Is "Deep Impact" a Possibility?

We've all been to the cinema and lots of people have seen movies like "Deep Impact" where a huge meteor slams into the Earth and devastates civilization.  How seriously should we take this threat to our very existence, and is it really likely that one of these monsters will strike the Earth in our lifetimes or in the lifetimes of our children?  Well that's a really difficult question to answer, but we can certainly give you our point of view, which is based on information carefully gleaned from scientists and latest information, combined with what we hope is a modicum of common sense.

Do they really hit?
The first question we can address is, "Do objects really hit the Earth?  The short answer to that is a resounding "Yes".  Things are hitting us all the time, and the meteor showers which people sit in their gardens or in fields in the countryside to watch are swarms of particles entering the atmosphere and burning up as the friction which their passage generates heats them to incandescence.  These objects are all very small, from grains of sand to pea sized objects, usually left behind in space after the passage of a comet or other object across the Earth's orbit.  Every time we reach that point in our orbit the Earth ploughs through the debris field and we have a meteor shower.  There are also numerous man-made objects constantly re-entering the atmosphere, and the return of the MIR space station in March 2001 was one example of how the bigger objects don't always burn up.  Largish chunks can survive all the way to the surface, where they can cause significant damage if they strike something vulnerable, like a city or perhaps a nuclear power station?

And it is these larger objects which should cause us the most concern.  Although small to medium sized objects can survive intact their encounter with the atmosphere, the damage they can cause on impact is limited to the object and immediate area which they hit.  But the larger objects have the capability to create a planet-wide catastrophe, through a mixture of huge tidal waves if they strike in the sea, or nuclear winter with the whole planet shrouded in thick clouds if they strike on land.  For this reason there is an increasing effort to detect Earth crossing objects and to predict whether they are likely to present a danger to Earth at any time in the future.  I remember being told only a few years ago that the number of astronomers dedicated to this search effort was about the same as the staff of a large McDonald's restaurant.  These days there are more, but amateur astronomers still play a major role in asteroid discoveries and subsequent orbital calculations.

How big do they have to be?
A diameter of perhaps a kilometer is sufficiently large to cause us real concern, and so far about 400 of these have been found.  It is estimated that perhaps 90% of them will have been found by the year 2015, and then at least the risk of annihilation will have been quantified.  History certainly has shown that they can and do hit our planet.  It is now known with a moderate degree of certainty that the demise of the dinosaurs 65 million years ago was caused by the impact in the Yucatan area in northern south America of an asteroid perhaps 15 kilometers wide.  It is also suspected that another major extinction event 251 million years ago was probably also caused by a large meteor impact.  This was the worst extinction event ever recorded, and it left the planet almost bereft of plants and animals.  It is thought that this event actually enabled the rise of the dinosaurs, only to have their existence terminated by a similar event 186 million years later.  These major extinction events, in all likelihood caused by major impacts, seem to happen about every 100 million years on average.

Are there any to worry us right now?
Well, not exactly.  But most people know that there have been, and continues to be, a string of discoveries of alarming objects out there.  In some cases the discovery of these objects has been accompanied by a scientist's announcement that a "close encounter" with Earth is likely at some point in the future, and in all cases these initial warnings have been retracted.  We will leave it to you to judge how much of the retraction was based on increased knowledge of the orbit following further examination and measurement, and how much based on a need to avoid raising panic in the general public.  Certainly both factors must be at play.

Some examples of objects which are "out there" are:

Object 1997XF11
This is a 1 to 2 mile wide asteroid, discovered in 1997 and announced on 10th March, 1998 as likely to pass within 30,000 miles in October 2028.  Only a day later NASA assured us that it will actually miss us by 500,000 to 600,000 miles, and it is important to note that this big difference is caused by minutely small differences in the orbital calculations.  Just the tiniest bit away from this course and things can change dramatically.  And of course there are so many things out in space which can cause just such tiny perturbations in a carefully calculated orbit, things like gravitational pull by planets and other bodies and encounters with uncharted objects.  We'd better hope that any slight orbital change is in our favour!
Object 2000SG344
This object is a good deal smaller - only between 30 and 70 meters - but it was initially given the very high probability of 1 in 500 that it will impact Earth on 21st September, 2030. This estimate has since been significantly changed, but it seems the orbit is really not very well known and so again, a small error can make a big difference.  This rock is small enough that it would only cause major problems if it actually hit something critical on impact.
Object 1999AN10
This one is a little more worrying.  Because of the nature of its orbit, this Earth crossing asteroid will undergo numerous encounters with the Earth for the next 600 years.  Although initial concerns that it could actually hit us have been discounted, it is scheduled to pass perhaps within 37,000 Km of us on 7th August, 2027.  However, there is a sting in the tail of this one, because there is a danger that if the object passes us in a particular way on the 2027 pass, then it could be deviated in its course by gravitational interaction with Earth itself.  There is an outside chance, if this happens, of an Earth impact in either 2034 or 2039.  See the diagram to see what we mean, and pay attention to the "most likely" comment.  No-one can really know for sure until it actually passes us and we can measure what has happened to its path.

Object 2004 MN4
This object is even more scary, mostly because it is the size of two and a half football fields - two-thirds the size of Devils Tower in Wyoming and packing the energy of 65,000 Hiroshima bombs.  It is the object which you may recall was first discovered in June 2004 when there was a bit of panic on Christmas Eve.  Calculations of its orbit at that time suggested that there was a one in 60 chance of it colliding with the Earth.  Well, as of 20th October, 2009 NASA has released updated orbital information which we can share with you, and the good news is that the refined path indicates a significantly reduced likelihood of a hazardous encounter with Earth in 2036.  Updated computational techniques and the newly available data indicate the probability of an Earth encounter on April 13, 2036 has dropped from one-in-45,000 to about four-in-a million.

A majority of the data that enabled the updated orbit of Apophis came from observations Dave Tholen and collaborators at the University of Hawaii's Institute for Astronomy in Manoa have made. Tholen pored over hundreds of previously unreleased images of the night sky made with the University of Hawaii's 2.2-meter (88-inch) telescope, located near the summit of Mauna Kea and incorporated measurements from the Steward Observatory's 2.3 meter (90-inch) Bok telescope on Kitt Peak in Arizona and the Arecibo Observatory on the island of Puerto Rico.  The information provided a more accurate forecast for Apophis' orbit well into the latter part of this century.  Among the findings is another close encounter by the asteroid with Earth in 2068, with chance of impact currently at approximately three-in-a-million.  As with earlier orbital estimates, where Earth impacts in 2029 and 2036 could not initially be ruled out due to the need for additional data, it is expected that the 2068 encounter will diminish in probability as more information about Apophis is acquired.

So, although initially Apophis was thought to have a 2.7 percent chance of impacting Earth in 2029, any possibility of an impact in 2029 have now been ruled out, but the object will still come within 29,450 kilometers (18,300 miles) of us on Friday, April 13 of that year.  In astronomical terms, that's a really close shave!  For a couple of hours after dusk, people in Europe, Africa and western Asia will see what looks like a medium-bright star creeping westward through the constellation of Cancer, making Apophis the first asteroid in human history to be clearly visible to the naked eye. And then it should be gone, having vanished into the dark vastness of space.  We will have dodged a cosmic bullet and will be safe - or will we?

Well, Maybe!  Scientists calculate that if Apophis passes at a distance of exactly 18,893 miles, it will go through a "gravitational keyhole."  This small region in space - only about a half mile wide, or twice the diameter of the asteroid itself - is where Earth's gravity would perturb Apophis in just the wrong way, causing it to enter an orbit seven-sixths as long as Earth's.  In other words, the planet will be squarely in the crosshairs for a potentially catastrophic asteroid impact precisely seven years later, on April 13, 2036.

Radar and optical tracking during Apophis's fly-by last summer put the odds of the asteroid passing through the keyhole at about 45,000-to-1.  Former astronaut Rusty Schweickart, now 71, knows a thing or two about objects flying through space, having been one himself during a spacewalk on the Apollo 9 mission in 1969. Through the B612 Foundation, which he co-founded in 2001, Schweickart has been prodding NASA to do something about Apophis—and soon. "We need to act," he says. "If we blow this, it'll be criminal."  If the dice do land the wrong way in 2029, Apophis would have to be deflected by some 5000 miles to miss the Earth in 2036.  Hollywood notwithstanding, that's a feat far beyond any current human technology.  The fanciful mission in the 1998 movie Armageddon - to drill a hole more than 800 ft. into an asteroid and detonate a nuclear bomb inside it - is about as technically feasible as time travel.  In reality, after April 13, 2029, there would be little we could do but plot the precise impact point and start evacuating people.  According to projections, an Apophis impact would occur somewhere along a curving 30-mile-wide swath stretching across Russia, the Pacific Ocean, Central America and on into the Atlantic. Managua, Nicaragua; San José, Costa Rica; and Caracas, Venezuela, all would be in line for near-direct hits and complete destruction. The most likely target, though, is several thousand miles off the West Coast, where Apophis would create a 5-mile-wide, 9000-ft.-deep "crater" in the water.  The collapse of that transient water crater would trigger tsunamis that would hammer California with an hour-long fusillade of 50-ft. waves.

BUT DON'T EVACUATE just yet.  Although we can't force Apophis to miss the Earth after 2029, we have the technology to nudge it slightly off course well before then, causing it to miss the keyhole in the first place.  According to NASA, a simple 1-ton "kinetic energy impactor" spacecraft thumping into Apophis at 5000 mph would do the trick.  We already have a template for such a mission: NASA's Deep Impact space probe - named after another 1998 cosmic-collision movie - slammed into the comet Tempel 1 in 2005 to gather data about the composition of its surface.  Alternatively, an ion-drive-powered "gravity tractor" spacecraft could hover above Apophis and use its own tiny gravity to gently pull the asteroid off course.  In 2005, Schweickart urged NASA administrator Michael Griffin to start planning a mission to land a radio transponder on Apophis.  Tracking data from the device would almost certainly confirm that the asteroid won't hit the keyhole in 2029, allowing everyone on Earth to breathe a collective sigh of relief. But if it didn't, there still would be time to design and launch a deflection mission, a project that Schweickart estimates could take as long as 12 years. It would need to be completed by about 2026 to allow enough time for a spacecraft's tiny nudge to take effect.

NASA, however, is taking a wait-and-see attitude.  An analysis by Steven Chesley of the Near Earth Object program at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., concludes that we can safely sit tight until 2013.  That's when Apophis swings by Earth in prime position for tracking by the 1000-ft.-dia. radio telescope in Arecibo, Puerto Rico.  This data could also rule out a keyhole hit in 2029.  But if it doesn't, the transponder mission and, if necessary, a last-resort deflection mission could still be launched in time, according to Chesley.  "There's no rush right now," he says. "But if it's still serious by 2014, we need to start designing real missions."

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