The Hale-Bopp comet was discovered in 1995 by two American astronomers, Alan Hale and Thomas Bopp, working independently.
Mass: 1.3 x 1016kg
Composition: Ice and rock
Orbital Period: 2,533 Earth years
Hale-Bopp is one of the brightest comets ever observed. Its closest approach to the Earth occurred in March 1997 at a distance of 1.3 astronomical units (AU), or 195 million kilometres. The comet’s coma, the dust and gases around its nucleus, could be seen by the naked eye for over 18 months. This was the brightest comet observed on Earth since Comet West in 1976. It was last detected by astronomers in the outer Solar System at 30.7 AU (4.6 billion kilometres), way beyond the orbit of Neptune around the Sun.
In April 1976, it passed within 0.7 AU (105 million kilometres) of Jupiter. The planet’s gravity could have affected the comet’s orbit. It last passed by Earth in July 2215 BC. Astronomers believe that it could have come close to colliding with Jupiter in this orbit. Hale-Bopp is expected to appear again within the inner Solar System in 4385. It is believed to be about the same age as the Sun, which is 4.5 billion years.
The comet’s brightness is due to its nucleus size and a dust cloud that extends 2.5 million kilometres across. One of the most interesting aspects of Hale-Bopp is its sodium tail. Astronomers have observed sodium emissions from the comas of other comets but not from their tails. In this case, the sodium source does appear to be the inner coma.
Hale-Bopp’s tail consists of neutral sodium atoms and extends for about 50 million kilometres. These sodium atoms are visible only with very powerful instruments and special filters. The sodium tail was positioned between Hale-Bopp’s white dust tail that followed the comet’s trajectory and the blue gas tail that pointed away from the Sun. The comet also has a concentration of deuterium (heavy water) that is twice the amount found in the Earth’s oceans. It is the first comet in which argon gas was detected.
There is only a remote possibility of the comet colliding with the Earth. But such an event would be about 44 times more powerful than the Cretaceous-Paleogene impact that caused the extinction of the dinosaurs. It would probably wipe out all life on Earth. A New Age group based in San Diego in California committed mass suicide in March 1997 when the comet was sighted in the belief that they would be transported to a higher level of existence.