2. Asteroids and meteorites


Trojan asteroids

Trojan asteroids

. Asteroids that share Jupiter’s orbit are known as the Trojan asteroids. They are located near the point where the gravitational force of Jupiter and the Sun are equal. The gravitational perturbations of the inner planets produce slight swinging motions, so the Trojan asteroids oscillate within the two shaded regions. Some of the Trojan asteroids may occasionally move close enough to be captured by Jupiter’s gravity, thereby accounting for the planet’s unusual outer satellites.


Near-Earth asteroids

Near-Earth asteroids

. The paths of three representative near-Earth asteroids, 1221 Amor, 1862 Apollo and 2062 Aten, all come closer to the Sun than most asteroids, located in the main belt beyond the orbit of Mars. Amor crosses the orbit of Mars, and almost reaches the Earth’s orbit. Apollo crosses the orbits of Mars, Earth and Venus (not shown). Aten is always fairly close to the Earth’s orbit.


Orbital resonance with Jupiter

Orbital resonance with Jupiter

. There are very few asteroids at certain distances from the Sun, due to resonance with the orbit of Jupiter. Vertical lines in this diagram of the first 400 numbered asteroids denote these vacancies, called Kirkwood gaps. The ratio 3/1 means that an asteroid at that distance makes 3 revolutions around the Sun for each 1 revolution completed by Jupiter. Asteroids are tossed out of such resonant orbits by Jupiter’s repeated gravitational perturbations. [Adapted from Charles T. Kowal’s Asteroids (New York: John Wiley 1996.)]


Chaotic asteroid orbit

Chaotic asteroid orbit

. Asteroid orbits can become chaotic under the gravitational influence of nearby massive Jupiter. Asteroids at certain locations in the main belt follow a trajectory that becomes increasingly off-center over thousands of orbits. They may eventually become interlopers with orbits that cross the Earth’s orbit. Some of these near-Earth asteroids may eventually collide with our planet.


Asteroid families

Asteroid families

. The orbital parameters of many asteroids are very similar, as shown in this diagram of orbital inclination (vertical axis) plotted as a function of orbital distance from the Sun (horizontal axis). Three families with common orbital elements are the Koronis, Eos and Themis families. The Flora family is sometimes subdivided into several separate ones. The Hungaria and Phocaea group of asteroids at high inclinations are separated from the main belt by resonance with Jupiter, that clears asteroids out of certain locations, and they are not true families. The Kirkwood gaps, also cleared by resonance, are noticeable by vertical white spaces; the one at 2.5 AU corresponds to an orbital period of 4 years and the 3:1 resonance. Another sharp break is present at 3.3 AU, corresponding to a period of 6 years and the 2:1 resonance. [Adapted from Charles T. Kowal’s Asteroids (New York: John Wiley 1996).]


Finding the composition of asteroids

Finding the composition of asteroids

. A prominent silicate absorption feature is present in the spectrum of sunlight reflected from the S-type asteroid 1685 Toro (dark dots with error bars). Asteroids like Toro may be the source of the stony meteorites recovered on Earth. The shaded spectrum is the reflection spectrum of a stony-chondrite meteorite. The wavelength is in units of micrometers, or 10-6 meters, designated as μm for short.


Asteroid distribution with distance

Asteroid distribution with distance

. The color, or surface composition, of the asteroids is correlated with distance from the Sun. In order of increasing distance, there are the white E asteroids, the reddish S or silicate ones, the black C or carbonaceous ones, and the unusually red D asteroids. This systematic change has been attributed to a progressive decrease in temperature with distance from the Sun at the time the asteroids formed. Simple temperature differences within the primeval solar nebula cannot, however, explain the rare metallic M asteroids found in the middle of the asteroid belt; they are probably the cores of former, larger parent bodies.


How fast do asteroids spin?

How fast do asteroids spin?

. Two groups of asteroids are indicated on this plot of their rotation rates (vertical axis) versus size (horizontal axis). There are the large asteroids that rotate at a wide variety of speeds, except at the fastest rates, and the small asteroids that spin the fastest. No known asteroid larger than 200 meters across rotates faster than once every 2.2 hours, perhaps because these asteroids are piles of rubble that fly apart if spun too fast. Smaller asteroids, which can turn once every few minutes, must be solid rocks.


Asteroid 1998 KY26

Asteroid 1998 KY26

. This lumpy, fast-spinning, near-Earth asteroid is just 30 meters across and it rotates with a period of just 10.7 minutes. These pictures of a computer-generated model were obtained by bouncing radar signals off the object when it passed within 800 kilometers from Earth, or at about twice the distance between the Earth and the Moon. The radar reflectivity showed similarities to carbonaceous chondrites, primordial meteorites that contain 10 percent to 20 percent water, suggesting that this asteroid might contain about a million gallons of water. The asteroid is too small for gravity to play a role in its shape, so its lumpy, round, nearly spherical shape is the result of collisions with other asteroids. (Courtesy of Steven J. Ostro, JPL.)


Asteroid 4179 Toutatis

Asteroid 4179 Toutatis

. These four views of a radar-derived model of 4179 Toutatis show shallow craters, linear ridges and a deep topographic “neck”. It may have been sculpted by impacts into a single, coherent body, or this asteroid might consist of two separate objects that came together in a gentle collision. Toutatis is about 4.6 kilometers long. (Courtesy of Steven J. Ostro, JPL.)


Asteroid 243 Ida

Asteroid 243 Ida

. A camera aboard the Galileo spacecraft captured this picture of the limb, or visible edge, of the S-type asteroid 243 Ida on 28 August 1993. Prominent in this view is a 2-kilometer deep valley seen in profile on the limb. Many small craters and some grooves are also seen on the surface, the scars of past collisions. (Courtesy of NASA and JPL.)


Asteroid 253 Mathilde

Asteroid 253 Mathilde

. An image mosaic of the C-type asteroid 253 Mathilde acquired by the NEAR Shoemaker spacecraft on 27 June 1997, when the spacecraft flew by the asteroid at a distance of 1.8 million meters. The part of the asteroid shown is about 59 kilometers by 47 kilometers across. The angular shape of the upper left edge is a large crafter viewed edge on. (Courtesy of NASA and the Johns Hopkins University Applied Physics Laboratory.)


Asteroid 433 Eros

Asteroid 433 Eros

. This global view of the S-type asteroid 433 Eros was obtained by the NEAR Shoemaker spacecraft on 29 February 2000 from a distance of 200 kilometers. This perspective highlights the major features of the asteroid’s northern hemisphere. The asteroid’s largest crater (top) measures 5.5 kilometers wide and sits opposite from an even larger 10-kilometer, saddle-shaped depression (bottom). (Courtesy of NASA and the Johns Hopkins University Applied Physics Laboratory.)


Craters on asteroid 433 Eros

Craters on asteroid 433 Eros

. The many craters on the surface of the asteroid 433 Eros are attributed to eons of collisions with other asteroids. Large boulders, perhaps broken off Eros during these impacts, are perched on one of the crater’s edge. The largest boulder, on the horizon in the center of the picture, is about 40 meters long. The two overlapping craters shown here were probably formed many millions of years apart. This picture, taken on 7 July 2000 from the NEAR Shoemaker spacecraft, is 1.8 kilometers wide. (Courtesy of NASA and the Johns Hopkins University Applied Physics Laboratory.)


Inside the giant gouge on Eros

Inside the giant gouge on Eros

. This mosaic is composed of images taken from the NEAR Shoemaker spacecraft on 15 February 2000, while the spacecraft was passing directly over the large gouge in the surface of the asteroid 433 Eros. Many narrow parallel troughs closely follow the shape of the gouge. Most of the asteroid’s surface is saturated with impact craters. Inside the gouge, however, only smaller craters are present, indicating that the area within the gouge is younger than the nearby surface. So the event that caused the gouge must have happened more recently than the formation of the rest of the surface of Eros. (Courtesy of NASA and the Johns Hopkins University Applied Physics Laboratory.)


Over the horizon of Eros

Over the horizon of Eros

. This incredible picture of the asteroid 433 Eros, taken from the NEAR Shoemaker spacecraft on Valentine’s day 14 February 2000, shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. House-sized boulders are present in several places; one lies on the edge of the giant crater separating the two ends of the asteroid. A bright patch is visible on the asteroid in the top left-hand part of this image, and shallow troughs can be seen just below this patch, running parallel to the asteroid’s long dimension. (Courtesy of NASA and the Johns Hopkins University Applied Physics Laboratory.)


Back in the saddle again

Back in the saddle again

. This image of the saddle region on 433 Eros was taken on 22 March 2000 by the NEAR Shoemaker spacecraft. The saddle is about 10 kilometers across. It may be the scar of an ancient crater, or somehow related to a different large crater on the opposite side of the asteroid. (Courtesy of NASA and the Johns Hopkins University Applied Physics Laboratory.)


The south saddle of Eros

The south saddle of Eros

. This mosaic of four images, photographed on 26 September 2000, was taken as the NEAR Shoemaker spacecraft looked down on the saddle region of the asteroid 433 Eros. A broad, curved depression stretches vertically across the image, as if scooped out by a cosmic sculptor. A boulder-rich area is seen in the lower right. (Courtesy of NASA and the Johns Hopkins University Applied Physics Laboratory.)


Touchdown

Touchdown

. The tip of the white arrow marks the location of the NEAR Shoemaker historic touchdown on the asteroid 433 Eros on 12 February 2001, when it became the first craft to land on an asteroid. This image mosaic was taken from the spacecraft on 3 December 2000 from an altitude of 200 kilometers. (Courtesy of NASA and the Johns Hopkins Applied Physics Laboratory.)


Close-up view of the surface of 433 Eros

Close-up view of the surface of 433 Eros

. This NEAR Shoemaker picture of the surface of asteroid 433 Eros was taken from a range of 250 meters on 12 February 2001, just before landing on the asteroid. The image is just 12 meters across, and the cluster of rocks at the upper right measures 1.4 meters across. (Courtesy of NASA and the Johns Hopkins Applied Physics Laboratory.)


Antarctica

Antarctica

. The midnight Sun illuminates the wind-swept ice at the bottom of the world. Numerous meteorites have been found embedded in the ice in this region near Allan Hills, Antarctica. These meteorites are probably fragments of asteroids that once had orbits between those of Mars and Jupiter, but a few of them may have come from the Moon or even Mars. (Courtesy of Ursula Marvin, Harvard-Smithsonian Center for Astrophysics.)


Stony meteorite

Stony meteorite

. Fragment of the stony, chondrite meteorite that fell near Johnstown, Colorado. (Courtesy of the American Museum of Natural History.)


Chondrules in Allende

Chondrules in Allende

. This photomicrograph of a thin section of the Allende meteorite shows numerous round silicate chondrules together with irregular inclusions. The meteorite section is 0.021 meters across and 0.027 meters high. (Courtesy of the Smithsonian Institution.)


Achondrite meteorite

Achondrite meteorite

. A photomicrograph of the achondrite meteorite that fell near Juvinas, France on 15 June 1821. It contains basalt, resulting from the melting and separation of material inside an asteroidal-sized parent body. The section shown here is 0.0032 meters across. (Courtesy of Martin Prinz, American Museum of Natural History.)


Meteorite from the Moon

Meteorite from the Moon

. Polarized light brings out the structure of a thin slice of a meteorite that probably came from the Moon. The abundance of several elements found in this meteorite are virtually identical with those found in rocks returned from the Moon, and unlike those found in any other meteorite or terrestrial rock. (Courtesy of Darrell Henry, NASA.)


Meteorite orbits

Meteorite orbits

. The calculated orbits of five meteorites, inferred from their trajectory before hitting the ground. All of them originated in the asteroid belt, indicating that these meteorites are chips off asteroids.


Widmanstätten pattern

Widmanstätten pattern

. When polished and etched with acid, an iron meteorite displays this distinctive Widmanstätten pattern produced by crystals of two different iron-nickel alloys. The pattern provides evidence that this meteorite was once buried within a parent body between 50 thousand and 200 thousand meters radius. This sliced specimen is about 0.05 meters across. (Courtesy of the Smithsonian Institution.)


Interplanetary objects

Interplanetary objects

. Repetitive collisions between interplanetary objects have produced many more smaller meteoroids than larger ones. Some of the largest asteroids are comparable in size to small moons, and ongoing collisions between asteroids have produced numerous smaller meteorites.


Summary Diagram

Summary Diagram

. Summary Diagram.