COMETS
From Ancient Harbingers to Modern-Day
Observing Targets
For much of human history, people were wary of comets. Early sky gazers had little understanding of what celestial objects really were. To them, comets weren’t like stars or planets. They seemed to appear and disappear without warning, and they changed shape over time. Some observers thought they were harbingers of bad fortune and bringers of doom, warning of catastrophic events about to happen.
Today, we know that comets are solar system objects in orbit around the Sun. They consist of a central core, called a nucleus—which is a combination of ices and dust. As a comet gets close to the Sun, those ices begin to vaporize—the technical term is sublimation. This is similar to what dry ice does if you expose it to air: It goes directly from an ice to a vapor. The materials in the comet form a cloud around the nucleus called a coma and also stream away from the comet in a pair of tails. The dust tail is made of dust from the comet, while the plasma tail consists of ionized (heated) gases that glow as they encounter the solar wind.
Interestingly, comets are known to leave behind streams of particles, which are eventually spread out along the path of the comet’s orbit. When Earth encounters one of those streams, the material gets caught up in our atmosphere. As particles from the stream fall to the surface, they vaporize in the atmosphere, creating meteors.
Comets have been called “dirty snowballs” and are thought to be a mixture of ices and rock and dust particles. As the comet gets closer to the Sun, the ices sublimate and rush out from the interior of the comet as jets of particles.
When a comet gets close to the Sun, dust particles flow away from the nucleus, forming a dust tail. The plasma tail comes from gases that are energized by the Sun and the solar wind.
Our Changing View of Comets
The earliest scientific thinking about comets began with the Greek philosopher Aristotle, who suggested they were the result of events happening in the upper atmosphere of our planet. There was a problem with that idea, however. If comets were very close to us, then they would look as if they were moving quickly through space. Comets change their positions very slowly, taking weeks or months to cross the sky. So what could they be, and where were they?
Tycho Brahe (1546–1601), a Danish astronomer, took immense interest in a bright comet that appeared in 1577. He and others carefully measured its position over several months and determined that it was well beyond Earth. A century later, the appearance of another bright comet allowed observers to determine its orbit by applying Kepler’s ideas about how objects moved in orbit around the Sun. Later on, Sir Isaac Newton (1642–1726) also showed that comets do move in such orbits.
The next bright comet to change our view was the great one: Comet Halley. Astronomer Edmond Halley (1656–1742) studied apparitions of comets noted in historical records and recognized that one particular comet kept showing up every seventy-six years. He used that information to predict its next appearance in 1758, although the comet actually showed up in 1759 due to changes in its orbit introduced by the gravitational pull of the outer planets. Comet Halley last appeared in our skies in 1985–86, and will make its next visit in the year 2061.
The twentieth century brought many advances in comet science, including Fred Whipple’s (1906–2004) idea that comets are essentially dirty snowballs in space. This is now the commonly accepted view. But where do comets come from, and how do they get flung into orbits around the Sun?
Treasures Across Time
Today we know that comets are relics of the materials that existed when the solar system began forming some 4.5 billion years ago. This makes them treasuries of solar system history. In recent years, several missions—including ICE, Vega 1 and 2, Suisei, Giotto, Deep Space 1, Deep Impact, and Stardust—have actually visited comets or captured samples of comet material for study. The European Space Agency Rosetta mission is aimed at a rendezvous with comet 67P/Churyumov-Gerasimenko and will land a probe on its surface.
During the formation of the solar system, the protosolar nebula was probably rich in icy materials throughout the cloud. Some ices were destroyed by the heat of the Sun or accreted into the planets. The rest probably migrated to the outer solar system, to create an icy shell of cometary nuclei called the Oort Cloud. There are also cometary nuclei in a region of the solar system called the Kuiper Belt. It extends out from the orbit of Neptune and contains many smaller frozen worlds and cometary nuclei.
A Headlong Rush to Glory Around the Sun
Comets travel long distances in their orbits—some from the most distant reaches of the solar system. It’s possible that some kind of gravitational interaction nudges a nucleus out of its comfortable orbit in the Oort Cloud. Perhaps a chunk of ice and dust gets moved by a close call with a nearby neighbor in the Kuiper Belt or by a brush with Neptune. That’s enough to change the object’s orbit toward the Sun. Gravitational nudges from other worlds along its path subtly alter the comet’s path. From there, it’s only a matter of time until the comet gets close to the Sun, grows a tail, and shows us the familiar shape that has fascinated observers throughout history.