Cosmic Collisions

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Updated March 14, 2011

Cosmic Collisions: Assessing the Risk

Earth under Bombardment

The Sun formed from the collapse of interstellar material in the solar nebula some 4.6 billion years ago. A tiny portion of its mass (0.2%) remained gravitationally bound but outside of the newly ignited body in a spinning disk of debris. Relatively quickly, smaller bodies called planetesimals started to gravitationally clump together from rings of material sharing a similar orbit. The early history of our solar system was a violent snowball fight. Indeed, a leading theory suggests that Earth's water supply originated from a vast reservoir of comets and other icy bodies impacting it. [This has application in Unit C: Environmental Chemistry: 'describe and illustrate processes by which chemicals are introduced to the environment or their concentrations are changed'.]

Over time the process of bombardment slowed as the number of smaller bodies dwindled. The crusts of rocky bodies cooled and the planets began to take on their unique identities. Earth's silent partner the Moon stands in mute testimony to the violence of bygone days. It bears many craters and scars from the first third of the solar system's history and a significant, but far lower number from the last three billion years. Earth surely underwent a similar bombardment, but due to the various processes of erosion - wind, water, glaciation, volcanism, plate tectonics, sedimentation - of a living planet, has healed its scars to a great degree. Only a few remain on the surface: Meteor Crater in Arizona and Lake Manicouagan in northern Quebec are good examples.

Nonetheless, there is much evidence to indicate the process of bombardment is not completely over. Indeed, it can be witnessed any night of the year as meteors. Earth accumulates thousands of tonnes of cosmic dust, or micrometeorites, on an annual basis.

Occasionally larger objects survive the passage through Earth's atmosphere to reach the ground as full-fledged meteorites. In 2000 a brilliant fireball split the sky over the Yukon Territory, depositing a treasure trove of scientifically valuable meteorites on the ice of Tagish Lake, BC. Through a combination of good fortune and quick action, Canadian scientists were able to collect specimens of an exceedingly rare form of carbonaceous chondrite. Perhaps the most primitive meteorite ever recovered, this fall is yielding valuable insights into the early history of the solar system.

More occasionally still, dangerously large objects fall from the sky. In 1908, a small asteroid some 30 metres in diameter exploded in the air over a remote forest in Tunguska, Siberia. This was the most powerful explosion of the 20th century, 100 times more powerful than the Hiroshima atomic bomb. The event was witnessed indirectly as far away as England, where unusually lit overnight skies were recorded. At ground zero directly beneath the blast, a few charred tree trunks stood resolutely along the downward line of force, their branches sheared away; outside of the blast centre trees fell away radially for some 30 km in all directions. Fortunately the area was so remote that only two people died, so remote in fact that a scientific expedition to survey the aftermath was not mounted until 1927 when the devastation was still apparent. Had this explosion occurred over a populated area, the extent of the catastrophe would be unimaginable.

Evidence does exist for global catastrophes, and it has been mounting in the past quarter century. The first real-time example of a significant body colliding with a planet in recorded history was Comet Shoemaker Levy 9. Torn apart by tidal stresses on a close pass to Jupiter in 1992, the comet was discovered in 1993 before meeting a spectacular demise in July 1994 when over 20, kilometre-scale comet fragments crashed into the planet at 60 km/s. The pieces of comet were like interplanetary flies on a cosmic windshield. But in the aftermath, mighty Jupiter bore huge, Earth-sized scars which took months to dissipate in its upper atmosphere. This was a sobering sight for anybody considering the implications of such objects striking Earth.

As recently as 1980, Luis Alvarez postulated that the sudden extinction of the dinosaurs 65 million years ago may have been caused by a natural disaster of extraterrestrial origin, based on an anomalous, globally distributed layer of the rare element iridium at the geologic stratum known as the K-T Boundary. Alvarez was not laughed out of court only because of his good name and reputation, as a Nobel Prize-winning physicist. Despite its cool reception, the evidence in favour of his theory quickly mounted. This culminated in the early 1990s in the discovery of the magnetic remains of a 180 km-wide crater where the Yucatan Peninsula meets the Gulf of Mexico whose origin was dated to exactly 65 million years ago. Although a few naysayers remain, the leading current theory for the K-T boundary mass extinction is the collision between Earth and a comet or asteroid some 10 km in diameter. Countless species were lost forever; even the 'survivors' lost most of their populations. Ever resilient, Earth recovered but under a new paradigm. The yellow brick road was paved for the ascendancy of humans.

What is the risk of Earth being smoked by a late planetesimal in the 21st century? And can we do anything to improve those odds?

Risk assessment

The chances of Earth being hit by an asteroid at some point in the future is 100%. How big an asteroid? How often? When?

There are as yet, no certain answers to these questions. However, students are encouraged to consider as many factors as possible that would be necessary to address this risk. [Again, this is consistent with the requirements of Unit C: Environmental Chemistry, specifically 'hazards, probabilities and risk assessment', as well as Unit E: Space Exploration, specifically 'investigate predictions about the motion, alignment, and collision of bodies in space'.]

The chances of an individual asteroid hitting Earth were recently compared (in a NASA release) to that of winning the lottery. Let's call it Canada's own 6/49, where an individual ticket has a 1 in 13,983,816 chance of being the right one. These are of course terribly long odds, but what would your chances be if you had 100,000 lottery tickets? 100 million? If you bought 1,000 tickets every week, how long would it take you to win? When it comes to cashing in The Big One on a global scale, the numbers aren't quite so well-defined, however these are the types of quantitative questions which must be considered.

Ever more frequently in the news these days are reports of asteroids perhaps hitting Earth at some finite date in the future, or narrowly missing in the here and now. There are currently over 450 objects on the list of Potentially Hazardous Asteroids (PHAs), each of which has a tiny but non-zero chance of some day encountering Earth. While probably less than 10% of the true total, this list is growing extremely rapidly; over half of its entries date since 2000. Earth is not suddenly encountering new perils, we are simply doing a much better job of identifying them. An entire program, the LIncoln Near Earth Asteroid Research, or Project LINEAR, is dedicated to discovering Earth-crossing asteroids, and has succeeded in discovering numerous comets as well. This low-budget program is proving the worth of remote sensing technology, and is in the early stages of building an important data set: what is the number and nature of Earth-crossing asteroids, and what are their trajectories?

Another question to consider is, what would happen if one hit? Asteroids and comets come in different sizes, densities, relative speeds, and impact angles. Earth offers a variety of possible impact sites: oceans, mountains, tundra, deserts, populated areas. Students are encouraged to do the following experiment:

Fill a shoe box with a few cm of flour. Sprinkle the top with an identifiable coloured substance, such as pepper or paprika. Drop a marble into the 'flour bed' from a height of 50 cm. Observe the results, including the crater created by the marble, the ejecta, the disturbance of the surface material, etc. Try different size objects. Try changing the impact velocity by dropping from different heights. (Ever the showman, Bill Nye the Science Guy did this experiment using a slingshot, although we prefer to leave decisions of this nature to paid-up members of the Teachers' Defence Fund!)

Some researchers have already compiled actuarial statistics of 'death by asteroid'. One study from the University of Arizona compiled the following estimated risks of death for an American over a 50-year period: Botulism 1 in 2,000,000 Fireworks 1 in 1,000,000 Tornado 1 in 50,000 Airplane crash 1 in 20,000 Asteroid impact 1 in 6,000 Electrocution 1 in 5,000 Firearms accident 1 in 2,000 Homicide 1 in 300 Auto accident 1 in 100

The probability of impact-related deaths may seem surprisingly large given they're hardly the stuff of everyday news items. (Indeed, students are encouraged to critique the study itself: are the conclusions realistic?) What tips the scales is that a single event can have global implications, unlike most forms of natural disaster - flood, hurricane, tornado, volcano, mudslide, earthquake - which are regional in nature. Another vital distinction is the potential to limit or prevent such a disaster, if the potential 'doomsday asteroid' is spotted early enough.

Again, students are asked to consider the options. Popular among these will be to 'nuke it', but it's not that simple; a nuclear device which breaks the asteroid apart could simply assure several smaller but still catastrophic impacts instead of one, actually increasing the area of destruction. Alternatively, a nuclear device set off to one side of the asteroid will alter its velocity by an infinitesimal amount, which with enough lead time would be sufficient to deflect the object out of harms way. Other possible solutions include lassoing the object, putting a rocket motor on it, putting an ion drive motor on it, putting a solar sail on it, or indeed any propulsion system which could influence the object's momentum. A large-scale mining operation could alter the asteroids mass and momentum while yielding valuable minerals. More conservatively, an accurate prediction of the time and location of an impact would allow for a mass evacuation of the target area. Brainstorm. This can be a fun activity in which the most improbable suggestion could in fact be the solution. Or, that same improbable suggestion could become the plot line of a major Hollywood motion picture! Consider the movies 'Deep Impact' and 'Armageddon', released nearly simultaneously in the summer of 1998. How realistically do they portray the threat of impact? The solution? Consider that the budget for each of these films dwarfs that of current PHA searches such as Project LINEAR.

Students may also consider the economic implications, including the costs of scaling up the PHA detection program and the cost of a mission or missions designed to avert an impact as compared to the potential cost of a regional or global catastrophe. While we're sure Earth will be impacted eventually, the chances of it occurring 'tomorrow' are vanishingly small. Should we continue to simply trust our luck?

Canadian involvement

Discovered by gravitational mapping of oil company surveyors, the Chicxulub crater on the Yucatan coast was subsequently analyzed by a team of scientists including Canada's Alan Hildebrand. Hildebrand was also project leader of the Tagish Lake Meteorite recovery expedition in the late winter of 2000, and of the subsequent scientific analysis at his home base in the University of Calgary where he is a professor in the Department of Geology and Geophysics

Peter Brown, originally from Fort McMurray, now works as a Professor of Physics and Astronomy at the University of Western Ontario where he is one of the world's leading experts on planetary bombardment, from meteoroids to asteroids. He too has been involved in the Tagish Lake Meteorite expedition and follow-up.

Comet Shoemaker-Levy 9 was co-discovered by Canadian amateur astronomer David Levy, working closely with the husband-and-wife team of Eugene and Carolyn Shoemaker, leading experts in the field of impact craters.

These are but three examples of Canadians who have made an 'impact' in one of the most exciting new fields in all of science.


The Whole Shebang: A State-of-the-Universe(s) report, by Timothy Ferris.

Rain of Fire and Ice: The Very Real Threat of Comet and Asteroid Bombardment, by John S. Lewis.

Cosmic Pinball: The Science of Comets, Meteors, & Asteroids, by Carolyn Sumners and Carlton Allen.

Cosmic Collisions, by Dana Desonie.

Doomsday Asteroid: Can We Survive?, by Donald W. Cox and James H. Chestek.

Internet resources:

"Tagish Lake Meteorite/Fireball Investigation"

"Earth Coorbital Asteroid 2002 AA29"

"1908 Siberia Explosion"

"Comet Shoemaker-Levy 9 Collision with Jupiter"

"Unit Plan: The Impact of Shoemaker-Levy 9"

"Mystery of the Chicxulub Crater"

"Potentially Hazardous Asteroids"

"Asteroid and Comet Impact Hazards"

"The Astronomy of 'Armageddon'"

"The Astronomy of 'Deep Impact'"


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We gratefully acknowledge the financial support of the 

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and the

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