13. Colliding worlds


Fragments of Comet Shoemaker-Levy 9

Fragments of Comet Shoemaker-Levy 9

. In July 1992, this comet passed so close to Jupiter that the icy material of its nucleus was torn apart by the differential gravitational forces of the giant planet. This panoramic image of the comet fragments was taken from the Hubble Space Telescope in January 1994, eight months after they were discovered and six months before they dived into the atmosphere of Jupiter. The length of the string of comet pieces is about 1.1 billion meters, three times the distance from the Earth to the Moon. The largest of the fragments in the string is about two kilometers across. Each fragment mimics a larger comet with a round coma and a dusty tail. (Courtesy of NASA and the Space Telescope Science Institute.)


Final orbit of Comet Shoemaker-Levy 9

Final orbit of Comet Shoemaker-Levy 9

. This comet was orbiting Jupiter for more than half a century, until it was ripped apart during a close encounter with the planet and collided with it two years later. The disruption occurred on 7 July 1992 when the comet passed within 0.0006 AU, or 90 thousand kilometers from the planetís center. Since Jupiter has a radius of just over 70thousand kilometers, the comet passed within about 20 million meters of the planetís cloud tops. Jupiterís unequal gravitational pull on the near and far sides of the comet nucleus then tore the object apart. Carolyn and Eugene Shoemaker and David Levy discovered the comet fragments on 23 March 1993, when the broken comet was almost at its farthest distance from Jupiter, at 0.31 AU. One by one the icy fragments exploded in Jupiterís cloud tops during impact week, from 16 to 22 July 1994.


Dark impact scars

Dark impact scars

. This Hubble Space Telescope image shows several dark spots (right) on Jupiter, each marking the impact site of a fragment of Comet Shoemaker-Levy 9. The Earth-sized scars remained visible for about five months, until the winds in Jupiterís outer atmosphere pulled them apart. A thin expanding ring of dark material, suggesting waves spreading out from the impact explosion, surrounded some of the dark central spots. Jupiterís Great Red Spot is also prominent, as it has been for centuries. (Courtesy of NASA and the Space Telescope Science Institute.)


Sunbathing comet

Sunbathing comet

. The short-period comet 96/P Machholz passed through the Sunís million-degree atmosphere, or corona, in January 2002, when it approached within about 18 billion meters of the Sun, or about 0.12 AU where one AU is the average Earth-Sun distance. The cometís coma and foreshortened tail can be seen above the occulting disk of the SOHO instrument that recorded this image. In a spectacular coincidence, a giant magnetic bubble, known as a coronal mass ejection, was explosively hurled from the Sun at about the same time (left). A CME tosses billions of tons of coronal gas into space, expanding to become much larger than the Sun. The white circle denotes the outer edge of the visible solar disk. Venus lies just outside the occulting disk (below right). The planet is so bright that its image is marred by a horizontal stripe. (Courtesy of the SOHO LASCO consortium. SOHO is a project of international collaboration between ESA and NASA.)


Fatal impact

Fatal impact

. This composite image records a comet plunging into the Sun on 23 December 1996. The innermost image (center) records the bottom of the million-degree solar atmosphere, known as the corona. The electrically charged coronal gas is seen blowing away from the Sun just outside the inner dark circle, which marks the edge of one instrumental occulting disk. Another instrument records the comet (lower left), as well as the coronal streamers at more distant regions and the stars of the Milky Way. (Courtesy of the SOHO EIT, UVCS and LASCO consortia. SOHO is a project of international collaboration between ESA and NASA.)


Death of a sungrazer

Death of a sungrazer

. This SOHO image shows a bright sungrazing comet (bottom center) headed into the inner atmosphere of the Sun on 22 October 2001. The partial white circle marks the outer edge of the visible solar disk, whose intense glare is hidden by the instrumentís occulting disk (opaque circular region at top center). The million-degree solar atmosphere can also be seen, steaming away from the Sun. (Courtesy of the SOHO LASCO consortium. SOHO is a project of international collaboration between ESA and NASA.)


Comet LINEAR falls apart

Comet LINEAR falls apart

. A composite of two Hubble Space Telescope, or HST, images (top panel), taken on 5 August 2000, shows the bright dust tail of Comet C/1999 S4 (LINEAR) subtending a region nearly 58 million meters across. The tail was created primarily during the complete destruction of part of the cometís nucleus about two weeks earlier. The rest of the nucleus broke up into fragments that are seen clustered near the tip of the tail (right). The European Southern Observatoryís Very Large Telescope image (bottom panel), taken about 35 hours after the HST image, was processed using unsharp masking which suppressed light from the tail and enhanced the visibility of the fragments. This image is about 103 million meters across at the comet, and the nearly vertical streaks are tracks of stars passing through the field near the comet. Since the Earthís orbit will not intersect the cometís orbit, the comet debris will not create future meteor showers on Earth. (Courtesy of Harold Weaver, the HST/VLT observing team, NASA/ESA, ESO, and the Space Telescope Science Institute.)


The Tunguska explosion

The Tunguska explosion

. On 30 June 1908, a giant blue-white ball of fire streaked across the daytime sky above the Tunguska River in Siberia, apparently becoming brighter than the Sun. The Tunguska fireball then exploded in the atmosphere, felling underlying trees like matchsticks. All of the toppled trees point away from a central location that must have been directly below the point of the explosion. Interpreting the extent and orientation of the tree-fall pattern, scientists concluded that the explosion above Tunguska released an energy equivalent to exploding a nuclear bomb between 5 thousand and 10 thousand meters above the Earthís surface.


Wolf Creek impact crater

Wolf Creek impact crater

. This relatively well-preserved crater near Wolf Creek, Australia, is partly buried under windblown sand. Iron meteorites have been found in the vicinity, as well as some impact glass. The rim diameter of this crater is 850 meters and the impact that created it occurred about 300 thousand years ago. (Courtesy of Virgil L. Sharpton and the Lunar and Planetary Institute.)


New Quebec impact crater

New Quebec impact crater

. A colliding asteroid or comet caused this impact crater, known as the New Quebec, or Nouveau Quebec, Crater after the region in Canada where it is located. The crater has a rim diameter of 3.4 thousand meters and an estimated age of 1.4 Ī 0.3 million years. (Image courtesy of George Burnside, Manotik, Ontario, Canada, and the Lunar and Planetary Institute.)


Roter Kamm impact crater

Roter Kamm impact crater

. Located in the Namib Desert, the raised rim of this impact crater makes a stark contrast with the darker background vegetation. Broad, shifting sand dunes, deposited by the wind, cover the crater floor, but rocks melted by the impact have been found on the crater rim. The rim diameter of this crater is 2,500 meters and its age is 3.7 Ī 0.3 million years. (Courtesy of Christian Koeberl and the Lunar and Planetary Institute.)


Shatter cones

Shatter cones

. The shatter cones that are found in the vicinity of many terrestrial craters provide evidence for shocks associated with the impact of a cosmic object. They point towards the direction of impact, like the cone-shaped plugs of glass that are often formed when a bullet strikes a window. The shatter cones shown here are about 0.05 meters in height. They are from the Wells Creek Tennessee Basin, a crater that is about 14 thousand meters across and roughly 200 million years old.


Site of impact that wiped out the dinosaurs

Site of impact that wiped out the dinosaurs

. The long-sought site of the scar left by a killer asteroid has been found near Chicxulub (filled circle), a small village at the tip of the YucatŠn Peninsula. Material from the submerged crater has been dated at about 65 million years, coinciding with a blast that triggered the eradication of most life on Earth. Thick sedimentary deposits laid down 65 million years ago in Haiti (big open circle) contain exceptionally large amounts of iridium, shocked quartz and glassy debris that are thought to be part of the impact across the Caribbean, almost two million meters away. The small open circles mark the sites of marine wave deposits associated with the same impact. [Adapted from Alan R. Hildebrand and William V. Boynton, Natural History 6, 47-53 (1991).]


Chicxulub impact crater

Chicxulub impact crater

. This image shows the variations in the magnitude of the gravity field at sea level in the vicinity of the buried Chicxulub impact basin in the northwestern corner of the YucatŠn Peninsula. The impact basin exhibits ring-like variations in the strength of the gravity. One interpretation of the craterís horse-shoe shape is that the asteroid struck the land at a low, oblique angle from the south-east (lower-right). The pronounced ring that separates the deep annular basin (magenta and deep blue regions) from the basin flanks (cyan and green) has a diameter of about 180 thousand meters. A discontinuous, subtle outer region with a diameter of 250 thousand to 280 thousand meters can be discerned particularly toward the southern (lower) part of the basin. (Courtesy of Virgil L. Sharpton and the Lunar and Planetary Institute.)


Doomsday rock

Doomsday rock

. This computer-generated image depicts a view of the Earth as seen from the surface of an asteroid or comet that might collide with our planet. If the impacting projectile is big enough, then the collision will severely disrupt human life and might even destroy it altogether. (Courtesy of JPL and NASA.)


Cosmic impact probabilities

Cosmic impact probabilities

. A hail of cosmic objects continually pelts the Earth from space. This plot shows the likelihood (vertical axis) that a member of the current population of Earth-crossing asteroids and comets will hit our planet. An object two kilometers in diameter, capable of producing certain worldwide damage, hits the Earth every million years on average. An impact like the one that wiped out the dinosaurs, giving rise to the Chicxulub crater, is estimated to occur every 100 million years. Since smaller asteroids and comets are much more numerous than larger ones, the smaller objects strike our planet more often. An impact like the Tunguska event, which occurred on 30 June 1908, might occur every 700 centuries or so. The explosive energy of the impact is also given (horizontal axis) in units of megatons of exploded trinitrotoluene, or TNT for short. One hundred megatons (1011 kilograms) of TNT is equivalent to about 4 x 1017 joules of energy, the amount released by a typical nuclear bomb on Earth.


Asteroid streak

Asteroid streak

. As the nearby asteroid 2100 Ra-Shalom moved along its orbit around the Sun, it produced an elongated steak or trail on this photograph, while the background stars retained their round shape. This 20-minute exposure was taken with an 0.46-meter (18-inch) Telescope on Palomar Mountain in California, during a pioneering search for Near-Earth Objects. (Courtesy of Eleanor F. Helin, Jet Propulsion Laboratory, California Institute of Technology.)


Summary Diagram

Summary Diagram

. Summary Diagram.