Hydrogen Cloud
Hydrogen clouds are expansive formations that occur around comets as they approach the sun. When a comet, comprised mainly of ice and carbon, gets closer to solar radiation, the heat causes the ice to sublimate, or turn directly into gas, creating a surrounding atmosphere known as the coma. This process not only generates gas and dust but also leads to the formation of a hydrogen cloud that can extend tens of millions of miles into space, often much larger than the comet itself. For instance, the Hale-Bopp comet's hydrogen cloud measured approximately 100 million kilometers in diameter, dwarfing the sun.
These clouds are primarily made up of hydrogen atoms, which are lighter than the other gases and particles in the coma, allowing them to disperse further into space. Although invisible to the naked eye, hydrogen clouds can be studied through ultraviolet radiation captured by spacecraft. The understanding of these clouds has advanced significantly since the first observations in the 20th century, which provided insights into the composition and activity of comets. Studying hydrogen clouds is vital for understanding the broader implications of comets, including their potential contributions to Earth’s water supply and their possible impacts on our planet.
Hydrogen Cloud
FIELDS OF STUDY: Astronomy; Observational Astronomy
ABSTRACT: A hydrogen cloud is an envelope of hydrogen that exists around the exterior of active comets. These clouds can be tens of millions of kilometers wide. They are not visible to the naked eye but can be detected in ultraviolet light. Scientists have learned more about hydrogen clouds through the exploration of active comets.
Hydrogen Cloud Formation
A comet is made up of a nucleus, a coma, and a tail. The nucleus is made mostly of ice and carbon. In deep space, far away from the sun, the comet is a frozen mass and does not have a coma or a tail. When the comet travels closer to the sun, however, the surface ice begins to sublimate, meaning that it turns from a solid directly into a gas without first becoming a liquid. When sublimation begins, it creates an atmosphere around the nucleus. This atmosphere is the coma. The tail of the comet forms when solar winds and radiation push particles from the coma toward the back of the comet.
The coma of the comet is made up of a gas and dust. Gas that forms during sublimation jets and fans away from the nucleus, and it forces dust out from the nucleus. Often sublimation and radiation from the sun also cause a hydrogen cloud to form outside the coma of the comet. These clouds can be massive, measuring up to tens of millions of miles across. Some hydrogen clouds even dwarf the sun. The Hale-Bopp comet had a hydrogen cloud that was 100 million kilometers (roughly 62 million miles) in diameter, while the sun is less than 1.4 million kilometers (roughly 870,000 miles) in diameter.
Attributes of Hydrogen Clouds
Hydrogen clouds extend far outside the coma because they are made up of hydrogen atoms. Because hydrogen atoms are much lighter than other atoms, they move out into space faster than the gas, dust, and particles that make up the coma. The hydrogen cloud that envelops a comet cannot be seen from Earth and is invisible to the naked eye. Instead, pictures of this phenomenon are taken with spacecraft. Scientists take images of the clouds using ultraviolet radiation.
Hydrogen clouds vary in shape. Some are symmetrical and some are asymmetrical. This is likely due to varied pressure from solar winds and radiation, which also move gas and dust to create a comet’s tail.
For many years, scientists suspected that water vapor would be present in comets’ gas comas due to the sublimation of ice. In 1985, this idea was verified through observation. Scientists believe that the sun’s ultraviolet radiation breaks down the water vapor molecules in the coma to separate the hydrogen atoms from the oxygen. The hydrogen atoms then form most of the surrounding hydrogen cloud. The breakdown of other molecules likely adds more hydrogen to the cloud.
Study of Comets and Hydrogen Clouds
Comets have been observed by humans for thousands of years, but for many years humans did not know what they were. Many believed that comets were bad omens or had supernatural powers. However, in the sixteenth and seventeenth centuries, scientists such as Tycho Brahe (1546–1601) and Isaac Newton (1642–1727) made observations that helped people better understand comets.
In 1705, astronomer Edmond Halley (1656–1742) noticed that comets that had been observed in 1531, 1607, and 1682 had basically the same orbits. Halley determined that the three comets were in fact three appearances of the same comet, following a regular path and schedule. This comet was later named Halley’s comet. Over the years, scientists identified more comets and developed theories about them.
Technological advances in the twentieth century allowed scientists to observe and study comets’ hydrogen clouds. The clouds were first discovered by the Orbiting Astronomical Observatory 2 (OAO-2), launched in 1968. In February 1986, the Pioneer Venus orbital spacecraft was able to view Halley’s comet with ultraviolet sensors. The images taken of the comet’s hydrogen cloud gave scientists important information about the activity and makeup of the comet. They also revealed the huge extent of hydrogen clouds for the first time. In 2014, the Rosetta space probe conducted further comet research, including landing a spacecraft on a comet’s surface for the first time.
Importance of Comets and Hydrogen Clouds
Understanding hydrogen clouds is important to better understanding comets. Based on their observations, scientists believe that much of the hydrogen in a hydrogen cloud comes from the breakdown of water molecules. Scientists use this fact to help them determine the overall composition of comets.
Studying comets in general is also important because comets carry water, hydrogen, and carbon, all of which are vital to human life. Some scientists believe that some of the water on Earth came from comets. Furthermore, comets have the ability to greatly impact life on Earth, and understanding more about them will help scientists better understand their movements and their potential impacts on Earth.
PRINCIPAL TERMS
- coma: the atmosphere that forms around a comet when it passes near the sun.
- comet: a small solar system body mostly made up of mostly carbon and ice.
- nucleus: the body of a comet, made up of ice, dust, and carbon.
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