OASI: Shoemaker-Levy 9

The last half of July 1994 witnessed much interest among astronomers and the wider public in the collision of comet Shoemaker-Levy 9 (SL9) with Jupiter. The comet was discovered on 25 March 1993 by Eugene and Carolyn Shoemaker and David Levy, using the 450 mm Schmidt camera at the Mount Palomar Observatory. The discovery was based on a photographic plate exposed two days earlier. The Shoemakers are particularly experienced comet hunters with 61 discoveries to their credit. Their technique relics on the proper motion of a comet to identify the object as a non-stellar body. They photograph large areas of the sky, typically with an eight minute exposure, and repeat the photograph 45 minutes later. Comparison of the two photographs with a stereo-microscope reveals any bodies which have moved against the background of fixed stars.

As so often in science, serendipity played a large part in the discovery of Shoemaker-Levy 9! The weather on the night of 23 March was so poor that the observers would not normally have bothered putting film into their camera. However, they had a box of old film to hand which had been partially exposed by accident some days previously, so decided to insert it into the camera rather than waste good film. Fortunately, two of the film plates, despite being fogged round the edges, captured the first image of a very strange, bar-shaped object. This object, which Carolyn Shoemaker first described as a squashed comet, later became known as comet Shoemaker-Levy 9.

Other, more powerful, telescopes revealed that the comet was, in fact, composed of 21 cometary fragments, strung out in a line, which accounted for the unusual shape. The term string of pearls was soon coined. Figure 1 is a mosaic taken by the Hubble Space Telescope (HST) during 24-27 January 1994. It shows the main fragments which at that time spanned a linear distance of approximately 600,000 km. Initially the fragments were surrounded by extensive dust clouds in the line of the nucleii but these later disappeared. Some of the nucleii also faded out (presumably due to disintegration), while others split into multiple fragments.

The use of large telescopes enabled the precise positions and velocities of the cometary fragments to be obtained and this facilitated calculation of their orbits. At the time of their discovery, the fragments were located approximately 50 million km from Jupiter, all co-orbital in a highly elliptical orbit. Calculations revealed that, prior to its disintegration, the comet had been in a periodic orbit around the Sun. It was captured by Jupiter at some unknown date and, on 07 July 1992, passed approximately 21,000 km above the Jovian cloud tops. The gravitational forces associated with such a close approach caused the comet to disintegrate, creating the 21 fragments visible at the time of its discovery.

The size of the original comet and each of the fragments was, and still is, something of a mystery. The first analysis of the orbital dynamics of the fragments suggested that the comet was originally some 2.5 km in diameter with an average fragment diameter of 0.75 km. Later work gave corresponding diameters of approximately 10 km and 2 km and these values are now considered more likely. There was considerable variation in the diameters of different fragments.

Further calculations revealed that the cometary fragments were on course to collide with Jupiter during July 1994, and that each fragment could deliver an energy equivalent to approximately 500,000 million tonnes of TNT. The prospect of celestial fireworks on such a grand scale immediately captured the attention of astronomers worldwide!

Each fragment was assigned an identity letter A-W (letters "I" and "O" were not used to avoid potential confusion with numbers "1" and "0") and a co-ordinated program of observations was put in place world-wide to track their progress towards impact with Jupiter. As the cometary fragments reached the cloud tops of Jupiter, they were travelling at approximately 60 km/s and the chain of fragments had spread out to cover approximately 30 million km. The impacts occurred during 16-22 July. All took place at a latitude of approximately 48°S which nominally placed them in the SSS Temperate Region however, visually, they appeared close to the Jovian polar region. Although the impacts all occurred some 10-15° round the limb on the far side of the planet as seen from Earth, the rapid rotation of the planet (a Jovian day is only some 10 hours long) soon carried them into the view of Earth-based telescopes. The collisions lived up to all but the wildest expectations and provided a truly impressive spectacle.

Jupiter is composed of a relatively small core of iron and silicates surrounded by hydrogen. In the depths of the planet (approximately 1000 km and more below the visible cloud tops) the hydrogen is so compressed that it is metallic in form; further from the centre, the pressure is lower and the hydrogen is in its normal molecular form. The Jovian cloud tops visible from Earth consist primarily of methane and ammonia with relatively small amounts of other elements and compounds which are thought to be responsible for the colours seen in the atmosphere.

The smaller cometary fragments plunged into Jupiter, rapidly disintegrated and left little trace; three of the smallest fragments, namely T, U and V left no discernible traces whatsoever. However, many of the cometary fragments were sufficiently large to produce a spectacular display. Each large fragment punched through the cloud tops, heated the surrounding gases to some 20,000 K on the way, and caused a massive plume or fireball up to 2000 km in diameter to rise. Before encountering thicker layers of the atmosphere and disintegrating in a mammoth shock wave, the large fragments raised dark dust particles and ultra-violet (UV) absorbing gases high into the Jovian cloud tops; in visible light, this material manifested itself as a dark scar surrounding the impact site.

The HST took many fine pictures of the collisions using its Wide Field Planetary Camera. Figure 2 is a view of the impact sites of fragments C, A and E (in that order from left to right across the disk). The satellite Io may also be seen, passing above the equator of the disc. The photograph was taken on 17 July. The impact sites, all near the planet's south polar region, are considerably more prominent in UV light than visible light (because of the presence of UV-absorbing gases). Some idea of the scale of the impact sites may be gained from comparison with the Great Red Spot, visible on the eastern limb of the planet, which has a major axis approximately equal to the diameter of the Earth.

Fragment G, one of the larger components of the comet, suffered a particularly impressive collision with Jupiter (at approximately 07:30 UT on 18 July.) Figure 3 shows the impact plume associated with the collision as seen by the HST, with views in several wavelengths. The plume is similar to the mushroom cloud associated with a nuclear explosion. In the figure, the collision site is still on the far side of the planet but the plume has reached sufficient altitude that it is visible round the limb. As explained above, the plume is most prominent in blue light. Figure 4 gives an Earth-based view of the plume taken by the Keck telescope on Mauna Kea (Hawaii). Figure 5 shows HST photographs taken once the impact site was carried by the planet's rotation into visibility from the Earth. The impact site bears a certain resemblance to a sunspot with a dark central region surrounded by lighter shading.

Some days after collision the impact sites began to evolve and fade as they became subject to the dynamics of Jupiter's atmosphere. No-one knows how long they will remain visible from Earth, but it is thought that the larger scars may persist for a year or more. The interest of professional astronomers in Jupiter is now waning and valuable work can therefore be performed by amateurs in tracking the evolution of the collision scars. The scars are easily visible in a modest telescope, and a large reflector will show them in some detail. There is scope for valuable observing work from now until Jupiter reaches conjunction with the Sun in November 2004.

Astronomers and archivists are now searching old records for possible previously unrecognised impacts on Jupiter. Several spots were reported from 1690 to 1872 by observers including William Herschel and Giovanni Cassini. The records of the BAA in 1927 and 1948 contain drawings of Jupiter with black dots or spots visible. It is possible that comet impacts have been observed before, without their identity being realised, but no-one can be sure.

Fig. 2. Impacts C, A & E (L-R). (HST, 17 July 1994.)

Fig. 3. Plume caused by impact G. (HST, 18 July 1994.)

Fig. 4. Fireball caused by impact G. (Keck Telescope, 18 July 1994.)

Fig. 5. Impact G scar. (HST, 18 July 1994.)

Observations at Orwell Park Observatory

During the evening of 20 July 1994, several members of OASI, observing with the 26 cm refractor at Orwell Park, witnessed the immediate aftermath of the collision of two fragments with Jupiter. The collisions occurred on the side of the planet facing away from the Earth (as indeed all did) but nonetheless, the rapid rotation of Jupiter soon carried the impact scars into view. Both were dark, almost black, and could not be missed. The observers judged that the scars were larger than the Great Red Spot, although the latter was not visible at the time.