Comet Shoemaker-Levy 9 Impact on Jupiter
Jupiter Explosion - A Comet's Tale

Comet Shoemaker-Levy 9 Impact on Jupiter
Jupiter Explosion - A Comet's Tale 

Orbital studies of the comet Shoemaker-Levy 9 soon revealed that it was orbiting Jupiter rather than the Sun, unlike all other comets known at the time.

Intense studies of the comet were undertaken, and as its orbit became more accurately established, the possibility of a collision became a certainty. 

Comet Shoemaker-Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of solar system objects.

This generated a large amount of coverage in the popular media, and the comet was closely observed by astronomers worldwide.

The collision provided new information about Jupiter and highlighted its role in reducing space debris in the inner solar system.

The comet was discovered by astronomers Carolyn and Eugene M. Shoemaker, David Levy and Philippe Bendjoya from France. Shoemaker-Levy 9, at the time captured by and orbiting Jupiter, was located on the night of March 24th, 1993, in a photograph taken with the 40 centimetres (1 ft 4 in) Schmidt telescope at the Palomar Observatory in California.

It was the first comet observed to be orbiting a planet, and had probably been captured by the planet 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 Roche limit, and Jupiter's tidal forces had acted to pull the comet apart.

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 16th and July 22nd, 1994, at a speed of approximately 60 km/s (37 mi/s). The prominent scars from the impacts were more easily visible than the Great Red Spot and persisted for many months.

While conducting a program of observations designed to uncover near-Earth objects, the Shoemakers and Levy and in company of a young French post-doc student Bendjoya discovered Comet Shoemaker-Levy 9 on the night of March 24th, 1993 in a photograph taken with the 0.4 m (1.3 ft) Schmidt telescope at the Palomar Observatory in California.

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, hence its name.

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 IAU Circular 5725 on March 27th, 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 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 line of sight 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.

Orbital studies of the new comet soon revealed that it was orbiting Jupiter
rather than the Sun, 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 apojove (furthest distance from Jupiter) of 0.33 astronomical units (49 Gm). Its orbit around the planet was highly eccentric.

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.

Several other observers found images of the comet in precovery images obtained before March 24th, including Kin Endate from a photograph exposed on March 15th, S. Otomo on March 17th, and a team led by Eleanor Helin from images on March 19th.

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 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 toward 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.

An SL-9 impact site on Jupiter, July 6, 1994. Photo by Hubble Space Telescope.

Shoemaker-Levy 9 (SL-9) was a short-period comet that was discovered by Eugene and Carolyn Shoemaker and David H. Levy. As the comet passed close by Jupiter, Jupiter's gravitational forces broke the comet apart.

Fragments of the comet collided with Jupiter for six days during July, 1994, causing huge fireballs in Jupiter's atmosphere that were visible from Earth.

The comet had apparently passed extremely close to Jupiter on July 7th, 1992, just over 40,000 km (25,000 mi) above the planet's 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 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 7th 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 (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.

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 (3.1 mi) 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.

This animation was produced by Walt Feimer in the Astronomy Visualization Laboratory at the Space Telescope Science Institute.

It is derived from a composite Hubble Space Telescope (HST) image taken in visible light showing the temporal evolution of the brightest region of comet P/Shoemaker-Levy 9.

In this false-colour representation, different shades of red colour are used to display different intensities of light.

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.

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 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 1995.

While the impacts took place on the side of Jupiter hidden from Earth, Galileo, then at a distance of 1.6 AU 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 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.

The first impact occurred at 20:13 UTC on July 16th, 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 (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 6 days, 21 distinct impacts were observed, with the largest coming on July 18th at 07:33 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 (600 times the world's nuclear arsenal).

Two impacts 12 hours apart on July 19th created impact marks of similar size to that caused by fragment G, and impacts continued until July 22nd, when fragment W struck the planet.