Comet Shoemaker-Levy; Collision with Jupiter

Between 17 and 22 July, over 20 fragments of Comet Shoemaker-Levy 9 will collide with Jupiter, the largest planet in the Solar System. The comet fragments will hit the atmosphere of Jupiter at speeds of over 200,000 km/hour.

Scientists have never had the opportunity to study such a collision, and the Anglo-Australian Telescope (AAT) in northwest NSW will play an important role in these observations.

This unusual comet was discovered by Carolyn and Gene Shoemaker and David Levy on 24 March 1993 using telescopes of the Palomar Observatory in California &emdash; subsequent calculations showed that the comet was on a collision course with Jupiter. While such collisions with Jupiter are thought to occur perhaps as often as once each century, the July event will be the first time such an impact has been predicted beforehand.

Before it was pulled apart during a close pass to Jupiter, the nucleus of the comet was probably about 10km in size. Since then, the nucleus has broken into more than 20 fragments. These fragments range in estimated size from as large as 1 to 2 km diameter down to particles the size of dust grains.

The fragments of the comet are spread out along slightly different orbits around Jupiter, and appear like a "string of pearls" in images already taken with the Anglo-Australian Telescope (AAT).

Scientists have determined the impact point on Jupiter to within a fraction of a degree in latitude and longitude, and the impact times are currently known to approximately 30 minutes. Impact times for the major fragments will be known to a precision of a few minutes in the week before they occur.

The impact point will not be directly visible from Earth, but plumes of material from the impact may be visible over the edge of the planet as soon as 3 minutes after impact. The disturbed impact region will rotate into view approximately 10 minutes later. It will then remain visible for approximately 5 hours before passing out of view as Jupiter rotates.

Jupiter does not have a solid surface in the way that the Earth does. It has an atmosphere that simply gets denser and hotter with depth. As the comet fragments enter the atmosphere, they will rapidly heat up as they are slowed down and will explode, releasing most of their energy into the surrounding atmosphere. The energy release sounds large (in the order of thousands of 100 megaton H-bombs for the largest fragments) but compared to the energy content of Jupiter's atmosphere it is a relatively minor disturbance.

What happens after the impact depends on the altitude at which the fragments explode. Smaller fragments may explode at higher altitudes and a flash of light may be reflected off the larger moons of Jupiter, particularly Europa. These flashes, if visible at all, will last only 1 to 2 seconds.

Larger fragments may explode at lower altitudes and the resulting fireball will carry gases normally found only at levels well below the visible cloud tops into higher layers, where they can be studied. Shock waves will travel outwards from the impact region and may disturb Jupiter's atmosphere enough to produce observable effects.

Like all the world's major telescopes the AAT will be trained on Jupiter for this event. In fact, the location of the AAT in longitude is crucial to ensure that Jupiter can be constantly monitored. The AAT will be in a prime position to observe the impacts of four of the fragments, including one of the largest. A team of astronomers from the Jet Propulsion Laboratory (JPL) in the USA and from the Anglo-Australian Observatory (AAO) will make observations using the AAT during the entire impact period. The Principal Investigator is David Crisp (JPL), and Vikki Meadows is the AAO team member.

Over the last few months, scientists have been using the AAT to make regular observations of the comet to help refine the predicted impact times for the major fragments. During the impact the investigators will use infrared equipment on the AAT to:

  1. study the comet fragments just before impact,
  2. study the material dredged up from Jupiter's deep atmosphere, and the effects on the atmosphere of the fireball following the impact,
  3. monitor Jupiter's moons for possible reflection flashes, and
  4. study possible changes in the brightness of Jupiter's faint ring that result from the influx of comet dust.

There will be no direct or indirect effects on the Earth &emdash; only the energy released by excited astronomers! The events are occurring simply too far away.

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