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Comet Hale-Bopp

Streaking across the heavens

In the spring of 1997, the night sky provided a spectacular light show as Comet Hale-Bopp, one of the brightest comets of the century, traversed the heavens. The comet was more closely studied than any before it, and scientists continue to make discoveries based on the data gathered by terrestrial and orbital telescopes and instrumentation.

Originating primarily from the Oort Cloud, a belt of stellar debris ringing our solar system, comets are balls of water ice interspersed with other elements and compounds. Occasionally, a passing star or one of the larger planets perturbs the orbit of one of the balls of debris, tipping it into the solar system toward the Sun on an elliptical orbit. As the comet approaches the Sun, the nucleus of the comet heats. Gas and dust begins to boil off, creating a bright coma around the head and trailing tails of dust and ionized gases, stretching for hundreds of thousands of kilometers behind the nucleus. Depending on the size of the orbit and whether it originates in the solar system or in the Oort cloud, a comet can return in a handful of years or in several thousand years.

On July 22, 1995, two men peered into their telescopes to view an obscure star cluster called M70. One was concrete company worker and amateur astronomer Thomas Bopp, who had taken a group of friends out into the Arizona desert for a star party. The other was professional astronomer Alan Hale, head of the Southwest Institute for Space Research, who was conducting his own observation session hundreds of mi (80-90 million km) away in New Mexico. Within minutes of the same time, it was later determined, each of the men noticed small fuzzy spot near galaxy M70, a spot where none had appeared previously. Both Hale and Bopp reported their findings by email to the International Astronomical Union, which verified the discovery of Comet 1995-01. Shortly thereafter, the object was renamed Comet Hale-Bopp.

Hale-Bopp was one of the most spectacular comets to appear this century, remaining visible to the naked eye for more than 19 months. At its peak, the comet was as bright as the most brilliant stars in the sky, with a tail that stretched 50-60 million mi (80–90 million km). Orbital analysis shows that the comet last passed through the inner solar system in 2213 B.C.; it will next visit in the year A.D. 4300. During its recent visit, it passed within 122 million mi (196 million km) of Earth and a mere 85 million mi (137 million km) from the Sun.

Boasting a cometary nucleus approximately 25 mi (40.25 km) in diameter, Hale-Bopp was more than four times as large as Halley's comet; later analysis showed that the primary nucleus may have a lesser companion of approximately half the size. As the comet streaked through the sky at about 98,000 MPH (157,789 km/h) at perihelion, the nucleus was rotating with a period of about 11.34 hours.

Most comets feature two tails. One, a streak of dust and debris emitted by the nucleus, trails behind the comet. The other, a stream of ionized gas stripped off by the solar wind, faces away from the sun. Hale-Bopp, however, boasted a third tail consisting of electrically neutral sodium atoms.

Astronomers from the Isaac Newton Telescope on La Palma, Italy, observed the tail while imaging the comet using a special filter that rejected all wavelengths of light but that emitted by sodium atoms. In most comets, sodium emission is observed only around the nucleus. Instead of observing the expected small halo of light around the nucleus of Hale-Bopp, however, the group discovered a tail 372,000 mi (600,000 km) wide and stretching out 31 million mi (50 million km) behind the comet. Though scientists do not fully understand the phenomenon, they believe that the tail was formed as sodium atoms released by the coma interacted with the solar wind and fluoresced.

From a scientific point of view, comet Hale-Bopp generated a tremendous amount of data and insight in the astronomical and astrophysical community. Soon after its discovery, scientists began to study the comet; by the time it made its pass around the sun, plans for observation and data gathering were well in place. To obtain ultraviolet data that is normally blocked by the earth's atmosphere, for example, NASA sent three instrument packages up on suborbital rockets to gather data in five-minute intervals.

Astronomers assert that comets were formed of the debris left behind during the early stages of the formation of the solar system. A study of hydrogen cyanide molecules ejected by Hale-Bopp provided astronomers with evidence linking icy nuclei of comets to the ices found in interstellar gas clouds, which are believed to condense into stars and planets. By studying the chemistry of the comet with techniques such as spectroscopy, astronomers hope to better understand the conditions under which the Sun and planets formed.

Using radio observations, scientists discovered eight molecules that had never before been seen in a comet, including sulfur monoxide, and other unique organic molecules and isotopes.

As the comet hurtles toward the outer solar system, astronomers continue to observe it, studying the differences in gas emission patterns, monitoring the evolution of the cooling nucleus, as well as the shrinking tail. New discoveries about the nature of comets and of our solar system will undoubtedly result from the analysis of this data and that collected during the spectacular transit of Hale-Bopp through our skies.

Kristin Lewotsky

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