When do comets form a tail

A few missions are intended to sample comets. NASA's Deep Impact mission will encounter Comet Tempel 1 in July , and will release a projectile into the comet surface to excavate a hole and expose a fresh surface on the nucleus. The spacecraft will collect data on comet emissions and will relay the data to scientists on Earth. While the data from these missions will be from only a few comets and might not be representative, the data will greatly improve our understanding of comet compositions.

What do the orbital paths of comets look like? Based on observations of how comets move through the sky, scientists have determined that comets travel around our Sun in highly elliptical oval-shaped orbits.

The time it takes to make a complete orbit is called a comet's period. Comet periods typically range from a few years to millions of years. Where do comets come from? Comets are divided into short-period comets and long-period comets. Short period comets — such as Comet Halley — revolve around our Sun in orbits that take less than years. Their orbital paths are close to the same plane of orbit as Earth and the other planets, and they orbit our Sun in the same direction as the planets.

Based on these orbital characteristics, short-period comets are believed to originate in the Kuiper belt , a disk-shaped region extending beyond Neptune. The Kuiper belt contains small, icy planetary bodies, only a few of which have been imaged. Occasionally the orbit of a Kuiper belt object will be disturbed by the interactions of the giant planets in such a way that it will have a close encounter with Neptune and either be flung out of the solar system or pushed into an orbit within our solar system.

Their orbital path is random in terms of direction and plane of orbit. Based on calculations from their observed paths, long-period comets are believed to originate in the Oort cloud. The Oort cloud is a spherical envelope that may extend 30 trillion kilometers approximately 20 trillion miles beyond our Sun.

Oort cloud objects have never been imaged. As a comet approaches our Sun, it begins to heat up and the ice begins to sublimate — to change from a solid to a gas with no liquid stage. Some of the dust is left behind as the ice sublimates. It forms a dark, protective crust on the surface of the nucleus and slows the melting.

In some places the protective layer is thinner, and jets of gas break through. The gas and dust form the cloud of the coma. Our Sun emits a solar wind, a constant flow of gas and particles mostly protons and electrons that streams outward at kilometers about miles per second.

Sunlight and solar wind sweep the dust and gas of the coma into trailing tails. Because sunlight and solar wind always flow outward from our Sun's surface, the tails always point away from our Sun no matter what direction the comet is moving in its orbit.

This means that the tails can be in front of the comet as the comet moves away from our Sun on its return to the outer part of its orbit. Two distinct tails develop — the plasma tail and the dust tail.

This is because the object is moving, so the direction of the pressure from sunlight is different from the actual direction of the sun. The sun's gravity is stronger than the component of the force trying to repel the dust away from the sun.

We know that that the direction of light from a star in space can seem to be changed because of the earth's rotation and revolution around the sun. This is called "phenomenon aberration. The position of a star seemingly changes because of the earth's motion, but the light itself goes directly to the earth. Aberration occurs for the same reason that rain seemingly pours down in front of you when you run, yet is actually dropping vertically.

It is also known that angles of aberration can be calculated. Chapter 1: The Mysteries of Light. Why Is the Sky Blue? How Do Rainbows Form? Why Light Fades in the Bathroom? Why Do Water Surfaces Shine? Why Do Comets Have Tails? Chapter 2: Making Light. Chapter 3: Applications of Light.

Chapter 4: Light and Its Future. A comet's appearance changes as it approaches the Sun. Solar heat warms the comet, giving it the following anatomy:.

As a comet accelerates towards the Sun, its surface temperature increases, and ices begin to sublimate into gaseous form. By the time the comet comes within about 5 AU of the Sun, sublimation has formed a noticeable atmosphere that easily escapes the comet's weak gravity.

The coma forms as the escaping atmosphere drags away dust particles that have been mixed with the sublimating ice. More ice turns into gas as the comet approaches the Sun.

Comet tails are expansions of the coma. Comet tails point away from the Sun, regardless of the direction in which the comet is traveling.

Comets have two tails because escaping gas and dust are influenced by the Sun in slightly different ways, and the tails point in slightly different directions. Gases escaping from the comet are ionized by the ultraviolet photons from the Sun. Once they are ionized, the solar wind carries them straight outward away from the Sun. These gases form the plasma tail. This picture of the comet Hale-Bopp was photographed over Boulder, Colorado during its appearance in Dust-size particles that escape from the comet experience a much weaker push from the Sun caused by the pressure of sunlight itself called radiation pressure , rather than by the charged particles of the solar wind.

While the dust tail also points generally away from the Sun, it has a slight curve back in the direction the comet came from.