TOEFL Reading Practice Test #14

Comets are intriguing objects in space that scientists have been studying for centuries. A comet, when distant from the Sun, appears as a small icy body, usually only a few kilometers in diameter. This icy mass is made of frozen gases (such as water, methane, and ammonia) and tiny particles of dust. As the comet travels closer to the Sun, the Sun’s heat causes the ice to vaporize, releasing the dust trapped inside. This forms a large cloud around the comet, known as the coma. The coma can extend tens of thousands of kilometers from the comet’s core, known as the nucleus. Sunlight reflecting off the dust particles in the coma makes the cloud visible from Earth. Additionally, ultraviolet radiation from the Sun breaks apart the vaporized gases, and as the particles return to their normal state, they emit light, adding to the brightness of the coma.

As the comet approaches the Sun even more closely, one of its most notable features, the tail, begins to form. There are two main types of tails: the dust tail and the ion tail. The dust tail forms when sunlight reflects off the dust in the coma. As sunlight strikes the dust particles, the energy transferred pushes them away from the coma, creating a long, curving tail. This tail can stretch for millions of kilometers and typically has a yellowish hue.

The ion tail, on the other hand, forms through a different process. The Sun’s ultraviolet radiation ionizes the gases in the coma, and the charged particles interact with the solar wind. These interactions push the ions directly away from the comet’s nucleus, forming a bluish tail. Unlike the dust tail, which curves gently, the ion tail extends straight out from the comet and always points directly away from the Sun. Both tails are influenced by the solar wind, which pushes particles away from the coma.

Although the nucleus of a comet is typically only a few kilometers in diameter, the coma can be much larger, sometimes hundreds of thousands of kilometers across, and the tails can stretch millions of kilometers into space.

As comets travel through the solar system, they leave behind debris along their path. Some of this material may cross Earth’s orbit. When Earth passes through this debris, it can cause small particles to enter Earth’s atmosphere, where they burn up and create a brief flash of light, known as a meteor.

With each pass by the Sun, comets lose some of their mass, and over time, they become less visible. Eventually, they may lose so much material that they can no longer be observed. Because of this process, the visible lifetimes of comets are quite short compared to the age of the solar system. This raises the question: where do new comets come from to replace the older, fading ones?

To explain the source of new comets, astronomer Jan Oort proposed that a vast cloud of icy debris exists far beyond the outermost planets, extending to about 50,000 times the distance between Earth and the Sun. This cloud, known as the Oort Cloud, contains icy bodies that, if disturbed by the gravity of a passing star, could be nudged into the inner solar system and become visible comets.