Meteor shower
A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at extremely high speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so almost all of them disintegrate and never hit the Earth's surface. Very intense or unusual meteor showers are known as meteor outbursts and meteor storms, which produce at least 1,000 meteors an hour, most notably from the Leonids. The Meteor Data Centre lists over 900 suspected meteor showers of which about 100 are well established. Several organizations point to viewing opportunities on the Internet. NASA maintains a daily map of active meteor showers.
Historically, meteor showers were regarded as an atmospheric phenomenon. In 1794, Ernst Chladni proposed that meteors originated in outer space. The Great Meteor Storm of 1833 led Denison Olmsted to show it arrived as a cloud of space dust, with the streaks forming a radiant point in the direction of the constellation of Leo. In 1866, Giovanni Schiaparelli proposed that meteors came from comets when he showed that the Leonid meteor shower shared the same orbit as the Comet Tempel. Astronomers learned to compute the orbits of these clouds of cometary dust, including how they are perturbed by planetary gravity. Fred Whipple in 1951 proposed that comets are "dirty snowballs" that shed meteoritic debris as their volatiles are ablated by solar energy in the inner Solar System.
Historical developments
Historical records suggest that Spartan observations of meteor showers occurred as early as 1200 BCE. There are detailed records from ancient archives showing they were observed from China, Japan, and Korea. There are multiple chronicles where Medieval Arabs recorded meteor showers, as they regarded them as good omens. A meteor shower in August 1583 was recorded in the Timbuktu manuscripts. The Lyrids meteor shower is the oldest such event to be continuously recorded, with records in China dating back to 687 BCE.In 1789, Antoine Lavoisier published the first modern chemistry textbook titled, Traité Élémentaire de Chimie. In it, he speculated that dust rising into the upper atmosphere could be consolidated into lumps of matter by lightning, forming fiery meteors as they plummeted to the ground. At the start of the 19th century, this became one of the most favored hypothesis for the formation on meteors. However, in 1794, German scientist Ernst Chladni proposed that meteorites originated in outer space, and as evidence he published a book linking fireballs to iron meteorites. This proposal was initially met with disbelief from some scientists, initially including Alexander von Humboldt, as it contradicted Isaac Newton's statement that space must be empty for planets to continue along their orbits.
In the modern era, the first great meteor storm was the Leonids of November 1833. One estimate is a peak rate of over one hundred thousand meteors an hour, but another, done as the storm abated, estimated more than two hundred thousand meteors during the 9 hours of the storm, over the entire region of North America east of the Rocky Mountains. American Denison Olmsted explained the event most accurately. After spending the last weeks of 1833 collecting information, he presented his findings in January 1834 to the American Journal of Science and Arts, published in January–April 1834, and January 1836. He noted the shower was of short duration and was not seen in Europe, and that the meteors radiated from a point in the constellation of Leo. He speculated the meteors had originated from a cloud of particles in space. Work continued, yet coming to understand the annual nature of showers though the occurrences of storms perplexed researchers.
The Italian astronomer Giovanni Schiaparelli ascertained the relation between meteors and comets in a series of letters to Angelo Secchi late in 1866. He was able to demonstrate that the Leonid meteor shower shared the same orbit as the Comet Tempel. Biela's Comet, discovered in 1772 and identified as periodic in 1826, was observed to have two components in 1846. During the 1852 return, both components were fainter and had a greater separation. In 1868, Edmund Weiss determined that the Earth would intersect the orbit of this comet in 1872, and a strong meteor shower was observed at that time. This meteor stream, now referred to as the Andromedids, further established the connection between comets and meteor showers.
In the 1890s, Irish astronomer George Johnstone Stoney and British astronomer Arthur Matthew Weld Downing were the first to attempt to calculate the position of the dust at Earth's orbit, taking into account the gravitational perturbations of Jupiter. They studied the dust ejected in 1866 by comet 55P/Tempel-Tuttle before the anticipated Leonid shower return of 1898 and 1899. Meteor storms were expected, but the final calculations showed that most of the dust would be far inside Earth's orbit. The same results were independently arrived at by Adolf Berberich of the Königliches Astronomisches Rechen Institut in Berlin, Germany. Although the absence of meteor storms that season confirmed the calculations, the advance of much better computing tools was needed to arrive at reliable predictions.
In 1981, Donald K. Yeomans of the Jet Propulsion Laboratory reviewed the history of meteor showers for the Leonids and the history of the dynamic orbit of Comet Tempel-Tuttle. A graph from it was adapted and re-published in Sky and Telescope. It showed relative positions of the Earth and Tempel-Tuttle and marks where Earth encountered dense dust. This showed that the meteoroids are mostly behind and outside the path of the comet, but paths of the Earth through the cloud of particles resulting in powerful storms were very near paths of nearly no activity.
In 1985, E. D. Kondrat'eva and E. A. Reznikov of Kazan State University first correctly identified the years when dust was released which was responsible for several past Leonid meteor storms. In 1995, Peter Jenniskens predicted the 1995 Alpha Monocerotids outburst from dust trails. In anticipation of the 1999 Leonid storm, Robert H. McNaught, David Asher, and Finland's Esko Lyytinen were the first to apply this method in the West. In 2006 Jenniskens published predictions for future dust trail encounters covering the next 50 years. Jérémie Vaubaillon continues to update predictions based on observations each year for the .
Radiant point
Because meteor shower particles are all traveling in parallel paths and at the same velocity, they will appear to an observer below to radiate away from a single point in the sky. This radiant point is caused by the effect of perspective, similar to parallel railroad tracks converging at a single vanishing point on the horizon. Meteor showers are normally named after the constellation from which the meteors appear to originate. This "fixed point" slowly moves across the sky during the night due to the Earth turning on its axis, the same reason the stars appear to slowly march across the sky. The radiant also moves slightly from night to night against the background stars due to the Earth moving in its orbit around the Sun. See for maps of drifting "fixed points".The geocentric velocity of the meteors can vary considerably between showers. For example, the velocity is around 27 km/s for the Taurids and 71 km/s for the Leonids. Incoming meteors produce a measureable light curve along their trajectory, which varies in brightness by the rate of ablation. The observed heights for meteor ionization is from, where the atmosphere is sufficiently dense to heat the projectiles. A typical meteor in a shower has a diameter of with a density of 2 g cm−3. It starts to melt at a temperature of around.
As the Earth rotates, the shower rate will be low when the radiant point is near the horizon, then it will rise to at least 50% of maximum when the radiant point reaches an altitude of 30° above the horizon. Optimum viewing is when the radiant point is at an angle of 45°, or half way up the sky, as the meteors are still passing through a thicker column of air. The longer, more prominent trails will then be observed 30–60° away from the radiant point. Most meteor showers improve their visibility after midnight, as the observer's position becomes more oriented toward the direction of the Earth's orbit around the Sun. For this reason, the best viewing time for a meteor shower is generally slightly before dawn — a compromise between the maximum number of meteors available for viewing and the brightening sky, which makes them harder to see.
Naming
Meteor showers are named after the nearest constellation, or bright star with a Greek or Roman letter assigned that is close to the radiant position at the peak of the shower, whereby the grammatical declension of the Latin possessive form is replaced by "id" or "ids." Hence, meteors radiating from near the star Delta Aquarii are called the Delta Aquariids. The International Astronomical Union's Working Group on Meteor Shower Nomenclature and the IAU's Meteor Data Center keep track of meteor shower nomenclature and which showers are established.Origin of meteoroid streams
A meteor shower results from an interaction between a planet, such as Earth, and streams of debris from a comet. Comets can produce debris by water vapor drag, as demonstrated by Fred Whipple in 1951, and by breakup. Whipple envisioned comets as "dirty snowballs", made up of rock embedded in ice, orbiting the Sun. The "ice" may be water, methane, ammonia, or other volatiles, alone or in combination. The "rock" may vary in size from a dust mote to a small boulder. Dust mote sized solids are orders of magnitude more common than those the size of sand grains, which, in turn, are similarly more common than those the size of pebbles, and so on. When the ice warms and sublimates, the vapor can drag along dust, sand, and pebbles.Each time a comet swings by the Sun in its orbit, some of its ice vaporizes, and a certain number of meteoroids will be shed. The meteoroids spread out along the entire trajectory of the comet to form a meteoroid stream, also known as a "dust trail".
Recently, Peter Jenniskens has argued that most of our short-period meteor showers are not from the normal water vapor drag of active comets, but the product of infrequent disintegrations, when large chunks break off a mostly dormant comet. Examples are the Quadrantids and Geminids, which originated from a breakup of asteroid-looking objects, and 3200 Phaethon, respectively, about 500 and 1000 years ago. The fragments tend to fall apart quickly into dust, sand, and pebbles and spread out along the comet's orbit to form a dense meteoroid stream, which subsequently evolves into Earth's path.