Giant Comets: Hyakutake and Hale-Bopp

By Hal Weaver, cometary scientist This past spring we were treated to a rare astronomical event. On March 25th, Comet Hyakutake, officially known as C/1996 B2, passed within 9.3 million miles of the Earth -- one-tenth the distance to the Sun, and became the most active comet to come this close to us in nearly 450 years. For those who could escape the glare of city lights, Hyakutake put on a fantastic visual display, sprouting a long narrow tail spanning more than thirty degrees across the sky. (If you make a fist and sight along the length of your arm, the angle covered by the width of your fist is roughly ten degrees.)

And the comet feast may not be over yet! A monster comet, possibly more than ten times larger than Hyakutake, is on its way to a rendezvous with the Sun on April Fool's Day of 1997. During the spring of next year Comet Hale-Bopp, also known as (C/1995 O1), promises to put on a show that should rival that of the recently departed Hyakutake.

Fortunately, neither Hyakutake nor Hale-Bopp pose any danger to the Earth. However, comets like them have certainly impacted the Earth in the distant past and will undoubtedly strike again in the future. The information below is provided as background for the NOVA special The Doomsday Asteroid, which is being broadcast on July 9th.

A comet is composed of a nucleus, a coma, and two kinds of tails. The nucleus, which is typically only about 1 to 10 miles across, is at the center of the comet. Fred Whipple, the Harvard astronomer who is the father of cometary science, dubbed the nucleus a giant ``dirty snowball''. That description was thought to be appropriate because cometary nuclei were known to contain a mixture of ices (the ``snowball'' component) and meteoritic material (the ``dirt''). However, based on recent infrared observations, some astronomers now think that the nucleus is more like an ``icy dirtball'' with a much higher proportion of dirt to ice than was originally thought. Except when various spacecraft flew near the nucleus of Comet Halley in 1986, no one has ever seen a comet nucleus directly.

As the nucleus moves closer and closer to the Sun, it gets warmer and warmer, which causes its ices to evaporate. When the ices evaporate, they drag Dust particles (i.e., the meteoritic material) off of the surface of the nucleus. Sunlight reflecting off of these dust particles produces a coma, which is primarily what you see when you look at a comet.

The dust particles leaving the nucleus are pushed by light from the Sun into a dust tail. The gas molecules (like water, carbon monoxide, and carbon dioxide) that evaporated from the ices are ionized by sunlight (the sunlight tears off one of their electrons) and are pushed by the solar wind into an ion tail. This beautiful picture of comet West, which became a spectacular visual object in 1976, shows both dust and ion tails.

Comets are members of our Solar System. But unlike the Earth and other planets, which always stay at approximately the same distance from the Sun, most comets are great travelers that spend most of their time on the outskirts of the Solar System, way beyond Pluto and then wisk in briefly for a close pass near the Sun.

The comets that pass close to the Sun originally came from one of two places: either the Oort Cloud or the Kuiper Belt. You can think of the Oort Cloud as a giant spherical shell surrounding the solar system that's filled with about 1 million million comets (a 1 followed by 12 zeros). Its inner and outer boundaries are not very well defined, but the cloud's extent is roughly from about 900 thousand million miles (a 9 followed by 11 zeros) from the Sun to about 9 million million miles (a 9 followed by 12 zeros) from the Sun. The outer boundary is almost halfway to the nearest star and, for all practical purposes, is the outside edge of our Solar System. Because they are so far from the Sun, the comets in the Oort Cloud take over 1 million years to make a single revolution around the Sun. Occasionally one of these comets feels a tug by another star in our galaxy and gets pushed closer to the Sun. Since such comets are probably making their first such trip near the Sun, they are called "new" comets. Approximately a dozen "new" comets are discovered every year.

The Kuiper Belt refers to a roughly disk-shaped region that lies in the same plane that contains the Earth and the other planets and which extends from just beyond Pluto's orbit out to about twice Pluto's orbit. (Pluto is about 40 times farther from the Sun than we are.) Astronomers believe that most of the comets that travel around the Sun in fewer than about 200 years originally came from the Kuiper Belt. These so-called "periodic," or "old," comets slowly drifted closer to the Sun over time. When they approach Jupiter, their paths are disturbed in a very chaotic way.

It is often difficult to tell whether a comet originally came from the Oort Cloud or the Kuiper Belt. For example, Halley's comet has a period of only 76 years, but many astronomers believe that Halley was once in the Oort Cloud.

Since comets have spent most of their existence in the outer regions of the solar system, they are essentially ``cosmic refrigerators.'' They are so cold that they have been able to preserve a record of the physical and chemical conditions of the solar system as it existed 4.6 billion years ago, during the time when the planets and Sun were formed. When we observe comets today, in a very real sense we are looking back into the past and learning about the origin of the solar system.

Although there are still many things about comets that we do not understand, scientific investigations over the past couple of decades have enabled us to identify at least some of the ices that are contained in comets. For example, we know that plain old water ice is the most abundant constituent of a comet. Since comets frequently collided with the Earth in the first billion years after the solar system was formed, it's quite possible that comets provided most of the water on our planet. Life as we know it requires abundant water in order to be viable, so cometary bombardments may have been key to the development of life on the Earth.

Of course, cometary bombardments can also be hazardous to life that already exists. A large comet or asteroid that impacted the Earth just off the Yucatan peninsula in Mexico approximately 65 million years almost certainly caused global-wide catastrophes, which probably contributed to the extinction of the dinosaurs. While such collisions are few and far between, efforts are being made to map all of the comets and asteroids that pass close to the Earth's orbit so that we can produce a reliable quantitative assessment of the impact hazard. Whether or not we can do anything to prevent catastrophe is a more difficult issue that we will ultimately have to face.

If you'd like to take a stab at making your own comet nucleus, here is a recipe that you can follow. This makes for a great demonstration in school classes and is fun to try at home as well. But PLEASE don't attempt this without adult supervision because the dry ice in this recipe can cause severe skin burns if not handled properly.

This comet was discovered on January 30th of this year by Yuji Hyakutake (pronounced "hyah-koo-tah-kay," with equal emphasis on all four syllables), who is an amateur astronomer from southern Japan. Here is Yuji's picture next to the giant pair of binoculars with which he made his discovery.

There are two unusual aspects to comet Hyakutake's orbit around the Sun. First, the plane containing its motion is almost perpendicular to the plane that contains the Earth and other planets (the latter is called the ECLIPTIC PLANE). This means that the comet probably originated from the Oort Cloud rather than from the Kuiper Belt. Secondly, and more importantly, Hyakutake passed VERY close to the Earth (astronomically speaking, anyway) on March 25th of this year. Although Hyakutake's miss distance was only the fifth closest of this century (comet IRAS-Araki-Alcock came three times closer in 1983), the other comets that came closer were all considerably less active, hence less bright, than Hyakutake. In fact, not since 1556 has a comet as active as Hyakutake passed as close to the Earth as Hyakutake.

Careful studies of Hyakutake's orbit indicate that it last passed this close to the Sun about 8,000 years ago and will make its next passage about 14,000 years from now. Unlike the case of comet Halley, there's no chance that any of us will ever see Hyakutake again.

In the spring of this year, shortly after comet Hyakutake was discovered, the scientific community swung into action. Hyakutake was the most intensively studied comet since Halley, and astronomical observatories all over the world focussed their sights on this now famous celestial visitor. The result was a bonanza of exciting new discoveries. Among the most surprising was the discovery of X-rays coming from the comet.

The Hubble Space Telescope (HST) produced the most detailed images of the comet. But the Hubble image shows a very different picture of the comet than what you are accustomed to seeing either through your own eyes, with binoculars, or through wide-field telescopes. The Hubble images emphasize the ``heart'' of the comet, the region near the nucleus, while the wide-field pictures are dominated by light from the outer coma and tails.

This processed Hubble image shows the bright jets of dust emanating from the nucleus more clearly.

A portion of the Hubble image shows three companions to comet Hyakutake. We believe that the surface of Hyakutake's nucleus is covered with a ``crust,'' and that a piece of this crust was lifted off and blown into the tail as it disintegrated.

Several new molecules were discovered in comet Hyakutake that should provide important insights into our understanding of the origin of our solar system. Infrared observations revealed that Hyakutake's nucleus is loaded with hydrocarbon ices, like methane, acetylene, and ethane, all present at roughly 1% of the water abundance. Hydrocarbons, of course, are the basis of life, so it is exciting to find so many of them in a comet. Current thermo-chemical models of the solar nebula (i.e., the gas and dust cloud from which the solar system formed) suggest that ethane and acetylene should be much less abundant than methane, which is not what we saw in Hyakutake. Either our understanding of the early solar suystem has to be revised, or maybe cometary matter consists primarily of interstellar material (i.e., the matter between the stars that eventually collapses into clumps that become new stars). The interstellar material retained its pristine form in the cold nuclei of comets. Upcoming observations of Hale-Bopp should illuminate these issues further.

This comet was discovered in July 1995 by Alan Hale and Thomas Bopp, who observed it the same night in neighboring states (New Mexico and Arizona, respectively). Hale-Bopp's orbital plane is even more tilted with respect to the Earth's orbit than was Hyakutake's. Thus, Hale-Bopp also probably originally came from the Oort Cloud. Hale-Bopp's path periodically takes it close to the orbits of both the Earth and Jupiter, which can cause significant changes in its trajectory over time. Hale-Bopp's period of revolution around the Sun is roughly 4000 years, so we will never see this comet again either.

Astronomers are very excited about Hale-Bopp because it was already relatively bright even when it was still well outside the orbit of Jupiter. For a comet to be as bright as Hale-Bopp at that distance implies that something very unusual is happening. Observations at radio wavelengths show that ~1 ton of carbon monoxide ice is evaporating from the nucleus every second! Careful analysis of a Hubble image indicates that Hale-Bopp is probably an exceptionally large comet. The nuclei of most comets are ~1-2 miles across. Halley's nucleus, which is considered to be pretty large, was only ~10 miles across in its largest dimension. Hale-Bopp's nucleus might be as large as 25 miles across. Assuming that the proportion of surface area on Hale-Bopp's nucleus that is active is comparable to what has been observed on other comets, the then Hale-Bopp must be very large which explains why it is so bright.

Since radar observations of comet Hyakutake indicated that its nucleus was probably less than ~2 miles across, at first glance one might expect Hale-Bopp to appear much brighter than Hyakutake. However, a comet's apparent brightness depends on other factors, the most important of which in this case is the distance from the Earth. Unfortunately, Hale-Bopp never gets very close to us. In fact, Hale-Bopp's minimum distance from the Earth is thirteen times farther away than Hyakutake's. So even though Hale-Bopp's activity level should dwarf Hyakutake's, the two comets will probably have comparable apparent brightnesses.

Hale-Bopp will not be of much interest to the general public until the spring of 1997, at which time we hope that the comet will be easily visible with the naked eye. Prior to March 21st you should look for the comet in the Eastern sky shortly before sunrise, while after that date you can see the comet in the western sky just after sunset. Here is a pictorial view of the observing conditions next spring.

If you go out to look for the comet, make sure that you:

• Dress warmly (even though it will be spring, you might get cold if you are outside for awhile)
• Go to as dark a site as possible
• Let your eyes become "dark-adapted" (if you're in the dark for about 15 minutes, your pupils will open completely and allow you to see faint things better)
• Use binoculars for the best view
• Sit in a lawn chair for maximum comfort while viewing

You might find it interesting to keep track of the comet's position and appearance from night to night.

Have fun!

DISCLAIMER

A final cautionary note: while observations this year indicate that Hale-Bopp will live up to our lofty expectations, we must keep in mind that comets are fickle objects. Accurately predicting Hale-Bopp's brightness nine months into the future is very difficult and highly uncertain. Remember how many people were disappointed in early 1974 when Comet Kohoutek fell far short of the brightnesses predicted by most astronomers!

Where Can I Get More Information?

A Few Interesting Comet WWW Sites:

Ron Baalke of NASA's Jet Propulsion Laboratory maintains two excellent WWW sites, one for comet Hyakutake and one for comet Hale-Bopp. Both sites are updated frequently with the latest news and contain lots of pretty pictures and animations. These sites also contain a fairly comprehensive list of links to other interesting comet WWW sites.

Charles Morris, also at JPL, maintains a Hale-Bopp Information Page that is aimed at a general, non-technical audience. He also has a Comet Home Page the provides a lot of useful general information on comets.

You might want to check out Sky and Telescope Magazine's online comet page for observing tips, nice finder charts, and up-to-date information on comet observations from around the world.