A smashing time

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Between July 16 and 22 this year, the planet Jupiter will be struck by the fragments of comet Shoe­maker-Levy 9, 1993e.

Moving at very high speed, these projectiles—perhaps up to 4 km in diameter—are expected to explode deep within Jupiter’s outer layers, providing us with a unique opportunity to learn more about this enigmatic planet.

Unlike Venus, which has a solid surface lurking beneath its dense clouds, Jupiter is a giant ball of gas and fluid. The upper layers are clouds composed of droplets or crystals of ammonia and water. Beneath the clouds is a transition zone of slush overlying a liquid core of hydrogen and helium, with only a small amount of rock in the very centre.

Over the last 50 years a great deal of observational work has been done on Jupiter: spectrographic analysis, radio emissions, infra-red radiation record­ings by spacecraft, and recording of the weather patterns in the upper atmosphere. Large spots, swirls and bands apparent in photographs are thought to represent storms and jet streams, but although their intensity changes, they appear remarkably stable. The Voyager 1 and 2 spacecraft provided much of this information in March 1979.

But for all this, our understanding of the planet is very far from complete, which is why the Galileo probe was launched in 1989. As well as orbiting Jupiter, Galileo has an instrumented capsule to release (probably within the next two years), which will para­chute down, transmitting data until it is crushed by the increasing atmospheric pressure.

All the solar system bodies which show signs of extensive cratering exhibit the occasional curious chain of craters—a phenomenon most plaus­ibly explained by a swift succession of strikes by bodies such as a recently fragmented cometary nucleus. Such combina­tions of fragmentation and collision with a planet probably only occur, on average, once in tens of thousands of years; per­haps just once since our species started making any sort of astronomical observations. Although on the scale of the universe’s putative age—five billion years—a once-in-20,000­ years event is common­place, compared with the 50-year working life of the average clean-living astronomer, it is unique!

Thus the prediction of a collision of a fragmented comet, Shoemaker-Levy 9, with Jupiter in mid-July has aroused intense interest amongst astrono­mers all over the world.

Discovered photographi­cally in March last year, Shoemaker-Levy, in orbit about Jupiter rather than the Sun, already presented an unusual appearance, and was described as “squashed.” In fact, by this time the comet had already been pulled apart by the Jovian gravitational field as it swung close above the planet’s cloud tops on its 1993 approach. (Unlike planets and moons, which are large enough to de­velop their own gravita­tional fields which then hold them together, comets are aggregations of low density material that easily disintegrate.)

Earlier photographs of the area inhabited by the comet show no sign of it, as the nucleus was still intact and too faint for detection. Photographs this year, particularly those taken with the recently repaired Hubble Space Telescope (one of NASA’s greatest technical feats), show 21 fragments lined up like beads on a neck­lace, with the brightest in the middle and the faintest at the ends. Keeping perfect formation, these fragments of the original nucleus are now hurtling towards Jupiter and their terminal encounter.

More recent photo­graphs show that most of the “dust” which normally accompanies a comet fragmentation has disap­peared. This is quite unexpected, and some astronomers now think that S-L9 may have originated as an asteroid rather than a comet. In any case, the fragments, the larger ones being at least a kilometre in diameter, and perhaps as much as four kilometres, will enter the upper reaches of the Jovian atmosphere at 60 km/s (216,000 km/h, equivalent to a 1 h 40 m flight to the Moon from Earth).

On average, each fragment will be arrested after penetrating up to 150 km below the top of the cloud layer and will release energy equivalent to hundreds of thousands of megatons of TNT. This will result in a fireball such as is seen in a nuclear explosion (although the largest atmospheric nuclear explosion was a mere 58 megatons, deto­nated by the Soviet Union in 1961) which will rise far above the cloud tops and spread out in the strato­sphere for 2000-3000 km. This fireball will be, for just over half a minute, as bright as the whole planet, and its light will shine on the inner moons, Io and Europa.

Because the comet has, at the time of writing, been observed for less than half of its orbit, and there are inherent uncertainties which limit the accuracy with which the position of such a body can be pre­dicted, the times of impact (see table) are only good to within ±45 minutes, although the collision site on Jupiter is already fairly certain. Impact times of the fragments will be deter­mined to within a few minutes closer to the events.

Sweeping in from the south—above, as we see Jupiter in our sky—the train of fragments will arrive in succession over a period of five-and-a-half days. all striking between Jovocentric latitudes 44°S and 45°S. All impacts will occur on the night side of the planet, and because of the Earth’s small angular distance from the Sun as seen from Jupiter, this means that the collisions will be just over the night-side horizon (see diagram).

Although we will not have a direct view, never­theless we may be able to observe the impacts indirectly. First, there is the short-lived flash which may be seen as a brighten­ing of either of the moons Io or Europa, which, together with their outer companions Ganymede and Callisto, are easily seen with binoculars. Alternatively, as the planet rotates (a complete Jupiter rotation takes just less than 10 hours) it may bring into view signs of the distur­bance caused by the collision. To see these effects, the observer will need to be familiar with the appearance of the planet beforehand, for the changes in appearance will probably be subtle.

The spacecraft Galileo will be 240 million kms from Jupiter (roughly one third of Earth’s mean distance from the planet) at the time of impact, and will have a direct view of events. Unfortunately, its camera can capture only one photograph every two­-and-a-half seconds, and relaying the images back to earth will take many weeks, on account of equipment failures that have shrivelled its operational capacities.

Each of the impacts will he, in effect, an astrophysical experiment. Because of remoteness, extreme temperatures and/ or size of the objects of their study, astronomers cannot perform experi­ments in the classical sense. Not for them the isolated sample subjected to carefully controlled changes in physical or chemical conditions.The astronomer is condemned to wait upon the course of events and to make the best of the observational data available.

The Shoemaker-Levy 9 fragments constitute a very dirty probe, and the event will be about as elegant as an experiment by William Brown of Just William notoriety. But this is all there is on offer, and during July there will be a worldwide effort by astronomers using every type of detector to monitor the events.

The sheer energy of the collisions (some claim hundreds of times that which was released in the collision that is thought to have been responsible for eliminating the dinosaurs 60 million years ago), the extent of their shock waves, and the depth within the Jovian atmos­phere from which they may eject material is far in excess of anything we could hope to engineer.

Just what will be seen and discovered as a result of these events is highly conjectural. In general terms, however, we can reasonably expect to discover a good deal about the chemistry of the lower levels of the Jovian atmos­phere, and hope that significant light will be shed on the physics of this extraordinary atmosphere, with its coloured cyclonic storm the Great Red Spot (first seen in 1664) and complex circumplanetary cloud bands.

It is paradoxical that Venus, with its mountain­ous surface, presents us with a surprisingly bland sheet of cloud wrapping the planet from pole to pole, whereas Jupiter, innocent of such impedi­ments to the smooth flow of its atmosphere, shows a spectacular complex of globe-engirdling jet streams and chains of nonstop hurricanes in its violent atmosphere.