Two astronomers, Alan Hale of New Mexico and Thomas Bopp of Arizona, observed a large and bright comet, seen just outside Jupiter's orbit. Discovered on July 23, 1995, this famous comet was carefully analyzed using Hubble Space Telescope. Hale-Bopp comet is famous for holding a record for the longest period of naked-eye visibility. It is predicted that this comet will not appear in another 2400 years.

Comet Hale-Bopp (formally designated C/1995 O1) was perhaps the most widely observed comet of the 20th century and one of the brightest seen for many decades. It was visible to the naked eye for a record 18 months, twice as long as the previous record holder, the Great Comet of 1811.

Hale-Bopp was discovered on July 23, 1995, at a great distance from the Sun, raising expectations that the comet would brighten considerably by the time it passed close to Earth. Although predicting the brightness of comets with any degree of accuracy is very difficult, Hale-Bopp met or exceeded most predictions when it passed perihelion on April 1, 1997. The comet was dubbed the Great Comet of 1997.

Sodium Tail
Comet Hale-Bopp's neutral sodium tail (the straight tail extending up to the left from the nucleus)
One of the most remarkable discoveries was that the comet had a third type of tail. In addition to the well-known gas and dust tails, Hale-Bopp also exhibited a faint sodium tail, only visible with powerful instruments with dedicated filters. Sodium emission had been previously observed in other comets, but had not been shown to come from a tail. Hale-Bopp's sodium tail consisted of neutral atoms (not ions), and extended to some 50 million kilometres in length.

The source of the sodium appeared to be the inner coma, although not necessarily the nucleus. There are several possible mechanisms for generating a source of sodium atoms, including collisions between dust grains surrounding the nucleus, and 'sputtering' of sodium from dust grains by ultraviolet light. It is not yet established which mechanism is primarily responsible for creating Hale-Bopp's sodium tail, and the narrow and diffuse components of the tail may have different origins.

While the comet's dust tail roughly followed the path of the comet's orbit and the gas tail pointed almost directly away from the Sun, the sodium tail appeared to lie between the two. This implies that the sodium atoms are driven away from the comet's head by radiation pressure.

Hale-Bopp, the most dazzling comet to grace our skies in the 20th Century, has drifted beyond the orbit of Neptune, over 30 AU from the sun (1AU = one sun-Earth distance).

Curious astronomers from Hungarian, U.S. and Australian research institutions decided to track the icy vagabond as it continued its outward journey on an epic 2,500 year orbit.

Observing the comet in 2007, Szabó’s team tracked Hale-Bopp when it was 25.7 AU distant. Back then, it still had a prominent coma suggesting that although the comet was so far away from the sun, its icy nucleus was still producing gas and dust.

This surprising observation raised the question where this activity will cease. It appears there is now has an answer: Compared to previous observations, the general appearance of the comet has drastically changed during the past number of years. The comet appears to have dimmed dramatically, potentially indicating that cometary activity has ceased. Although recent observations suggest there is some coma-like material surrounding the nucleus, it might be a cloud of left-over dust from previous outbursts being dragged with the comet’s passage through the solar system.

Interestingly, the researchers calculated the comet’s surface temperature during the 2007 observation campaign, at 53.1 K (-220 Celsius or -364 Fahrenheit). So, if cometary activity has completely ceased, they are able to put constraints on the temperature at which comets, like Hale-Bopp, become frozen to death.

“Since Hale-Bopp seems to turn into inactive state, it can be described or inferred that the temperature of activity cessation is somewhere between 50-53 K for a Hale-Bopp type comet.”

Understanding how comets act as they pass through the solar system is of great importance, especially if one should be passing near-Earth orbit. Outgassing of ices can cause deviations in the comet’s path potentially making them unpredictable chunks of ice should they steer in our direction. Knowing at what distance (indeed, what temperature) cometary activity stops will help characterize future comet sightings.

On March 22, 1997, a super bright comet by the name of Hale-Bopp made its closest approach to Earth. It was bright enough for people to see without telescopes or binoculars for over 18 months. Comet Hale-Bopp still holds the record for being visible to the naked eye for longer than any other comet, and it was probably the most-viewed comet in history. It passed by Earth at a safe distance of about 120 million miles before continuing its orbit around the sun. As it got closer to the sun, its two blue and white tails grew bigger and brighter. By the time it made it to the sun on April 1, it was shining brighter than every star in the sky except for Sirius.