Halley's Comet - astronomy.

Halley's Comet - astronomy. I INTRODUCTION Halley's Comet, comet, or ball of ice and dust orbiting the Sun, that can be seen from Earth once about every 76 years. It travels in a very elongated path around the Sun, in a direction opposite to that of the planets in the solar system. The path of the comet extends from near the Sun to beyond the planet Neptune. The periods between its approach to Earth usually vary from 75 to 79 years because the gravitational forces of other planets change the comet's orbit slightly. The comet's scientific designation is 1P/Halley, the first comet listed in the official catalog of comets. II CHARACTERISTICS OF HALLEY'S COMET Halley's Comet has a rocky, irregularly shaped core, or nucleus, about 10 km (about 6 mi) across. The nucleus is composed of rock and dust held together by ice. Ice on the comet sublimes, or changes stage directly to gas, as the comet approaches the Sun. As this occurs, material escapes the nucleus, forming a coma, a distinctive cloud of gas and dust around the nucleus. A tail of gas and dust also forms as material begins to escape the nucleus. As Halley's Comet approaches the Sun, it loses rocky material. These chunks of rock continue orbiting the Sun in the same orbit as the comet. When Earth passes through the orbit, some of the particles enter Earth's atmosphere, creating the Orionid meteor shower every year in October and the Eta Aquarids meteor shower every year in May (see Orionids). Astronomers classify Halley's Comet as a short-period comet, meaning that its orbital period is less than 200 years. Halley's Comet belongs to the group of short-period comets with periods 20 years or longer, a group called Halley-type comets or intermediate-period comets. According to current theories, Halley's Comet likely originated in part of the Oort Cloud, a vast sphere of icy bodies theorized to exist at a distance from the Sun of 20,000 to 200,000 astronomical units (AU). (An AU is the mean distance between Earth and the Sun, or about 150 million km [93 million mi].) Some calculations suggest that Halley's Comet may have come from an inner, disklike region of the Oort Cloud. When it first became a comet and entered the inner solar system thousands of years ago, Halley's Comet may have been a long-period comet. Over time the gravitational pull of planets in the solar system may have reduced its orbital period, turning it into a short-period comet. III OBSERVATION AND EXPLORATION OF HALLEY'S COMET Humans have recorded the appearances of Halley's Comet since the year 239 BC, when Chinese astronomers recorded the appearance of a "broom star." English astronomer Edmond Halley published a treatise on the nature of comets in 1705. Halley was the first person to demonstrate that comets revolve in elliptical orbits around the Sun. He also predicted that the comet now known as Halley's Comet would return in 1758. After his prediction proved accurate, the comet was given his name. Five space probes flew by and studied Halley's Comet on its pass near Earth in 1986. The Soviet probes Vega 1 and 2 passed within about 8,000 km (about 5,000 mi) of the comet. The Vegas' solar panels, which provided the crafts' energy, were so damaged by the dust and particles that accompanied the comet that ground controllers abandoned the mission a few days after the Vega probes crossed Halley's orbit. Two Japanese spacecraft, Suisei and Sakigake, also passed close to the comet in 1986. Suisei came within 151,000 km (93,800 mi) of the comet and continued on to planned meetings with comets Temple-Tuttle and Giacobini-Zimmer for 1998. Unfortunately, Suisei ran out of fuel in 1991 and shut itself off. Sakigake crossed Halley's orbit about 7 million km (about 4 million mi) from the comet and continues to orbit the Sun. Sakigake is expected to run out of fuel before making other comet flybys. The fifth spacecraft was Giotto, launched by the European Space Agency (ESA) in 1986. Giotto was equipped with cameras, telescopes, microphones, and other sensors. Engineers on Earth used data returned from the other spacecraft to position Giotto as close to the comet as possible. It flew within 550 km (340 mi) of the comet's nucleus and relayed pictures to Earth. Some of Giotto's instruments were damaged by dust particles, but the spacecraft survived remarkably well. Giotto's pictures of Halley's Comet showed that the comet's nucleus was much darker and less round than expected, being about 15 km (9 mi) long by 7 km (3.6 mi) wide. After its encounter with Halley's Comet, Giotto was put into hibernation. In 1992 the spacecraft was revived for an encounter with comet 26P/Grigg-Skjellerup and passed within 100 to 200 km (60 to 120 mi) of its nucleus. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.