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00:00Every year, rocks and debris from space collide with Earth in spectacular fashion.
00:05They light up the sky, even smash into Earth's surface.
00:09But what happens when something enormous comes knocking?
00:13Would it trigger a worldwide ice age?
00:15Or raise temperatures that boil the oceans?
00:18Could a monster be headed right for us?
00:21Could it be breathing down our necks?
00:26Could Earth be headed for a collision?
00:30Could Earth be headed for a collision?
00:32Could Earth be headed for a collision?
00:42San Francisco, sometime in the near future.
00:46High above the skyline of this city by the bay, lurks a monster.
00:52A monster made of rock.
00:56A giant asteroid a quarter of a mile across is bearing down on Earth.
01:06Scientists have known of this monster for decades.
01:09They have had fair warning, but ultimately there is no stopping it.
01:14No escape.
01:15Evacuations leave the city a ghost town.
01:21From the Embarcadero to the Golden Gate Bridge, a sense of foreboding curls around the buildings like fog.
01:29Miles above, the asteroid looms.
01:33As the asteroid hits the Earth's atmosphere, the city is rocked.
01:39First by violent shock waves.
01:43Then as sheared off pieces pummeled the landscape.
01:49Finally, San Francisco is swallowed by a devastating tsunami.
01:55Caused by the main body of the asteroid, which hit miles off the California coast.
02:12San Francisco is destroyed in a matter of moments.
02:26One more Hollywood special effects fantasy?
02:29Yes and no.
02:31As far as we know, a giant asteroid will not plow into San Francisco.
02:37But this much of the story is true.
02:40There is a giant asteroid speeding dangerously close to Earth.
02:45And for a few days in 2004, scientists around the world believed that just this kind of scenario could actually happen.
02:53Earth is under attack, and the evidence is all around us.
02:58It bears the scars of objects like this crashing into it.
03:03This is Apophis, an asteroid one quarter of a mile wide.
03:10When Apophis emerged from the black vastness of space in 2004,
03:16scientists struggled to predict where it was headed.
03:20For a while they thought Apophis was headed straight toward Earth.
03:25What would an asteroid the size of Apophis do?
03:29It depends.
03:31Asteroid astronomers say we know it's an S-type asteroid.
03:35That means it's similar to stony meteorites.
03:39That's where the S comes from.
03:41They're most similar to stony meteorites.
03:44If something is rocky, it's much more likely to break up into smaller pieces
03:49and possibly explode just above the surface of the Earth.
03:54These atmospheric explosions can be violent, sending shock waves into the Earth.
04:00But some stony asteroids create even more devastation by remaining intact.
04:06A lot depends upon just how aerodynamically it's shaped and the orientation of the object as it enters the Earth's atmosphere.
04:15It may be that there are some protrusions that could break off of the parent asteroid during atmospheric entry.
04:24A disaster of such scale would have far-reaching implications.
04:29Are we able to do anything about a massive space rock headed our way?
04:34What we could do if there was an asteroid heading our way is going to depend a lot on how much time we have.
04:41And we'd need lots of time to deal with a potential Earth-killing asteroid.
04:46Currently, no program is in place to deal with an asteroid or comet colliding with Earth.
04:53Of course, in this country, NASA is much involved with this problem.
04:58But even they aren't involved with preventing impacts.
05:04In other words, if astronomers discover an object today that will impact the Earth within 10 years, we might be sitting ducks.
05:15This is not just fanciful theorizing small objects really slam into planets and could slam into the Earth.
05:24And small is relative.
05:27Apophis is dwarfed by the asteroid that tore into Earth 65 million years ago,
05:33wiping out the dinosaurs.
05:38Even scarier, Apophis is larger than the asteroid that carved out Meteor Crater in Arizona 50,000 years ago.
05:48Dangerous asteroids and comets are flying around the solar system,
05:52just waiting to puncture Earth's atmosphere and lay waste to its inhabitants.
05:58Is anybody watching out for them?
06:01How often do they swing into our path?
06:04And what sort of impact have they made in the past?
06:08The science of understanding what these objects are is the same science as understanding what to do about them.
06:15In other words, there are wonderful scientific questions about how they relate to the beginning of the solar system.
06:21How did they come together?
06:22How did they relate to life on Earth, the formation of our planet?
06:26All the things we want to learn as scientists about these objects are the same sorts of things that are very useful if we were to decide what to do about it.
06:36Even as we sort out these questions, Apophis hides in the sky, just out of sight.
06:42Apophis has surely been orbiting the sun for millions, possibly billions of years.
06:49But the story of how Apophis was discovered began at Kitt Peak, outside of Tucson, Arizona, in June 2004.
06:57Astronomers visiting from the University of Hawaii search for objects orbiting near the Earth using the University of Arizona's 90-inch telescope.
07:07Their search is part of an overall effort to find and track asteroids a kilometer or larger within the solar system.
07:16This instrument was completed in something like 2002, and one of the first actual users was Roy Tucker for his asteroid survey to discover asteroid Apophis.
07:29We use the 90-inch telescope because the camera has a wide field, excellent light gathering capability,
07:37and also it's well suited for looking at parts of the sky that are relatively close to the sun in the sky.
07:45The principal interest was looking for asteroids that are at small solar elongations.
07:53More commonly they'll be in the daytime sky where they're not observable at all,
07:58or they'll be close to the sun after sunset or before sunrise.
08:03This instrument was well suited to Roy Tucker's task of searching for near-Earth asteroids.
08:09Near-Earth asteroids are usually divided up into three groups.
08:14Normally the Earth is described as having an orbit one astronomical unit away from the sun.
08:24Basically it just means that an astronomical unit is the average distance from the sun to the earth in its orbit.
08:29If an asteroid at its closest point to the sun can come within 1.3 astronomical units,
08:36but as long as it doesn't cross the orbit of the earth, it's called an AMOR.
08:40It's not a threatening object.
08:42The next class of objects are the Apollo asteroids.
08:46The Apollo asteroids have average distances from the sun more than one astronomical unit,
08:52but because orbits around the sun are actually ellipses, basically stretched out circles,
08:58at the near point to the sun they can actually approach the sun more closely than the earth does.
09:04They cross the orbit of the earth.
09:06These objects, if the geometry and the inclinations and such are correct,
09:11these can pose a threat of impact.
09:13They can be dangerous objects.
09:15The next class are the Otten asteroids.
09:19Now the Otten class asteroids have orbits that, an average distance that's entirely inside the earth,
09:25but because their orbits are stretched out ellipses at the most distant part of their orbit from the sun,
09:32they can actually be outside the orbit of the earth.
09:35They can be seen in a dark nighttime sky opposite the sun.
09:39The astronomers hope to find previously unobserved objects in a region of the sky facing the sun within earth's orbit.
09:48The question we had was, are there asteroids interior to the earth's orbit
09:54that could potentially sneak up from our blind side and hit us?
09:59They didn't wait long to find out.
10:02Astronomers seeking new near-earth objects take photographs of the sky with powerful telescopes.
10:08Then look for white streaks on the image, the tell-tale sign of a fast-moving object.
10:15When you're looking for near-earth asteroids, you're looking for something usually that's moving pretty quickly.
10:20The images that we collect, we collect three images, and we essentially make a little three-frame movie.
10:27So if all the stars more or less just sit there while you're blinking the images,
10:32but if you see something moving, blink, blink, blink, blink, blink, blink, blink, something's moving,
10:37you figure, aha, I've got an asteroid here.
10:39And if you see something go streak, streak, streak, you know it's very exciting
10:43because that's really moving quickly.
10:47We found a moving object that we thought might be the same object
10:52that we had seen just a couple nights earlier.
10:54And I told Dave, here's your guy, this is the one you're looking for.
10:59And we measured the position, and Dave did some orbital computations
11:05and quickly realized this couldn't possibly be the object.
11:08The team carefully plotted the movement of the object for two nights over a relatively short arc.
11:14To secure a preliminary designation as it's called for an asteroid,
11:20you need two nights of observations of a new object.
11:24They don't have to be consecutive nights, but they have to be, you know,
11:27within no more than ten days apart.
11:30It's a very difficult process to calculate where that orbit's going to be in the future
11:34because it's almost like baseball.
11:36And so the ball leaves the bat.
11:38You don't know where it's going to land.
11:40You can't quite tell is it in the park or out of the park.
11:44And you have to keep watching the trajectory
11:46and watching the trajectory and watching the trajectory.
11:49And then eventually, after you see enough of the path,
11:52you can predict whether it's going to land fair or foul
11:55or on your head in the bleachers.
11:58The Hawaiian team couldn't plot enough observations
12:01to accurately predict the object's far future path,
12:05but did confirm that this was nothing they'd previously seen.
12:09We reported it to the Minor Planet Center as a new object.
12:13The Minor Planet Center in Cambridge, Massachusetts,
12:16is a clearinghouse of data on the many objects besides the eight major planets,
12:21from Mercury to Neptune, that orbit our sun.
12:24Astronomers send their observations to the Minor Planet Center
12:28to be filtered through the center's vast database of previous observations.
12:33Most of what we do at the Minor Planet Center
12:37is just process the observations of minor planets.
12:40So people all over the world observe asteroids.
12:43They send the positional measurements to the Minor Planet Center.
12:46We put them together with either being known asteroids
12:50or new objects,
12:52and anything that might be a near-Earth asteroid,
12:54we pay a special attention to.
12:56The Minor Planet Center initially named the object
13:00the team in Tucson had found, 2004 MN4,
13:04based on the year and month it was first discovered.
13:07Six months later, on December 18th, the object turned up again.
13:13Once we were able to link those two data sets together,
13:16we announced the object as a new near-Earth asteroid,
13:19and once the folks at JPL got a hold of it
13:23and did the impact calculations,
13:25that's when things got really interesting.
13:27Interesting because it behaved like no other near-Earth object
13:31in the center's 60-year history.
13:332004 MN4 seemed to be headed for a collision with Earth.
13:47Winter 2004.
13:49Astronomers were watching an unusual stadium-sized asteroid
13:53labeled 2004 MN4 in near-Earth orbit.
13:57In order to understand what made it so unusual,
14:00it helps to know what asteroids are
14:03and where they came from.
14:06And that means going way back in time
14:09to when the solar system first formed.
14:12Four to five billion years ago,
14:15the solar system slowly condensed
14:17out of the nebula that surrounded the sun,
14:19and this was a nebula of gas and other elements,
14:23dust particles.
14:25Due to the force of gravity,
14:31this nebula condensed into the planets.
14:34Most of it actually condensed into Jupiter.
14:38So the planets condensed slowly out of the solar nebula,
14:42and then the little pieces left over
14:44became the comets and the asteroids,
14:47and all of the little fragments of rubble
14:49in the solar system.
14:50These little pieces consist of two types of objects,
14:58asteroids and comets.
15:01Asteroids tend to be rockier and metallic,
15:05and comets tend to have a mixture of rocks and ices.
15:09There's kind of a blend between the two.
15:12Earth deals with asteroids more often than comets.
15:16These are rocks from outer space,
15:19probably from the asteroid belt.
15:21The asteroid belt is this debris that exists
15:24between the orbit of Mars and Jupiter.
15:27We forget that Jupiter is the 800-pound gorilla.
15:30It outweighs all the other planets put together.
15:33Its gravitational field is so great
15:35that a planet wandering too close to Jupiter
15:38would be ripped apart.
15:40That's why we see this trail of debris.
15:44Over eons, objects in the asteroid belt have collided,
15:49producing debris of various sizes,
15:52much like a hammer pounding rocks.
15:54The solar system has had four billion years
15:58of further processing, during which the main bodies
16:04of the asteroid belt have been colliding with each other.
16:07The hammer blows represent the collisions,
16:09and you can see what happens.
16:11You end up with a few large chunks,
16:15several intermediate-sized chunks,
16:17but a whole lot of very small ones.
16:21This is called a size frequency distribution.
16:25So, in the case of the near-Earth objects
16:29that have potential for collision,
16:31you can see why the smaller ones hit more frequently.
16:35There's simply more of them.
16:37As asteroids collide,
16:39they occasionally fall into the paths of the planets.
16:42Over billions of years,
16:45the planets have absorbed most of this material through collisions,
16:49but there is still plenty of debris left.
16:52Even relatively small objects can make big impacts
16:57when they hit the Earth.
16:58But some of these asteroids are bigger.
17:12The largest is about 20 miles long.
17:15If an object 20 miles across were to slam into the Earth,
17:21it would probably destroy all life as we know it.
17:26Luckily, right now,
17:28there doesn't seem to be any Earth killer on the horizon.
17:31But a cosmic collision here on Earth isn't always bad.
17:36No one knows how or when life first formed on Earth,
17:42roughly 3.8 billion years ago.
17:44But cosmic collisions very likely could have played a role.
17:48The precursors of life are called amino acids.
17:52Amino acids eventually create proteins,
17:55and that's what we are made of.
17:57In fact, if you crack open a meteor,
18:00oftentimes you'll find evidence of amino acids
18:02in the center of a meteorite.
18:04And that's why some people believe
18:06that perhaps the very chemicals of life
18:08came from outer space.
18:10Ever since life was established on Earth,
18:14large-scale impacts can have devastating effect,
18:17possibly creating mass extinctions.
18:28Was 2004 MN4 on the same path?
18:33When you combine the December observations
18:35with the June observations,
18:37that is when the orbit became known well enough
18:39to determine, hey, this thing's going to come close
18:42to the Earth in the year 2029.
18:45Exactly how close?
18:47Scary close.
18:49On December 25th,
18:51NASA raised the probability of an impact
18:53to 1 in 45.
18:56As additional observations came in,
18:58that impact probability peaked
19:00at about 1 in 30.
19:02Now, 1 in 30 will get most people's attention.
19:05Only it didn't.
19:07Nobody seemed to pay any attention
19:09to the discovery of the near-Earth object
19:12with the highest probability of impact
19:14ever recorded.
19:17Instead, the eyes of the world
19:19were focused on another global disaster.
19:24This one not theoretical, but very real.
19:35Our planet is under attack.
19:37Luckily, almost all of the extraterrestrial material
19:40that hits the Earth is smaller than a grain of sand.
19:44Space dust formed from the jostling impacts
19:47of rocky asteroids or clouds of debris
19:50left in the wake of a comet that produce a meteor shower.
19:54Meteor showers are one of the most spectacular events
19:58in the night sky.
19:59And the reason is because the Earth actually goes
20:02through the tail of a comet.
20:04A comet is like a litter bug.
20:06It has a tremendously long tail containing debris
20:10of ice crystals and rock and dust.
20:13And as the Earth goes through this trail of debris,
20:17we see literally hundreds, hundreds of shooting stars
20:20throughout the night sky.
20:22In 2004, David Tholen, asteroid 2004 MN4 co-discoverer,
20:32was given the honor of naming it.
20:34He named it Apophis,
20:36the Greek name of an Egyptian god Apep, the Destroyer.
20:41If Apophis hits the Earth,
20:43it will live up to its namesake.
20:45Estimated at approximately a quarter mile across,
20:48and almost certainly a stony object,
20:51its impact would be huge.
20:54Apophis might damage the entire ozone layer
20:58of the Earth's atmosphere around the globe.
21:04It could also spew super-hot, poisonous gases into the air.
21:09It might be like Pinatubo
21:12or some of the other volcanoes that have gone off recently.
21:15NASA's Christmas Day 2004 press release,
21:18announcing the likelihood of an impact with Apophis,
21:21was historic.
21:22But the story was buried by news of the earthquake
21:25and tsunami that rocked Asia the same day.
21:34That's when the big tsunami had occurred in the Indian Ocean.
21:39So that was the big news item of the day, not Apophis.
21:42Despite its obscurity,
21:45Apophis became the highest-rated near-Earth object
21:48on the Torino scale,
21:49a system for rating the danger of asteroid impacts.
21:53The Torino scale basically gives you a sound bite, if you will,
21:57of whether or not it merits any public attention.
22:01Zero and one on the scale are pretty much routine discoveries
22:04that there may be some chance of hitting the Earth.
22:09If you get up into the high end of the scale,
22:12eight, nine, or ten, it means you've discovered an object
22:14that you are certain is going to hit the Earth.
22:17So eight, nine, and ten are really bad.
22:19Achieving a four out of ten on the Torino scale
22:22indicated a significant impact danger.
22:25How concerned would the general public really be
22:28about the threat of an asteroid?
22:30Nobody alive today has any direct experience
22:33of one colliding with Earth.
22:36But Earth has lots of experience.
22:40In the old days, geologists would say,
22:42ha, bah, humbug.
22:44There's no way that, hey, a rock from space
22:47could cause destruction on the Earth.
22:49This was called the giggle factor.
22:51Geologists would giggle any time an astronomer would say,
22:55but look in the heavens.
22:57Look at the moon.
22:58Look at all the debris we see in outer space.
23:02Geologists in particular were used to events
23:05taking place over millions of years,
23:07not fractions of a second.
23:09But in the mid-20th century,
23:11a renegade group of scientists,
23:13including geologist Eugene Shoemaker,
23:16believed Earth had been pummeled by asteroid impacts
23:19in the past.
23:20And there was every reason to think
23:22there would be more in the future.
23:28Shoemaker's evidence at the time
23:30was the cratered surface of the moon.
23:32But when Gene got going,
23:34the canonical wisdom was that the craters were volcanic.
23:40He did some sketches and some photographs
23:43that convinced him that they were impact craters.
23:46The problem with this theory
23:48was that if the moon was covered with impact craters,
23:51then the Earth, four times that size,
23:54ought to be two.
23:56But apparently, it wasn't.
23:58The Earth is a very dynamic planet.
24:01Of course, we have an atmosphere and we have an ocean
24:04and then we have a plate tectonics.
24:06So, on the Earth, impact craters get eroded very quickly.
24:11It's very hard to see them.
24:13The Earth didn't have plate tectonics
24:15and didn't have an atmosphere, didn't have erosion.
24:18The Earth would look like that.
24:19It would have craters all over.
24:21It's a dangerous solar system out there.
24:24There's lots of stuff coming at us.
24:27Shoemaker understood this,
24:29but he needed to find a fresh impact crater to prove it.
24:32A 50,000-year-old scar in Arizona proved fresh enough.
24:39Gene and Carolyn were driving around
24:41and they approached Meteor Crater at the last minute,
24:46long after it had closed for the day.
24:48They climbed up the back wall
24:50and they got to the edge and they looked over
24:53and Gene said, this is an impact feature.
24:56Investigated by the U.S. Geological Survey in 1891,
25:01Beringer Crater, as the crater was later called, was a mystery.
25:05While some did believe it was an impact crater,
25:08the leading theory was exploding underground steam vents.
25:11Shoemaker said no.
25:13He proved it was created by a rock from the sky.
25:17We're walking along the rim of Meteor Crater
25:22near Winslow, Arizona.
25:24This huge excavation was caused by the impact
25:29of an iron nickel meteorite.
25:32If we were standing here about 50,000 years ago,
25:36just prior to this impact,
25:38suddenly up in the sky you would have seen
25:40this bright point of light,
25:42but you wouldn't have had time to react to that
25:45or even look at it for very long.
25:47Certainly no time to run away from it.
25:49Within four seconds, it would be here.
25:52It would have hit the ground at a tremendous velocity
25:55and generated this incredible explosion.
26:00Anything in the immediate vicinity of the impact door,
26:04the projectile, would have been heat-melted
26:07or even vaporized in the impact.
26:10At increasing distances from that point of impact,
26:14we have hurricane-force winds that start as much as 2,000 miles an hour
26:21and peter out up to a distance of some 20 miles or so.
26:26Anything standing on the rim where I am
26:28would not have survived this event.
26:31Today, the evidence is still plain to see
26:34that Meteor Crater is an impact crater.
26:38There are several layers of rock that you can see
26:40in the walls of the crater here,
26:42and the fact that they're all dipping away
26:45from the center of the structure
26:48is indicative of an impact feature.
26:51The 100-foot diameter iron-nickel body,
26:55as it touched the ground,
26:57would probably have continued on penetrating the ground
27:01to something like its total diameter
27:05before all that kinetic energy was transformed into a shockwave.
27:10And as the shockwave decayed,
27:13it accelerated the rock mass down and out and then up and out.
27:20A young crater should have a raised rim,
27:25and this rim is made up of beds that have been overturned.
27:31Gene Shoemaker's theory that asteroids and other minor planets
27:37plowed into Earth regularly was a tough sell for many scientists,
27:41even with the geologic evidence Shoemaker was accumulating.
27:45I really believe that it was the beginning of the space program
27:50that showed craters on the surface of Mars,
27:54showed craters on the surface of Mercury.
27:56You can't evade the idea that it's a dangerous place
28:01and there's things flying around the entire solar system
28:03and the Earth would not be left out.
28:06I think that that caused the revolution,
28:09that there's no disagreement among living geologists now
28:13that these are meteor craters.
28:15But NASA's space program provided another piece of evidence
28:20to bolster Shoemaker's theory,
28:22a unique perspective for looking at planet Earth.
28:27Earth is a hard place to find craters
28:29because you have erosion processes,
28:32you have vegetation that will hide them,
28:34and so it's taken a lot of sleuthing by geologists
28:38with satellite images to try to identify crater features.
28:43With the invention of satellites,
28:45it was soon revealed that the Earth actually is covered in craters.
28:49Despite erosion, vegetation, and plate tectonics,
28:53Earth's impact scars were hiding in plain sight.
28:57Now that scientists were able to identify suspected craters,
29:01they needed to confirm the impacts by closely examining the nearby rocks
29:06for evidence of violent collision.
29:10Just because you see a hole in the ground that's round
29:12doesn't mean it's a crater.
29:14Scientists have required that when you go and you find something
29:17that might be a crater, you must find the geologic evidence
29:20that it is actually due to an impact.
29:22And the key evidence of that is the shocked minerals,
29:26evidence that the feature was formed by a sudden instantaneous pressure
29:31and heat wave that changes the mineral structures of the surrounding rocks.
29:35One concealed impact crater in particular
29:38held the clue to what killed the dinosaurs 65 million years ago.
29:43If you dig down into the Earth, down to what is called the K-T boundary,
29:48you find this very thin layer of soil which represents a boundary
29:53between the dinosaur era and the post-dinosaur era,
29:57between the Cretaceous and the tertiary era.
30:00And if you look at that debris,
30:02you find evidence of an extraterrestrial impact.
30:06In particular, you find evidence of the element iridium.
30:09Iridium is quite rare on the Earth,
30:12but it's rather common in outer space.
30:15And bingo!
30:16When you analyze these soil samples,
30:19you find large quantities of iridium.
30:22In other words, something from space brought lots of iridium
30:26to Earth's surface right at the moment when the dinosaurs went extinct.
30:32In the 1970s, the father and son team of scientists
30:36Luis and Walter Alvarez proposed that a massive asteroid hit the Earth,
30:41causing global devastation, plummeting temperatures,
30:45and a massive extinction event.
30:53The problem with the theory was that nobody could find a crater old enough
30:57and large enough to connect to such a literally earth-shaking event.
31:02But engineers working for Mexican oil company Pemex
31:05discovered something strange in the Gulf of Mexico.
31:09They found an arc, a gigantic arc that cut through the Yucatan.
31:14Maybe this arc was part of a circle.
31:17Who put it all together was Alan Hildebrand.
31:20He then put two and two together and said,
31:22well, maybe it was in fact a gigantic meteor or comet
31:27that hit Mexico 65 million years ago.
31:30Maybe we can use iridium as a tell-tale sign to prove it
31:34and to nail the theory to the wall.
31:37The wide acceptance of the Chicxulub impact by the 1990s
31:41was evidence that at least occasionally,
31:43sudden unforeseen events in Earth's history
31:46can have catastrophic effects.
31:49The Earth could be changed overnight, even in an instant.
31:53Earth got a reminder of this fact when in 1994
31:57it was witness to one of the solar system's most spectacular impacts.
32:02A comet discovered by the shoemakers
32:05and amateur astronomer David Levy
32:08plowed into the planet Jupiter with spectacular results.
32:12As the world watched through high-powered telescopes,
32:17the largest impact ever witnessed in the modern era played out in real time.
32:22No one believes anything until they see it sometimes.
32:27And so for us, particularly for Gene,
32:31the culmination of our work was in this comet striking Jupiter.
32:37It was the living proof, if you will,
32:40that comets or other objects do strike planets.
32:46When the Shoemaker-Levy comet barreled into Jupiter,
32:49it forced the world to confront the very real possibility
32:52that life might be exterminated from a looming object headed our way.
32:58This was enough for the U.S. Congress to take action.
33:02Using land-based and orbiting telescopes,
33:05astronomers, both professional and amateur,
33:08began searching the skies for an Earth-killer.
33:11The question was, what could they do if they found one?
33:15In the wake of the Shoemaker-Levy multiple impact on Jupiter in 1994,
33:28Congress funded research for a space guard program
33:32to search for other objects that might be out in space,
33:35determine whether any of them might devastate the Earth,
33:39and to come up with options for how to deter such an event.
33:43NASA got involved, and there were a number of meetings at that time
33:48where we discussed what we wanted to do by way of searches.
33:53Due to funding issues, the space guard program never really materialized
33:57at the scale Congress originally anticipated.
34:00But the scientific community began searching the skies anyway
34:04for objects that might threaten Earth.
34:06Scientists were able to implement programs
34:09for finding and tracking asteroids beginning in the mid-1990s.
34:15For instance, the U.S. Air Force
34:17and MIT's Lincoln Near-Earth Asteroid Research, or Linear Survey,
34:22use satellite tracking technology to search for near-Earth objects.
34:27Right now, a linear survey has probably discovered the most asteroids in.
34:33The question is, is there anything out there that has a chance of hitting us
34:39in a timescale that we care about, the timescale of our lives,
34:42the timescale of our children's or our grandchildren's lives?
34:45And so that's the main question we want to know.
34:48Could Apophis be that object?
34:52Throughout the early days of 2005, astronomers gathered data on Apophis.
34:58Apophis orbits the sun once every 323 days, crossing Earth's path twice.
35:05During these close encounters, the light Apophis reflects
35:09reveals clues about its makeup.
35:11When we discover an object and we like to know what its composition is,
35:14we can do that by looking at its color and breaking up that color
35:19into its spectral fingerprints and then matching those spectral fingerprints
35:23with the spectral fingerprints that we measure from meteorites in the laboratory.
35:27And in this way, it gives us the best idea of what it is that object is made of
35:32and how we might deal with it if it proved to be an actual threat.
35:36It appears that Apophis is made out of a common kind of meteoritic material
35:41that we have samples of in our museums and laboratories
35:45and has about the strength of ordinary rock.
35:48But Apophis is no ordinary rock.
35:51NASA estimates the energy release of an impact with Apophis
35:55to be roughly 880 megatons of TNT,
36:00or roughly 65,500 times more energy
36:04than that released by the Hiroshima nuclear bomb Little Boy.
36:08For comparison, Meteor Crater in Arizona was formed by an object
36:13that released between 10 and 20 megatons.
36:17The risk will shift around depending on the size of the object.
36:22At this size range, it's a terrestrial impact that one ought to be concerned about
36:28because this has potential for destroying a city-sized area.
36:32Even small Earth impactors can be incredibly violent.
36:39A strange and mysterious event in Siberian Russia a century ago
36:43may provide a clue to the sort of destruction possible
36:46when a fairly small near-Earth object collides with the Earth.
36:51In June of 1908, there was in a very remote part of Siberia
36:57a massive explosion in the sky.
37:00People a long way away from this thing saw a huge flash of light,
37:06more brilliant than the sun coming in, and it exploded
37:10and frightened people, threw its debris into the atmosphere
37:16and caused bright nights for many days and weeks.
37:21The Russian Revolution intervened before any scientist could venture
37:25to the remote impact location along the Tunguska River.
37:29It was nearly 20 years before anyone saw what happened.
37:34But when they did, it was spectacular.
37:38The Tunguska event devastated a forested area covering more than 2,150 square kilometers.
37:46It remains the largest impact event on Earth in recent history.
37:51But here is where things get strange.
37:54There is no impact crater at Tunguska.
37:58It's a complex set of things that happens to an object
38:02during the couple of seconds when it goes from outer space
38:05to hitting the dense part of the atmosphere.
38:08It starts to break up, but into sort of a pancake-shaped object.
38:12And this, of course, has more area and hits still more atmosphere.
38:16And it generates a shockwave, and the entire atmosphere
38:20above and below this flying object is badly disturbed.
38:25The friction is so great as the object enters the atmosphere,
38:29it creates a superheated plasma-like material that can glow in the sky for days and weeks.
38:35Scientists have long suspected, and computer modeling has confirmed,
38:40that the downward thrust of the shockwave through the atmosphere is so strong
38:45that a relatively small object, even one that breaks up high above the Earth,
38:50can do great damage at ground level.
38:53So it's possible that the object that made Tunguska is not really as big as we thought it was.
38:58Maybe it's only 30 meters across, or 40 meters across.
39:02And if that's the case, that a small object can do as much damage as happened in Tunguska,
39:07the Earth is struck by small objects much more frequently.
39:10So actually, it means that it's a more dangerous environment than we thought.
39:20Apophis will still call on Earth on Friday, April 13, 2029,
39:25for a very close encounter, closer than some of our geosynchronous satellites.
39:30Yes, it's going to get close enough to become a naked eye object,
39:34and it should get about as bright as one of the stars in the handle of the Big Dipper.
39:42And when Apophis comes this close to Earth,
39:45our planet's gravitational pull will tug on it,
39:48pulling it slightly out of its current orbit.
39:51No one's exactly sure how much this will affect Apophis,
39:54but it could be enough to pull the asteroid through a keyhole in space.
39:59The use of the word keyhole is a rather unfortunate term.
40:04And speaking of it as a keyhole,
40:07whenever I think of a keyhole, I also think of a door.
40:11There's a whole sequence of doors, if you like.
40:14So it's much more of a tube that the object has to go through,
40:19rather than a keyhole in a door.
40:23This tube is not much bigger than Apophis itself.
40:27The chances of it lining up with this tube and bending its orbit are low,
40:32about one in 45,000.
40:34But if it does, Apophis will be back, headed straight for Earth,
40:39possibly seven years later in 2036.
40:42So it's a precision of the bending of the orbit that brings it back an exact number of years in the future,
40:49so that it's there when the Earth is there on April 13th of whatever year.
40:54Having Apophis and the Earth in the same place is extremely bad for Apophis,
41:02and pretty bad for the fragile surface of our planet.
41:07A disaster of such scale would have far-reaching implications.
41:13Are we able to do anything about a massive space rock headed our way?
41:17What we could do if there was an asteroid heading our way is going to depend a lot on how much time we have.
41:24If we had many decades of warning time, which is not at all unlikely,
41:30there are a whole lot of different ways that we could move the asteroid.
41:34A very powerful rocket, perhaps an ion engine, might slowly nudge it out of the way.
41:45Or if you need something with a little bit less time, there's a lot of talk that perhaps a nuclear explosive
41:51may be something that doesn't actually hit the rock.
41:55But if you could do maybe a standoff nuclear explosion that would just have a little blast
42:00and just push it just a little ways you could deflect it, just enough to make it not touch the Earth.
42:08Scientists are less enthusiastic about planting a nuclear bomb directly on an asteroid.
42:15The problem being that a nuclear explosion might turn one giant problem
42:20into thousands of smaller but still pretty big problems.
42:24But there are even more options.
42:26Another example is a mirror that would focus the sunlight down on the surface and melt the rocks
42:31which would then stream particles away and push the asteroid in the other direction.
42:37There's been a suggestion of painting one side of the asteroid a color
42:41and then the various forces of sunlight might produce enough slight movement of an asteroid
42:47so it would miss the Earth.
42:49The problem with all of these proposals is that they require decades of lead time,
42:55lots of money, and there's no guarantee of success.
42:59But none of these options will be exercised.
43:02None of these plans executed until the public recognizes the threat involved.
43:07If we wanted to talk about objects the size of the meteor crater impactor,
43:1430 to 50 meters in diameter, we could be talking about as many as 1 to 10 million objects
43:21currently crossing the Earth's orbit.
43:23Anywhere from 1 to 10 million objects the size of the meteor crater impactor,
43:2810% of them solid iron, all crossing Earth's path and virtually none of them accounted for.
43:37Any one of them at any time could be headed for Collision Earth.