Halley's Comet

A bright comet visits us.

It is a big ball of dust and ice. This ball of ice is like a dirty snowball. It comes back every 75 to 80 years.

People have seen it for a long time. A man named Edmond Halley found its path. He knew it would come back. It was last here in 1986. It will visit us again in 2061. We can watch it in the sky!

Halley's Comet is a special object in space. It is a short-period comet. This means it visits the inner Solar System often. It appears every 72 to 80 years.

People have seen it since at least 240 BC. An English astronomer named Edmond Halley studied it. He used math to show it was the same comet returning. He predicted it would return in 1758. He was right!

A comet is like a dirty snowball. It is made of dust and ices. These ices include water, ammonia, and carbon dioxide.

In 1986, a spacecraft named Giotto flew close to it. It saw the nucleus, which is the solid center. The surface is mostly dusty. Only a small part is icy. The comet orbits the Sun in a very long oval. This shape is called an ellipse. It even moves in the opposite direction of the planets. This is called a retrograde orbit. It will visit us again in mid-2061.

Halley's Comet is a very famous object in our Solar System. It is a short-period comet, which means it visits the inner parts of space often. Most comets take thousands of years to orbit the Sun. However, Halley returns every 72 to 80 years.

This makes it the only short-period comet we can always see with our eyes. It last visited us in 1986. You can look forward to seeing it again in mid-2061.

A comet works like a giant, dirty snowball traveling through space. It is made of dust and different types of ices. These ices include water, carbon dioxide, and ammonia.

As the comet gets close to the Sun, these ices turn into gas. This process creates a coma and a long tail. Some scientists once thought the comet was mostly ice. However, spacecraft showed that the surface is actually mostly dusty material. Only a small part of the comet is made of ice.

For a long time, people did not know comets were predictable. In the past, some thought they were just clouds in our atmosphere. An astronomer named Tycho Brahe proved they were far beyond the Moon in 1577. Later, Edmond Halley used the laws of gravity from Isaac Newton. He looked at records from the years 1531, 1607, and 1682. He realized these were all the same object. He predicted it would return in 1758. A German farmer named Johann Georg Palitzsch saw it return just in time.

Spacecraft have given us many amazing facts about this comet. In March 1986, the Giotto mission flew very close to it. This was the first time a spacecraft studied a comet in detail.

Two other ships, Vega 1 and Vega 2, also visited. Vega 2 got as close as 8,030 km to the comet. These missions helped us learn about the shape and temperature of the nucleus. The nucleus is the solid center of the comet. We also learned that the comet's orbit is retrograde. This means it moves in the opposite direction of the planets.

Halley's path is a very long, stretched-out oval called an ellipse. It travels from near the Sun all the way out to Pluto's distance.

Because its path crosses near Earth, we see two meteor showers. These are called the Eta Aquariids and the Orionids. The comet might have started in a huge area called the Oort cloud. It is also possible it came from the Kuiper belt. Scientists think the comet might eventually disappear or split apart. It has been in its current path for a very long time.

Halley's Comet, officially known as 1P/Halley, is a unique object in our Solar System. It is a short-period comet, meaning its orbit around the Sun lasts less than 200 years. Most comets have much longer orbits that take thousands of years. Halley is the only short-period comet that humans can consistently see with the naked eye. It appears in the inner Solar System every 72 to 80 years. Most of its recorded appearances have occurred in intervals of 75 to 77 years. The comet last visited us in 1986. It is scheduled to return in mid-2061.

The comet follows a highly elliptical orbit, which is a very stretched-out oval shape. Its orbital eccentricity is 0.967, which is close to a 1 that would indicate a parabolic path. The comet reaches perihelion, its closest point to the Sun, between the orbits of Mercury and Venus. At its farthest point, called aphelion, it reaches a distance roughly equal to Pluto's orbit. Unlike the planets, Halley has a retrograde orbit. This means it orbits the Sun in the opposite direction of the planets. Its orbit is also inclined by 18 degrees to the ecliptic, the plane of the planets.

For centuries, people did not understand what comets actually were. Before the Renaissance, many believed comets were just disturbances in Earth's atmosphere. In 1577, Tycho Brahe used parallax measurements to prove comets lie beyond the Moon. Later, Sir Isaac Newton published his laws of gravity and motion in 1687. However, Newton's own work on comets was incomplete. He suspected two comets from 1680 and 1681 were the same, but he could not fully explain them.

Edmond Halley used Newton's laws to solve the mystery of the comet's path. In 1705, he studied 24 different comet observations. He noticed that the comet of 1682 had an orbit nearly identical to comets seen in 1531 and 1607. Halley concluded these were all the same object returning every 76 years. He even predicted it would return in 1758. He died in 1742 before seeing it, but his prediction was correct. A German farmer named Johann Georg Palitzsch saw it return on December 25, 1758.

Modern spacecraft have allowed us to see the comet's structure in detail. In March 1986, the ESA's Giotto mission became the first spacecraft to observe a comet closely. The Giotto mission provided data on the nucleus, which is the solid center of the comet. The Vega 1 and Vega 2 missions also visited the comet in 1986. Vega 2 flew as close as 8,030 km to the surface. These missions helped confirm the "dirty snowball" model proposed by Fred Whipple. This model suggested comets are made of dust and volatile ices like water, carbon dioxide, and ammonia.

While the "dirty snowball" model was mostly correct, the spacecraft found surprises. The surface of Halley is actually mostly made of dusty, non-volatile materials. Only a small portion of the comet is actually icy. As the comet approaches the Sun, these ices undergo sublimation. Sublimation is when ice turns directly into gas. This process creates a coma and a long tail. This gas also creates jets that can push the comet slightly off its path. These non-gravitational effects cause the comet to arrive a few days later than expected.

Scientists are still studying where Halley came from and how long it will last. It may have originated in the Oort cloud, a distant sphere of icy bodies. It could also have come from the Kuiper belt or a new population of objects. Halley is a Halley-type comet, which is different from the more common Jupiter-family comets. Because it loses mass as it evaporates, it will not last forever. Some calculations suggest it may disappear or split in two within tens of thousands of years. Others suggest it could be ejected from the Solar System entirely.