THE HALE-BOPP COMET

WE CAN SEE A COMET!

The Hale-Bopp comet was discovered in July 1995 but it has not become very well visible with the naked eye up to now. On other hand, the Hale-Bopp is believed to be the most active comet of the last 400 years. These two reasons have attracted not only the attention of the astronomical community but of the general public too.

The brightness of a comet is not dependent on its activity, but on its distance from the Sun and the Earth. When the Hale-Bopp is at its closest point to the Earth, it will still be further away than the Hyakutake comet which we were able to see during the spring of 1996. For this reason it is speculated that the Hale-Bopp will be slightly brighter than the Hyakutake. Anyway, what may seem spectacular for an astronomer may not be more than interesting for the general public. Gary Kronk, the great comet observer, recollects when he observed a comet with a friend and his friend questioned: 'Is that all?. Thanks for getting me up in the middle of the night to see a smudge in the sky'.

WHERE DO WE HAVE TO LOOK?

The comet is visible now at dawn, when it is still dark enough and it is brighter than the stars of the Great Bear. The Hale-Bopp is at nearly 20 degrees in the northeast sky, from 6:00 am to 6:30 am from La Palma. Initially it looks just like another star, no brighter than the other stars in the same area. To identify the comet it is best if you use an optical instrument. This optical aid does not need to be too powerful, binoculars are sufficient. With the binoculars the star will appear as a diffuse object with a very intense spot in the center and two tails. It is important to stress the fact that observations must be carried out in a dark area, without lights or full moon and that the horizon towards the northeast should be free of obstacles (buildings, mountains, etc).

In mid March the comet will be closer to the horizon and so its visibility will be worse. At the end of this month, beginning of April, the Hale-Bopp will be at its brightest moment, and will be visible at dusk, after the Sun has disappeared and the sky becomes dark. From the 26th of March to the 12th of April the conditions for the observation of the comet will be extraordinary. The tail of the comet is expected to have a length of 20 to 30 degrees, 40 to 60 times the diameter of the Moon.

From the end of April up to June, the Hale-Bopp will move towards the south, getting closer and closer to the Sun. As it approaches the Sun its brightness will gradually diminish. During the last days of April the visibility will not be as good due to the Full Moon. The tail will be lager in May and June, but not as bright as in March. From June onwards, the comet will no longer be visible in the Northern Hemisphere.

WHAT IS A COMET?

A comet is a small body that orbits around the Sun in a similar manner to how the Earth and the other planets of the Solar System also go around the Sun. This part is called the nucleus and is composed of ice, dust and rocks. Fred Whipple, an expert on comets, defined a comet in 1950 as a 'dirty snowball'. As the comet approaches the Sun, the heat sublimes the ice, and the ice goes from the solid to gas, dragging atoms and molecules with it. As the material is expelled it give place to two phenomena both visible in the comet: the coma (in Latin hair) and the tails. It is impossible to distinguish the nucleus of a comet with the naked eye. The coma is a type of atmosphere that evolves the nucleus, and that can reach up to an extension of millions of kilometers. The tail is made up of particles that the comet leaves behind it. Generally it is possible to observe two types of tails: dust tails and gas tails. The gas tail is the more frequent of the two. The tail points away from the sun caused by the wind which blows from the Sun (this is so even when the comet is traveling away from the Sun!), and in some cases an anti-tail can be observed in the opposite direction.

The Hale-Bopp follows a parabolic orbit with a period between 4000 and 3000 years, reaching the closest point to the Earth within its orbit on the 22nd of March and will pass by the closest point to the Sun on the 1st of April. The Hale-Bopp has both gas and dust tails.

It is important to remember that comets are not the same as meteorites or falling stars. While meteorites are small particles falling into our atmosphere from a height of 100 km and that are seen for one or two seconds, comets are further away than the Moon and move very slowly in relation to other stars making it almost impossible to appreciate the movement. As in the case of other celestial bodies, a comet rises and sets everyday. Nowadays it is considered possible that most meteor showers are produced by residual particles from comets. For example, the showers of Perseids in the summer months are related to the Swift-Tuttle comet. If these particles are large, they are called meteorites. It is considered possible that some meteorites may have been part of ancient comets.

HOW FREQUENTLY CAN WE OBSERVE SPECTACULAR COMETS?

It depends on what we call spectacular comets, but the average is about one every 20 years, i.e. about three in a life time. The increase of light pollution will make observations of the Hale-Bopp difficult. The light pollution problem is such that it will be extremely difficult to see the comet in a large town. The observing conditions in La Palma will be extraordinary as it is one of the few places in the world with laws that regulate the light pollution and external lighting. It will be a very good chance to enjoy the sky we protect.

Even so, many people will feel defrauded. This is because people think it's possible to see the comet with naked eye with the same appearance as in pictures. But these pictures are taken with special detectors that register light during long intervals of time, very different to what happens with the human eye. It's curious to think that, due to the fast evolution of science and technology, the same image that does not attract out attention today caused terror and anguish in out ancestors.

The main comets of this century have been:

1910 Halley
1927 Skjellerup-Maristany
1965 Ikeya-Seki
1970 Bennett
1976 West
1996 Hyakutake
1997 Hale-Bopp?

It seems that we were born in the right year!

WILL IT BE THE COMET OF THE CENTURY?

It is difficult to define what the comet of the century is. In 1985 and 1996 the Halley comet attracted a lot more attention and could be studied at a few kilometers away thanks to echo sounders. Another important cometary event was the impact of the Shoemaker-Levy 9 comet with Jupiter in 1994, which was observed by ING telescopes in life. On the other hand, Hyakutake and Hale-Bopp? have not been the brightest comets of the century. In 1976, for example, the West comet gave a greater spectacle. The question is that each time a new comet is visible there are better technical equipment for the observations and more effective means of communication. This has given a greater importance to comets like Hyakutake and even more so to the Hale-Bopp.

It is difficult to say if the Hale-Bopp will be very bright, i.e. as bright as Jupiter, or not. David Levy, a greatest comet enthusiast of all times, once said that 'comets are like cats: both have tails and do exactly as they please'.

THE ING TELESCOPES PLAY AN IMPORTANT PART IN OBSERVATIONS OF THE COMET

Once again the ING telescopes will play an important part in an international campaign of observations. The ING telescopes together with several others situated also in the Canary Islands, will be used by the European Group to study the Hale-Bopp. This group is formed by a total of approximately 40 astronomers from 10 different countries. As has been happening in the last months, and more intensely during the months of March and April, astronomers have been taking turns in coming to La Palma to take images and spectra of the comet.

WHY IS IMPORTANT FOR US TO STUDY THE HALE-BOPP?

There are mainly two reasons: firstly because we believe that comets are ancestral bodies that were created during the origins of the Solar System. Comets are thus considered as fossils that let us study the physical and chemical conditions that existed initially in the Solar System. Secondly, it is common knowledge that some comets have collided with the Earth and it is believed that it was due to one of these collisions that molecules were deposited on the Earth and were the cause of life. It is also believed that comets played an important role in the formation of the atmosphere and primitive seas. So studying the chemical composition of the comets we can discover more things about the origins of the Solar System and life on Earth. It is probable that our knowledge of the chemical composition of comets will have increased greatly after the Hale-Bopp comet has gone by because this comet will become one of the best studied due to its brightness and how long it has been visible.

Currently most comets are found outside the solar system, in a cloud of gas and dust that has been practically intact during hundreds of thousands of years. These regions are known as the Oort Cloud and the Kuiper Belt. The Oort Cloud is formed of long orbit comets. Due to collisions between themselves o due to gravitational forces, the comets abandon the cloud and penetrate in the solar system, becoming visible from Earth if the orbit is near to ours. This cloud has not been detected directly. The Kuiper belt is nearer than the Oort Cloud and is composed of comets with shorter orbit periods. The first time an object from the Kuiper Belt was detected directly was in 1992. Since then the Isaac Newton Group of Telescopes has contributed in a crucial way to the discovery of other objects from the belt and in the determination of their orbits. We know that the Hale-Bopp comet does not belong to either the Oort Cloud nor to the Kuiper belt.

HOW BIG IS THE HALE-BOPP

Most comets have a nucleus with dimensions between 1 and 10 km. The Halley comet was an irregular body of 8x13 km. It is very difficult to calculate the dimensions of the nucleus of a comet form Earth as it is hidden behind the coma of the comet. Some indirect estimates indicate that Hale-Bopp is not larger than 10 or 15 km, even though other more optimistic calculations done by the Space Telescope calculate 40 km. Hyakutake was 5 km in diameter.

HOW FAR AWAY FROM THE EARTH WILL THE HALE-BOPP WHEN CLOSEST TO THE EARTH?

The Hale-Bopp will be 197 million kilometers away from the Earth when it's orbit takes it to the nearest point to the earth. This will happen on the 22nd of March. The Hyakutake comet was 15 million kilometers away when closest to the Earth.

For a comet to collide with the Earth, two factors need to happen: for the orbits of the objects to coincide in a point, and for both object to be at that point at the same time. The Hale-Bopp could never collide with the Earth as it has no common point with the Earth's orbit.

HOW WAS THIS COMET DISCOVERED?

This comet was seen for the first time on the 23rd of July 1995, when two amateur astronomers were observing a group of stars called M70 in Sagittarius. The two American astronomers were called Alan Hale, PhD in Astrophysics, and Thomas Bopp. As it is tradition to call the comets by the name of the person/s who discover them, the comet C/1995 O1 (this is the astrophysical nomenclature) is known as Hale-Bopp. When Alan and Thomas made their discovery, the comet was further away from the Earth than any other comet ever discovered by amateur astronomers.

The Isaac Newton Group of Telescopes is run by The United Kingdom and The Netherlands and consists of the William Herschel (4.2 m), the Isaac Newton (2.5 m) and the Jacobus Kapteyn (1.0 m) telescopes. William Herschel telescope is the most powerful telescope of the Roque de Los Muchachos Observatory, La Palma, Spain.