Comets: Crash Course Astronomy #21

Summary

Crash Course Astronomy explores the mysteries of comets, from their historical significance as omens to their scientific composition of ice and rock. Phil Plait explains how comets, dubbed 'dirty snowballs,' undergo sublimation near the Sun, creating fuzzy comas and distinctive tails. The script delves into comets' potential role in delivering water and the building blocks of life to Earth, suggesting an extraterrestrial influence on our origins. It also highlights space missions that have brought us closer to understanding these celestial bodies, hinting at the secrets of life's origins.

Takeaways

• 🌌 Comets have been observed since antiquity and were often considered omens or harbingers of human events.
• 🌠 Comets are similar to asteroids but are a balanced mixture of ice and rock, sometimes referred to as 'dirty snowballs'.
• 🔥 The ice in comets sublimates when they approach the Sun, creating a fuzzy appearance and forming a coma.
• 🌈 Comets have two types of tails: an ion tail influenced by the solar wind and a dust tail pushed by sunlight.
• 🌍 The Earth passed through the tail of Comet Halley in 1910, causing public concern but no harm due to the extremely low density of the tail.
• 📊 Comets are classified by their orbital periods, with short-period comets having orbits less than 200 years and long-period comets taking over two centuries to orbit the Sun.
• 🌑 There is a repository of comets beyond Neptune, possibly the source of both short and long-period comets.
• 🚀 Spacecraft missions, such as Giotto and Rosetta, have provided close-up observations of comets, revealing their irregular shapes and active surface features.
• 💧 Comets may have contributed significantly to Earth's water supply billions of years ago through impacts.
• 🧬 The Stardust mission found complex organic molecules, including amino acids, in comet samples, suggesting comets could have delivered the building blocks of life to Earth.
• 🔮 Studying comets is akin to investigating our own origins, as they are like time machines providing insights into the early solar system and the potential beginnings of life.

Q & A

What role have comets historically played in human culture?

Historically, comets have been used for prognostication, with people interpreting them as omens or harbingers of human events, sometimes as good omens and sometimes as bad ones.

How is Comet Halley depicted in the Bayeux Tapestry?

Comet Halley is shown in the Bayeux Tapestry as an astronomical event that coincided with the Norman invasion of the British Isles in 1066.

What are the main components of a comet?

Comets are a mixture of ice and rock. The 'ice' includes frozen water, carbon dioxide, carbon monoxide, methane, and ammonia, while the 'rock' consists of rocks, gravel, and dust.

Why are comets sometimes referred to as 'dirty snowballs'?

Comets are sometimes called 'dirty snowballs' because they are made up of a mix of ice and rock, with the ice component including various frozen gases and the rock component including rocks, gravel, and dust.

What happens to a comet when it is heated by the Sun?

When a comet is heated by the Sun, the ice on it undergoes sublimation, turning directly into gas and creating a cloud around the comet, known as the coma.

What are the two types of tails that a comet can have, and what causes them?

A comet can have two types of tails: an ion tail and a dust tail. The ion tail is formed by ionized gas molecules that are influenced by the Sun's magnetic field and the solar wind, while the dust tail is formed by dust particles pushed away by the pressure of sunlight.

Why do the tails of a comet usually point in different directions?

The tails of a comet usually point in different directions because the ion tail, composed of ionized gas, is influenced by the solar wind and points directly away from the Sun, while the dust tail, composed of dust particles, is pushed by sunlight and tends to lag behind the comet in its orbit.

How are short-period comets and long-period comets classified based on their orbits?

Short-period comets have orbital periods of less than 200 years and tend to orbit the Sun in the same plane as the planets. Long-period comets have orbital periods longer than 200 years and can have orbits that are tilted in various directions.

What was the significance of the 1910 event when Earth passed through the tail of Comet Halley?

In 1910, Earth passed through the tail of Comet Halley, causing public fear due to the detection of cyanogen, a deadly gas, in the tail. However, no harm occurred because the density of the tail was extremely low.

What insights did the Rosetta mission provide about the surface and structure of comet 67P/Churyumov-Gerasimenko?

The Rosetta mission revealed that comet 67P has a double-lobed shape resembling a cosmic rubber ducky, with a surface devoid of craters, indicating a very young surface. It also showed jets of gas emitted from specific places and a hard surface that was tougher than expected, suggesting a possible crust formed by warming ice near the surface.

How might comets have contributed to the origin of life on Earth?

Comets, along with asteroids, may have brought a significant amount of water to Earth billions of years ago. Additionally, analysis of samples from comet Wild 2 by NASA's Stardust mission found the presence of organic molecules, including amino acids, which are the building blocks of life.