Minor Space Bodies

Minor Space Bodies


Comets

A comet is a relatively small Solar System body. Comet nuclei are loose collections of ice, dust, and small rocky particles, ranging from a few hundred meters to tens of kilometers across. When a comet is close enough to the Sun, it displays a thin, fuzzy, temporary atmosphere called a coma, and sometimes also has a tail. These phenomenon are the result of the solar radiation and the solar wind interacting with the comet nucleus. Comets have been observed since ancient times and have historically been considered bad omens. Particularly bright or notable examples are called "Great Comets".


Halley's Comet

A color image of Halley's Comet

Halley's Comet or Comet Halley (officially designated 1P/Halley)[1] is the best-known of the short-period comets, and is visible from Earth every 75 to 76 years.[1][10] Halley is the only short-period comet that is clearly visible to the naked eye from Earth, and thus the only naked-eye comet that might appear twice in a human lifetime.[11] Other naked-eye comets may be brighter and more spectacular, but will appear only once in thousands of years. Halley's returns to the inner Solar System have been observed by astronomers since at least 240 BC, and recorded by Chinese, Babylonian, and medieval European chroniclers, but were not recognized as reappearances of the same object. The comet's periodicity was first determined in 1705 by English astronomer Edmond Halley, after whom it is now named. Halley's Comet last appeared in the inner Solar System in 1986 and will next appear in mid-2061.[12]

During its 1986 apparition, Halley's Comet became the first to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation.[13][14] These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly surmised that Halley would be composed of a mixture of volatile ices—such as water, carbon dioxide and ammonia—and dust. However, the missions also provided data which substantially reformed and reconfigured these ideas; for instance it is now understood that Halley's surface is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.



Comet Hale Bopp

Image of Hale Bopp comet taken on 4th April 1997

Comet Hale-Bopp (formally designated C/1995 O1) was arguably the most widely observed comet of the twentieth century, and one of the brightest seen for many decades. It was visible to the naked eye for a record 18 months, twice as long as the previous record holder, the Great Comet of 1811.

Hale-Bopp was discovered on July 23, 1995 at a great distance from the Sun, raising expectations that the comet would brighten considerably by the time it passed close to Earth. Although predicting the brightness of comets with any degree of accuracy is very difficult, Hale-Bopp met or exceeded most predictions when it passed perihelion on April 1, 1997. The comet was dubbed the Great Comet of 1997.

The passage of Hale-Bopp was notable also for inciting a comet-related panic. Rumours that an alien spacecraft was following the comet gained remarkable currency, and inspired a mass suicide among followers of the Heaven's Gate cult.


Comet McNaught

Comet McNaught taken from Swift's Creek, Victoria, Australia

Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic comet discovered on August 7, 2006 by British-Australian astronomer Robert H. McNaught.[2] It was the brightest comet for over 40 years, and was easily visible to the naked eye for observers in the Southern Hemisphere in January and February 2007.

With an estimated peak magnitude of -5.5, the comet was the second brightest since 1935.[3] Around perihelion on January 12, it was visible worldwide in broad daylight. Its tail measured an estimated 35 degrees in length at its peak.[4]

Given its estimated eccentricity over 1.0, suggesting a hyperbolic trajectory, it is believed McNaught will leave the Solar System and never be seen again.


Comet Holmes

Comet Holmes on November 4, 2007,
showing the blue ion tail on the right taken from Hungary

Comet Holmes (official designation: 17P/Holmes) is a periodic comet in our solar system, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its October 2007 return when it temporarily brightened by a factor of about half a million, in what was the largest known outburst by a comet, and became visible to the naked eye. It also briefly became the largest object in the solar system, as its coma (the thin dissipating dust ball around the comet) expanded to a diameter greater than that of the Sun (although its mass remained minuscule

Comet Holmes was discovered by Edwin Holmes on November 6, 1892 while he was conducting regular observations of the Andromeda Galaxy (M31). Its discovery in 1892 was possible because of an increase in its magnitude similar to the 2007 outburst; it brightened to an approximate magnitude of 4 or 5 before fading from visibility over a period of several weeks.[3]


The comet's discovery was confirmed by Edward Walter Maunder (Royal Observatory, Greenwich, England), William Henry Maw (England), and Kidd (Bramley, England) and independent discoveries were made by Thomas David Anderson (Edinburgh, Scotland) on November 8 and by Mike Brown , (Wilkes) , (USA) and by John Ewen Davidson (Mackay, Queensland, Australia) on November 9.[4]

The first calculations of the elliptical orbits of 17P/Holmes were done independently by Heinrich Kreutz and George Mary Searle. Additional orbits eventually established the perihelion date as June 13 and the orbital period as 6.9 years. These calculations proved that the comet was not a return of Biela's Comet.

The 1899 and 1906 appearances were observed, but the comet was lost (see Lost comet) after 1906 until it was recovered on July 16, 1964 by Elizabeth Roemer (US Naval Observatory Flagstaff Station, Arizona, USA). Aided by the computer predictions of Brian G. Marsden, the comet has been observed on every subsequent return.