Visions of the Cosmos

Planetary Science

The Solar System

Asteroids and Meteors

The Asteroids -Mountains in the Sky and Meteorites - the Rocks that Fall to Earth

In the vast depths of interplanetary space, the asteroids orbit the fire-ball of our Sun - the Day-star.  A few are small planets and are roughly spherical.  The majority are irregular bodies and are more like mountains in the sky.  Even smaller are the meteors - rocks boulder and pebbles the orbited the Sun even before the planets were formed - remnants from those turbulent times when the Solar System was being born from the protoplanetary disc

During the eighteenth century the search for a planet between the orbits of Mars and Jupiter had begun.   On the first of January 1801, the Italian astronomer  G.Piazzi of the University of Palermo discovered the largest of the asteroids.  It was only about 900 kilometers in diameter and far too small to fill the role of a major planet.  Three small worlds  - Pallas, Juno and Vesta were discovered between 1801 and 1808 but it was not until 1845 that Astrea, the next asteroid, was discovered.  From then on more and more 'small planets' were discovered.  A total number was listed in the 'Dictionary of Minor Planet's names by Lutz Schmadel and published 1997 by Springer Verlag was 7,041.  Every asteroid except Hermes was given both a name and a number.   At the recent conference held in Prague on 24 August 2006 the International Astronomical Union which relegated  Pluto to the category of a dwarf planet also included Ceres in the new classification.  To a large extent this this was an arbitrary classification which was by no means unanimous - the voting on changing the category of Pluto was 237 in favour with 157 against with 17 abstentions of the astronomers present.   However Ceres certainly fits the classification since it is more or less spherical in shape which is not true of most of the other asteroids.

When it was clear that there was no large planet but only a large number of 'small planetary bodies' they were given the inappropriate name ASTEROIDS meaning star-like.  Unfortunately the name has stuck.   Many of them are very small and are of irregular shapes and are more like 'mountains in the sky'.  Smaller still are the meteorites and even smaller boulders and tiny pebbles that were formed even before the major planets existed They are remnants from those turbulent times when the Solar System was first being formed.

The Asteroid Belt

The majority of asteroids orbit the Sun between Mars and Jupiter.  Their distribution however is not simply random but has been affected by the powerful gravitational effect of Jupiter.  The illustration below gives a general picture of the asteroid belt

Most of the asteroids are rocky bodies that orbit the Sun between Mars and Jupiter in the "Main Asteroid Belt" that is centered around 2.7 times the Earth-Sun distance (astronomical unit or AU) from the Sun. Two "clouds" of icy asteroids 60° ahead and behind Jupiter (and at or near Jupiter's orbital distance from the sun) are called Jupiter Trojans while two similar objects in the Mars orbit are called Martian Trojans.  A few called Centaurs are located beyond the orbit of Saturn.

Some asteroids have been found inside Earth's orbit.  Some of these are described as near Earth objects.    The Amor asteroids cross the orbit of Mars but never come closer to the Sun than 1.017 A.U.s which is the Earth's greatest distance from the sun.   The two moons of Mars, Phobos and Diemos, are believed to be captured asteroids.  The Apollo asteroids distances from the Sun greater than 1 A.U.  The Aten asteroids are defined as those which orbit the Sun at average distances less than 1 A.U. but are never nearer the Sun than 0.983 AU which is the nearest that the Earth ever gets to the Sun.   Ther definitions are somewhat arbitrary since small perturbations can cause changes in the orbits of these tiny planetoids..  It is estimated that there are  at least 1,500 bodies that belong to these three groups that are over one kilometer in diameter which cross the Earth's orbit. 

Almost half the asteroids occur in groups which have similar orbits.  These are referred to as HINAYAMA FAMILIES and it is possible that in some cases members of the same group may have formed from one large planetoid which broke up by collisions into a number of smaller bodies.

Some of the most studied groups and families are listed below

Classification of Families or Groups according to their astronomical distances

Name

Group or Family

Mean Distance from the Sun in AUs

Hungaria

Group

1.95

Flora

Group

2.2

Phocaea

Group

2.36

Koronis

Hirayama Family

2.88

Eos

Hirayama Family

3.02

Themis

Hirayama Family

3.13

Hilda

Group

4.00

Thule

Solitary

4.26

Trojans

Group

5.2  Same Orbit as Jupiter

There are a few asteroids which do not orbit within the main belt.

Hidalgo has a highly elliptical orbit between 2.0 and 9.7 A.U.

Charon which orbits between 8.5 and 18.9 A.U. as far as Uranus.

The other extremes are the AMOR, APOLLO and ATEN asteroids.  They concern us deeply since they represent hazards to our own planet.  Ther danger of Earth-crossing asteroids and comets are discussed in the chapter of the web-site on Great Extinctions.

The Table below lists a number of representative asteroids

 The Table below lists a number of representative asteroids

 

Number and Name

Min Distance in A.U.s

Max Distance  in A.U.s

Approx 'Diameter' or in dimensions in kilometres

Type

Remarks

1      Ceres

2.55

2.77

940

C

Largest - now classified as a 'Dwarf Planet'

4      Vesta

2.15

2.37

576

V

 

2      Pallas

2.12

2.77

559 X 525 X 532

CU

 

10    Hygeia

2.76

3.13

430

C

 

704  Interamnia

2.61

3.06

388

E

 

52    Europa

2.79

3.11

292

C

Not to be confused with Jupiters Moon

87    Sylvia

3.19

3.50

282

P

 

3      Juno

1.98

2,87

248

S

 

15    Psyche

2.53

2.92

248

M

 

24    Themis

2.71

3.13

228

C

 

153   Hilda

3.41

3.97

222

P

 

279   Thule

4.22

4.27

130

D

 

624   Hector

5.05

5.17

150 X 300

 

A Trojan - possibly two asteroids stuck together

253   Ida

2.73

2.86

56 X 24 X 21

S

Member Koronis Family Photographed on Galileo Mission

         Dactyl

2.73

2.86

1 kilometer

S

Satellite of Ida - orbits at 100 kilometers

336   Lacadiera

2.04

2.47

about 69 km across

D

Discovered by August Charlois 19 September 1892

951   Gaspa

1.83

2.21

18 X 11 X 9

S

Photographed from Galileo Space craft

         Phobus

 

 

 

 

Satellite of Mars

         Diemos

 

 

 

 

Satellite of Mars

1221 Amor

 

 

 

V

Discovered March 1932.  possible V type similar to Vesta

1862 Apollo

 

 

 

 

Discovered April 1932

         Hermes

 

 

 

 

In 1937 passed 800.000 kilometers from Earth

         XM1

 

 

 

 

9 Dec 1994  passed 100.000 kilometers from Earth

2062 Aten

 

 

 

 

Discovered January 1976

The Classification of Asteroids

The classification of asteroids is not possible without considering them together with METEORITES.  The importance both of asteroids and meteorites in a study of their astrochemistry and their relevance to the original state of the Solar System can not be overemphasised.

The oldest rocks on the Earth are about 3,800 million years old - those on  Venus are probably only a few hundred million because of the volcanic resurfacing.   Even Mars rocks are scarcely older than Earth.   About 3,900 million years ago even the original surfaces of Mercury, the Moon and the moons of the outer planets would have been virtually obliterated by the late heavy bombardments.  This only left the asteroids and the Kuiper Belt objects virtually unchanged since the very early time when the Solar System was formed.    It is therefore to the asteroids and meteorites that scientists have to turn in order to understand the original building of the Solar System.

C type asteroids  The reflectance spectra of C type asteroids resemble meteorites called carbonaceous chondrites.   Both Ceres and Pallas belong to this group.  Ceres shows a strong spectral absorption band at 3μm in the infra red indicating the presence of hydrated minerals (minerals containing water). 

D type asteroids are reddish in colour and have surfaces consisting largely of clays.  An example of this type is Lacadiera

S type asteroids  Members of this class are reddish in colour and consist of silicate minerals. They are very similar to each other in colour, spectra and albedo and may have been derived for the same parent body.  Lacrimosa, Ida and its satellite Dactyl belong to this group.

E type asteroids resemble Enstatite chondrites which are meteorites containing the mineral enstatite MgSiO3.  They are fairly rare and the largest is Interamnia with a mean diameter of 388 kilometers.

V type asteroids   The only large one is Vesta.  At some stage Vesta is believed to have melted and produced igneous rock.

M type asteroids resemble metallic meteorites and consist largely of iron  together with nickel and other metals.  They may have been derived from the metal rich core of larger asteroids before they broke up   Psyche with a mean diameter of 248 kilometers is the largest of the group.

A type asteroids seem to be almost pure olivine  (FeMg)2SiO4. An example is Aeternitas

P type asteroids are similar to M types.  Examples are Sylvia and Hilda. 

In addition to asteroids in the above groups there are a number such as Feronia which have so far eluded classification.

C and D type asteroids resemble carbonaceous chondrite meteorites are derived and are generally further from the Sun than the other types.

Reflectance Spectroscopy

Until it is possible to send space probes to the asteroids the chemical and physical nature of these bodies can only be investigated by observations of the reflected light from their surfaces and by direct studies of meteorites.  The study of substances by reflected light is called reflectance spectroscopy.

When light falls on a surface the colour of the surface depends on the degree of absorption of light of various wavelengths.  Pure white surfaces reflect all the light to the same extent.   If a substance appears in a specific colour this is because some wavelengths of light have been absorbed more than others, so that the percentage of light absorbed varies with the wavelength.   Minerals present in asteroids contain metals.   Some of them such as sodium, potassium, magnesium and calcium form a number of compounds in which the ions are colourless.  Others however such as those containing transition metals are usually are usually coloured.  Typical transition metals are TITANIUM, VANADIUM, CHROMIUM, MANGANESE, IRON, COBALT, NICKEL and COPPER.   Each individual compound will have its own specific light absorption curve and therefore characteristic colour.   This is not only true of visible light but also applies to the ultra-violet and infra-red parts of the spectrum.  Because asteroids are composed of more than one substance, the light curves obtained in practise are a composite of all the substances present.  Nevertheless the light curve for an asteroid can be used to help in its  classification and in its relationship to known meteorites.  Thus the study of the amount of light reflected by an asteroid at various wavelengths in the visible and near infra-red is a most useful diagnostic tool in astrochemistry.  Most of the information , which is known about the surfaces of asteroids, is derived from the measurement of reflected sunlight at wavelengths  between 0.3 and 3.5μm in the visible and near infra-red regions of the spectrum.  The amount of visible or infra-red radiation reflected by a surface depends upon the chemical and physical nature of that surface.

The two illustrations below show reflectance spectra for some well known minerals – olivine, pyroxenes, diopside and bronzite.   The illustrations are credited from the web-site on Spectroscopy of Rocks and minerals and Principles of Spectroscopy by Roger N Clark USGeological Survey MS 964 Box 25046 Federal Center Denver ColoraDO 80225-0046 (http://speclab.cr.usgs.gov/PAPERS.refl-mrs/refl4.html

Most asteroids contain a mixture of different minerals and interpretation of the reflectance spectra requires expert interpretation.

Most asteroids contain a mixture of different minerals and interpretation of the reflectance spectra requires expert interpretation.

 

.

The graphs were obtained by plotting the readings of ther reflectance on the y axis against the wavelength in micrometers on the x axis

The light reflected from an asteroid is measured at a number of different wavelengths and compared to 'the colour of sunlight'.  This gives a pattern over a whole wavelength range as to how much sunlight is reflected from the surface of the asteroid.  Originally this was done using a spectrum scanner which had the disadvantage that the light intensities were being measured over a time period during which conditions such a the opacity of the sky could vary.  This method has been superseded by multichannel spectrophotometers in which the reflectivity of the surface of the asteroid is simultaneously measured at a number of wavelengths using a large number of photomultipliers.  The bank of photomultipliers operate over a wide range of wavelengths all at the same time.  Even more recently solid state detectors have come into use.  The most sophisticated ones have millions of pixels by which a whole picture can be obtained over the whole wavelength range all at the same time.  These are known as CCDs (charge-couple devises).

Albedo

The ALBEDO of surface is defined as the percentage of light reflected from the surface.  Since it varies with wavelength and at any given wavelength it is characteristic of different substances it is really another way of talking about reflectance spectra.   One of the problems in measuring the albedo is that fact that the surfaces of many asteroids are often covered .like the Moon with aregolith which obscures the nature of the real surface..  The depth of the regolith can sometimes be measured by radar methods.  Circumstantial evidence can sometimes be derived from the meteorites which are believed to have been derived from certain families of asteroids.  From the jig-saw puzzles derived fro meteorites, regolith depth and the albedo at several different wavelengths much information has been gained about the nature of asteroids.

VESTA

Vesta was the fourth asteroid to be discovered.  Vesta has a unique surface feature which scientists look forward to peering into. At the asteroid's south pole is a giant crater - 460 kilometers (285 miles) across and 13 kilometers (8 miles) deep. The massive collision that created this crater gouged out one percent of the asteroid's volume, blasting over one-half million cubic miles of rock into space

It is therefore possible to study its 'Stratigraphy'.  Hubble telescope images of the asteroid Vesta are providing astronomers with a glimpse of the oldest terrain ever seen in the solar system and a peek into a broken-off section of the "mini-planet," which exposes its interior. Hubble telescope images of the asteroid Vesta are providing astronomers with a glimpse of the oldest terrain ever seen in the solar system and a peek into a broken-off section of the "mini-planet," which exposes its interior.   Hubble's pictures provide the best view yet of Vesta's complex surface, which has geologic features similar to those of terrestrial worlds such as Earth or Mars. NASA's Hubble Space Telescope images of the asteroid Vesta are providing astronomers with a glimpse of the oldest terrain ever seen in the solar system and a peek into a broken off section of the "mini-planet" that exposes its interior.

On 10 October 1995 the Hubble Space Telescope released pictures of the surface of Vesta

"The Hubble observations show that Vesta is far more interesting than simply a chunk of rock in space as most asteroids are," said Ben Zellner of Georgia Southern University. "This qualifies Vesta as the 'sixth' terrestrial planet."

No bigger than the state of Arizona, Vesta offers new clues to the origin of the solar system and the interior makeup of the rocky planets. "Vesta has survived essentially intact since the formation of the planets," Zellner said. "It provides a record of the long and complex evolution of our solar system."

Hubble reveals a surprisingly diverse world with an exposed mantle, ancient lava flows and impact basins. It once had a molten interior. This contradicts conventional ideas that asteroids essentially are cold, rocky fragments left behind from the early days of planetary formation.

 Vesta has a unique surface feature which scientists look forward to peering into. At the asteroid's south pole is a giant crater - 460 kilometers (285 miles) across and 13 kilometers (8 miles) deep. The massive collision that created this crater gouged out one percent of the asteroid's volume, blasting over one-half million cubic miles of rock into space. 

Most of the identified meteorites from Vesta are in the care of the Western Australian Museum. This 1.4 pound (631 gm) specimen comes from the New England Meteoritical Services. It is a complete specimen measuring 3.7 inch x 3.1 inch x 3.4 inch (9.6 cm x 8.1 cm x 8.7 cm) showing the fusion crust, evidence of the last stage in its journey to Earth.

Illustration Credits Russell Kempton New England Meteoritical Services and B Zellner Georgia Southern University

Astronomers also believe that fragments gouged out of Vesta during ancient collisions have fallen to Earth as meteorites, making Vesta only the fourth solar system object, other than Earth, the Moon and Mars, where scientists have a confirmed laboratory sample. (About 50-60 other meteorite types are suspected to have come from asteroids, but positive identifications are more difficult to make.A number of meteorites are thought to have originated from Vesta after the massive impact have been studied.

This meteorite is a sample of the crust of the asteroid Vesta, which is only the third solar system object beyond Earth where scientists have a laboratory sample (the other extraterrestrial samples are from Mars and the Moon).

Hubble's pictures provide the best view yet of Vesta's complex surface, with a geology similar to that of terrestrial worlds such as Earth or Mars. The asteroid's ancient surface, battered by collisions eons ago, allows astronomers to peer below the asteroid's crust and into the past.

Astronomers also believe that fragments gouged out of Vesta during ancient collisions have fallen to Earth as meteorites, making Vesta only the fourth solar system object, other than Earth, the Moon and Mars, where scientists have a confirmed laboratory sample. (About 50-60 other meteorite types are suspected to have come from asteroids, but positive identifications are more difficult to make.)

The meteorite is unique because it is made almost entirely of the mineral pyroxene, common in lava flows. The meteorite's 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. The meteorite's chemical identity points to the asteroid Vesta because it has the same unique spectral signature of the mineral pyroxene.

The meteorite also has the same pyroxene signature as other small asteroids, recently discovered near Vesta, that are considered "chips" blasted off Vesta's surface. This debris extends all the way to an "escape hatch" region in the asteroid belt called the Kirkwood gap. This region is swept free of asteroids because Jupiter's gravitational pull removes material from the main belt and hurls it onto a new orbit that crosses Earth's path around the Sun.

The meteorite probably followed this route to Earth. It was torn off Vesta's surface as part of a larger fragment. Subsequent collisions broke apart the parent fragment and threw pieces toward the Kirkwood gap and onto a collision course toward Earth. The fragment's journey ended in 1960 when it fell in Western Australia.

Vesta seems to be the only large asteroid that has undergone extensive melting and igneous activity similar to a larger planet.  It seems possible that it may have formed in a region of the protosolar disc that was sufficiently rich in the fairly short lived isotope of aluminium 26Al to have caused the small planet to melt..  This would have caused the small planet to have formed a nickel-iron core surrounded by several zones of magma surrounding the core.   As the magma cooled it would have differentiated into a number of rock forms.

The first mineral which began to crystallise was OLIVINE (Mg/Fe)2SiO4.  This appears to have been followed by the formation of a layer of lighter rock -a pyroxene called DIOGENITE (Mg/Fe)2(SiO3)2.  This appears to have been followed by basaltic flows resulting in the formation of basaltic flows, forming EUCRITE CaAl2Si2O9 and PIGEONITE, similar chemically to diogenite.  This seems to be the pattern found in the impact crater on Vesta where olivine has been exposed at the center of the crater.

The Dawn Mission

The NASA Dawn Mission was originally scheduled to be launched on the 'lucky day' 07/07/07 but has been rescheduled for September 2007.  The mission is aptly named since during its nearly decade-long mission, the Dawn mission will study the asteroid Vesta and dwarf planet Ceres, celestial bodies believed to have accreted early in the history of the solar system. The mission will characterize the early solar system and the processes that dominated its formation. During the earliest epochs of our solar system, the materials in the solar nebula varied with their distance from the sun. As this distance increased, the temperature dropped, with terrestrial bodies forming closer to the sun, and icy bodies forming farther away.   The asteroid Vesta and the recently categorized dwarf planet Ceres have been selected because, while both speak to conditions and processes early in the formation of the solar system, they developed into two different kinds of bodies. Vesta is a dry, differentiated object with a surface that shows signs of resurfacing. It resembles the rocky bodies of the inner solar system, including Earth. Ceres, by contrast, has a primitive surface containing water-bearing minerals, and may possess a weak atmosphere. It appears to have many similarities to the large icy moons of the outer solar system.  By studying both these two distinct bodies with the same complement of instruments on the same spacecraft, the Dawn mission hopes to compare the different evolutionary path each took as well as create a picture of the early solar system overall. Data returned from the Dawn spacecraft could provide opportunities for significant breakthroughs in our knowledge of how the solar system formed.  The launch has been postponed until September 2007 and is due for a gravity sling shot from Mars in Summer 2009.  Its planned arrival close to Vesta was September 2011 and Ceres February 2015.  These times will probably be a little later due to the late launch

Dawn Spacecraft Tests Ion Engine                                                              En Route to Shed Light on Asteroid Belt

10.09.07 NASA's Dawn spacecraft successfully completed the first test of its    09.27.07 Dawn had a beautiful ride to space on September 27 and ison its way

 ion propulsion system over the week-end                                                       to study a pair of asteroids

Illustrations Courtesy of NASA

Dawn is unique in the sense [that] it’s the first major use of an ion propulsion system for a deep space mission, and it is that system that had some of the technical challenges that caused the mission to be stood-down and looked at by the independent assessment team. Because of that, very special attention has to be paid to make sure we technically answer all the questions regarding the ion propulsion system, and do what’s right before we deliver that hardware for integration. 
To carry out its scientific mission, the Dawn spacecraft will carry three science instruments whose data will be used in combination to characterize these bodies. These instruments consist of a visible camera, a visible and infrared mapping spectrometer, and a gamma ray and neutron spectrometer. In addition to these instruments, radiometric and optical navigation data will provide data relating to the gravity field and thus bulk properties and internal structure of the two bodies

The Shapes of Asteroids

The brightness of an asteroid over a number of observations can be used to determine its rate of rotation and its shape.  The apparent brightness depends upon:

Size

Shape

Distance from the Earth ( or a space satellite observing the asteroid)

Reflectivity or Albedo

 

Extra Paragraphs on Asteroids and Meteorites are in Preparation

 

 

 Solar System

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