A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently bright, a comet may be seen from Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.
The Oort cloud, sometimes called the Öpik–Oort cloud, first described in 1950 by Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 au. It is divided into two regions: a disc-shaped inner Oort cloud and a spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud.
Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–76 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the only naked-eye comet that can appear twice in a human lifetime. Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.
The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars.
Jean-Louis Pons was a French astronomer. Despite humble beginnings and being self-taught, he went on to become the greatest visual comet discoverer of all time: between 1801 and 1827 Pons discovered thirty-seven comets, more than any other person in history.
In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy.
Comet Morehouse (modern formal designation: C/1908 R1) was a bright, non-periodic comet discovered by US astronomer Daniel Walter Morehouse and first observed on September 1, 1908. It was unusual in the rapid variations seen in the structure of its tail. At times, the tail seemed to split into up to six separate tails; at others, the tail appeared completely detached from the head of the comet. The tail was further unusual in that it formed while the comet was still 2 AU away from the Sun (where distances of 1.5 AU are more usual), and that there was a high concentration of the CO+ ion in its spectrum.
Comet 322P/SOHO, also designated P/1999 R1, P/2003 R5, P/2007 R5, and P/2011 R4, is the first periodic comet to be discovered using the automated telescopes of the SOHO spacecraft, and second to be given a numbered designation, after 321P/SOHO. JPL Horizons next predicts 322P to come to perihelion at 2019-Aug-31 12:25 UT.
Comet Lulin is a non-periodic comet. It was discovered by Ye Quanzhi and Lin Chi-Sheng from Lulin Observatory. It peaked in brightness and arrived at perigee for observers on Earth on February 24, 2009, at magnitude +5, and at 0.411 AU from Earth. The comet was near conjunction with Saturn on February 23, and outward first headed towards its aphelion, against the present position of background stars, in the direction of Regulus in the constellation of Leo, as noted on February 26 and 27, 2009. It was expected to pass near Comet Cardinal on May 12, 2009. The comet became visible to the naked eye from dark-sky sites around February 7. It figured near the double star Zubenelgenubi on February 6, near Spica on February 15 and 16, near Gamma Virginis on February 19 and near the star cluster M44 on March 5 and 6. It also figured near the planetary nebula NGC 2392 on March 14, and near the double star Wasat around March 17. According to NASA, Comet Lulin's green color comes from a combination of gases that make up its local atmosphere, primarily diatomic carbon, which appears as a green glow when illuminated by sunlight in the vacuum of space. When SWIFT observed comet Lulin on 28 January 2009, the comet was shedding nearly 800 US gallons (3,000 l) of water each second. Comet Lulin was methanol-rich.
Comet Beljawsky is also known as 1911 IV and 1911g according to the naming conventions at the time. The comet was discovered by the Russian astronomer Sergei Ivanovich Beljawsky September 29, 1911 and shortly thereafter it was independently seen by four or five other observers in the United States and probably by others throughout the world. At the time it was discovered, the comet was near to the Sun which made observations difficult. However, several days after discovery it was a naked eye object for a few days in the morning sky and later, after perihelion, as an evening object. It faded rapidly, becoming visible only in telescopes and was last seen on February 17, 1912. The comet sported a tail 8 to 10 degrees in length. In mid-October, the comet was visible in the evening sky together with another bright comet, C/1911 O1 Brooks.
Comets have been observed by humanity for thousands of years, but only in the past few centuries have they been studied as astronomical phenomena. Before modern times, great comets caused worldwide fear, considered bad omens foreboding disaster and turmoil, for example the 1066 passage of Halley's Comet depicted as heralding the Norman conquest of England. As the science of astronomy developed planetary theories, understanding the nature and composition of comets became a challenging mystery and a large area of study.
Comets have been observed for over 2,000 years. During that time, several different systems have been used to assign names to each comet, and as a result many comets have more than one name.
C/1807 R1, also known as the Great Comet of 1807, is a long-period comet. It was visible to naked-eye observers in the northern hemisphere from early September 1807 to late December, and is ranked among the great comets due to its exceptional brightness.
C/2012 S4 (PANSTARRS) is a non-periodic comet with the largest listed aphelion of any object on the JPL Small-Body Database. But the listed aphelion distance of 500,000 AU (8 ly) from the Sun is a generic near-perihelion unperturbed two-body solution that assumes the Sun and comet are the only two objects in the Universe. Other comets, such as C/2015 TQ209 (LINEAR) and C/2017 A3 (Elenin) have had epoch-dependent solutions with aphelia of more than 1,000,000 AU (16 ly). As a comparison, the closest star, Proxima Centauri, is about 4.24 light years from the Sun, and Wolf 359 is 7.78 light years away. Any comet more than about 150,000 AU (2 ly) from the Sun can be considered lost to the interstellar medium. Using an epoch when C/2012 S4 (PANSTARRS) is beyond the planetary region of the Solar System, it is clear C/2012 S4 (PANSTARRS) will not be ejected from the Solar System.
