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Shoemaker–Levy 9, on a collision course(total of 21 fragments, taken on July 1994)DiscoveryDiscovered byDiscovery dateMarch 24, 1993Orbital characteristics94.2°1.8 km (1.1 mi)Comet Shoemaker–Levy 9 ( D/1993 F2) was a that broke apart in July 1992 and collided with in July 1994, providing the first direct observation of an extraterrestrial of objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by worldwide. The collision provided new information about Jupiter and highlighted its possible role in reducing in the.The comet was discovered by astronomers and and in 1993. Shoemaker–Levy 9 (SL9) had been captured by Jupiter and was orbiting the planet at the time. It was located on the night of March 24 in a photograph taken with the 46 cm (18 in) at the in. It was the first active comet observed to be orbiting a planet, and had probably been captured by Jupiter around 20–30 years earlier.Calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992.
At that time, the orbit of Shoemaker–Levy 9 passed within Jupiter's, and Jupiter's had acted to pull apart the comet. The comet was later observed as a series of fragments ranging up to 2 km (1.2 mi) in diameter. These fragments collided with Jupiter's southern hemisphere between July 16 and 22, 1994, at a speed of approximately 60 km/s (37 mi/s) (Jupiter's ) or 216,000 km/h (134,000 mph). The prominent scars from the impacts were more easily visible than the and persisted for many months. Contents.Discovery While conducting a program of observations designed to uncover, the Shoemakers and Levy discovered Comet Shoemaker–Levy 9 on the night of March 24, 1993, in a photograph taken with the 0.46 m (1.5 ft) at the in. The comet was thus a serendipitous discovery, but one that quickly overshadowed the results from their main observing program.Comet Shoemaker–Levy 9 was the ninth periodic comet (a comet whose orbital period is 200 years or less) discovered by the Shoemakers and Levy,. It was their eleventh comet discovery overall including their discovery of two non-periodic comets, which use a different nomenclature.
The discovery was announced in 5725 on March 26, 1993.The discovery image gave the first hint that comet Shoemaker–Levy 9 was an unusual comet, as it appeared to show multiple nuclei in an elongated region about 50 long and 10 arcseconds wide. Of the noted that the comet lay only about 4 from Jupiter as seen from Earth, and that although this could be a line-of-sight effect, its apparent motion in the sky suggested that the comet was physically close to the planet.
Jupiter-orbiting comet Orbital studies of the new comet soon revealed that it was orbiting rather than the, unlike all other comets known at the time. Its orbit around Jupiter was very loosely bound, with a period of about 2 years and an (the point in the orbit farthest from the planet) of 0.33 (49 million kilometres; 31 million miles). Its orbit around the planet was highly ( e = 0.9986).Tracing back the comet's orbital motion revealed that it had been orbiting Jupiter for some time. It is likely that it was captured from a solar orbit in the early 1970s, although the capture may have occurred as early as the mid-1960s.
Several other observers found images of the comet in images obtained before March 24, including from a photograph exposed on March 15, on March 17, and a team led by from images on March 19. An image of the comet on a Schmidt photographic plate taken on March 19 was identified on March 21 by M. Lindgren, in a project searching for comets near Jupiter.
However, as his team were expecting comets to be inactive or at best exhibit a weak dust coma, and SL9 had a peculiar morphology, its true nature was not recognised until the official announcement 5 days later. No precovery images dating back to earlier than March 1993 have been found.
Before the comet was captured by Jupiter, it was probably a short-period comet with an just inside Jupiter's orbit, and a interior to the.The volume of space within which an object can be said to orbit Jupiter is defined by Jupiter's (also called the Roche sphere). When the comet passed Jupiter in the late 1960s or early 1970s, it happened to be near its aphelion, and found itself slightly within Jupiter's Hill sphere. Jupiter's gravity nudged the comet towards it.
Because the comet's motion with respect to Jupiter was very small, it fell almost straight toward Jupiter, which is why it ended up on a Jove-centric orbit of very high eccentricity—that is to say, the ellipse was nearly flattened out.The comet had apparently passed extremely close to Jupiter on July 7, 1992, just over 40,000 km (25,000 mi) above its cloud tops—a smaller distance than Jupiter's radius of 70,000 km (43,000 mi), and well within the orbit of Jupiter's innermost moon and the planet's, inside which are strong enough to disrupt a body held together only by gravity. Although the comet had approached Jupiter closely before, the July 7 encounter seemed to be by far the closest, and the fragmentation of the comet is thought to have occurred at this time.
Each fragment of the comet was denoted by a letter of the alphabet, from 'fragment A' through to 'fragment W', a practice already established from previously observed broken-up comets.More exciting for planetary astronomers was that the best orbital calculations suggested that the comet would pass within 45,000 km (28,000 mi) of the center of Jupiter, a distance smaller than the planet's radius, meaning that there was an extremely high probability that SL9 would collide with Jupiter in July 1994. Studies suggested that the train of nuclei would plow into Jupiter's atmosphere over a period of about five days.
Predictions for the collision The discovery that the comet was likely to collide with Jupiter caused great excitement within the astronomical community and beyond, as astronomers had never before seen two significant Solar System bodies collide. Intense studies of the comet were undertaken, and as its orbit became more accurately established, the possibility of a collision became a certainty. The collision would provide a unique opportunity for scientists to look inside Jupiter's atmosphere, as the collisions were expected to cause eruptions of material from the layers normally hidden beneath the clouds.Astronomers estimated that the visible fragments of SL9 ranged in size from a few hundred metres (around 1,000 ft) to two kilometres (1.2 mi) across, suggesting that the original comet may have had a nucleus up to 5 km (3.1 mi) across—somewhat larger than, which became very bright when it passed close to the Earth in 1996. One of the great debates in advance of the impact was whether the effects of the impact of such small bodies would be noticeable from Earth, apart from a flash as they disintegrated like giant. The most optimistic prediction was that large, asymmetric would rise above the limb of Jupiter and into sunlight to be visible from Earth.Other suggested effects of the impacts were waves travelling across the planet, an increase in haze on the planet due to dust from the impacts, and an increase in the mass of the. However, given that observing such a collision was completely unprecedented, astronomers were cautious with their predictions of what the event might reveal.
Jupiter in, Shoemaker–Levy 9 collision (left), (right)Anticipation grew as the predicted date for the collisions approached, and astronomers trained terrestrial telescopes on Jupiter. Several space observatories did the same, including the, the -observing, and significantly the, then on its way to a rendezvous with Jupiter scheduled for 1995.
Although the impacts took place on the side of Jupiter hidden from Earth, Galileo, then at a distance of 1.6 AU (240 million km; 150 million mi) from the planet, was able to see the impacts as they occurred. Jupiter's rapid rotation brought the impact sites into view for terrestrial observers a few minutes after the collisions.Two other space probes made observations at the time of the impact: the, primarily designed for observations, was pointed towards Jupiter from its location 2.6 AU (390 million km; 240 million mi) away, and the distant probe, some 44 AU (6.6 billion km; 4.1 billion mi) from Jupiter and on its way out of the Solar System following its encounter with in 1989, was programmed to look for radio emission in the 1–390 range and make observations with its ultraviolet spectrometer. Hubble Space Telescope images of a from the first impact appearing over the limb of the planetThe first impact occurred at 20:13 on July 16, 1994, when fragment A of the nucleus entered Jupiter's southern hemisphere at a speed of about 60 km/s (35 mi/s).
Instruments on Galileo detected a that reached a peak temperature of about 24,000 (23,700 °C; 42,700 °F), compared to the typical Jovian cloudtop temperature of about 130 K (−143 °C; −226 °F), before expanding and cooling rapidly to about 1,500 K (1,230 °C; 2,240 °F) after 40 seconds. The plume from the fireball quickly reached a height of over 3,000 km (1,900 mi). A few minutes after the impact fireball was detected, Galileo measured renewed heating, probably due to ejected material falling back onto the planet. Earth-based observers detected the fireball rising over the limb of the planet shortly after the initial impact.Despite published predictions, astronomers had not expected to see the fireballs from the impacts and did not have any idea in advance how visible the other atmospheric effects of the impacts would be from Earth.
Observers soon saw a huge dark spot after the first impact. The spot was visible even in very small telescopes, and was about 6,000 km (3,700 mi) (one Earth radius) across. This and subsequent dark spots were thought to have been caused by debris from the impacts, and were markedly asymmetric, forming crescent shapes in front of the direction of impact.Over the next six days, 21 distinct impacts were observed, with the largest coming on July 18 at 07:33 UTC when fragment G struck Jupiter. This impact created a giant dark spot over 12,000 km (7,500 mi) across, and was estimated to have released an energy equivalent to 6,000,000 (600 times the world's nuclear arsenal).
Two impacts 12 hours apart on July 19 created impact marks of similar size to that caused by fragment G, and impacts continued until July 22, when fragment W struck the planet. Observations and discoveries Chemical studies. Brown spots mark impact sites on 's southern hemisphereObservers hoped that the impacts would give them a first glimpse of Jupiter beneath the cloud tops, as lower material was exposed by the comet fragments punching through the upper atmosphere. Studies revealed in the Jovian spectrum due to (S 2) and (CS 2), the first detection of either in Jupiter, and only the second detection of S 2 in any.
Other molecules detected included (NH 3) and (H 2S). The amount of sulfur implied by the quantities of these compounds was much greater than the amount that would be expected in a small cometary nucleus, showing that material from within Jupiter was being revealed.bearing molecules such as were not detected, to the surprise of astronomers.As well as these, emission from heavy such as, and was detected, with abundances consistent with what would be found in a cometary nucleus. Although a substantial amount of water was detected spectroscopically, it was not as much as predicted beforehand, meaning that either the water layer thought to exist below the clouds was thinner than predicted, or that the cometary fragments did not penetrate deeply enough. Waves As predicted beforehand, the collisions generated enormous waves that swept across Jupiter at speeds of 450 m/s (1,476 ft/s) and were observed for over two hours after the largest impacts. The waves were thought to be travelling within a stable layer acting as a, and some scientists thought the stable layer must lie within the hypothesised water cloud.
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However, other evidence seemed to indicate that the cometary fragments had not reached the water layer, and the waves were instead propagating within the. Other observations. A sequence of images, taken several seconds apart, showing the appearance of the of fragment W on the dark side of JupiterRadio observations revealed a sharp increase in emission at a wavelength of 21 cm (8.3 in) after the largest impacts, which peaked at 120% of the normal emission from the planet. This was thought to be due to, caused by the injection of —electrons with velocities near the speed of light—into the Jovian by the impacts.About an hour after fragment K entered Jupiter, observers recorded emission near the impact region, as well as at the of the impact site with respect to Jupiter's strong. The cause of these emissions was difficult to establish due to a lack of knowledge of Jupiter's internal and of the geometry of the impact sites. One possible explanation was that upwardly accelerating from the impact accelerated charged particles enough to cause auroral emission, a phenomenon more typically associated with fast-moving particles striking a planetary atmosphere near a.Some astronomers had suggested that the impacts might have a noticeable effect on the, a of high-energy particles connecting Jupiter with the highly moon. High resolution spectroscopic studies found that variations in the ion, and temperatures at the time of impact and afterwards were within the normal limits.Voyager 2 failed to detect anything with calculations showing that the fireballs were just below the craft's limit of detection.
Ulysses also failed to detect anything. Post-impact analysis. A reddish, asymmetric ejecta patternSeveral models were devised to compute the density and size of Shoemaker–Levy 9. Its average density was calculated to be about 0.5 g/cm 3 (0.018 lb/cu in); the breakup of a much less dense comet would not have resembled the observed string of objects.
The size of the parent comet was calculated to be about 1.8 km (1.1 mi) in diameter. These predictions were among the few that were actually confirmed by subsequent observation.One of the surprises of the impacts was the small amount of water revealed compared to prior predictions. A on, probably caused by a similar impact event. Main article:On July 19, 2009, exactly 15 years after the SL9 impacts, a new black spot about the size of the Pacific Ocean appeared in Jupiter's southern hemisphere. Thermal infrared measurements showed the impact site was warm and spectroscopic analysis detected the production of excess hot ammonia and silica-rich dust in the upper regions of Jupiter's atmosphere. Scientists have concluded that another impact event had occurred, but this time a more compact and strong object, probably a small undiscovered asteroid, was the cause. Jupiter as a 'cosmic vacuum cleaner' The impact of SL9 highlighted Jupiter's role as a 'cosmic vacuum cleaner' for the inner Solar System.
The planet's strong gravitational influence leads to many small comets and colliding with the planet, and the rate of cometary impacts on Jupiter is thought to be between 2,000–8,000 times higher than the rate on Earth.The extinction of the dinosaurs at the end of the period is generally thought to have been caused by the (along with its antipodal ), which created the, demonstrating that impacts are a serious threat to life on Earth. Astronomers have speculated that without Jupiter to mop up potential impactors, extinction events might have been more frequent on Earth, and complex life might not have been able to develop. This is part of the argument used in the.In 2009, it was shown that the presence of a smaller planet at Jupiter's position in the Solar System might increase the impact rate of comets on the Earth significantly. A planet of Jupiter's mass still seems to provide increased protection against asteroids, but the total effect on all orbital bodies within the Solar System is unclear. This and other recent models call into question the nature of Jupiter's influence on Earth impacts. See also., a near-Earth comet in the process of disintegratingReferences Notes.
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