This is an enlargement of a NASA Hubble Space Telescope image of the "brightest nucleus" in a string of approximately 20 objects that comprise comet P/Shoemaker-Levy 9. The comet is hurtling toward a July 1994 collision with the giant planet Jupiter.
Hubble's high resolution shows that this bright region is actually a group of at least four separate pieces. The HST images allow for the best separation of the individual nuclei and their surrounding halo of dust (comae), which results in a better estimate of the nuclear sizes.
The image was taken with the Wide Field and Planetary Camera (WFPC), in PC mode, on July 1, 1993. The Hubble observations show that the cometary nuclei are probably less than three miles (5 km) across, as opposed to earlier estimates of nine miles (14 km).
The new Hubble data show that the impacts will unleash only 1/10th to 1/100th as much energy as thought previously. However, even with these new size estimates, the total energy of the July 1994 collisions will be equivalent to 100 million megatons of TNT.
Comet Shoemaker-Levy 9, torn into pieces as a result of a close approach to Jupiter in July 1992, will collide with Jupiter during the third week of July 1994. Of tremendous scientific importance, the impacts of the cometary fragments will release more energy into Jupiter's atmosphere than that of the world's combined nuclear arsenals. Because the impacts will occur on the night side of Jupiter, the explosions will not be directly observable from Earth. However, professional and amateur astronomers may observe the impact light flashes reflected off the inner satellites of Jupiter. Any lasting effects on Jupiter, such as atmospheric clouds, ejecta plumes, or seismic thermal disturbances, may be observable an hour or so later when the rotation of Jupiter brings the impact sites into Earth's view.
Analysis of high-resolution images of the comet taken by the NASA's Hubble Space Telescope in July 1993 suggests that the major cometary fragments range in size from one to a few kilometers. The large fragments are embedded in a cloud of debris with material ranging in size from boulder-sized to microscopic particles. Although comet-like outgassing of the fragments has not been observed, the fragile nature of the object suggests that it is indeed a comet rather than a more compact asteroid.
Comet Shoemaker-Levy 9 was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. It was first detected on a photograph taken on the night of March 24, 1993 with the 0.4-meter Schmidt telescope on Palomar Mountain in California. Subsequent observations were forthcoming from observers at the University of Hawaii, the Spacewatch telescope on Kitt Peak in Arizona and McDonald Observatory in Texas. These observations were used to demonstrate that the comet was in orbit about Jupiter, and had made a very close approach (within 1.4 Jupiter radii from Jupiter's center) on July 7, 1992. During this close approach, the unequal Jupiter gravitational attractions on the comet's near and far sides broke apart the fragile object. The disruption of a comet into multiple fragments is an unusual event, the capture of a comet into an orbit about Jupiter is even more unusual, and the collision of a large comet with a planet is an extraordinary, millennial event.
The impact of comet Shoemaker-Levy 9 onto Jupiter represents the first time in human history that people have discovered a body in the sky and been able to predict its impact on a planet more than seconds in advance. The impact will deliver more energy to Jupiter than the largest nuclear warheads ever built, and up to a significant fraction of the energy delivered by the impact which is generally thought to have caused the extinction of the dinosaurs on Earth, roughly 65 million years ago. Earth-bound observers are taking this opportunity to observe and study the comet's collision with a planet to gain more understanding of one of the fundamental physical processes within the solar system, impacts. The discovery has spawned scientific thinking about the frequency with which comets fragment and implications related to the inventory of small bodies in the Solar System and how they modify the surface and atmospheres of the planets.
The fast approaching collision of segmented Periodic Comet Shoemaker-Levy 9 with the planet Jupiter has peaked the interest of professional and amateur astronomers worldwide. Scientists expect a spectacular 51/2-day event from July 16-22 and anticipate some observations. For the first time in history, scientists have advance notice of such a collision and the technological capabilities to observe it.
Astronomers predict the comet's 20+ segments will hit Jupiter's dark night side, where they will be hidden from telescopes on Earth. Some observers may be able to view the phenomenon indirectly in light reflected from Jupiter's inner moon or off ring particles. Other observers anticipate viewing the impacts and expected explosions through observations from NASA's Galileo and other spacecraft or by studying the aftereffects on Jupiter's atmosphere.
Thousands of planet-watchers are readying observatories on the ground and in space for what they hope will be a remarkable encounter. For comet experts and planetary specialists around the world, this may be the most important event of their careers because of the discoveries they may make about the nature of comets and the makeup of Jupiter's atmosphere and magnetosphere. This knowledge may help them explain similar high-energy events on Earth.
The fragmented comet was discovered by Eugene and Carolyn Shoemaker and David Levy on March 24, 1993. It was the ninth periodic comet found by this team of professional and amateur astronomers. They identified the comet through a photograph taken with the 18-inch Schmidt telescope at Mt. Palomar Observatory near Los Angeles, California. Subsequent imaging conducted by James Scotti at the Spacewatch Telescope on Kitt Peak in Arizona, and by Jane Luu and David Jewitt at the Mauna Kea Observatory in Hawaii revealed the comet's peculiar form: it is actually a string of numerous fragments of comet, "a string of pearls."
P/Shoemaker-Levy 9 probably split apart during July 1992, when scientists think it traveled within 113,000 kilometers of Jupiter's center. During this pass in its orbit around Jupiter, the planet's tidal forces tore it apart. The comet is designated, "P," for "periodic," because even before its capture in a death grip by Jupiter, its original orbit around the Sun was closed and contained within our solar system.
Its fragments vary in size, with about six relatively large pieces, a dozen medium-sized ones, and assorted smaller debris. The average chunk is estimated to be two kilometers in diameter, although no one knows for certain. The size or mass of the fragments however, will determine the nature of their impact on Jupiter's atmosphere.
As the comet string nears Jupiter, its associated dust coma will be bombarded by charged particles trapped in Jupiter's magnetosphere. Gas and dust ejected from the comet may be swept up by Jupiter's magnetic field, possibly causing large changes in the density and composition of Jupiter's aurora. The comet fragments will plummet into the planet one by one, like a freight train falling off a bridge. The explosions within Jupiter's atmosphere may inject atmospheric ingredients into the magnetosphere, altering Jupiter's radio emissions. As the comet fragments enter Jupiter's stratosphere, they will be heated and lose some mass and energy by aerodynamic forces. Because they are fragile, they may break up after penetrating about 300-400 kilometers into the atmosphere. The largest and strongest fragments will descend another 50-200 kilometers. At this point, the fragments will release the majority of their kinetic energy in a spectacular explosion in a little over a second. This explosion may create a fireball, like a nuclear burst, that could rise above Jupiter's cloud tops in a matter of minutes.
Ordinarily, events occurring 150-200 kilometers below visible cloud tops would be invisible beyond the planet. In this case, however, the shock wave from the airburst may blow through the planet's atmosphere carrying the gases far above Jupiter's clouds. This deeper gas contains volatile materials that ultimately will condense high in the atmosphere. The gas may form unusual clouds, which could last a long time if the comet's ice particles are small. The impacts could create a thermal anomaly or a tremendous storm, similar to Jupiter's Red Spot but not as big. This new turbulence or spot might be visible through the most powerful Earth-based telescopes.
When these comet fragments dissipate in the atmosphere, they could be very bright - possibly as bright or brighter than Jupiter itself. The light from the impact of the largest fragments could brighten a well-placed inner moon of Jupiter enough to be detected by powerful Earth telescopes. The Hubble Space Telescope is scheduled to make more pictures of Jupiter during the days of impact. Thousands of Earth-bound astronomers throughout the world will point their telescopes towards Jupiter to look for some evidence of the crash.
In the United States, NASA and The National Science Foundation have jointly funded a coordinated program to support research efforts for this event, using many ground-based observatories and several spacecraft: Galileo, Hubble Space Telescope, the International Ultraviolet Explorer, the Extreme Ultraviolet Explorer, Ulysses, and Voyager 2. The program includes listening for radio signals, visible and thermal imaging, modeling, theory, and data analysis. Since 1993, an electronic bulletin board on the Internet, organized by astronomer Michael A' Hearn at the University of Maryland, has kept the world's planetary-scientific research community advised of the latest information on the pending collision. In January 1994, 175 astronomers from the United States and Europe met to identify the specific physical phenomena they would try to observe to gain the most knowledge from the event. They established a continuous worldwide series of Jupiter observations during the collision's six-day time frame. They also agreed to disseminate their scientific information within three to six months after obtaining it. Throughout the planning process, scientists in varied U.S. and international organizations have displayed a spirit of cooperation in their quest for discoveries. Astronomers hope to gain more knowledge about the composition of comets and the makeup of Jupiter's atmosphere. Analysis of the new data may teach us more about the role of comets, meteors, and other space objects in the disappearance of the dinosaurs more than 65 million years ago. Additional measurements and observations may test theories of other mass extinctions on Earth, the behavior of high-energy shock waves and cloud formation in planetary atmospheres, the makeup of comets, and even the origin of planets.
Most bodies in the solar system with a visible solid surface exhibit craters. On Earth we see very few because geological processes such as weathering and erosion soon destroy the obvious evidence. On bodies with no atmosphere, such as Mercury or the Moon, craters are everywhere. Without going into detail, there is strong evidence of a period of intense cratering in the solar system that ended about 3.9 billion years ago. Since that time cratering appears to have continued at a much slower and fairly uniform rate. The cause of the craters is impacts by comets and asteroids. Most asteroids follow simple circular orbits between the planets Mars and Jupiter, but all of these asteroids are perturbed, occasionally by each other and more regularly and dramatically by Jupiter. As a result some find themselves in orbits that cross that of Mars or even Earth. Comets on the other hand follow highly elongated orbits that often come close to Earth or other major bodies to begin with. These orbits are greatly affected if they come anywhere near Jupiter. Over the eons every moon and planet finds itself in the wrong place in its orbit at the wrong time and suffers the insult of a major impact.
The Earth's atmosphere protects us from the multitude of small debris, the size of grains of sand or pebbles, thousands of which pelt our planet every day. The meteors in our night sky are visible evidence of this small debris burning up high in the atmosphere. In fact, up to a diameter of about 10-meters (33 feet), most stony meteoroids are destroyed in the atmosphere in thermal explosions. Obviously some fragments do reach the ground, because we have stony meteorites in our museums. Such falls are known to cause property damage from time to time. On October 9, 1992, a fire ball was seen streaking across the sky all the way from Kentucky to New York. A 27-pound stony meteorite (chondrite) from the fireball fell in Peekskill, New York, punching a hole in the rear end of an automobile parked in a driveway and coming to rest in a shallow depression beneath it. Falls into a Connecticut dining room and an Alabama bedroom are well documented incursions in this century. A 10-meter body typically has the kinetic energy of about five nuclear warheads of the size dropped on Hiroshima, however, and the shock wave it creates can do considerable damage even if nothing but comparatively small fragments survive to reach the ground. Many fragments of a 10-meter iron meteoroid will reach the ground. The only well-studied example of such a fall in recent times took place in the Sikhote-Alin Mountains of eastern Siberia on February 12, 1947. About 150 US tons of fragments reached the ground, the largest intact fragment weighing 3,839 pounds. The fragments covered an area of about 1 x 2 kilometers (0.6 x 1.2 miles), within which there were 102 craters greater than 1 meter in diameter, the largest of them 26.5 meters (87 feet), and about 100 more smaller craters. If this small iron meteoroid had landed in a city, it obviously would have created quite a stir. The effect of the larger pieces would be comparable to having a car suddenly drop in at supersonic speeds! Such an event occurs about once per decade somewhere on Earth, but most of them are never recorded, occurring at sea or in some remote region such as Antarctica. It is a fact that there is no record in modern times of any person being killed by a meteorite. It is the falls larger than 10 meters that start to become really worrisome. The 1908 Tunguska event was a stony meteorite in the 100-meter class. The famous meteor crater in northern Arizona, some 1219 meters (4,000 feet) in diameter and 183 meters (600 feet) deep, was created 50,000 years ago by a nickel-iron meteorite perhaps 60 meters in diameter. It probably survived nearly intact until impact, at which time it was pulverized and largely vaporized as its 6-7 x 1016 joules* of kinetic energy were rapidly dissipated in an explosion equivalent to some 15 million tons of TNT! Falls of this class occur once or twice every 1000 years.
There are now over 100 ring-like structures on Earth recognized as definite impact craters. Most of them are not obviously craters, their identity masked by heavy erosion over the centuries, but the minerals and shocked rocks present make it clear that impact was their cause. The Ries Crater in Bavaria is a lush green basin some 25 kilometers (15 miles) in diameter with the city of Nordlingen in the middle. Fifteen million years ago a 1500-meter (5000 feet) asteroid or comet hit there, excavating more than a trillion tons of material and scattering it all over Europe. This sort of thing happens about once every million years or so. Another step upward in size take us to Chicxulub, an event that occurs once in 50-100 million years. Chicxulub is the largest crater known which seems definitely to have an impact origin, but there are a few ring-like structures that are 2-3 times larger yet about which geologists are uncertain. There are now more than 150 asteroids known that come nearer to the Sun than the outermost point of Earth's orbit. These range in diameter from a few meters up to about 8 kilometers. A working group chaired by Dr. David Morrison, NASA Ames Research Center, estimates that there are some 2,100 such asteroids larger than 1 kilometer and perhaps 320,000 larger than 100 meters, the size that caused the Tunguska event and the Arizona Meteor Crater. An impact by one of these larger meteors in the wrong place would be a catastrophe, but it would not threaten civilization. However, the working group concluded that an impact by an asteroid larger than 1-2 kilometers could degrade the global climate, leading to widespread crop failure and loss of life. Such global environmental catastrophes, which place the entire population of the Earth at risk, are estimated to take place several times per million years on average. A still larger impact by an object larger than about 5 kilometers is damaging enough to cause mass extinctions. In addition there are many comets in the 1-10 kilometer class, 15 of them in short-period orbits that pass inside the Earth's orbit, and an unknown number of long-period comets. Virtually any short-period comet among the 100 or so not currently coming near the Earth could become dangerous after a close passage by Jupiter.
This all sounds pretty scary. However, as noted earlier, no human in the past 1000 years is known to have been killed by a meteorite or by the effects of one impacting. (There are ancient Chinese records of such deaths.) An individual's chance of being killed by a meteorite is small, but the risk increases with the size of the impacting comet or asteroid, with the greatest risk associated with global catastrophes resulting from impacts of objects larger than 1 kilometer. NASA knows of no asteroid or comet currently on a collision course with Earth, so the probability of a major collision is quite small. In fact, as best as we can tell, no large object is likely to strike the Earth any time in the next several hundred years. To be able to better calculate the statistics, astronomers need to detect as many of the near-Earth objects as possible. It's likely that we could identify a threatening near-Earth object large enough to potentially cause catastrophic changes in the Earth's environment, and most astronomers believe that a systematic approach to studying asteroids and comets that pass close to the Earth makes good sense. It's too late for the dinosaurs, but today astonomers are conducting ever-increasing searches to identify all of the larger objects which pose an impact danger to Earth.
* joule: a unit of measurement, the amount of energy corresponding to one watt
acting for one second.
Foley, Theresa M., "Comet heads for collision with Jupiter," Aerospace America,
pp 24-29, April 1994.
Levy, David H., "Pop! Pow! Smash!," The Sciences, pp 31-35, May/June 1994.
Smith, Douglas L., "When a Body Hits a Body Comin' Through the Sky," Engineering & Science, pp 3-13, Fall 1993.