Astronomy

A. Dayle Hancock

adhancock@wm.eduSmall 239

Office hours: MTWR 10-11am

http:// physics.wm.edu/~hancock/171/

Vagabonds of the Solar System

AsteroidsWhy the asteroid never form into a planetA look at asteroidsMeteorites and asteroidsMeteorites and the origins of the solar systemCometsComets come from the outer solar system.

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Asteroids In the eighteenth century astronomers using a 'rule of thumb' though there should be a planet between Mars and Jupiter.After searching they found the first small object Ceres at 2.77 AU. Note its size compared to the Earth and Moon. Shortly afterward astronomer found other even smaller objects Pallas, Vesta and Juno. Today over a million of these objects are know. They are called asteroids and orbit between Mars and Jupiter.

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Asteroids

Asteroids orbit mainly between the orbits of Mars and Jupiter. They range in size between Ceres (975 km) to less than a 1 km. A few (Apollo and Icarus) are in highly eccentric orbits which bring them inside of Earth's orbit. Computer models suggest Jupiter ejected planetesimals from

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this region with only 0.1% of the original material remaining. There was not enough material to collect into a fifth terrestrial planet.

Asteroids

Jupiter influences the asteroids today. There are gaps in the asteroid 'belt' where a there are few asteroids. These are known as Kirkwood gaps. Consider a circular orbit of 5.93 years (½ of Jupiter's period). Once every two orbits the asteroid is directly between Jupiter and the Sun. This ejects the asteroid from it's 5.93

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year orbit. In fact any small integer ratio of the the periods forms an orbital resonance which clears that orbital radius.

Asteroids Close Up

The largest asteroid is Ceres (top). It was probably once molten and is large enough to have pulled itself into a spherical shape. Its most likely underwent chemical differentiation.

The radar image of the 'dog bone' asteroid shows a non-spherical object which is only gravitational bound.

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The Dawn spacecraft orbited Vesta in 2011. Vesta is the second largest asteroid. It is probably differentiated although it is not large enough to pull itself into a sphere. The bottom image shows where a large impact formed the equatorial grooves. Meteorites have been identified that come from Vest.

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Asteroids Close Up

After mapping Vesta, the Dawn spacecraft moved on to orbit Ceres in 2015. Ceres is the largest asteroids. It is spherical in shape and is differentiated.The bright spots in the impact crater are not understood.

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Asteroids Close Up

The Japanese probe Hayabusa landed on the asteroid Itokawa in 2005. A small sample in a return canister returned to Earth in 2010. The sample contained about 1500 small grains. Itokawa is thought to be a 'rubble pile' held together by gravity. It shows few craters.

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Asteroids Close Up

Like Itokawa, most larger asteroids have been shattered by repeated impacts. The impacts were not strong enough to disperse the material so it eventually reforms into the 'rubble pile' held together by gravity.

Trojan Asteroids

The Trojan Asteroids are asteroids captured by the gravitational forces of Jupiter and the Sun. The asteroids are captured at the stable Lagrange points

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Asteroids Have Hit the Earth

Near Earth Objects (NEOs) have impacted the Earth. Meteor crater in Arizona is 1.2 km and hit Earth about 50,000 years ago. It was the result of an iron rich meteor about 50 m across traveling at a speed of 11 km/s (25,000 miles per hour).

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The element iridium is common in asteroids but rare on Earth. Luis Alvarez and his son Walter discovered in the 1970s that a thin layer in the geological record shows an exceptionally high level of iridium. This layer corresponds to a time 65 million years ago when the dinosaurs suddenly became extinct. In 1992 a team of geologist suggested a 180 km diameter create near the north coast of the Yucatan in Mexico was the impact that killed the dinosaurs. 11

Asteroids Have Hit the Earth

In 1908 a stony asteroid hit in Siberia near Tunguska. The asteroid is believed to have exploded before reaching the ground at an altitude of 7 km. The explosion released the equivalent of 1015 joules of energy or a blast the size of a several hundred kilotons nuclear bomb. The event is thought to have been from a asteroid 80 m in diameter and traveling at 22 km/s (50,000 miles per hour).

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Asteroids Have Hit the Earth

In 2013 a 15 m asteroid with a mass of 10,000 tons entered the atmosphere over Russia. It fell into a lake (Chebarkul) but the blast wave cause extensive damage in the area. It is the largest strike since the Tunguska event.

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Asteroids Have Hit the Earth

A major strike by an asteroid could cause very serious damage to a city, country or even the entire Earth as the one that wiped out the dinosaurs shows. A large strike could throw up 1013 kg of dust which could block out the Sun a cause major world wide crop failures threatening civilization.

Fortunately, large object strikes are very infrequent. An object 5-10 m strike Earth every year but burn up in theAtmosphere without causing serious problems. An object like the Tunguska event (50 m) occur about once a century.Kilometer size object would be a major disaster but the last known event of this size was the strike that wiped out the dinosaurs.

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Asteroids Have Hit the Earth

Meteoroids, Meteors and Meteorites

A meteoroid is a piece of rock or metal like an asteroid. A meteor is a brief flash of light as a meteoroid burns up in the atmosphere, a 'shooting star' These fast moving meteoroids are often heated to 3000 K. Most completely burn up in the atmosphere. If a piece of the meteoroid is large enough to not completely burn up and to fall to the surface, it is called a meteorite.

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Types of Meteorites

There are three types of meteorites. One type is the stony meteorite. 95% of all meteorites are stony type. They normally have a 'fusion crust' from the high heat. The resemble ordinary Earth rocks.

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The second type of meteorite is the 'stony iron' type. They consist of about ½ iron and ½ rock. Only about 1% of meteorite are stony iron.

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Types of Meteorites

The third type of meteorite is the iron type. The account for about 4% of meteorites. They are mainly iron but can contain 10-20% nickel. When cut and polished iron meteorites show the metal crystal structure called Widmanstatten patterns.

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Types of Meteorites

Meteorites and the Early Solar System

A rare class of stony meteorites called carbonaceous chondrite show no sign of having been melted in space. The rare meteorites contain H

2O as well as carbon compounds

(organics) including amino acids which are the building blocks of proteins and DNA. These could have provided the original building blocks of life on Earth.

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Comets Comets are 'dirty snowballs' that come from outside the orbit of Neptune. They are normally in highly elliptical orbits. As they approach the Sun, the Sun vaporizes the ices and dust. The gases glow to produce a tail in the opposite direction of the Sun. A dust trail is left behind the path of the comet. The coma is a large bright area around the actual solid nucleus.

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Before 1986 when a probe flew by comet Hally, no one had seem the nucleus of a comet because of the glare of the coma. These images from the ESA Giotto mission show the dark potato shaped nucleus of comet Hartley. It is about 15 x 8 km. The dark color of the nucleus is due to carbon based compounds after the ice has vaporized. The lower image shows jets which are emitting water and CO

2 at the rate of about 200

kg/s21

Comets

In 2006 the US Stardust mission flew by comet Wild 2. It collected samples of comet dust in aerogel which were returned to Earth. Amino acids (the building blocks of life) were discover when the material was analyzed. The bottom image is of a 2μ piece of mineral found in the comet dust.

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Comets

In 2015 the ESA Rosette mission orbited the comet 67P/Chryumov-Gerasimenko. It landed a small probe Philea on the comet but the probe failed because its solar panels were in shade. The probe was design to send back images as well as detect organic molecules. Rosetta sent back data comet the comet until recently.

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Comets

These images show what happened when the US Deep Impact mission fire a 372 kg projectile Traveling at 37,000 km/h into comet Temple 1. It ejected 11,000 tons of material. This confirmed that comet is only held together by gravity. It also showed the comet

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Comets

had a low density 600 kg/m3 and could not be solid. The ejected material was also analyzed by spectroscopic means.

The hydrogen envelope of a comet is never visible from Earth. The Earth's atmosphere absorbs the UV from the hydrogen. The hydrogen is formed when UV light from the Sun breaks up H

2O.

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Comet Envelopes and Tails

The image of the hydrogen envelope was made with an ultraviolet camera on a rocket above the Earth's atmosphere.

As we saw earlier, a comet has two tails. The ionized gases are easily pushed away from the Sun by the solar wind. This forms the ion tail. Dust tail is pushed by radiation pressure from light from the sun. The dust trail is not effected by this force as much so the dust trail ends up being curved.

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A Comet's Two Tails

The ion tail points away from the Sun as the comet approaches perihelion. As it moves away form the Sun, its ion tail still points aways from the Sun. Since comets are in highly elliptical orbits, after they move away from the Sun it gets colder and eventually freezes and the tails disappear. The orbits can take the comet out passed the orbit of Neptune like Halley's comet or even further. Some have semimajor axis of 2000 AU.

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Comets Orbits

Collision of objects in the Kuiper belt can launch pieces into highly eccentric orbits that come near the Sun. These are Jupiter family comets since they can be strongly influenced by Jupiter's gravity. These comets are short period comets with periods of 20-200 years. Halley's comet is a short period comet with a period of 86 years.

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Comets from the Kuiper Belt

Long period comets are thought to come from the Oort Cloud. The Oort cloud extends from the Kuiper belt out to 50,000 AU. Because it is so large there is a long period comet discovered every month, it is thought there must be 5 trillion comets with a size of 1 km or large in the Oort cloud. Jupiter's large gravity ejected these object 4.5 billion years ago

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Comets from the Oort Cloud

Each time a comet comes near the Sun at perihelion, it some of its mass is vaporized. A comet loses about 0.5-1.0% of its mass. After 100-200 orbits it will break up and end up a swarm of dust and pebbles. Sometimes a comet may come too close to a large object and break up from the gravity of the large object. This happened to comet Shoemaker-Levy 9 when it come close to Jupiter in 1992.

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Comet Break ups

When a comet 'burns out' leaving pebbles and dust, the remaining material can result in a seasonal meteor shower. If the comet has only recently ended it life, the fragments will be concentrated along the original orbit of the comet. Over time the debris will spread out over the original orbit.When Earth's orbit intersects this ring of material, we see a meteor show.

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Comet and Meteor showers