Unit 41 Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The Asteroid Belt • •

Using Bode’s Rule (a simple mathematical formula that seemed to predict where planets would be found), the asteroid Ceres was discovered between the orbits of Jupiter and Mars Soon many more small bodies were discovered

Bode’s Rule

(0 + 4)/10 = (3 + 4)/10 = (6 + 4)/10 = (12 + 4)/10 = (24 + 4)/10 = (48 + 4)/10 = (96 + 4)/10 = (192 + 4)/10 = (384 + 4)/10 = (768 + 4)/10 = (1536 + 4)/10 = 5.2 10.0

19.6 38.8

77.2

154.0

Number

0.4

0.7

1.0 1.6 2.8

Planet

Mercury Venus Earth Mars Ceres (dwarf) Jupiter Saturn Uranus Neptune Pluto (dwarf) Eris (dwarf) Bode’s Rule

True Distance

0.39

0.72

1.00

1. 52 2.78

5.20

9.58

19.2

30.1

39.5

67.7

• The Asteroid Belt

Now we know of 210,453 asteroids, located in the asteroid belt

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Types of Asteroids (by location)

The Asteroid Belt •

Near-Earth Asteroids (NEAs): ones that closely approach the Earth

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Amors : perihelion distances between 1.017 and 1.3 AU;

»

Have orbits that cross the orbit of Mars, but not Earth, at perihelion

»

Example: Eros

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Types of Asteroids (by location)

The Asteroid Belt •

Near-Earth Asteroids (NEAs): ones that closely approach the Earth

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Atens : semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU;

»

have orbits that NEVER cross the orbit of Earth at perihelion

The Asteroid Belt –

Types of Asteroids (by location)

•

Near-Earth Asteroids (NEAs): ones that closely approach the Earth

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Apollos : semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU

»

have orbits that cross the orbit of Earth at perihelion

»

Example: Apollo

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Types of Asteroids (by location)

The Asteroid Belt •

Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun ;

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Further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).

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Located between mars and Jupiter

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Types of Asteroids (by location)

The Asteroid Belt •

Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun ;

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Between the main concentrations of asteroids in the Main Belt are relatively empty regions known as the Kirkwood gaps.

»

These are regions where an object's orbital period would be a simple fraction of that of Jupiter .

»

An object in such an orbit is very likely to be accelerated by Jupiter into a different orbit.

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Types of Asteroids (by location)

The Asteroid Belt •

Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit).

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Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether.

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Types of Asteroids (by location)

The Asteroid Belt •

Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit).

–

Curiously, there are many more in the leading Lagrange point (L4) than in the trailing one (L5). (There may also be a few small asteroids in the Lagrange points of Venus and Earth (see Earth's Second Moon ) that are also sometimes known as Trojans ; 5261 Eureka is a "Mars Trojan".)

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Types of Asteroids (by location)

The Asteroid Belt •

Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit).

»

Located orbiting 60 degrees ahead or behind the planet

»

Martian

»

Jovian

»

Neptunian

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Types of Asteroids (by location)

The Asteroid Belt •

There also a few "asteroids" (designated as "Centaurs") in the outer solar system: 2060 Chiron (aka 95 P/Chiron) orbits between Saturn and Uranus ;

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The orbit of 5335 Damocles ranges from near Mars to beyond Uranus;

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5145 Pholus orbits from Saturn to past Neptune .

–

–

Types of Asteroids (by location)

The Asteroid Belt •

There also a few "asteroids" (designated as "Centaurs") in the outer solar system: 2060 Chiron (aka 95 P/Chiron) orbits between Saturn and Uranus ;

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There are probably many more, but such planet-crossing orbits are unstable and they are likely to be perturbed in the future.

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The composition of these objects is probably more like that of comets or the Kuiper Belt objects than that of ordinary asteroids. In particular, Chiron is now classified as a comet.

The Asteroid Belt •

The Main Asteroid belt was once believed to be the remains of a shattered planet due to Jupiter, or once a part of Mars. But…

The Asteroid Belt •

The asteroids are not the remains of a shattered planet or a part of Mars.

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All the asteroids mass added together is ~1% of the Earth’s mass

The Shapes and Sizes of Asteroids •

Asteroids come in all shapes and sizes

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Ceres is the largest, only 578 miles across

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Ceres is massive enough to pull itself into a sphere

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Most asteroids are not massive enough

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Eros is potato shaped

The Shapes and Sizes of Asteroids •

Most asteroids are very small but would still cause tremendous damage if one impacted Earth!

•

Spacecrafts have only recently visited asteroids

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NEAR landed on Eros

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Discovered craters and a regolith-covered surface

The Shapes and Sizes of Asteroids

Artist’s concept of the Dawn spacecraft (NASA) http:// dawn.jpl.nasa.gov

/ • NASA’s Dawn Mission will be the first to orbit a main belt asteroid, doing a detailed and extensive study of the two largest asteroids Ceres and Vesta • Scientists will study their surface features and gain insights on their internal structure

• Dawn is scheduled for launch on September 27 th at the Cape Canaveral Air Force Station on a Delta II 7925H launch vehicle • With a Mars gravity assist in February 2009, it will arrive at Vesta in October 2011

• It will depart Vesta in May 2012 and arrive at Ceres in August 2015 • The end of the mission is planned for January 2016

NASA technicians install the instruments aboard the Dawn spacecraft The instruments aboard the spacecraft are: • A visible camera infrared mapping and an spectrometer to reveal surface minerals • A gamma ray and neutron spectrometer to determine the elements that make up the asteroids • The spacecraft will also measure the gravity field of each asteroid

Dawn spacecraft shown with Ceres (right) and Vesta (left) in an artist’s image •

Dawn’s ion propulsion system allows it to undertake a mission that would have been unaffordable using other technologies

Dawn spacecraft shown with Ceres (right) and Vesta (left) in an artist’s image •

Two large solar panels stretching 19.7 meters (65 feet) from tip to tip harness the Sun’s energy and power the ion engines

Dawn spacecraft shown with Ceres (right) and Vesta (left) in an artist’s image •

The energy ionizes the onboard fuel (xenon), accelerates the ions, which in turn accelerate the spacecraft

Hubble Telescope images of Ceres (top) and Vesta (bottom) •

These asteroids were chosen because they are two contrasting planetesimals — Ceres is wet and icy and may have subsurface water. Vesta is dry and rocky and may have been formed by volcanoes

Hubble Telescope images of Ceres (top) and Vesta (bottom) •

Ceres is about 578 miles in diameter and Vesta is about 329 miles in diameter

•

Both bodies formed early in the history of the solar system

•

Exploring a new frontier, Dawn will journey “back in time,” so to speak, over 4.5 billion years to the beginning of our solar system

•

Many thousands of bodies in the main asteroid belt between Mars and Jupiter formed at the same time and in similar environments as the rocky planets (Mercury, Venus, Earth, and Mars)

•

Scientists theorize that asteroids were budding planets that were never given the chance to grow due to the gravitational effects of Jupiter

•

By investigating these two asteroids, Dawn hopes to unlock some of the mysteries of planetary formation: the conditions and the building blocks under which they were formed

Artist’s concept of Dawn spacecraft (NASA) •

Dawn will also contrast the formation and evolution of these two small planetesimals that followed very different evolutionary paths so we can understand what controls that evolution

Artist’s concept of Dawn spacecraft (NASA) •

This can help us in our observation of exoplanets in other planetary systems

Asteroid Composition •

Asteroids can be grouped into three compositional types:

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Carbonaceous bodies

•

Carbon rich, coal-like substance

•

Located in the outer part of the asteroid belt

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Silicate bodies

•

Composed primarily of silicates (low-density rock)

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Metallic iron-nickel bodies

•

Composed mostly of dense metals

•

Located in the inner part of the asteroid belt

• •

Classification of Asteroid’s

Asteroid Composition

Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo : By examining the spectra of light reflected from these objects, we can classify asteroids as follows

•

Classification of Asteroid’s

– Asteroid Composition

C - Dark, probably carbon-containing (carbonaceous) C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles ; would

appear darkest and reflect the least light

• • •

Classification of Asteroid’s

– Asteroid Composition

S - Twice as bright as C, probably made of stony iron S-type, 17%: relatively bright (albedo .10-.22); metallic nickel iron mixed with iron- and magnesium-silicates; Silicaceous M - Similar to iron meteorites

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M-type, most of the rest: bright (albedo .10-.18); pure nickel iron; Metallic; densest P and D - Low brightness, reddish

–

There are also a dozen or so other rare types.

•

Where did these three kinds of asteroids come from?

Asteroid Composition

Origin of Asteroids • •

Asteroids are probably fragments of planetesimals The planetesimal had collected a mixture of rock and metals, and then differentiated, creating a dense metallic core and a lighter, silicate-rich outer shell

Origin of Asteroids • A collision with another asteroid could have shattered the planetesimal – Fragments of the inner core would form the iron nickel asteroids – Fragments of the outer shell would form the silicate asteroids

• Asteroid Orbits

It is likely that the asteroids were unable to form a planet (however small) due to the gravitational influence of Jupiter

• • Asteroid Orbits

Jupiter “stirs up” the asteroids, keeping them apart There are empty regions in the asteroid belt, called Kirkwood Gaps

• Asteroid Orbits

These gaps are present at orbital resonances of Jupiter

• Asteroid Orbits

Asteroids with an orbital resonance get periodic tugs from Jupiter, pulling them out of position

October 13, 2004

DAWN Asteroids: 1 Ceres and 4 Vesta

By Christina O. Lee

Astro 249

DAWN Mission Goals

•

To characterize conditions and processes of the solar system's earliest epoch by investigating in detail the two largest protoplanets remaining intact since their formations. The growth of these bodies (and others in the belt) were presumably interrupted by the formation of Jupiter, whose gravitational forcing countered the accretionary process.

October 13, 2004 Astro 249

Targets: 1 Ceres & 4 Vesta

•

Two largest protoplanets residing between Mars and Jupiter.

•

Each followed a very different evolutionary path constrained by diversity of processes that operated during the very first few Myrs of solar system evolution October 13, 2004 Astro 249

Relative sizes

• •

Ceres is 933 km diameter, 870000 x 10 Vesta is 510 km diameter, 300000 x 10 October 13, 2004 Astro 249 15 15 kg kg

October 13, 2004

Vesta Quick Facts

Astro 249

• • • • • •

Named after the Roman Goddess of Hearth The only asteroid visible with the naked eye 3rd largest asteroid (510 km diameter) Distance: 2.361 AU Rotation rate: 5.342 hr Evolved and dry, differentiated body

Vesta – some properties

• • •

Have rocks more strongly magnetized than Mars (> 100s nT)

–

alter ideas of how and when dynamos arise Have basaltic surface composition

–

possibly possessing an early magma ocean like our Moon Experienced significant excavating events

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indicated by huge crater near southern pole October 13, 2004 Astro 249

Vesta meteorite

Almost entirely made of pyroxene, commonly found in lava flows on Earth Mineral grain structure also indicates it was once molten, and its oxygen isotopes are unlike oxygen isotopes found for all other rocks of the Earth and Moon.

• •

Radioisotope chronology (i.e. cosmic ray dating) from the howardite, eucrite, and diogenite (HED) meteorites believed to be from Vesta suggests it accreted in only 5-15 million years (Vesta has same spectral signature as pyroxene)

–

similar evidence indicates that Mars continued to accrete for close to 30 Myrs and Earth for 50 Myrs Dating also indicate that impacts have released meteoritic material at least 5x in the last 50 Myrs.

October 13, 2004 Astro 249

October 13, 2004

Ceres – Quick Facts

• • • • •

Named after the Roman Goddess of Agriculture Largest asteroid (933 km diameter) Distance: 2.769 AU Rotation rate: 9.075 hr Very primitive surface w/ water bearing minerals!

Astro 249

Ceres – Some properties

•

Have active hydrological processes leading to seasonal polar caps of water frost

•

Have thin permanent atmosphere of frost

•

Have dry clay surface

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indicated by microwave studies) October 13, 2004 Astro 249

No Ceres Meteorite

• •

…But it is expected to be like Vesta and formed ~ 10 Mya. Possibly the excavating events or dynamic that provided the HED meteorites did not occur, but it's possible that the reflectance spectrum of the surface is not indicative of Ceres' crustal rocks

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on Vesta, the basaltic dust layer reflects its crustal composition October 13, 2004 Astro 249

Why are Ceres and Vesta so different?

It appears that a rather short additional radial separation allowed Ceres (further away) to accrete wet and stay cool while early heat sources in the accreting material melted Vesta (slightly closer) October 13, 2004 Astro 249

Go DAWN mission!

Will determine internal structure, density and by measuring their mass, shape, volume and spin rate with both imagery, laser altimetry and gravity October 13, 2004 Astro 249