Discovery

Comets in the Sky

This month sees a hail of comet fragments rain down upon Jupiter, as the remains of Comet Shoemaker-Levy drop into the planet's atmosphere. Between July 16th and 22nd, more than 20 pieces of the shattered comet will hit the huge planet, producing bright fireballs hundreds of kilometres in height.

The impacts will occur on the side of Jupiter facing away from Earth, but astronomers will get a view of the aftermath of each hit some 30-60 minutes later. Jupiter is well-placed in southern skies at present, being bright and high in the western sky, and southern observatories will be waiting to see how Jupiter's thick atmosphere of methane and ammonia reacts to the massive impacts.

Comet watching has a long distinguished history, dating back more than 2,000 years in written records. Halley's Comet was observed by the ancient Chinese, it shone above the battlefield of Hastings in 1066, and stretched across New Zealand skies in 1910.

Comets are conglomerations of solid rock, dust, ice and frozen volatile gases such as ammonia and methane. As they fall towards the Sun, the gases heat up and vaporise in spurts and jets. This "outgassing" produces a reflective coma or head, which is followed by a million kilometres or more of tail streaming behind it.

Comet Shoemaker-Levy may have looked like this in previous passages through the solar system, but in 1992 it was caught in Jupiter's gravitational field. It won't escape.

How to Make a Comet

Line a large plastic mixing bowl with a thick plastic bag.
Add two cups of water, two tablespoons of sand or dirt, a small rock, a dash of ammonia, and a dash of organic material (molasses or golden syrup works well).
To this mixture, add two cups of crushed dry ice (frozen carbon dioxide).
Stir vigorously until the mixture is almost totally frozen.
Use the bowl liner to shape the "comet" into a ball and unwrap for display.

As it begins to warm up, the carbon dioxide (CO₂) starts to sublimate, going directly from a solid to a gas. This mirrors what happens to comets when they are heated by the Sun. Small jets of gas escape where the CO₂ is vented through small holes in the still-frozen water -- outgassing.

After several hours, the comet develops a series of craters as the CO₂ sublimates away, through the water ice. Eventually, all the frozen material will melt, leaving the rocky heart behind.

A potentially more interesting version, if somewhat less active, is the comet-as-sundae approach:

Put half a banana in a glass dish.
Mix two tablespoonfuls of whole and finely chopped nuts with a cup of softened Neapolitan ice-cream (any multi-coloured ice-cream will do to represent the different frozen gases).
Pack this symmetrically around the banana.
Eat it while thinking about comets.