KILLER ASTEROIDS By: Peter Starr Date: 20th September 2003 1.0 Introduction It was 1989 and the day was March the 23rd. It was a quiet sunny morning in Sydney, Australia, no different from any other in early autumn. Unknowingly to Sydney’s population, a mountain half a kilometre in diameter was hurtling through space at 74,000km/hr on a collision course with the Earth. Just before impact, the sky ignited and split in two. In an instant, the city became a dust cloud that circled the Earth and blocked out the Sun. The resultant shockwave travelled around the earth several times. Plant life started to die and 50% of life on Earth was to be no more. Should the Earth have been six hours further around in its orbit for that day, the opening paragraph of this essay is, in a nutshell, what would have happened. As fate would have it, the 45 million tonne asteroid tumbled harmlessly on in its journey around the sun. It was not until a week later that Geologist and part time Astronomer Henry Holt noticed a streak (the asteroid) on a photographic plate. The asteroid was named 1989 FC. (1 p52) Comets and asteroids threaten all life on Earth. Global catastrophe is a statistical inevitability. It is just a matter of when….. 2.0 Asteroids and Comets Asteroids are common inhabitants of our solar system, found predominantly in the region between Mars and Jupiter. More than 1 million of these objects revolve harmlessly around the sun in the Asteroid Belt [1]. Asteroids range in size from that of a pebble to hundreds of kilometres in diameter [12]. The asteroid belt is a remnant from the formation of our solar system from a failed planet that never accreted during the formation of the solar system [12] due to the gravitational influence of Jupiter. [14]. [12] The comets, composed mainly of ice and dust, originate in the outer suburbs of the solar system, beyond the orbit of Neptune in the Kuiper Belt. Longer period comets are thought to have originated in the Oort Cloud at the perimeter of the solar system some 100,000AU [1]. Scientists have postulated that there could be a trillion comets in this region. [2] Comets, like asteroids, are left over from the formation of the solar system. 2.1 How do comets and asteroids find themselves in the inner solar system? Current theory is that the Oort cloud and Kuiper belt are disturbed by tidal forces of nearby stars or large gas clouds [2]. Many are ejected out of the solar system and into the interstellar medium. However, some are pushed inwards towards the Sun. Long period comets may have orbits of thousands of years and are unlikely to come close to the Earth. However a few may stray too close to a gas giant like Jupiter. Some may fall into Jupiter, but others have their orbits converted by the gravitation of Jupiter and they become short period comets. The latter scenario is probably what happened to Halley’s comet [1]. Shorter period comets are likely to be more dangerous i.e. they are more likely to encounter an inner planet. After a few thousand orbits, the comets evaporate to leave a trail of dust and enter the earth’s atmosphere as a harmless but spectacular meteor shower. It is estimated that there are about 2000 long period comets greater than a kilometre in diameter crossing the orbit of Jupiter every year [2]. It is estimated that a comet will come as close to the moon every 5000 years [2 ]. Should a long period comet collide with the Earth, it would do more damage than an asteroid of similar size, as its momentum is much greater. Encounters with long period comets occur significantly less often than asteroid impacts [19]. Unlike comets, asteroids exist as intact bodies until collision with a planet. Asteroids made from iron do not evaporate like comets. An asteroid may be perturbed from its orbit by the gravity of Jupiter as it orbits the sun, sending it on a collision course with another asteroid to break into fragments. These fragments may be propelled into the inner solar system on a collision course with the Earth. There are many asteroids meeting this description. Those crossing the Earth’s orbit are called Near Earth Objects (NEOs), the potential Killer Asteroids. Asteroids have existed since the creation of the solar system [11]. The earth was formed by the accumulation of asteroid size objects and has been bombarded by them ever since. The largest ever impact happened during this era [7]. A Mars-sized object collided with the earth almost smashing Earth to pieces and formed the moon. A collision of such intensity would end all life. 2.2 Collisions with the Earth – what is the evidence? Objects strike the earth every day. They are observed as falling stars streaking across the night sky. What we observe are grain-sized objects, remnants of old comets. Observations of the Earth’s Moon have led to the documentation of thousands of craters of all sizes. Mercury and Mars are similar, both pocked from asteroids and comets [11]. Earth’s dynamic atmosphere, oceans and mountain building have wiped evidence of most craters. There is much evidence to demonstrate the history of Earth’s collisions with asteroids. The Barringer Crater in Arizona (USA) was the result of impact with a fifty metre-wide asteroid travelling at 11km/second around 50,000 years ago. The crater produced is 200m deep and 1.2km wide. [16] The explosion was equivalent to a 4 megaton bomb. Imagine such an asteroid colliding with a modern day city. This asteroid was relatively quite small but disastrous at the local level. An asteroid two hundred times larger is thought to have collided with the Earth sixty five million years ago and is attributed to the mass extinction of the dinosaurs [15]. Strong evidence for this includes the layer of clay on the Earth’s surface containing up to 100 times more iridium [21], a common element in iron-rich asteroids, than any other layer. Below this layer are many dinosaur fossils and above it are none. [2] Scepticism among the scientific and broader communities was rife as no crater was found until the discovery of the Chicxulub Crater on the Yucatan peninsula near Mexico. The crater measures 180km in diameter and is thought to have been created by impact with an object 10km across. Impact threw up enough dust into the atmosphere to block light for several years. Earth’s surface temperature dropped to tens of degrees below zero and most plants could not photosynthesise. Plant life died resulting in dramatic changes to the food chain and extinction of 70% of all animal species [15]. An event of this magnitude is thought to occur every 100 million years, causing mass extinctions and redirecting the course of evolution. There are five known mass extinctions in the Earth’s history and these were probably caused by large asteroid impacts. 200 million years ago, an asteroid hit the earth marking the end of the Triassic era and beginning the 135 million year reign of the dinosaurs [3]. “This three-dimensional satelite map of the Chicxulub crater showing local gravity and magnetic field variations” [21] More recent evidence includes impacts observed over the last century. In Siberia on June 30, 1908 [22], a giant fireball was seen moving rapidly through across sky before an enormous explosion occurred. This is known as the Tunguska Event. A rocky asteroid exploded in mid-air with the energy of a 1 megaton bomb [2, p258]. It levelled 2000 km2 of forest and produced a shock wave that twice circled the Earth. So much dust was expelled into the air, one could read of the scattered light in London at night 10,000km away for two days. There was no impact crater. The only trace of the asteroid was a large number of tiny diamonds strewn over the site. Such diamonds are already known to exist in meteorites. [4] The event coincided with the beta tau rid meteor shower, connected with the orbit of Comet Encke. Perhaps this was a chunk of that comet. A Tunguska-like event happens every 1000 years. [4] On June 25, 1178, several British monks saw “…a bright new moon and its horns were tilted towards the East. Suddenly the horn split in two. From the midpoint, a flaming torch sprang up spewing fire-hot coals and sparks” [4]. A new crater was found in this area that maybe attributed to what the monks saw. In fact the moon is librating with a period of 3 years and amplitude 3 metres consistent with the moon being struck within the last 1000 years. These events remind us that not all catastrophic impacts occurred in the early days of the solar system. [4] On July 16, 1994, a string of comets named Shoemaker Levy 9 crashed into Jupiter. The resultant dust cloud was bigger than earth. This was the first time in human history that a comet has been observed hitting a planet. There have also been a number of near misses. In 1972 a 100m-wide asteroid, twice the size of that which produced the Arizona crater, entered the upper atmosphere over Canada and travelled back into space. [7] On the 7th of January 2002, an asteroid 300 metres wide missed the Earth by twice the distance of the Moon. It was discovered twelve days before the event. If the Earth had been its target, there would have been too little time to do anything. [5] On the 12th of March 2002, EM7, a 60m-wide asteroid, was discovered. It had passed within 385,000km of the Earth and came from the direction of the Sun. 3.0 Collision forecasts and aversion There is a 1 in 300 chance an asteroid 1km in diameter will collide with the Earth on March 16, 2880. The asteroid is named 1950 DA. [6], [13]. There is time to deflect the asteroid but the question remains: How could an asteroid of this size or bigger be deflected? The method described by Hollywood in several recent films is to nudge the asteroid off course with nuclear explosions. This may work if the asteroid is solid however many asteroids are porous and others, loose collections of rubble [8]. An explosion would prove dangerous: it could fragment the asteroid into smaller asteroids. Perhaps the safest way to avert disaster would be to alter the surface of the asteroid resulting in a reduction of radiated heat. Over a period of many years, this maybe enough to alter the orbit of the asteroid and avoid collision with the Earth. This is described as the Yarkovsky Effect [6]. Surface alteration could be achieved by adding a layer of dirt to the asteroid, or painting the asteroid’s surface white. A spacecraft could be docked onto the surface of the asteroid and, using its thrusters, push the asteroid into a new trajectory [8]. Scientists are powerless to alter asteroids’ orbits until asteroids are located, their orbits calculated and determine if and when asteroids will impact the Earth. Spaceguard, an international affiliation of groups designed to locate and identify 90% of asteroids of diameter 1km or greater by 2008. [8],[17] 1km objects were chosen as they are capable of global destruction [9]. It is estimated that there are over 900 1km sized NEOs. At present, 50% of them have been discovered. None of these asteroids are presently known to be travelling towards earth. Disappointingly, there are no telescopes looking from the southern hemisphere leaving a hole in the project. Spaceguard will later extend to the smaller asteroids capable of destroying cities. [10] 4.0 Conclusion Killer asteroids are lurking in the depths of space waiting for opportunity. Fortunately, the odds of a collision in our lifetime are extremely small. Ongoing surveys should detect an Earth-bound asteroid allowing scientists to devise a way or ways in which to avert disaster. Humankind however must not be too complacent. History has illustrated the power of an asteroid impact to not only give the dinosaurs their reign but also take it away. 5.0 References. [1] Comets, asteroids and Meteorites, Time Life Books, 1990, pages 24-26. [2] Sagan C, Druyan A, Comet, Headline Book Publishing, first published 1985, p104 - 204 [3] Mayell H, Comets May Have Led to Birth and Death of Dinosaur Era , for National Geographic News, May 16, 2002 [4] Sagan C, Cosmos, Abacus Books, first published 1981. [5] Blade T, U.S. Summons Experts to Draft Asteroid Defense Plan, The Cincinnati Post, June 7, 2002 [6] Braun D, Is a Large Asteroid Headed for Impact With Earth in 2880? , National Geographic News, April 4, 2002 [7] Was Moon Born From Planet's Crash Into Earth? , By Ben Harder for National Geographic News, August 20, 2001 http://news.nationalgeographic.com/news/2001/08/0820_moonimpact.html [8] “Incoming”, Gregory Mone, September 2003, http://www.popsci.com/popsci/aviation/article/0,12543,473545-3,00.html [9] (search for large asteroids nears completion, experts ponder gap s in program Micheal Paine - talk at General Assembly of the IAU david morrison from nasa s ames research centre, alan harris of the space science institute in Colorado) [10] Earth escapes brush with asteroid killer, Richard Stenger, January 7, 2002, http://www.cnn.com/2002/TECH/space/01/07/killer.asteroid/ [11] Armageddon Times, Paul Davies, http://bulletin.ninemsn.com.au/bulletin/EdDesk.nsf/0/9ede60adf34830aaca256a0d00067a77?OpenDocument [12] Asteroid introduction, Denis Gabor, http://www.solarviews.com/eng/asteroid.htm [13] Killer Tsunamis if asteroid hits Earth in 2880, News in Science, June 2, 2003, Anna Salleh, http://www.abc.net.au/science/news/stories/s867646.htm [14] Jupiter, a double edged sword, January 23, 2003, http://www2b.abc.net.au/science/k2/stn/archives/archive50/newposts/307/topic307681.shtm [15] “Dinosaur killer”, asteroid crater imaged for first time, National Geographic News, March 7, 2003, http://news.nationalgeographic.com/news/2003/03/0307_030307_impactcrater.html [16] Meteors, Meteorites, and Impacts, Bill Arnett, May 6, 2003, http://seds.lpl.arizona.edu/nineplanets/nineplanets/meteorites.html [17]Killer asteroid hunt reaches a milestone, Space.com, Michael Paine, August 7,2000, http://www1.tpgi.com.au/users/tps-seti/neo_milestone.pdf [18] 65 MILLION YEAR OLD PUZZLE - THE DINOSAUR KILLER http://www.pip.com.au/~paceman/THE%20DINOSAUR%20KILLER.html [19] Comets: How Big a threat To Earth?, John Roach, National Geographic News, Jamuary 28, 2003, http://news.nationalgeographic.com/news/2003/01/0128_030128_comets.html [20] Barringer, http://www.wa.gov.au/tiac/forum/2001/hocking/famous/barr.htm [21]Chicxulub, http://www.wa.gov.au/tiac/forum/2001/hocking/famous/Chix.htm [22] The Tunguska Meteorite: a dead-lock or the start of a new stage of inquiry? ? Part I, N.V.Vasilyev http://omzg.comcen-1.nsk.su/tunguska/en/articlese/vasiljeve.html