Wednesday, 24 July 2013

Death from Above: Impacts in our Solar System

Wednesday July 24th, 15:30

As I write this I’m sat on Deck 9 looking out over the thick fogs of the Grand Banks.  This is the third of a series of public lectures for the guests on Queen Mary 2 as part of the ‘Cunard Enrichment Programme’ for this transatlantic crossing, and concerns a natural phenomenon with wide-ranging implications for the continued survival of our species. 

On a bright, cold February 15th morning in the southern Ural region of Russia, an asteroid hurtled into the Earth’s atmosphere at speeds exceeding 41,000 mph.  It was totally unexpected, and no one saw it coming.  It became a brilliant meteor, outshining the light of the morning Sun to cast shadows across the town of Chelyabinsk.  This airburst explosion had the energy of 440 kilotonnes TNT, and the resulting shockwave injured 1500 people and caused an estimated $33 million in property damage.  It’s possible that fragments of the meteorite smashed a 6-m wide hole in Lake Chebarkul’s frozen surface.  All this from an asteroid thought to have been around 17-20 m wide weighing around 10,000 tonnes.  So why didn’t we have any warning?  How frequent are these events?  It left us all wondering – what would it have been like if this had been my town, my home, my place of work?  And history tells us that the consequences could have been much, much worse.

A Hierarchy of Impact Events

First, some terminology.  A meteoroid is a small grain, maybe up to 1-m in size, and possibly left behind by a passing asteroid or comet.  We call these objects meteors when they enter the Earth’s atmosphere to burn up due to friction, resulting in ‘shooting stars’ and regular meteor showers throughout the year.  If the object survives the heat of entry and makes it to the surface, we have a meteorite.  The terms bolide and superbolide are sometimes used but have no precise astronomical definition, they refer to the brightness of the resulting fireball.  An airburst, such as that at Chelyabinsk, is a mid-air explosion that makes no contact with the ground.  Our best example of an airburst occurred in the remote Siberian region of Tunguska in 1908, when a small asteroid or comet (60-190 m wide) exploded 5-10 km above the Earth’s surface, knocking over 80 million trees over an area 830 square miles in size.  The dust and particles from the explosion were reported to produce sky glows in the evenings over Europe for many days afterwards.  Given the upheavals of the first decades of the 20th century, no expeditions were sent to the region for almost ten years, so our knowledge of this airburst comes from the devastated landscape and eyewitness accounts of the event.  The Tunguskan airburst was the largest impact event in recent history.

When meteorites do reach the ground, the consequences can be enormous.  Our solar system bears the remnant scars of this bombardment, and there is no finer example than the Barringer crater 43 miles east of Flagstaff, Arizona.   Estimated to be 50,000 years old, this crater was created when a nickel-iron meteorite 60-m wide impacted with the Colorado plateau with an energy of 10 Megatonnes TNT.  The meteorite itself was vapourised as it blasted a hole 1200 m wide and 170 m deep.  It is named after Daniel Barringer, who first suggested that it had been caused by an impact, which was later verified by Eugene Shoemaker as the first evidence of an extraterrestrial impact on Earth. 

Larger collisions can have consequences for the existence of life on our planet.  Buried beneath the Yucatan peninsula of Mexico lies the Chicxulub meteorite crater.  An oil survey had first detected an underwater ring beneath the Gulf of Mexico, and mapping of gravitational anomalies later confirmed the presence of an enormous crater 110 miles wide.  The 65-million year age of the crater coincides with the end of the Cretaceous period, the start of the Paleogene (the K-Pg boundary) and the end of the age of the dinosaurs.  This impact, and the environmental conditions that followed, triggered mass extinctions across our planet and ushered in the age of the mammals, and ultimately the human species.   Impact processes have shaped our evolution and could one day threaten our survival, but thankfully extinction level events are rare.  To understand the nature of the impactors and the frequency of such events, we shall now look outwards to the other bodies in our solar system.

Impactors in Our Solar System

The birth of our planet was complex and violent, as small rocky protoplanets grew larger and larger over time through collisions and mergers, gravity clumping material together until the planets as we know them today had formed.  But this process was chaotic and remains incomplete, with vast fields of debris remaining throughout our solar system that was never incorporated into the forming protoplanets.  Gravity serves as the great meddler, and as the planets moved and jostled to assume their present-day locations, they sent material flying in all directions, leading to a period of heavy cratering on all the planets known as the Late Heavy Bombardment around 3.9 billion years ago.  We need only look at the full moon tonight to witness the scars left by this bombardment.

Some of that solar system debris can be found in the broad asteroid belt between the orbits of Mars and Jupiter.  Jupiter’s immense gravity prevented this material from forming a planet, but large objects do exist – Ceres is known as a dwarf planet some 950 km in diameter, and soon to be visited by NASA’s Dawn spacecraft.  Other large asteroids include Vesta, Pallas and Hygiea.  We say that these objects are ‘volatile poor’, as many of their light molecules (water included) have been lost to leave dry and airless bodies.  This contrasts them with the icy comets, rich in volatiles and icy material.  For much of their existence, comets exist as dirty snowballs, only forming their characteristic tails when they wander into the inner solar system.   As the cometary nucleus is warmed, they produce tails of dust and plasma that produce incredible spectacles in the night sky.  We know of around 5000 cometary bodies in our solar system, but models suggest there could be as many as a trillion out there in the distant solar system.  And the line between comets and asteroids can sometimes be blurred, as an extinct comet that has lost all its ice and volatiles would resemble an asteroid.

Comets are broken down into categories depending on how frequently they visit the inner solar system.  Short period comets come from the realms of the giant planets and the Kuiper belt beyond Neptune.  If a comet has it’s maximum orbital distance near Jupiter, then it would be known as a ‘Jupiter-family’ comet (of which there are around 450 with orbital periods less than 20 years).  Comet Hartley 2 is a good example, and its nucleus was imaged in 2010 by NASA’s EPOXI mission.  Halley-type comets have orbits that last between 20 and 200 years.  At the other extreme, longer period comets all appear to come from an extremely distant cloud of cometary objects, taking hundreds of years between their visits.  The hypothesised spherical Oort cloud (which has never been directly observed) could extend 50,000 AU from the Sun, and makes its presence known when gravitational perturbations kick an icy comet onto a trajectory into the inner solar system.

The size and brightness of a cometary tail depends on a multitude of factors, but those comets that put on a particularly magnificent display are known as the ‘Great Comets.’  Examples include Halley’s Comet, a frequent visitor to the inner solar system and known since ancient times (e.g., it was recorded in the Bayeux tapestry in 1066), last seen in February 1986 and returning again in July 2061; and more recently Hale Bopp in April 1997, which featured a tail 50 million km long and visible for 18 months to the naked eye.  Some comets wander too close to the Sun and become ‘dirty snowballs in hell’ as they pass through the solar corona.  One such comet, Lovejoy, was observed to pass through the corona in December 2011 and emerge on the other side intact.  Finally, this year attention has been focused on two comets – PANSTARRS and ISON.  The first arrived in the spring, but was difficult to identify in the sky.  The second, ISON, should arrive in December, and has the potential to be a spectacular great comet.  It’s a new arrival to the inner solar system, with pristine volatiles ready to vent, but although a tail is forming it’s hard to tell how bright it will become. 

Jupiter Strikes

My own interest in impact events stems from a brief period of time in 2009 when my research took a very exciting and unexpected turn. But first rewind to 1994.  You may remember the fate of comet Shoemaker-Levy 9, the 9th comet discovered by Gene and Carolyn Shoemaker with David Levy.  In 1992 Jupiter’s immense gravity had disrupted this icy comet and broken it into fragments A-W, each 100-500 m in diameter and setting them on a course to collide with the giant planet in 1994.  They slammed into Jupiter, creating plumes of material and dark bruises on the atmosphere that we’re visible for many weeks afterwards.  These are classic examples of airbursts, even though they took place on a distant world, and allowed us to explore the detailed physics of the collisions.  The superheated entry column left by the cometary fragments caused an ejection of the atmosphere (a mix of Jupiter’s atmospheric soup and vapourised comet), high above the clouds.  The ejecta travelled ballistically, then slammed back down on the atmosphere to create the dark debris fans that characterised each impact site.  This phenomenon was the chance of a generation to witness the aftermath of a cosmic collision.

In the summer of 2009, on a Sunday afternoon in Pasadena California, I was having a barbeque with some friends.  I was due in the office that night to help my boss run a telescope on the summit of Mauna Kea, Hawaii, to observe the interactions of some giant storms on Jupiter.  That afternoon, we received an email from a talented amateur observer, Anthony Wesley of Australia, who had spotted a new dark scar in Jupiter’s south polar region.  He suggested that a fresh impact had occurred, for the second time in recorded history.  So when we arrived at work that Sunday night, we immediately chose techniques to identify the smoking gun of an impact – the high altitude debris left by the collision – and it came into view in startling clarity.  The discovery propelled us all into the world’s media for a few hours, as our image was reproduced around the globe – it was a terrifically exciting time!

Scientists scrambled for telescope time to diagnose what had happened.  By studying the impact debris across a wide range of wavelengths, from the infrared to the ultraviolet, we could piece together the events like a detective story.  Ultimately, the chemistry of the impact debris suggested that whatever had hit Jupiter that night had been depleted in ice and volatiles – no water-related chemistry was observed, and all the signs have since pointed to a rocky asteroid colliding with the giant in 2009.  We even managed to persuade the Hubble Space Telescope to track the evolution of the impact debris – Hubble was still in testing and verification after the final servicing mission, so this was quite a feat and showed how the debris was blown around by Jupiter’s powerful winds.  Evidence of the impact was still present 5 months on.

Since that time, the amateur community has become adept at impact monitoring on Jupiter, using webcam movies to detect the telltale flashes of meteors exploding in the jovian atmosphere.  We’ve now witnessed three such events since 2010, but none have left the dramatic atmospheric scarring of either the cometary impacts in 1994 or the asteroidal impacts in 2009.   In essence, these are no different to what happened over Chelyabinsk in 2013, and the objects are even of a similar size.  The frequency of impacts can be used to understand the rates of impacts in our solar system at the present time.

Preparing for the Future

So impact processes are commonplace in our solar system, having shaped planetary evolution in the past and still causing dramatic events today.  So what is being done, and what can be done, to minimise the risks associated with these collisions?

The first task is to understand the frequency of impacts, which can be done via a chart showing the object size and energy versus the number of impacts over the years, centuries and millennia.  As a benchmark, a 4-m stony asteroid is predicted to hit the Earth once a year or so; a Chelyabinsk-sized object every few hundred years; a football field sized asteroid every 2000 years; and a 1-km sized object every half million years or so, on average.  Such a collision would have both regional and globally devastating consequences.  But a 10-km impactor, like that which triggered the extinction of the dinosaurs, might be expected once every 100 million years or so.  These are all averages and present an idea of probability, and should not be seen as a timeline for such events!

The second task is to assess the nature of the impact hazards today, by monitoring all ‘Near Earth Objects’ (NEOs) and assessing their potential to collide with Earth.  For example, NASA has a congressional mandate to characterise all NEOs over 1-km in size, and this supports the international ‘SpaceGuard’ project scanning the skies for these objects.  Almost 10,000 NEOs are known, the majority of them asteroids, and our models suggest that this represents over 90% of the population out there.  But recall that Chelyabinsk was far smaller (<20 m), and would never have been picked up by such surveys.  All objects are assigned a number on the Torino scale, which balances the probability of impact versus the amount of damage it would cause.  When an object is first spotted it is assigned a high number, but as we refine our knowledge of the orbital parameters it is downgraded to zero.  Today, only one NEO still has a nonzero probability of impact, 2007VK184, which has a 1/1820 chance of hitting us in June 2048.  But as our knowledge of the orbit of this body improves with time, we fully expect this hazard to be downgraded too.

Finally, if we do spot a hazard heading our way, what safeguards are in place to prevent collision?  It certainly depends on many factors, including the size of the threat and the amount of time we’d have to prepare.  That’s precisely what projects like SpaceGuard is designed for.  The method ultimately chosen to prevent a collision will depend on cost, performance, operations and risk, but can be loosely broken into two categories – destruction or delay.  Destructive methods would break the impactor into smaller, safer fragments to remove the threat.  Delay tactics would tweak the orbit of the impactor so that it passes us safely.  Nuclear detonations; using ballistic impactors; or mass drivers and gravity tractors; focused solar energy or ion beams; or even attaching conventional rockets to the impactor, could all be viable options for destruction and delay.  Our readiness will improve with our technology, but its our duty to be ready for the challenge when, not if, it finally arrives.  I finish with a final quote from Sagan:  “Extinction is the rule.  Survival is the exception.”  Something worth bearing in mind when we see the next Chelyabinsk.

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