Chapter 13, 14.1 & 14.2 “Asteroids, Comets & Meteoroids” Astronomy 4 Asteroid Gaspra

  Asteroids are small rocky objects containing little if any ices.

Comets are dirty, iceballs a few miles in diameter Comets are dirty, iceballs a few miles in diameter. They are most visible when they come close to the Sun and develop heads and tails that may stretch tens of millions of miles.

There are three families of asteroids: Asteroid Belt Trojan Asteroids Earth-Approaching Asteroids.   Most asteroids lie in Asteroid Belt.

Discovery: 1801 - Giovanni Piazzi discovered the first asteroid named Ceres. 2.8AU from Sun. By 1890 more than 300 discovered. More than 10,000 asteroids now have well-determined orbits. One million asteroids with a diameter of less than a mile. Total mass less than that of Moon.

Asteroids are named after characters in Greek and Roman mythology originally   Ceres is the largest asteroid (620 miles in diameter) Pallas and Vesta are next largest. (300 miles). Most asteroids orbit in a belt 2.2 to 3.3 AU from Sun with periods from 3.3 to 6 years.

Chapter 25 “Asteroids, Comets & Meteoroids”

 There are three classes of asteroids according to composition: Class C: Some asteroids are dark objects. Albedos of 3-4%. Primitive objects of rock mixed with dark organic carbon compounds. Ceres and Pallas are examples.   Class S: Some asteroids are stony. No carbon compounds so higher reflectivity. Class M: A few asteroids are metallic.

  Asteroid Ida

Vesta is the brightest asteroid: 300 miles diameter. Distance = 2.4 AU. Albedo = 30%. Surface covered with basalt indicating that it was once volcanic.

Vesta

Trojan Asteroids: Associated with Jupiter. Located at two points 60d ahead of and behind Jupiter. Some to 120 miles in diameter. Class M primitives. In 1990 Trojan class of asteroids were discovered associated planet Mars.

Earth-Approaching Asteroids: Asteroids and comets that fall into this category are called Near-Earth Objects (NEOs)

LONDON, England (Reuters) -- A giant asteroid is heading for Earth and could hit in 2014. U.S. astronomers have warned British space monitors. But for those fearing Armageddon, don't be alarmed -- the chances of a catastrophic collision are just one in 909,000. Asteroid "2003 QQ47" will be closely monitored over the next two months. Its potential strike date is March 21, 2014, but astronomers say that any risk of impact is likely to decrease as further data is gathered.

Chapter 25 “Asteroids, Comets & Meteoroids”

Comet West

Comet Hale-Bopp

Comet Bradfield

Sungrazer

The word comet is from "cometes" meaning "long-haired star". Appearance of starry nucleus, head and tail.   Comet orbits range from circular with extremely short-periods of only a few years to highly elliptical orbits with periods of millions of years. Halley's Comet has a period of 76 years. It will return in 2061 and some of you will see it.

A comet's nucleus is an icy body a few miles in diameter with embedded dust. There are three parts to a comet: nucleus coma or head tail.

Chapter 25 “Asteroids, Comets & Meteoroids”

As a cometary “nucleus” approaches the Sun, ices sublimate releasing dust to form a “coma” or head and a tail. The Giotto spacecraft photographed Comet Halley's nucleus in 1986. Gas is mostly H2O.

Nucleus of Halley’s Comet

The Stardust Spacecraft took this picture of the nucleus of Comet Wild 2 on January 2, 2004 from a distance of 500 kilometers (about 311 miles).

Origin of Comets: Oort's Comet Cloud & Kuiper’s Belt: Astronomer Jan Oort proposed existence of a cloud of comet nuclei at a distance of 50,000 AU or 1000X Pluto's distance from the Sun. This is known as Oort's Comet Cloud. One trillion comet nuclei in Oort Cloud. Another source of comet material is the Kuiper’s belt just beyond the orbit of Pluto. 60 objects discovered within belt.

Chapter 25 “Asteroids, Comets & Meteoroids” Distance of the Oort Cloud Chapter 25 “Asteroids, Comets & Meteoroids”

Death of Comets: Comets may end their lives by Impacting a planet. Being ejected from the Solar System. Being perturbed into a short-period object and exhausting all of their volatile materials.

Chapter 25 “Asteroids, Comets & Meteoroids” Astronomy 4 Asteroid Gaspra Review of New Material for Final Exam will be this Mon 6/23 which is also The Deadline for Extra Credit. Final will be Thu 6/26 at 7:00 a.m.

Meteoroids are bits of cosmic debris that travel through the Solar System and are not visible because they are far too small. Most meteoroids are smaller than a pea 100 tons of meteoric materials strikes the Earth's atmosphere each day.

Meteoroids, Meteors and Meteorites: We see meteoroids when they plunge into the Earth's atmosphere and burn up in a brief streak of light called a meteor. Meteors are popularly called a "shooting stars" or "falling stars". Most meteors vaporize at altitudes of 50-80 miles and the meteoric dust they release may produce rare high altitude clouds called nacreous or noctilucent clouds.

On any clear moonless night an observer may see as many as six sporadic meteors an hour. Meteoroids the size of a golf ball produce very bright meteors called bolides or fireballs.

If a meteoroid survives being a meteor and reaches the Earth's surface intact, it is called a meteorite.

Many meteoroids are parts of narrow streams or rivers of cosmic material that orbit the Sun and are associated with parent comets. When Earth crosses through one of these streams of meteoric material we witness a meteor shower.   Meteors visible as part of a meteor shower appears to emanate from a very specific point in the sky called a radiant. This apparent convergence of meteor paths, when traced in reverse, is an illusion of perspective.

Rarely meteor showers produce extreme bursts of very high numbers of meteors. Such an event is called a meteor storm (i.e. Leonid meteor shower)   The most dependable current meteor shower is the Perseid meteor shower, which reaches a maximum every year about August 11th.

Meteorites fall into three categories: 1) Irons, composed of metallic nickel-iron 2) Stones, composed of silicate material, 3) Stony-irons, made of a mixture of both materials. Most meteorites that reach Earth's surface and are found are stones.