Asteroids - Meteorites Visitors from the solar system

This chapter covers all of the solar system that is not the planets or the Sun: meteorites, asteroids, and comets. Some group them all together as ``solar system fluff'' because the objects are much smaller than planets and most moons. The ``fluff'' may be small in size but certainly not in importance. We get clues of the origin of the solar system from these small objects.

There are millions of small rocks that orbit the Sun, most of them between the orbits of Mars and Jupiter. About one million of them are larger than 1 kilometer across. This diagram shows the positions of all numbered asteroids and comets within the orbit of Jupiter as of July 1, 2002. Asteroids are yellow dots; comets are white, sunward-pointing arrowheads.

Those smaller than about 300 kilometers across have irregular shapes because their internal gravity is not strong enough to compress the rock into a spherical shape. The largest asteroid is Ceres with a diameter of 1000 kilometers. Pallas and Vesta have diameters of about 500 kilometers and about 15 others have diameters larger than 250 kilometers.

The combined mass of all of the asteroids is less than the Moon's mass The combined mass of all of the asteroids is less than the Moon's mass. Very likely the asteroids are pieces that would have formed a planet if Jupiter's strong gravity had not stirred up the material between Mars and Jupiter.

Unlike the movie The Empire Strikes Back, where the spacecrafts flying through an asteroid belt could not avoid crashing into them, real asteroids are at least tens of thousands of kilometers apart from each other. Several spacecraft sent to the outer planets have traveled through the asteroid belt with no problems.

Three basic types of asteroids: C = Carbonaceous S = Silicates M = Metals (iron and nickel)

C-type Made of silicate materials with a lot of carbon compounds so they appear very dark. They reflect only 3 to 4% of the sunlight hitting them. You can tell what they are made of by analyzing the spectra of sunlight reflecting off of them. This reflectance spectra shows that they are primitive, unchanged since they first solidified about 4.6 billion years ago. The asteroid called Mathilde, recently explored by the NEAR spacecraft is an example of this type

S-Type They are made of silicate materials without the dark carbon compounds so they appear brighter than the C types. They reflect about 15 to 20% of the sunlight hitting them. Most of them appear to be primitive and they make up a smaller fraction of the asteroids than the C types. Gaspra and Ida, explored by the Galileo spacecraft on its way to Jupiter, are examples of this type.

M-type They are made of metals like iron and nickel. These rare type of asteroids are brighter than the S and C types. We think they are the remains of the cores of differentiated objects. Large objects were hot enough in the early solar system so that they were liquid. This allowed the dense materials like iron and nickel to sink to the center while the lighter material like ordinary silicate rock floated up to the top. Smaller objects cooled off quicker than larger objects, so they underwent less differentiation.

When asteroids collide with one another, they can chip off pieces from each other. Some of those pieces, called meteoroids, can get close to the Earth and be pulled toward the Earth by its gravity.

Using imagery collected from NASA shuttle missions, scientists have been studying a large, multi-ringed crater centered near the town of Puerto Chicxulub on the Yucatan peninsula. It is believed that the crater was formed by an asteroid or comet which slammed into the Earth more than 65 million years ago. It is this impact crater that has been linked to a major biological catastrophe where more than 50 percent of the Earth's species, including the dinosaurs, became extinct.

A view of what a large asteroid might look like as it hits the Earth.

Missions to Asteroids and Comets Deep Impact - NASA Flyby of Comet P/Tempel 1 (2005) Rosetta - ESA Mission to Rendezvous with Comet Churyumov-Gerasimenko (2004) Hayabusa (Muses-C) - ISAS (Japan) Sample Return Mission to Asteroid 25143 Contour - NASA Mission to fly by three comet nuclei (2002) Stardust - NASA Discovery Mission to Comet P/Wild 2 (1999) Deep Space 1 - NASA Flyby Mission to asteroid 1992 KD (1998) Galileo - NASA Mission to Jupiter via asteroids Gaspra and Ida Giotto - ESA mission to Comets Halley and Grigg-Skjellerup ICE (ISEE-3) - NASA Mission to Comet Giacobini-Zinner NEAR - NASA Near-Earth Asteroid Rendezvous with 433 Eros Sakigake - Japanese ISAS mission to Comet Halley Suisei - Japanese ISAS mission to Comet Halley Vega 1 - Soviet mission to Venus and Comet Halley Vega 2 - Soviet mission to Venus and Comet Halley

NEAR Shoemaker The Near Earth Asteroid Rendezvous - Shoemaker (NEAR Shoemaker), renamed in honor of Gene Shoemaker, was designed to study the near Earth asteroid Eros from close orbit over a period of a year. The mission was the first-ever to orbit an asteroid and to touch down on the surface of an asteroid. The ultimate goal of the mission was to study the near Earth asteroid 433 Eros from orbit for approximately one year. Eros is an S-class asteroid approximately 13 x 13 x 33 km in size, the second largest near-Earth asteroid. The mission ended with a touchdown in the "saddle" region of Eros on 12 February 2001.

NEAR Descent Image NEAR Shoemaker took this image of asteroid 433 Eros from a range of 3,773 feet

Launched in January 2005, NASA's Deep Impact spacecraft will deploy an impactor that will essentially be "run over" by the nucleus of comet Tempel 1 on, July 4. Before, during and after the demise of this 820-pound impactor, a "flyby" spacecraft will be watching the 4-mile wide comet nucleus from nearby, collecting pictures and data of the event. The impactor's impact with comet Tempel 1 is expected to form a football-field-sized crater, seven stories deep. Ice and dust debris will be ejected from the crater. This glimpse beneath the surface of a comet, where material and debris from the formation of the solar system remain relatively unchanged, will answer basic questions about the formation of the solar system.

Your opinion matters! Should the government, prepare a meteorite defense plan for earth? What are your greatest global concerns?