The near-Earth approach of a disintegrating comet in May 2006 gave scientists from the Johns Hopkins University Applied Physics Laboratory a rare opportunity to study its chemical composition.

Comet 73P/Schwassman-Wachmann 3 is one of thousands of comets originating in the Kuiper belt, a large cloud of ice and dust beyond the orbit of Neptune. Chunks of ice grow in the clouds of the Kuiper belt in much the same way hail grows in the clouds of Earth's atmosphere.

Occasionally, a large ball of ice and dust - now a full-fledged comet - escapes the Kuiper belt and takes up a years-long elliptical orbit through the solar system. Comet 73P, which was discovered in 1930, takes a little more than five years to complete an orbit of the Sun. Every 16 years it makes a close approach to Earth.

Beginning in 1995, astronomers observed comet 73P breaking up into several large fragments. Each time a comet passes near the sun, its temperature rises substantially, causing ice to fragment and vaporize into the bright tail of the comet. Many comets eventually disintegrate or melt away over a period of centuries or millenia.

Astronomers from APL and the Observatoire de Meudon in Paris took advantage of the comet's most recent flight past Earth to take detailed spectroscopic readings of two of the largest fragments of the comet. These fragments offered an unprecedented view by exposing the comet's deepest layers. The results were reported in the July 12, 2007, issue of Nature.

Observations were made over two nights from several telescopes at the Keck Observatory, located at the summit of the dormant volcano Mauna Kea in Hawaii.

Scientists were able to observe the telltale chemical signatures of water. They also find a large amount of hydrogen cyanide, a simple compound containing a single molecule each of hydrogen, carbon and nitrogen.

Surprisingly for a Kuiper belt object, however, there were only trace amounts of other molecules, especially the carbon-containing compounds that make up much of the Kuiper belt's dust cloud. This may indicate that some comets originating deep in the solar system never incorporate many of these substances as they are formed.

The researchers had expected to see substantial differences between the chemical makeup of the two fragments because most comets are thought to grow in fits and starts, creating an effect similar to the layers within a tree. Interestingly, however, the two fragments were nearly identical, indicating that comet 73P was relatively homogeneous internally.