The Universe in the Classroom

The Comet About to Smash into Jupiter

What will happen when the fragments hit Jupiter?

The impacts will be centered on June 19, 1994; the first is expected late at night on July 16, with an impact, on average, about every six hours. All the fragments will enter Jupiter's atmosphere at an angle of 42 degrees from the vertical and impact near a latitude of 44 degrees south, but on the back side of the planet as seen from Earth (about 10 degrees in longitude behind the edge of Jupiter, as seen from Earth). However, because Jupiter spins so rapidly (a day on Jupiter lasts only 9 hours 50 minutes), the sites will rotate into view from Earth within about 20 minutes of each impact.

Exactly what will happen as the fragments enter the atmosphere of Jupiter is very uncertain, though there are many predictions. Any body moving through an atmosphere is slowed by atmospheric drag, by having to push the molecules of that atmosphere out of the way. The kinetic energy (energy of motion) lost by the body is given to the air molecules. They move a bit faster (become hotter) and in turn heat the moving body. The drag increases roughly as the square of the velocity. In any medium, a velocity is finally reached at which the atmospheric molecules can no longer move out of the way fast enough and they begin to pile up in front of the moving body. This is the speed of sound (Mach I -- 331.7 meters/second or 741 mph in air on Earth at sea level). A discontinuity in velocity and pressure is created which is called a shock wave. Comet Shoemaker-Levy 9 will enter Jupiter's atmosphere at about 60 kilometers per second. which would be about 180 times the speed of sound on Earth (Mach 180!) and is about 50 times the speed of sound even in Jupiter's very light, largely hydrogen atmosphere.

At high supersonic velocities (much greater than Mach 1) enough energy is transferred to an intruding body that it becomes incandescent and molecular bonds begin to break. The temperature may rise to 50,000 kelvin (90,000 degrees Fahrenheit.) or more for very large bodies such as the fragments of Shoemaker-Levy 9. The effect of increasing temperature, pressure and vibration on an intrinsically weak body is to crush it and cause it to flatten and spread. Meanwhile the atmosphere is also increasing in density as the comet penetrates to lower altitudes. All of these processes occur at an ever increasing rate. The net result is that the fragile Shoemaker-Levy 9 fragments will suffer almost immediate destruction. The only real question is whether each fragment will break into several pieces immediately after entry, and therefore exhibit multiple smaller explosions, or whether it will survive long enough to be crushed, flattened and obliterated in one grand explosion and terminal fireball.

Astronomer Zdenek Sekanina, of the Jet Propulsion Laboratory in Pasadena, California, calculates that about 93 percent of the mass of a 1013-kilogram fragment, still moving at almost 60 kilometers per second, remains one second before the terminal explosion. During that last second, the energy of perhaps 10,000 100-megaton bombs is released. Much of the cometary material will be heated to many tens of thousands of degrees, vaporized, and ionized along with a substantial amount of Jupiter's surrounding atmosphere. The resulting fireball should balloon upward, even fountaining clear out of the atmosphere, before falling back and spreading out into Jupiter's atmosphere, imitating in a non-nuclear fashion some of the atmospheric hydrogen bomb tests of the 1950s.

One of the more difficult questions to answer is just bow bright these explosions will be. Sekanina calculates that a 1013--kilogram fragment, a reasonable value for the largest piece, will reach an apparent visual magnitude of -10 during the terminal explosion. This is 1,000 times Jupiter's normal brilliance and only 10 times fainter than the full Moon! However, Sekanina calculates that the explosions will occur above the clouds. There is much controversy as to exactly how deep into the atmosphere the fragments will penetrate before exploding, with other astronomers arguing that the fragments will explode beneath the visible clouds. The brightness of explosions occurring below the clouds would be attenuated by a factor of at least 10,000, making them most difficult to observe.
Final orbit of Shoemaker-Levy 9
Jupiter-centered final orbit of Shoemaker-Levy 9 as viewed from the Sun. (Courtesy P.W. Chodas)

The fireball created by the terminal explosion will spew vaporized comet material to very high altitudes as it expands and balloons upward. It may carry with it atmospheric gases that are normally to be found only far below Jupiter's visible clouds. Hence the impacts may give astronomers opportunity to detect gases which have been hitherto hidden from view. As the gaseous fireball rises and expands it will cool, with some of the gases it contains condensing into liquid droplets or small solid particles. If a sufficiently large number of particles form, then the clouds they produce may be visible from Earth-based telescopes after the impact regions rotate onto the visible side of the planet. These clouds may provide the clearest indication of the impact locations after each event.

Large regular fluctuations of atmospheric temperature and pressure will be created by the shock front of each entering fragment and travel outward from the impact sites, somewhat analogous to the ripples created when a pebble is tossed into a pond. These may be observable near layers of existing clouds in the same way that regular cloud patterns are seen on the leeward side of the mountains. Jupiter's atmosphere will be sequentially raised and lowered. creating a pattern of alternating cloudy areas where ammonia gas freezes into particles (the same way that water condenses into cloud droplets in our own atmosphere) and clear areas where the ice particles warm up and evaporate back into the gas phase.

Whether or not these "wave'' clouds appear, the ripples spreading from the impact sites will produce a wave structure in the temperature at a given level that may be observable in infrared (or thermal) maps. In addition there should be compression waves, alternate compression and rarefaction in the atmospheric pressure, which could reflect and refract within the deeper atmosphere, much as seismic waves reflect and refract due to density changes inside Earth.

The phenomena directly associated with each impact from entry trail to rising fireball will last perhaps three minutes. The fallback of ejecta over a radius of a few thousand kilometers will last for about three hours. Seismic waves from each impact might be detectable for a day, and atmospheric waves for several days. Vortices and atmospheric hazes could conceivably persist for weeks. New material injected into the Jovian ring system might be detectable for years. Changes in the magnetosphere (Jupiter's magnetic field is much stronger than that of Earth and affects an area of space tens of millions of kilometers from the planet) and/or the Io torus (particles ejected from Io's volcanoes are ionized and trapped by Jupiter's magnetic field into a donut-shaped torus completely circling the planet) caused by the sudden influx of large amounts of cometary dust might also persist for some weeks or months. There is the potential to keep planetary observers busy for a long time!

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