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Comet Shoemaker-Levy 9

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Hubble Space Telescope image of Comet Shoemaker-Levy 9, taken on May
   17, 1994.
   Enlarge
   Hubble Space Telescope image of Comet Shoemaker-Levy 9, taken on May
   17, 1994.

   Comet Shoemaker-Levy 9 (SL9, formally designated D/1993 F2) was a comet
   which collided with Jupiter in 1994, providing the first direct
   observation of the collision of two solar system objects. This
   generated a large amount of coverage in the popular media, and SL9 was
   closely observed by astronomers worldwide. The comet provided many
   revelations about Jupiter and its atmosphere and highlighted Jupiter's
   role in reducing space debris in the inner solar system.

   Astronomers Carolyn and Eugene M. Shoemaker and David Levy discovered
   the comet, which was the eleventh one they had found. It is called
   "Shoemaker-Levy 9" because only periodic comets are numbered in this
   way; the Shoemaker-Levy team has discovered four non-periodic comets in
   addition to its nine periodic ones. Shoemaker-Levy 9 was located on the
   night of March 24, 1993, in a photograph taken with the 0.4- metre
   Schmidt telescope at the Mount Palomar Observatory in California.
   Unlike all other comets discovered before then, it was orbiting Jupiter
   rather than the Sun.

   SL9 was in pieces ranging in size up to 2  kilometres in diameter, and
   is believed to have been pulled apart by Jupiter's tidal forces during
   a close encounter in July 1992. These fragments collided with Jupiter's
   southern hemisphere over a period of time between July 16 and July 22,
   1994, at a speed of approximately 60  kilometres per second (37  miles
   per second). The prominent scars from the impacts could be seen on
   Jupiter for many months after the impact, and observers described them
   as more easily visible than the Great Red Spot.

Discovery

   Comet Shoemaker-Levy 9 (SL9) was discovered on the night of March 24,
   1993 by the Shoemakers and Levy, who were conducting a program of
   observations designed to uncover near-Earth objects. The comet was thus
   a serendipitous discovery, but one that quickly overshadowed the
   results from their main observing program. The discovery was announced
   in IAU Circular 5725 on March 27, 1993. Subsequently, several other
   observers found the comet in images obtained before March 24, including
   K. Endate from a photograph exposed on March 15, S. Otomo on March 17,
   and a team led by Eleanor Helin from images on March 19 .

   The discovery image gave the first hint that SL9 was an unusual comet,
   as it appeared to show multiple nuclei in an elongated region about 50
   arcseconds long and 10 arcseconds wide. Brian Marsden of the Central
   Bureau for Astronomical Telegrams noted that the comet lay only about 4
   degrees from Jupiter as seen from Earth, and that while this could of
   course be a projection effect, its apparent motion suggested that it
   was physically close to the giant planet . Because of this, he
   suggested that the Shoemakers and David Levy had discovered the
   fragments of a comet that had been disrupted by Jupiter's gravity.

A Jupiter-orbiting comet

   A montage of images of Jupiter and the comet, showing the relative
   scale and angle of impact.
   Enlarge
   A montage of images of Jupiter and the comet, showing the relative
   scale and angle of impact.

   Orbital studies of the new comet soon revealed that, unlike all other
   comets discovered before then, it was orbiting Jupiter rather than the
   Sun. Its orbit around Jupiter was very loosely bound, with a period of
   about 2 years and an apojove (furthest distance from Jupiter) of 0.33
   Astronomical Units (AU) (49.4 million km). Its orbit around the planet
   was highly eccentric (e = 0.9986).

   Tracing back the comet's orbital motion revealed that it had been
   orbiting Jupiter for some time. It seems most 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 . No precovery images
   dating back to earlier than March 1993 have been found so far. Before
   the comet was captured by Jupiter, it was probably a short-period comet
   with an aphelion just inside Jupiter's orbit, and a perihelion interior
   to the asteroid belt .

   The volume of space within which an object can be said to orbit Jupiter
   is defined by Jupiter's Hill sphere (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 into Jupiter, which is why it ended up on a
   Jupiter-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 above the planet's cloud tops — a smaller
   distance than Jupiter's radius of 70,000 km, and well within the orbit
   of Jupiter's innermost moon Metis and the planet's Roche limit, inside
   which tidal forces 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
   solutions suggested that the comet would pass within 45,000 km of the
   centre 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 plough into Jupiter's atmosphere over a period of about five
   days.

Predictions for the collision

   Astronomers at STSCI await the first images from the impact of fragment
   A.
   Enlarge
   Astronomers at STSCI await the first images from the impact of fragment
   A.

   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 to at most a couple of kilometres across,
   suggesting that the original comet may have had a nucleus up to 5 km
   across – somewhat larger than Comet Hyakutake, 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 meteors.

   Other suggested effects of the impacts were seismic waves travelling
   across the planet, an increase in stratospheric haze on the planet due
   to dust from the impacts, and an increase in the mass of the Jovian
   ring system. However, given that observing such a collision was
   completely unprecedented, astronomers were cautious with their
   predictions of what the event might reveal.

Impacts

   Jupiter in Ultraviolet (about 2.5 hours after R's impact)
   Enlarge
   Jupiter in Ultraviolet (about 2.5 hours after R's impact)

   Anticipation was high as the predicted date for the collisions
   approached, and astronomers trained their telescopes on Jupiter.
   Several space observatories did the same, including the Hubble Space
   Telescope, the ROSAT X-ray observing satellite, and significantly the
   Galileo spacecraft, then on its way to a rendezvous with Jupiter
   scheduled for 1996. While the impacts would take place on the side of
   Jupiter hidden from Earth, Galileo, then at a distance of 1.6 AU from
   the planet, would be able to see the impacts as they occurred.
   Jupiter's rapid rotation would bring the impact sites into view for
   terrestrial observers a few minutes after the collisions.

   Two other satellites made observations at the time of the impact: the
   Ulysses spacecraft, primarily designed for solar observations, was
   pointed towards Jupiter from its location 2.6 AU away, and the distant
   Voyager 2 probe, some 44 AU from Jupiter and on its way out of the
   solar system following its encounter with Neptune in 1989, was
   programmed to look for radio emission in the 1–390  kHz range.
   HST images of a fireball from the first impact appearing over the limb
   of the planet.
   Enlarge
   HST images of a fireball from the first impact appearing over the limb
   of the planet.

   The first impact occurred at 20:15 UTC on July 16, 1994, when fragment
   A of the nucleus slammed into Jupiter's southern hemisphere at a speed
   of about 60 km/s. Instruments on Galileo detected a fireball which
   reached a peak temperature of about 24,000  K, compared to the typical
   Jovian cloudtop temperature of about 130 K, before expanding and
   cooling rapidly to about 1500 K after 40 s. The plume from the fireball
   quickly reached a height of over 3,000 km . 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 .

   Astronomers had expected to see the fireballs from the impacts, but did
   not have any idea in advance how visible the 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 (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 6 days, 21 discrete impacts were observed, with the
   largest coming on July 18 at 07:34 UTC when fragment G struck Jupiter.
   This impact created a giant dark spot over 12,000 km across, and was
   estimated to have released an energy equivalent to 6,000,000  megatons
   of TNT (750 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 Jupiter's southern hemisphere.
   Enlarge
   Brown spots mark impact sites on Jupiter's southern hemisphere.

   Observers 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. Spectroscopic
   studies revealed absorption lines in the Jovian spectrum due to
   diatomic sulfur (S[2]) and carbon disulfide (CS[2]), the first
   detection of either in Jupiter, and only the second detection of S[2]
   in any astronomical object. Other molecules detected included ammonia
   (NH[3]) and hydrogen sulfide (H[2]S). 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. Oxygen-bearing molecules such
   as sulfur dioxide were not detected, to the surprise of astronomers .

   As well as these molecules, emission from heavy atoms such as iron,
   magnesium and silicon was detected, with the abundances of these atoms
   being consistent with what would be found in a cometary nucleus. While
   substantial 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.

Seismic waves

   As predicted beforehand, the collisions generated enormous seismic
   waves which swept across the planet at speeds of 450 km/s and were
   observed for over two hours after the largest impacts. These waves
   seemed to be gravity waves, but their location was subject to debate.
   The waves were thought to be travelling within a stable layer acting as
   a waveguide, and some scientists believed the stable layer must lie
   within the hypothesised tropospheric water cloud. However, other
   evidence seemed to indicate that the cometary fragments had not reached
   the water layer, and the waves were instead propagating within the
   stratosphere .

Other observations

   A sequence of Galileo images, taken several seconds apart, showing the
   appearance of the fireball of fragment W on the dark side of Jupiter.
   Enlarge
   A sequence of Galileo images, taken several seconds apart, showing the
   appearance of the fireball of fragment W on the dark side of Jupiter.

   Radio observations revealed a sharp increase in continuum emission at a
   wavelength of 21  cm after the largest impacts, which peaked at 120% of
   the normal emission from the planet. This was thought to be due to
   synchrotron radiation, caused by the injection of relativistic
   electrons into the Jovian magnetosphere by the impacts .

   About an hour after fragment K entered Jupiter, observers recorded
   auroral emission near the impact region, as well as at the antipode of
   the impact site with respect to Jupiter's strong magnetic field. The
   cause of these emissions was difficult to establish due to a lack of
   knowledge of Jupiter's internal magnetic field and of the geometry of
   the impact sites. One possible explanation was that upwardly
   accelerating shock waves from the impact accelerated charged particles
   enough to cause auroral emission, a phenomenon more typically
   associated with fast-moving solar wind particles striking a planetary
   atmosphere near a magnetic pole .

   Some astronomers had suggested that the impacts might have a noticeable
   effect on the Io torus, a torus of high-energy particles connecting
   Jupiter with the highly volcanic moon Io. High resolution spectroscopic
   studies found that variations in the ion density, rotational velocity,
   and temperatures at the time of impact and afterwards were within the
   normal limits .

Post-impact analysis

   Fragment G impact site, showing asymmetric ejecta pattern.
   Enlarge
   Fragment G impact site, showing asymmetric ejecta pattern.

   One of the surprises of the impacts was the small amount of water
   revealed compared to prior predictions. Before the impact, models of
   Jupiter's atmosphere had indicated that the break-up of the largest
   fragments would occur at atmospheric pressures of anywhere from 300
   kilopascals to a few megapascals (from three to a few hundred bar), and
   most astronomers expected that the impacts would penetrate a
   hypothesised water-rich layer underneath the clouds.

   Astronomers did not observe large amounts of water following the
   collisions, and later impact studies found that fragmentation and
   destruction of the cometary fragments in an 'airburst' probably
   occurred at much higher altitudes than previously expected, with even
   the largest fragments being destroyed when the pressure reached 250
   kPa (2.5 bar), well above the expected depth of the water layer. The
   smaller fragments were probably destroyed before they even reached the
   cloud layer .

Longer-term effects

   The visible scars from the impacts could be seen on Jupiter for many
   months after the impact. They were extremely prominent, and observers
   described them as more easily visible even than the Great Red Spot. A
   search of historical observations revealed that the spots were probably
   the most prominent transient features ever seen on the planet, and that
   while the Great Red Spot is notable for its striking colour, no spots
   of the size and darkness of those caused by the SL9 impacts have ever
   been recorded before .

   Spectroscopic observers found that ammonia and carbon sulfide persisted
   in the atmosphere for at least fourteen months after the collisions,
   with a considerable amount of ammonia being present in the stratosphere
   as opposed to its normal location in the troposphere .

   Counterintuitively, the atmospheric temperature dropped to normal
   levels much more quickly at the larger impact sites than at the smaller
   sites: at the larger impact sites, temperatures were elevated over a
   region 15,000–20,000 km wide, but dropped back to normal levels within
   a week of the impact. At smaller sites, temperatures 10  K higher than
   the surroundings persisted for almost two weeks . Global stratospheric
   temperatures rose immediately after the impacts, then fell to below
   pre-impact temperatures 2–3 weeks afterwards, before rising slowly to
   normal temperatures  .

Frequency of impacts

   A chain of craters on Ganymede, probably caused by a similar impact
   event. The picture covers an area approximately 120 miles across.
   Enlarge
   A chain of craters on Ganymede, probably caused by a similar impact
   event. The picture covers an area approximately 120 miles across.

   Since the impact of SL9, two further very small comets have been found
   to be orbiting Jupiter. Studies have shown that the planet, by far the
   most massive in the solar system, can capture comets from solar orbit
   into Jovian orbit rather frequently.

   Cometary orbits around Jupiter are generally unstable, as they will be
   highly elliptical and likely to be strongly perturbed by the Sun's
   gravity at apojove (the furthest point on the orbit from the planet).
   Studies have estimated that comets probably crash into Jupiter once or
   twice per century, but the impact of comets the size of SL9 is much
   less common – probably no more often than once per millennium.

   There is very strong evidence that comets have previously been
   fragmented and collided with Jupiter and its satellites. During the
   Voyager missions to the planet, planetary scientists identified 13
   crater chains on Callisto and three on Ganymede, the origin of which
   was initially a mystery. Crater chains seen on the Moon often radiate
   from large craters, and are thought to be caused by secondary impacts
   of the original ejecta, but the chains on the Jovian moons did not lead
   back to a larger crater. The impact of SL9 strongly implied that the
   chains were due to trains of disrupted cometary fragments crashing into
   the satellites.

Jupiter as a "cosmic vacuum cleaner"

   The impact of SL9 highlighted Jupiter's role as a kind of "cosmic
   vacuum cleaner" for the inner solar system. Studies have shown that the
   planet's strong gravitational influence leads to many small comets and
   asteroids colliding with the planet, and the rate of cometary impacts
   on Jupiter is thought to be between two and eight thousand times higher
   than the rate on Earth

   If Jupiter were not present, these small bodies could collide with the
   inner planets instead.

   The extinction of the dinosaurs at the end of the Cretaceous period is
   generally believed to have been caused by the Impact event which
   created the Chicxulub crater, 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 much more frequent on Earth, and complex life might not have been
   able to develop . This is part of the argument used in the Rare Earth
   hypothesis.
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