Confusion about Comets

Confusion About Comets

Some confusion about comets continues to our own time. In 1957, Carl Sagan was a graduate student at the University of Chicago’s Yerkes Observatory. Alone in the observatory late one night, he heard the telephone ring persistently. When he answered, a voice betraying a well-advanced state of inebriation, said: Lemme talk to a shtrominer”. “Can I halp you?” “Well, see, we are having this garden party out here in Wilmette, and there is something in the sky. 

The funny part is, though, if you look straight at it goes away. But if you don’t look at it, there ii is”. The most sensitive part of the retina is not in the center of the field of view. You can see faint stars and other objects by averting your vision slightly. Sagan knew that, barely visible in the sky at this time, was a newly discovered comet called Arend-Roland. So he told the man that he was probably looking at a comet. There was a long pause, followed by a query: “Wash’a comet? “A comet, Carl Sagan replied, is a snowball one mile across”. There was a long pause after which the caller requested : Lemme talk to a real shtronomer. 

When Halley’s comet reappears in 2062, we wonder what political leaders will fear the apparition, what other silliness will then be upon us. While the planets move in elliptical orbits around the Sun, their orbits are not very elliptical. At first glance they are, by and large, indistinguishable from circles. It is the comets, especially the long period comets, that have dramatically elliptical orbits. The planets are the old-timers in the inner solar system; the comets are the newcomers. Why are the planetary orbits nearly circular and neatly separated one from the other? Because if planets had very elliptical orbits, so that their paths intersected, sooner or later there would be a collision. In the early history of the solar system.

Halley’s comet. Once around the Sun is a long time if you live in the outer reaches of the solar system. Image A Light Blue Pond by © Megan Jorgensen

Those with elliptical crossing orbits tended to collide and destroy themselves. Those with circular orbits tended to grow and survive. The orbits of the present planets are the orbits of the survivors of this collisional natural selection, the stable middle age of a solar system dominated by early catastrophic impacts.

 In the outermost solar system, in the gloom for beyond the planets, there is a vast spherical cloud of a trillion cometary nuclei, orbiting the Sun no faster than a racing car at the Indianapolis 500. (The Earth is r=1 astronomical unit, 150,000,000 kilometers from the Sun. Its roughly circular orbit then has a circumference of 2 пr = 10(9) km. Our planet circulates once along the path every year. One year = 3 x 10(7) seconds. So the Earth’s orbital speed is 10 (9) km/3 x 10(7) sec. = 30 km/sec. Now consider the spherical shell of orbiting comets that many astronomers believe surrounds the solar system at a distance = 100,000 astronomical units, almost halfway to the nearest star. 

From Kepler’s third law it immediately follows that the orbital period about the Sun of any one of them is about (10(5))3/2 = 10 (7.5) = 3×10(7) or 30 million years. Once around the Sun is a long time if you live in the outer reaches of the solar system. The cometary orbit is 2пa= 2п x 10 (5) x 1.5 x 10(8) km = 10(14) km around, and its speed is therefore only 10(14)km/10(15) sec – 0,1 km/sec = 220 miles per hour. Halley's comet