13. Colliding worlds

A comet hits Jupiter

Discovery of Comet Shoemaker-Levy 9

Eugene Shoemaker, his wife Carolyn, and the amateur astronomer, David Levy, were involved in routine observations the evening of 23 March 1993, a dark and cloudy night with stormy weather on its way. They were continuing a ten-year search for comets and asteroids that might be headed toward the Earth using the small 0.46-meter (18-inch) wide-field photographic telescope at Palomar Observatory in California. Good and expensive film couldnít be wasted during the poor weather conditions, so some fogged film, which had been partially exposed to light, was used to photograph a clear place in the night sky near Jupiter before the clouds covered it up. Two days later, when Carolyn Shoemaker examined the images taken on the flawed film, she saw an elongated feature that looked to her like a ďsquashed cometĒ. When the discovery was confirmed with better telescopes, the stretched-out blur of comet light was resolved into several objects aligned along a single straight line projected in the sky, like pearls on a string. In accordance with tradition, it was named Comet Shoemaker-Levy 9, after the discoverers, ninth in a series of objects the trio found traveling around the Sun in short-period orbits.

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When a comet nears the Sun, the ices in the comet nucleus turn directly from solid to gas and release dust to form a round, fuzzy coma, sometimes accompanied by a tail that points away from the Sun. But instead of a single coma and tail, powerful telescopes revealed a train of baby comets, each with its own invisible nucleus, nearly spherical coma and elongated dust tail.

Breakup and Collision

Comet Shoemaker-Levy 9, abbreviated SL9, consisted of the pieces of a former, single comet that had been trapped in a two-year orbit around Jupiter for decades. But when it traveled too close to Jupiter in 1992, the comet was shredded apart and launched on a trip to oblivion. It passed within about 20 million meters of Jupiterís cloud tops, and 90 million meters from the planetís center. So there was a modest difference between the planetís gravitational attraction on the near and far side of the comet, enough to rip the fragile comet into at least 20 observable pieces. Most of these fragments remained visible over the entire subsequent lifetime of the comet.

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What made SL9 unique was that the broken comet was inexorably hurtling along a path to total destruction, doomed to collide with Jupiter. Orbital calculations indicated that the train of comet fragments would plunge into the giant planet in July 1994, two years after the former single cometís disruption and more than one year after the discovery of its pieces.

Impact

The collision of Comet Shoemaker-Levy 9 with Jupiter in July 1994 was perhaps the most widely witnessed event in astronomical history. Practically every telescope in the world was trained on Jupiter during impact week, between 16 and 22 July 1994. Infrared heat detectors were placed at the focal point of the Keck Observatoryís giant 10-meter telescope atop Mauna Kea in Hawaii, and the Hubble Space Telescope was poised to record the event at visual wavelengths. Every other major astronomical observatory participated, as did numerous amateur astronomers from their own backyard.

Detailed calculations indicated that the collisions would be on the dark ďbackĒ side of Jupiter, hidden from the Earthís view by the body of the giant planet. So it might be something like watching a World Series ball game from a seat behind a stadium post. The comet fragments would nevertheless strike Jupiter close to the side facing Earth, so astronomers hoped that something would be seen when the planetís rapid rotation, of once every 9 hours 55.5 minutes, brought the impact sites into view. Moreover, the Galileo spacecraft, on its way to Jupiter, had a direct view of the actual collisions from its unique position in space.

Instruments aboard the Galileo spacecraft measured temperatures that soared to 10 or 20 thousand degrees kelvin when the fragments plunged into the clouds of Jupiter. Thatís at least twice as hot as the Sunís visible disk, at 5.28 thousand degrees kelvin. Rising plumes of hot gas were hurled three thousand meters above Jupiterís clouds, each generating a bright flash of infrared light.

What incredible luck, to have a comet break into pieces, hit Jupiter, and generate brilliant bursts of infrared light so close to edge of the planetís backside that they could be seen from on or near our planet. And the good fortune didnít end there, for the arching plumes of hot gas produced great dark scars when they cascaded back down into the giant planet.

Fallout Fig. .. 

The comet fragments dove into Jupiter, one after another, like the cars of a train when its locomotive is derailed. After generating a bright ball of light, each fragment disfigured Jupiter with a black scar that had never been seen before, twice as large as the Earth and spanning tens of millions of meters. Meanwhile waves swept across the impact site and reverberated deep within the planet, which seemed to shudder from the impacts.

Consumed by the Sun

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Some comets plunge deep into the Sunís thin, million-degree outer atmosphere, or corona. Instruments aboard the Solar and Heliospheric Observatory satellite, abbreviated SOHO, have recorded their death-defying trip around the Sun. One of its instruments uses an occulting disk to block out the bright light of the visible solar disk, enabling it to detect the comets as they move through the inner corona. A comet often pays a heavy price for this trip, sometimes breaking apart because of the Sunís forces.

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Other comets are hurtling toward complete meltdown, passing so close to the Sun that the encounter is fatal. Though rarely, if ever, hitting the visible solar disk, or photosphere, these comets can come closer than 50 million meters from it. They are unlikely to survive the Sunís intense heat and gravitational forces at that range. Amateur astronomers from all over the world have examined SOHOís real-time images posted on the Internet, discovering hundreds of previously unknown comets on their death-dive into the Sun.

Most of the comets discovered by SOHO, about 90 percent of them, are small cometary fragments known as the Kreutz sungrazers, which closely approach the Sun from one direction in space. They are named after the German astronomer Heinrich Kreutz (1854-1907) who found that many of the comets, which had come closest to the Sun in the 19th century, seemed to have a common origin with similar orbits. It turned out that they are all fragments of a single large comet that first broke up when passing very close to the Sun thousands of years ago. The original fragmentation may have been witnessed in 321 BC by the Greek philosopher Aristotle (384-322 BC) and by the Greek historian Ephorus (405-330 BC), but it may have occurred much later. The break up gave rise to two main comets, perhaps with orbital period of 350 years and about 700 years, and the two parts were split into more pieces during return visits to the Sun.

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When a member of the Kreutz sungrazer group moves around its orbit and returns to our vicinity, it can dive into the inner corona and disappear forever. Spectroscopic observations from SOHO indicate that each comet fragment can be very small, just 6 to 12 meters across, despite their spectacular display. Such a tiny object, falling so close to the Sun, would vaporize completely away, like the proverbial snowball in hell.

In its six years of service, SOHO has spotted more than four hundred comets, making it by far the most prolific comet finder in the history of astronomy. Aside from the numerous Kreutz sungrazers, SOHO has found more than forty new comets, which is comparable to the number of comet discoveries during almost any decade throughout the previous two centuries. The other main impetus for recent comet discoveries has been the Lincoln Near Earth Asteroid Research, abbreviated LINEAR, Program, operated by MITís Lincoln Laboratory. It consists of a pair of telescopes dedicated to detecting and cataloging Near-Earth Objects that threaten Earth. LINEAR has completed millions of observations, finding more than seven hundred confirmed near-Earth asteroids or comets. One of them, dubbed Comet LINEAR and also known as C/1999 S4, has been caught breaking up on its way into the Sun, vanishing much further out than the Kreutz sungrazers.

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When discovered in September 1999, Comet LINEAR was exceptionally bright at a relatively large distance of about 4 AU, which has often happened to other comets during their first trip through the inner solar system. But then something unexpected happened. As it moved closer to the Sun, the comet broke apart into numerous parts. But the comet was as far as 0.8 AU from the Sun when it disintegrated.

Impacts with the Earth

The solid surfaces of almost all planets and satellites, from Mercury and the Moon to Jupiterís satellite Ganymede, are marked with impact craters; the one exception is Jupiterís moon Io whose volcanic outpourings of lava have erased all the craters from its surface. The impact craters are the scars of past collisions with cosmic objects speeding through space. The terrestrial planets originated by the coalescence of these objects, and one of them tore enough material out of our planet to forge the Moon. Large comets or asteroids might even have brought water to the young Earth. And the cosmic barrage continues today. A hail of cosmic objects is now pelting the Earth as it sweeps through space. Some of them are tiny, and burn up in the atmosphere. Every month at least one house-sized object is blowing up when it enters our air, producing a blast as forceful as a nuclear bomb. Now and then a bigger one gets through, gouging a crater out of the ground and even threatening the inhabitants of Earth.

Explosions in the atmosphere

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The largest object to strike the Earth in the 20th century wasnít quite big enough to reach the ground. It disintegrated between 5 thousand and 10 thousand meters up, over the Podkamennaya Tunguska River in central Siberia. The shock wave generated by the ensuing explosion leveled trees over 2 trillion (2 x 1012) square meters of the underlying land, an area larger than New York City and surrounding suburbs. The energy produced was equivalent to the aerial explosion of the nuclear bomb that leveled Hiroshima. So much devastation, yet it failed to produce a crater.

The terrestrial impact record

Even the largest craters, produced by the biggest comets or asteroids, will gradually disappear from sight with the passage of time. The same forces that erode mountains, deposit sediments, eject lava and shift continents are erasing the craters and removing them from sight. If not for these dynamic forces, the craters accumulated over the ages would be as densely distributed and prominent as the overlapping craters on the Moon.

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Only about 160 terrestrial impact craters have managed to survive the ravages of time. They have been identified on images taken from space, using airplanes, the Space Shuttle, or satellites such as Landsat. These craters can be first identified from aerial photographs, by their circular shapes and uplifted and overturned rims. But since other processes, such as volcanism and erosion, can also leave circular holes, confirming evidence of an impact origin must be gathered from rocks in and around the crater.

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How do geologists know that some terrestrial craters are due to the explosions of projectiles coming from space? They look for rocks that have been transformed under the conditions of extreme temperature, pressure, and shock associated with a high-velocity, external impact. The most apparent shock effect is the formation of conical structures called shatter cones, which point toward the center of the impact. Other evidence includes glassy, previously molten material formed at high temperature, and minerals with a deformed crystal structure produced by a shattering, high-pressure impact. Roughly ten percent of the craters also contain meteorites that had to come from space.

Demise of the dinosaurs and other mass extinction

Catastrophe from the sky

Collisions by objects from outer space have always been a menace to life on Earth. During the planetís first billion years, the barrage was probably so intense that living things could not exist on the Earthís surface. After those early times, the rate of bombardment slowed down, so impacts of exceptionally large cosmic projectiles became less frequent. But these giant impacts continued every once in a while, with devastating consequences. The most recent death rock arrived 65 million years ago, resulting in the wholesale removal of life on Earth. Such an abrupt destruction of an entire species of living things by a force of nature is known as a mass extinction.

A thin, worldwide layer of clay, just 0.01 meters thick, provided the initial evidence that an asteroid or comet collision wiped out the dinosaurs. The clay layer was deposited at the right time, and it contained unusual amounts of the rare element iridium that had to come from outer space.

Walter and Luis Alvarez, and their colleagues concluded in 1980 that the iridium deluge came from outside the Earth, delivered by a large asteroid or huge comet that struck the Earth and vaporized about 65 million years ago. According to their hypothesis, the iridium was lofted into the atmosphere along with other debris by the fireball of hot gas created during the collision, and then carried by the winds over much of the globe. The worldwide cloud of iridium-rich dust then slowly filtered back down to the ground where it produced a thin global layer that contained relatively large amounts of iridium. They estimated that a layer 0.01 meters thick covering the entire Earth would be deposited by an asteroid about 10 thousand meters in diameter.

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After years of searching, the telltale crater was found straddling the northern coastline of the YucatŠn Peninsula. It is located below the Mayan village of Chicxulub (pronounced Cheek-shoe-lube, a Maya phase for ""horns of the devil""), and is hence known as the Chicxulub impact basin. The discovery of this crater and the subsequent confirmation of its age at 65 million years led most scientists to accept the impact hypothesis for the demise of the dinosaurs.

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High-resolution gravity maps in the 1990s revealed the size and structure of the buried Chicxulub crater. Regions of high density and greater gravitational pull are distributed in several concentric rings, with an outermost diameter of at least 180 thousand meters and perhaps as much as 250 thousand to 280 thousand meters. Its size and bulls-eye pattern is similar to the largest impact basins on the Moon and Mercury, created by cosmic collisions during the early days of the solar system about four billion years ago.

Probability of death from above

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The Earth does not occupy a secure niche in space. Our planet is instead immersed in a cosmic shooting gallery, subject to a steady bombardment by lethal, Earth-approaching asteroids and comets. Somewhere in space, one of them is hurtling toward a future collision with Earth. And if it is large enough, the impact will severely disrupt terrestrial life upon impact. Itís only a matter of time.

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Thus, to estimate the risk of being hit in a way that matters, the potential impacting projectiles first have to be sorted according to size. Fragments smaller than a few tens of meters across burn up in the atmosphere and rarely reach the ground Ė except for the exceedingly tiny particles of cosmic dust that drift down into your hair. Asteroids or comets a hundred meters in diameter are expected to strike Earth every thousand years on average. They could take out a city and cause severe local damage, but pose no threat to the Earth as a whole.

Breaking a date with doomsday

Finding the hidden threat

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While we know that asteroids and comets have collided with the Earth in the past, and that they will inevitably hit our planet in the future, we do not yet know if any them are now headed for a deathly collision with our solitary outpost of life. Astronomers are therefore taking a census of everything out there that is big enough and close enough to threaten us. Once all of these near-Earth objects are located, and their current trajectories known, astronomers can use computers and refined observations to determine their precise future paths and establish whether and when any of them will strike the Earth.

Doing something about the threat

Sooner or later we will discover an asteroid or comet headed toward collision with the Earth. But unlike most natural disasters, the impact can be forestalled once the killer object has been fingered and we can see it coming. Evasive action will depend on the lead-time available and the expected physical nature of the object, including whether the object is a binary system.

And what do we do if we find a large asteroid or comet headed our way? The Earth cannot be moved out of the way, but we could launch an intercept mission to redirect the objectís course. If the impact is many years away and the threatening object relatively far away from us, all you have to do is give it a little nudge. By the time the asteroid or comet reaches the Earthís vicinity, that small change in trajectory will make a big difference, enabling it to bypass the planet.

If the warning time is only a matter of months or less, the sole recourse might be to send a high-powered rocket armed with a bomb powerful enough to redirect the object or blow it up. Such a possibility has sparked the interest of bomb designers and some members of the military. A conventional nuclear weapon might be used to deflect or destroy a small, solid, rocky asteroid, but a much larger explosion could be needed to divert or pulverize a loosely bound object, like some large asteroids and most comets.

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(Summary Diagram)