365DaysDate: March 5, 2009


Title: Cool Comets

Podcaster: Martin Ratcliffe

Description: Comets stir the imagination, and have for thousands of years. Comets and larger icy bodies on the outer solar system, the Kuiper Belt Objects, provide insight into the origin of our solar system. In this podcast I combine a personal reflection of observing famous comets like Halley, Hyakutake and Hale-Bopp, with the fascinating insight astronomers now have of how our solar system came to appear as it does today, with a set of eight major planets whose orbits are arranged near a single plane and a huge array of icy bodies in highly inclined orbits, KBO’s and comets.

Bio: Martin Ratcliffe received his BSc in Astronomy from the University College London, England. Martin has published 4 books, including The Night Sky Revealed (Barnes and Noble) and State of the Universe 2008 (Praxis-Springer). Although his final year research project was published, he pursued a career in Planetariums, working at theaters in Armagh, Northern Ireland, Buhl Planetarium in Pittsburgh and the CyberDome Theater in Wichita, Kansas. A former President of the International Planetarium Society, Martin is a contributing Editor for Astronomy magazine, co-writing the Sky Show monthly column for the past 13 years. He is now Director of Professional Development for Sky-Skan, a leading digital planetarium manufacturer.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Mark Duwe


Comets are cool. Distant vagabonds. Icy flotsam from the far reaches of our solar system. These brief visitors from the depth of space are, to me, the most captivating of all objects in astronomy. If there’s a bright comet around, I don’t get much sleep. Why? I don’t really know. There’s simply a strong connection to these objects that stir the soul as well as the mind. I have to record their image. Odd I know, but let me describe a picture for you to illustrate.

Back in early 1997 I was standing on a dry desert in southern Utah. No lights, no signs of habitation, only the stark outline of dramatic geological features that had stood for millions of years. Hanging in the sky was the bright comet Hale-Bopp – its delicate arching tail projected against the stars of Perseus. This comet was stunning. I could easily imagine, four thousand years earlier, near this very spot, a family of cave dwellers living in the cliffs of Utah wanders outside at night, viewing the same comet on its previous visit.

Those early humans had no idea what the object was, but on its next return in 1997, astronomers are measuring the constituent ices and gases that the comet is made of using modern telescopes and spectroscopes. In a mere snapshot of cosmic time, Hale-Bopp rounds the Sun one more time and the inhabitants of Earth have advanced another tiny step in their evolution on Earth. Who knows what Earth will be like the next time Hale-Bopp appears.

Comets connect us to our origins, as does viewing a stellar birthplace, or a starry night. We’ve known since the 1940’s that all the chemical elements heavier than helium are manufactured inside stars or generated in supernova explosions – now a fact so fundamental it should be taught in every chemistry class.

But what of our solar system’s origin? Well, as modern astronomers have discovered, comets contain pristine material that has changed very little since the formation of the solar system. After a star forms from a collapsing gas cloud, the residue of heavier material becomes confined to a rotating disk. The heat of the star drives volatile material, that material that melts easily, away from the inner region to a great distance far from the star. Ices and interplanetary dust collect into a variety of dirty snowballs, large and small. We can see such debris disks surrounding many stars, such as Beta Pictoris, or the bright star, Fomalhaut. This frozen material remains unchanged until one of the dirty snowballs falls in towards the Sun, melts, gas and dust expands away from the nucleus, is swept back by the ever-present wind of charged particles from the Sun, and a comet is formed.

So let’s return to Hale-Bopp. Noble gases like Argon are easily driven from comets even at low temperatures, and they don’t react chemically. Measuring their presence gives vital clues, as sure as a DNA trace at a crime scene, to the chemical content of the solar nebula and a peak into its early history. Argon was first detected in Comet Hale-Bopp. Its abundance was so high that it indicated the comet had always been quite cold and probably formed in the deep outer reaches of the solar system.

Comet’s represent a fundamental connection to the content of the outer solar system. Since they come into the inner solar system, we get free samples to study. These ethereal wispy visitors come from a region known as the Oort Cloud, spelt o-o-r-t, after a famous dutch astronomer.

There are larger icy objects in the solar system. Beyond Neptune, the Kuiper Belt is filled with almost planet-sized icy balls of various sizes. So far the largest is Eris, followed by Pluto and hundreds of other objects. Check out the orbits of the largest members, some four dozen larger than 400 miles diameter, and they all cluster around the orbit of Neptune, but no closer. We see evidence of such belts of material around other stars that have been studied by space telescopes, especially the Spitzer Space Telescope.

How can a forming planetary system filled with objects of all sizes become so split in distribution, with nearly all the icy objects lying outside Neptune’s Orbit?

Migration is the answer. In the early solar system, filled with icy objects, the larger ones that fell inwards toward the Sun became so strongly affected by Jupiter’s large gravity that they were ejected from the main solar system. Bodies that passed close to Saturn, Uranus and Neptune (and at least one was captured, it’s called Triton) transmitted energy to each outer planet, causing their orbits to migrate outwards. The end result was a cluster of icy bodies lying beyond Neptune, and few of them inside the orbit of Neptune. Many have a resonance with Neptune’s orbit, receiving a small gravitational kick on each orbit, in some cases the kick causes the inclination of the orbit to increase, explaining why the Kuiper belt is filled with highly inclined orbits, like Eris, Pluto, and hundreds of other objects.

The outer solar system carries vast clues to the origin of our solar system. Jupiter turns out to be a great protector of the inner solar system, a good thing for Earth, because it either ejected the icy bodies, or broke them apart, as we experienced with Comet Shoemaker Levy 9 in 1993 and 1994.

The smaller icy bodies are called comets when they enter the inner solar system and produce a coma, and sometimes a tail. The nucleus of a comet typically spans all sizes up to perhaps a hundred miles across. They represent the bottom end of a size scale of pristine icy objects from beyond Neptune. Comets have captured the imagination of humanity for centuries, but only now do we recognize the vital clues they hold for the origin of the solar system.

Seeing a comet in the night sky offers not only astronomers to discover the chemical content of the outer solar system

So let me paint some personal experiences of comets I’ve seen. I first saw Halley’s comet in October 1985 using the 10″Grubb Refractor at the Armagh observatory in Northern Ireland. This was a huge personal thrill – the previous person to see Halley with that scope was J.L.E. Dreyer in 1910, who used the same scope to complete his New General Catalog of non-stellar objects. Five months later, I stood atop a hill in Kenya with two colleagues from Armagh Planetarium. From across the valley – we heard a lion roar – at a suitable distance not to be worried. Raising my eyes from the valley below to the Milky Way above, their hung Halley’s Comet at its best, with a seven degree long tail. There are other stories from Kenya, including a sudden thunder of hoofs from a herd of water buffalo scarring the life out of us while we tried to observe the comet. The raw nature of Earth combined with the raw nature of the solar system.

A month later I was recording Halley’s ever changing comet tail from central Australia, within a mile of Ayers Rock. The rock band, Dire Straights, was due to give a concert here but had cancelled. Staff from the hotel held a wake for the group, playing Dire Straights music under a clear desert sky. I sat with them and pointed out Halley’s Comet – now when I here the song, So Far Away” with Mark Knoppfler’s soulful guitar riffs, I am transported back to the desert skies of Australia.

There was the surprise comet, Hyakutake, that came within 2 million miles of Earth in 1996, a year before Hale-Bopp. Unlike Hale-Bopp, an intrinsically giant comet, Hyakutake was only bright because it came close to Earth. I lived in Pennsylvania at the time, known for cloudy nights in March. I first saw Hyakutake with colleagues from the Buhl Planetarium in Pittsburgh as we raced into Ohio for a clear sky – through broken cloud the greenish glow adjacent to the orange star Arcturus was stunning. later, snapping the comet with my 5 and a half inch Schmidt camera that I had used for Halley, I got six clear nights in a row, and magical views of the 40 degree long tail stretching from Ursa Major back into Virgo.

Halley, Hyakutake, and Hale-Bopp each had their own character, unique appearance, and unique experiences for me as an observer of these rare objects from the depths of our solar system.

Whenever a comet appears, like the current Comet Lulin visible in our March skies, I recall these great memories from great comet’s in the past, and wonder when the next bright one will come, because for sure, I’ll be preparing for more sleepless nights.

Martin Ratcliffe

End of podcast:

365 Days of Astronomy
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