Halley's Comet

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.