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Astronomy Section - Asteroids

Introduction

Although neglected scientifically and publicly for a long time, in the past ten years asteroids have been the subject of much interest and debate. This has caused by announcements that the extinction of the dinosaurs 65 million years ago was caused by an asteroid that came to Earth off the coast of Mexico and also by movies such as Deep Impact and Armageddon. NASA Administrator Daniel Goldin's "faster, better, cheaper" motto has also been a boon to asteroid science mainly because a small asteroid relatively close to Earth is a lot cheaper to visit than one of our larger planetary neighbours.

Types of Asteroids

The main asteroid belt in our solar system lies between Mars and Jupiter but some cross Earth's orbit and indeed, sometimes collide with us. The inner belt ( closer to Mars ) consists mostly of stone and iron. These are known as S class. The outer belt is mainly darker, more carbonaceous asteroids called C class. There are also the so-called "Trojan" asteroids that revolve in Jupiter's orbit. These are D class and are similar in composition to C class asteroids. There are also a few V class bodies that are made of igneous material.

Asteroid Composition

The current hypothesis on the composition of asteroids was put forward around twenty years ago by Don Davis and Clark Chapman and has been refined since. It states that most asteroids larger than one kilometre across are actually composites of smaller pieces that vary in size from over a kilometre to boulders a few metres across. This fits in with observations about size and spin rate. It has been found that no asteroid larger than 200 metres in diameter rotates faster than about once every two hours, or around ten times per day. If these asteroids are made of rubble then if they rotate any faster than this they will fly apart. Smaller asteroids, that can rotate once every few minutes are have to be made of solid rock.

Asteroids Gallery:

Vesta
Catalogue No.: 4
Dimensions: 525 km in diameter.
Density: 3.3g/cm cubed
Class: V
Rotation: 5.34 hours.
The largest of the V class asteroids ( it is made of basaltic rock ) and also one of the first discovered.

Ida & Dactyl
Catalogue No.: 243
Dimensions: 56x24x21 km
Density: 2.5
Class: S
Rotation:4.63 hours
This asteroid was passed by by the Galileo probe. This asteroid is notable for having a satellite, Dactyl that is only 1.4 km in diameter. It is not known how Dacyl was formed and how it became a satellite to Ida.

Mathilde
Catalogue No.: 253
Dimensions: 66x48x46 km
Density: 1.3
Class: C
Rotation: 17.4 days
This is the largest asteroid yet visited and also one of the slowest rotators. Its low density can only be explained by its porous composition.

Toutatis
Catalogue No.: 4179
Dimensions: 4.5x2.4x1.9 km
Density: 2.1
Class: S
Rotation: Two separate periods ( 5.41 and 7.35 days )
Due to Earth's gravity it's orbit is chaotic. It also wobbles along different axes that combine and create nonperiodic rotation. On Septermber 29 2004 it will come within four lunar distances, close enough to see with binoculars.

Castalia
Catalogue No.: 4769
Dimensions: 1.8x0.8x0.8 km
Density: 2.1
Class: S
Rotation: 4 hours
Castalia was the first ever asteroid to be imaged. This occurred in 1989 when passed within eleven lunar distances of Earth and was imaged by radar from the Arecibo observatory in Puerto Rico.

Asteroid Missions:

NEAR (Near Earth Asteroid Rendezvous):
Currently orbiting Eros ( Catalogue No. 433 ), this probe was renamed for Eugene Shoemaker and has also visited Mathilde. Cameras are mapping the asteroid to a few meters resolution, spectrometers are analysing the composition and a magnetometer is searching for a magnetic field and also for reactions with the solar magnetic field.

Galileo:
Although not primarily an asteroid probe Galileo was the first spacecraft to visit an asteroid Gaspra ( Catalogue No. 951 ) in 1991 on its way to Jupiter

MUSES-C:
A Japanese probe that will hopefully be launched in 2002 collect material from asteroid Nereus. It will also release a nanorover which will hop across the surface
(see spaceplace.jpl.nasa.gov/muses3.htm
for plans to build your own).

Deep Impact:
This will be the first probe to perform a geomechanical experiment on an asteroid. If all goes according to plan Deep Impact will shoot a 500 kilogram ( 1100 pound ) copper projectile into Comet Tempel 1.

            

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