confirmed to have entered the Earth's atmosphere at a speed 12.8 km/s above northern Sudan, its path observed over a wide area and from aircraft.  To search for fragments of the asteroid 2008 TC3, students and staff from the University of Khartoum lined up to comb the desert and have so far found some 280 meteorites having a combined weight of 5 kg.

The Comet that crashed into Jupiter

In 1993, the Shoemakers, Eugene and Caroline, and David Levy were using the 0.46m Schmidt telescope at the Palomar Observatory in California to search for near-Earth asteroids.  These, as we will see, are asteroids that come within the orbit of the Earth and hence, in principle, could impact with the Earth. 

Eugene Shoemaker was a brilliant geologist who had hoped to become one of the astronauts who explored the Moon in the early 1970's, but was rejected because of a medical condition.  He is probably most famous for proving that the huge geological depression in Arizona was actually an impact crater which is now named after him. Shoemaker was perhaps the first person to bring to other scientists' and the public's attention the danger of the impacts of comets and asteroids on the Earth.  He was involved in several US space missions, including the Apollo missions when he taught the astronauts about the geology of lunar craters. He and his wife Carolyn - a great team later joined by David Levy - discovered about 800 asteroids and 20 comets.

On the night of March 24th, they took an image which showed what appeared to be comet having multiple nuclei whose proximity and motions suggested that it was associated with the planet Jupiter.  The existence of this object was soon confirmed by James V. Scotti of the Spacewatch program at the University of Arizona and the comet was named Shoemaker-Levy 9 as it was the 9th short period comet that the team had discovered during their NEO search.  

Further observations revealed that it was orbiting Jupiter rather than the Sun in a highly eccentric orbit with a period of about 2 years.  It appeared that in the late 60's or early 70's Jupiter had captured it from its earlier orbit around the Sun and the comet had become, in effect, a temporary satellite of Jupiter.  Orbital calculations showed that on the 7th July, 1992 it had come within 40,000 km of Jupiter's cloud tops.  This is within what is called the planets Roche limit within which a body can be torn apart by its tidal forces.  As a result, the comet's nucleus had been broken up into 23 fragments which were labelled 'A' to 'W'.   Each of these fragments had a slightly different orbit and formed a 'train' which was gradually becoming more spread out.  The visible fragments of SL9 were estimated to range in size from a few hundred metres up to a couple of kilometres across, suggesting that the original comet may have had a nucleus up to 5 km across.

The train of fragments making up comet Shoemaker-Levy 9 on its final orbit around Jupiter.

As the orbits of the fragments were refined it became apparent that they would collide with Jupiter in July the following year - the train of nuclei ploughing into Jupiter's atmosphere over a period of about five days!  This naturally caused great excitement as astronomers had never before seen solar system bodies collide and provided a unique opportunity for scientists to look inside Jupiter's atmosphere, as the collisions were expected to cause eruptions of material from the layers normally hidden beneath the clouds. 

It became apparent that the impact site would lie just beyond the Jovian limb, and so beyond direct view from Earth, but that within a short time Jupiter's rotation would make the effects of the impacts appear on its visible face.  However, the Galileo spacecraft, on its way to investigate Jupiter, would be able to observe the impacts as they occurred.  The Hubble Space Telescope (HST) was trained on Jupiter hoping to observe the plume of material that was expected to rise well above the cloud tops and perhaps become visible beyond the limb. 

The first impact occurred at 20:13 UTC on July 16, 1994, when fragment A of the nucleus slammed into Jupiter's southern hemisphere at a speed of about 60 km/s.  Instruments on Galileo detected a fireball which reached a peak temperature of about 24,000 K, compared to the typical Jovian cloud top temperature of about 130 K. It then expanding and cooled rapidly to about 1500 K. The plume from the fireball quickly reached a height of over 3,000 km and was observed by the HST.

Astronomers had expected to see the fireballs from the impacts, but did not have any idea in advance how visible the atmospheric effects of the impacts would be from Earth. Observers soon saw a huge dark spot after the first impact. The spot was visible even in very small telescopes, and was about 6,000 km (one Earth radius) across.

Over the next 6 days, 21 distinct impacts were observed with the largest, resulting in a giant dark spot over 12,000 km across, resulting from the impact of fragment G on July 18th.  This impact was estimated to have released an energy equivalent to 6,000,000 megatons of TNT (600 times the world's nuclear arsenal).

The impact sites of fragments F and G - appearing like a pair of eyes on the surface of Jupiter.

Hopes that the impact would give further information about the atmosphere were realised as spectroscopic studies revealed absorption lines in the Jovian spectrum due to diatomic sulphur (S2) and carbon disulphide (CS2), the first detection of either in Jupiter, and only the second detection of S2 in any astronomical object. Other molecules detected included ammonia (NH3) and hydrogen sulphide (H2S) but, to astronomers's surprise, oxygen-bearing molecules such as sulphur dioxide were not detected.  The amount of water detected was also less than predicted indicating that either the water layer thought to exist below the clouds was thinner than predicted, or that the cometary fragments did not penetrate deeply enough.

The scars from the impacts were easily visible on Jupiter for many months so a trawl was made of early observations of Jupiter to see if such an event had occurred before but none were found.  However such events must have happened before in the life of the solar system and evidence was found in the Voyager spacecraft observations of crater chains on the surfaces of Ganymede (three) and Callisto (thirteen). 

The impact of SL9 highlighted Jupiter's role as a kind of "cosmic vacuum cleaner" for the inner solar system. The planet's strong gravitational influence leads to many small comets and asteroids colliding with the planet, and if Jupiter were not present, the probability of impacts with the Solar System's inner planets would be much greater. Without Jupiter, extinction events such as that at the end of the Cretaceous 65 million years ago would be much more frequent and may not have allowed intelligent life to have developed here on Earth! 

There is a sad, yet poignant, end to this story.

In 1997, at the age of 69, Eugene Shoemaker was killed in a car crash during an annual trip to Australia in search for asteroid craters.  A small vial of Shoemaker's ashes was loaded aboard the spacecraft Lunar Prospector, and now rests with the craft on the surface of the moon. He is thus the first person to be buried on another planet.  Around the capsule is wrapped a piece of brass foil inscribed with an image of Comet Hale-Bopp, an image of Shoemaker Crater in northern Arizona, and a passage from William Shakespeare's "Romeo and Juliet":

And, when he shall die,

Take him and cut him out in little stars,

And he will make the face of heaven so fine

That all the world will be in love with night,

And pay no worship to the garish Sun.


Near Earth Objects (NEOs): their discovery and potential threats to Earth.

 These are objects orbiting the Sun whose orbit brings them to within 1.3 astronomical units of the Sun many of whose orbits will cross that of the Earth.  The vast majority are asteroids (NEAs), but a few are short period comets.  By the summer of 2009, 6,244 near-Earth asteroids were known, ranging in size up from ~50m to ~32 km (1036 Ganymed) in diameter.  The second largest is 433 Eros which was the target for the Near-Earth Asteroid Rendezvous (NEAR) mission in 2000.  The number of near-Earth asteroids over one kilometre in diameter is estimated to be 500 - 1,000. NEAs only survive in their orbits for a few million years until they collide with the inner planets, are ejected from the solar system by close-approaches with the planets or fall into the Sun.  This implies that, to account for the currently observed number of NEAs, asteroids must be being perturbed from their orbits within the asteroid belt between Mars and Jupiter by the gravitational effects of Jupiter - thus providing a continuing supply of near-Earth asteroids.

Every 100 years or so, rocky or iron asteroids larger than about 50 meters are expected to impact the Earth's surface and cause local disasters or produce tidal waves.  Every few hundred thousand years or so, we would expect asteroids larger than a kilometre to impact the Earth giving rise to a global disasters such as that caused by the Chicxulub Event.  The obvious consequence of the impacts of Comet Shoemaker-Levey 9 on Jupiter alerted governments to the possible threat to Earth and so programs were instituted to discover NEOs, characterize their sizes and predict their future trajectories and so assess any potential threat.

Programmes to detect NEOs:

LINEAR  - Lincoln Near-Earth Asteroid Research

 The LINEAR programme began in 1996 initially using a telescope designed for observing satellites orbiting the Earth.  It now uses two one-metre telescopes and one half-metre telescope based at Socorro in New Mexico and by 2004 was discovering tens of thousands of objects each year and accounting for 65% of all new asteroid detections. The camera uses a large format CCD array to observe the telescopes 2 degree field of view.  A large area of sky can be observed each night with each target patch of sky being observed 5 times each night - largely searching along the ecliptic (the plane of the solar system) where most NEOs would be expected to be found.

Linear Telescope at Socorro


Near-Earth Asteroid Tracking (NEAT)

 NEAT uses a 1.2 m telescope now located on Maui, Hawaii.  It began observing in December 1999 and now observed for the six nights each month prior to New Moon. Each 10 hour observing session yields 15 Mbytes of information (compressed down from 26 Gbytes!) which is transmitted to the Jet Propulsion Laboratory for analysis.  Detected objects are immediately reported to the world-wide observing community via the Minor Planet Centre (MPC).


Spacewatch, originally set up in 1980, uses the 0.9 m telescope sited at the Kitt Peak Observatory in Arizona to hunt for NEOs.  The project has recently acquired a 1.8 m telescope, also at Kitt Peak which allows them to search 0.7 magnitudes fainter.   The 0.9 m telescope has been upgraded with a mosaic of CCDs which enables it to cover the sky at least six times faster.  This has increased the rate of detection of Earth-approaching asteroids to around 300 per year.


Spaceguard is the overall name for these affiliated search programmes, designed to detect 90% of near-earth asteroids over 1 km diameter by 2008.  The name was coined by Arthur C. Clarke in his novel Rendezvous with Rama where SPACEGUARD was the name of an early warning system created following a catastrophic asteroid impact!   The number of known near-Earth asteroids larger than 1 km diameter has now passed the 800 mark. If the population of NEAs larger than 1 km is 1000 (as predicted from several studies), 800 represent 80 percent completeness - so the project has not quite reached its target.

How is it done?

It is impossible to calculate the orbit of an object from a single observation.  With a single observation little can be determined - the asteroid is only known to lie within a cone whose angular dimension is given by the error of the measured position.  A second observation will enable a position to be found and three or more observations will enable an approximate orbit to be determined.  Once the approximate orbit of an NEO has been found, two further techniques can be used to refine its orbit: firstly, whilst powerful radars can not provide accurate positions for the NEO, they can give an accurate speed of approach or recession which is a significant help and secondly, it is possible to examine archive (earlier) sky images on which the object might be expected to appear to see if the object can be spotted on them.  If so, their accurate position at some time in the past is a considerable aid to defining the precise orbit.

Assessment of Risk for NEOs

 There are two scales that astronomers use to assess the risk posed by individual NEOs.  The simpler is called the Torino Scale which uses a range of integers from 0 to 10.  A 0 indicates an object has a negligibly small chance of collision with the Earth or is too small to penetrate the Earth's atmosphere intact (and hence pose no threat) whilst 10 indicates that a collision is certain, and the impacting object is large enough to precipitate a global disaster.  The value that an NEO is assigned is based on its collision probability and its kinetic energy (expressed in megatons of TNT).  The Palermo Scale is similar but more complex.

Torino Scale

The current record for highest Torino rating was held by the 270m NEO, 99942 Apophis, which in December 2004 was given a rating of 2 which was then upgraded to 4.  It is now expected to pass quite close to the Earth on Friday, April 13, 2029 but, as there is no possibility of an impact, its rating has been downgraded to a 0.  Its orbit may be perturbed as it passes the Earth in 2029 so its rating may increase in the future.  This was the first NEO to have been given a Torino Scale value higher than 1.  In February 2006, the rating for 2004 VD17 was initially given a value of 2 due to a possible encounter in the year 2102, but further observations have again reduced it to 0.  2007 VK184, discovered on November 12, 2007 by the Catalina Sky Survey, has a Torino Scale value of 1.  Observations suggest that 2007 VK184 has a probability of 1 in 31,300 of hitting the Earth during June 2048. The asteroid is estimated to have a diameter of 130 m, and so could cause a significant, but not world-wide, threat to Earth.

Impact predictions often make the news!  Initial observations tend to show an increasing chance of impact, but then further observations rule one out.  At first, when there are only a few observations, the error ellipse around the NEOs path is very large and may include the Earth.  This leads to a small, but non-zero, impact probability. Further observations may well shrink the error ellipse and will, if it still includes the Earth, raise the impact probability as the Earth now covers a larger fraction of the error region.  Finally, as described above, radar observations or the discovery of previous sightings of the object on archival images allow its orbit to be predicted more accurately.  This shrinks the error ellipse so that usually the Earth is not included and so the impact probability returns to near zero.

As a result, there have often been problems due to exaggerated press coverage of asteroids that might (on the basis of a limited number of observations) impact the Earth.  These are usually shown not to be a threat when further observations are made.  Reporting of possible NEO threats is now 'managed' to prevent false alarms and the Torino scale might be abandoned in favour of the Palermo scale (which is more obtuse!).  Only two NEOs currently have positive ratings on this scale: (89959) 2002 NT7 has a one in a million chance of impact on February 1st, 2019 and Asteroid (29075) 1950 DA which has a diameter of 1 km (so could cause a global catastrophy) has a one in 300 chance of impacting the Earth on March 16, 2880

What could we do to avert disaster?

As nearly all the NEO's that could present a global threat to the Earth have now been discovered, it is likely that we would know that an object was on a collision course several years in advance.  If so, our present technology could be used to deflect the threatening object away from Earth.  The key requirement is to intercept the object when it is furthest from the Sun.  At this point its kinetic energy, speed and (importantly) momentum are at a minimum and hence less energy is required to alter its orbit - the blowing up of an object just before impact as in 'Armageddon' will simply cause millions of smaller objects to rain down on the Earth rather than one large one - with broadly similar effects.

Both the Earth and the impactor are in orbit around the Sun and an impact can only occur when both reach the same point in space at the same time.  The Earth is approximately 12,750 km in diameter and is moving at 30 km per second around its orbit.  It thus travels a distance of one planetary diameter in just over seven minutes. So, if we can delay or advance NEO's passage to the Earth by times of this order (depending on the geometry of the two orbits), it would then miss the Earth.  It will thus be obvious that the NEO's time of arrival must be known to this precision in order to forecast the impact and determine what alteration to its orbit is necessary.

Assuming that a rocket can reach the NEO when far from the Sun there are several ideas for making a small change in its orbit, for example, a nuclear fusion weapon set off above the surface would produce high speed neutrons that would impact on material on the surface of the asteroid facing the explosion.  This material would then expand and blow off, thus producing a recoil upon the asteroid that would nudge it towards a fractionally different orbit out of harms way - it does not have to be a big change, a very modest velocity change in the asteroid's motion would cause the asteroid to miss the Earth entirely.  It is important that the explosions are not so energetic so as to break up the NEO so numbers of small explosions over a period of time would be likely to be used.

The European Space Agency is already producing the design of a space mission, named Don Quijote, which would attempt to alter the momentum of the NEO by a simple collision.  In the case of one possible asteroid threat, 99942 Apophis, a spacecraft weighing less that one ton could give the required deflection of its orbit.  Another idea is to use a 'gravity tractor', a heavy spacecraft that hovers over the NEO which is thus gravitationally attracted towards it.  By slowly moving the tractor spacecraft over a number of years, perhaps using an 'ion thruster', the orbit could be sufficiently changed.  This has the attraction that it would work with spinning NEOs or those that are 'rubble piles' where the nuclear explosions would probably break them apart.  A neat idea is called a 'mass driver' which is an automated system on the asteroid to mine and eject material into space thus giving the object a slow steady push and, at the same time, decreasing its mass.  Finally, the Sun's radiation pressure could perhaps be used with a large solar sail attached to the NEO so that the pressure of sunlight could eventually redirect the object away from its predicted Earth collision.

Gravity Tractor to alter NEO orbit.

The Problem with Comets

There is no doubt that at some time in the future a comet will impact on Earth - but we have no idea when.  The problem is this.  In the case of asteroids, it is likely that before long we will know the orbits of all those that are a major threat to Earth and so would be able to predict a possible impact event many years before it would happen so enabling us to take suitable action.  The same is true for the short period comets whose orbits could also be modified.  But comets are continuously coming into the inner solar system which have never been observed before, and it is unlikely that we will have sufficient time to take appropriate action.  Such an event could easily bring about the end of human civilisation but we should not be too alarmed.  The likelihood is that such a cometry impact will only occur once every 300 million years or so and thus the chances of this happening in our lifetime is exceedingly small.

But there is a non-zero chance and the author (who suspects that intelligent life is very rare in our galaxy and thus that the human race is rather special) feels that something should be done in order to preserve us in the event of such an eventuality.  Perhaps two underground retreats could be set up on (not quite) opposite parts of the world where diverse ethnic groups could go to spend a fortnights holiday (rather like Centre Parks but underground).  In the case of a global catastrophe, there would be power and provisions to support them for several years until the dust in the atmosphere has cleared after which they could emerge into a new future.  Is that so stupid an idea?        

The Future

NASA has now been directed to set up a follow-on project from Spaceguard whose objective is to find 90% of NEOs whose diameters are greater than 140 metres by the end of 2020.  As these are considerably fainter, a new generation of significantly larger telescopes will be required, and these are likely to include the enormous LSST (The Large Synoptic Survey Telescope) which will use an 8.4 m wide field telescope equipped with a 3200 Mpixel camera!   Other new telescopes will include the 4.2 m DCT (The Discovery Channel Telescope) and Pan-STARRS (The Panoramic Survey Telescope & Rapid Response System).


Artists Impression of the LSST


A simple rule for life: enjoy every day to the full!


©Ian Morison, 17/12/2009

This event was on Thu, 17 Dec 2009

Professor Ian Morison

Professor Ian Morison

Professor of Astronomy

Ian Morison began his love of astronomy when, at the age of 12, he made a telescope out of lenses given to him by his...

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