The gap between Mars and Jupiter was of great historical interest due to the Bode-Titus relation. In 1801, first minor planet (later termed asteroid) between Mars and Jupiter was discovered. Then followed an increasing number of further discoveries of minor planets.
date asteroid sizeBy 1890, over 300 asteroids had been discovered. By 1990, over 100,000 known asteroids had been found, most with sizes ranging from 10 meters to 100 km and most found in orbits between the Earth's and Saturn.
1801 - Ceres 500 km 1802 - Pallas 290 km 1804 - Vesta 260 km 1806 - Juno 150 km
Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona and Clearwater, Canada.
Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets made from material left over from the formation of the solar system that never coalesced into a planet, probably due to the gravitational effects of Jupiter. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers across, which is less than half the diameter of our Moon.
Asteroids are gathered into ``families'' based on their orbital characteristics:
Trojan Family:
The Trojan family of asteroids are located at either the leading or trailing Lagrangian points in Jupiter's orbit. These are two stable points in the gravitational attraction between Jupiter and the Sun. Both points are populated with a cluster of asteroids that have be captured in these stable points over the life of the Solar System.
Apollo Family:
The Apollo family of asteroids are those with near-solar orbits. These are objects with highly eccentric orbits at are Earth-crossing. Earth-crossing, of course, means the potential for impact with the Earth (i.e. the objects that cause mass extinctions). A 1 kilometer sized asteroid would impact with the Earth releasing the same energy as a 20,000 megaton blast. It would leave a 13 km sized crater at the impact point.
Past near misses:
object name date closest distance ------------------------------------------- Eros 1931 - 23 million km Icarus 1968 - 6.4 million km unnamed 10km object 1972 - 60 km (!) unnamed 20km object 1988 - 1.1 million kmAmor Family:
The Amor family are asteroids with Mars-crossing orbits. We believe they evolve by interaction with Mars into Apollo asteroids.
Hirayama Family:
The Hirayama family are groups of asteroids that travel in a cluster along the same orbit. They are probably remnants of a single, large body that was broken into a group of smaller asteroids.
Asteroid Composition:
Asteroids come in two flavors, C-type or carbonaceous and S-type or silicate. C-types tend to have a large fraction of carbon in their make-up and, thus due to radiation darkening, are the least reflective of the two groups. S-type asteroids are rich in rocky or silicate materials and are more reflective. About 75 percent of all asteroids are C-types, and the C-types are more common in the outer Asteroid Belt. S-types are rarer and occupy the inner Belt.
Gaspra:
Gaspra was discovered by Grigoriy N. Neujamin in 1916. Gaspra was imaged by the Galileo spacecraft and found to have an elongated shape with a rotational period of 7.04 hours.
As Galileo approached Gaspra, it produced a mosaic of images that were used to determine that it is an irregular body with dimensions of about 20 x 12 x 11 km. Its surface reflects approximately 20 percent of the sunlight striking it. Gaspra is classified as an S-type asteroid and is likely composed of metal-rich silicates and perhaps blocks of pure metal.
Several craters are visible on Gaspra, but none approach the scale of the asteroid's radius. The fact that Gaspra is irregular in shape and lacks any large craters suggests that it has a comparatively recent origin, most likely from the collisional breakup of a larger body. Gaspra has probably been in its present state for the last 300 to 500 million years.
Ida/Dactyl:
Ida is a heavily cratered, irregularly shaped asteroid in the main asteroid belt. Ida is classed as a S-type. It is a member of the Koronis family, which we believe was created when a larger body perhaps 200 to 300 kilometers in diameter was smashed relatively recently.
On August 28, 1993 Galileo came within 2,400 kilometers of Ida, the second asteroid ever encountered by a spacecraft. They passed each other at a relative velocity of 12.4 km/sec (28,000 mph). At the time of the encounter, Ida and Galileo were 441 million kilometers from the Sun.
Ida is about 56 x 24 x 21 kilometers in size, more than twice as large as Gaspra. It has a period of rotation of 4 hours, 38 minutes. Its density has been estimated to be between 2.2 and 2.9 gm/cc. Ida's age is somewhat baffling. Its surface is heavily cratered suggesting that it has existed in its present form for at least a billion years and perhaps much longer. It is also considerably older than estimates for the Koronis breakup.
The greatest discovery from the Galileo fly-by was that Ida has a natural satellite. The moon has been named Dactyl by the International Astronomical Union. Dactyl is the first natural satellite of an asteroid ever discovered and photographed. The tiny moon is about 1.2 by 1.4 by 1.6 km across. The name is derived from the Dactyli, a group of mythological beings who lived on Mount Ida. The Dactyli protected the infant Zeus after the nymph Ida hid and raised the god on the mountain.
Dactyl is made more or less from the same kind of material as Ida. As an S-type asteroid, Ida is composed mostly of silicate rocks. Galileo scientists believe the moon may have been created at the same time as Ida when an older, larger asteroid was shattered in a collision with another asteroid, giving birth to dozens of smaller asteroids. Alternatively, it is possible that Ida was hit by a smaller object even more recently, leaving a crater on the asteroid and throwing off the material that became the small moon.
Galileo scientists also believe it is virtually impossible that the moon is a captured object, something created completely separately from Ida that happened to wander near the asteroid and be caught by its gravitational field. According to the laws of celestial mechanics, such an event would deflect the smaller object, but it would not be captured into orbit unless a third force of some kind slowed it down.
Toutatis:
In 1989, asteroid 4179 was discovered by French astronomers and named Toutatis after a Celtic god that was the protector of the tribe in ancient Gaul. Its eccentric, four-year orbit extends from just inside Earth's orbit to the main asteroid belt.
In December 1992, Toutatis made a close approach to Earth. At the time, it was an average of about 4 million kilometers from Earth. Images of Toutatis were acquired using radar carried out at the Goldstone Deep Space Communications Complex in California's Mojave desert. The radar images of Toutatis reveal two irregularly shaped, cratered objects about 4 and 2.5 kilometers in average diameter which are probably in contact with each other. These "contact binaries" may be fairly common since another one, 4769 Castalia, was observed in 1989 when it passed near the Earth. Numerous surface features on Toutatis, including a pair of half-mile-wide craters, side by side, and a series of three prominent ridges -- a type of asteroid mountain range -- are presumed to result from a complex history of impacts.
Toutatis is one of the strangest objects in the solar system, with a highly irregular shape and an extraordinarily complex tumbling rotation. Both its shape and rotation are thought to be the outcome of a history of violent collisions. One consequence of this strange rotation is that Toutatis does not have a fixed north pole like the Earth. Instead, its north pole wanders along a curve on the asteroid about every 5.4 days. The rotations of hundreds of asteroids have been studied with optical telescopes. The vast majority of them appear to be in simple rotation with a fixed pole and periods typically between one hour and one day, the scientists said, even though the violent collisions these objects are thought to have experienced would mean that every one of them, at some time in the past, should have been tumbling like Toutatis.
Vesta:
Vesta has a diameter of 525 kilometers and is smaller than the state of Arizona. It rotates about its axis in 5.34 hours. Vesta is the most geologically diverse of the large asteroids and the only known one with distinctive light and dark areas -- much like the face of our Moon. Hubble images have revealed a diverse world with ancient lava flows and a gigantic impact basin that is so deep, it exposes the asteroid's subsurface, or mantle. Vesta's surface shows a geology similar to that of terrestrial worlds such as Earth and Mars. Ground-based spectroscopy of Vesta indicates regions that are basaltic, which means lava flows once occurred on its surface. This is surprising evidence that the asteroid once had a molten interior, like Earth does. This contradicts conventional ideas that asteroids are essentially cold, rocky fragments left behind from the early days of planetary formation.
One possibility is that Vesta agglomerated from smaller material that includes radioactive debris that was incorporated into the core. These hot isotope may have melted the core, causing the asteroid to differentiate: heavier, dense material sank to the center while lighter rock rose to the surface. This is a common structure for the terrestrial planets. After Vesta's formation, molten rock flowed onto the asteroid's surface. This happened more than four billion years ago. The surface has remained unchanged since then, except for occasional meteoroid impacts.
Meteors:
The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the Solar System falls into Earth's atmosphere creating temporary incandescence resulting from atmospheric friction. This typically occurs at heights of 80 to 110 kilometers above Earth's surface. The term is also used loosely with the word meteoroid referring to the particle itself without relation to the phenomena it produces when entering the Earth's atmosphere. A meteoroid is matter revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. Even smaller particles are called micrometeoroids or cosmic dust grains, which includes any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized.
Meteor's come in a range of sizes, from dust-sized which we see as reflected sunlight in the orbital plane of the Solar System (called zodiacal light) to house-sized. When a meteor enters the atmosphere friction causes ablation of its surface (i.e. it burns up). If the meteor is small (fist-sized) it vaporizes before hitting the ground. If larger it survives to impact on the ground, although it will be reduced in size during entry into the atmosphere. About 25 million meteors enter the Earth's atmosphere every day (duck!). Most burn up and about 1 million kilograms of dust per day settles to the Earth's surface.
Meteorites have proven difficult to classify, but the three broadest groupings are chondrites, pallasites, and iron. The most common meteorites are chondrites, which are stony meteorites. Radiometric dating of chondrites has placed them at the age of 4.55 billion years, which is the approximate age of the Solar System. They are considered pristine samples of early solar system matter, although in many cases their properties have been modified by thermal metamorphism or icy alteration.
Other meteorite types which have been geologically processed are irons and pallasites. Iron meteorites are classified into thirteen major groups and consist primarily of iron-nickel alloys with minor amounts of carbon, sulfur, and phosphorus. These meteorites formed when molten metal segregated from less dense silicate material and cooled, showing another type of melting behavior within meteorite parent bodies. Thus, meteorites contain evidence of changes that occurred on the parent bodies from which they were removed or broken off, presumably by impacts, to be placed in the first of many revolutions. Pallasites are stony iron meteorites composed of olivine enclosed in metal.
Meteors often occur in showers and swarms. A meteor shower or swarm is visible as an increase in the number of ``shooting stars'' (meteors burning up as they hit the Earth's atmosphere) during certain times of the year. The showers often last for a couple weeks, with peaks of a couple days. The reason is shown below. Old comets breakdown into individual rocks over time (several thousand years). These rocks are clustered together at first as a swarm, then later spread out along the old comet orbit. When the Earth passes through the old orbit it encounters a fraction of the meteors causing a shower or swarm.
Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto. Historically, comets were thought to be atmospheric phenomena to early man, rare and objects of great curiosity. Tycho Brahe showed there was no parallax and that they must be located at distances greater than the Earth's orbit from the Sun. Comets at various times where thought to be ghosts, filled with poisonous gas, demons or cosmic signs; but are actually just asteroids covered in ice.
Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size (up to hundreds of km in diameter) and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.
As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.
The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun. Note that the ion tail points away from the Sun regardless of the orbital motion, thus a comet tail may move in front of the comet's orbit when a comet is leaving the inner Solar System. Each time a comet visits the Sun, it loses some of its volatiles. Eventually, it dissolves into a meteor swarm. For this reason, comets are said to be short-lived, on a cosmological time scale.
Comets have their origin in the giant Oort cloud that exists outside our Solar System. Perturbations from nearby stars cause the comets in the Oort cloud to fall into the Solar system in hyperbolic orbits (i.e. orbits with only a one time encounter with the inner Solar System). If a comet passes to close to Jupiter, its orbit is converted into a highly eccentric ellipse and the comet becomes periodic (i.e. it comes back to the inner Solar System on a regular basis, like Halley's Comet with a period of 76 years).
Comet Shoemaker-Levy 9:
Comet Shoemaker-Levy 9 was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. It was first detected on a photograph taken on the night of March 24, 1993, with the 0.4-meter Schmidt telescope on Palomar Mountain in California. The magnitude of the comet's brightness was reported as 14, more than a thousand times too faint to be seen with the naked eye.
Through the observations, the comet's orbit was demonstrated to be around Jupiter and that it had made a very close approach to Jupiter on July 7, 1992. During this close approach, the unequal Jupiter gravitational attractions on the comet's near and far sides broke the fragile object apart.
Since it was not at all obvious where the center of mass of this new comet lay, most observers were using the position of what appeared to be the center of the train. This made an accurate orbit (or orbits) difficult to determine. It was eventually determined that the comet train would again pass within 25,000 kilometers of the center of Jupiter, on July 19, 1994. This distance was less than the radius of Jupiter. In other words, the comet, or at least parts of it, would hit Jupiter.
By December 9 1993, the probability of impact for all the large fragments of Shoemaker-Levy 9 was calculated to be greater than 99.99%. The fragments would hit over a period of several days, centered on July 19, on the night side of Jupiter. Unfortunately, this was the back side of Jupiter as viewed from Earth. The impact site would be close to the limb of Jupiter, near 750 from the midnight meridian and only a few degrees beyond the dark limb as seen from Earth. The disruption of a comet into multiple fragments is an unusual event, the capture of a comet into an orbit about Jupiter is even more unusual, and the collision of a large comet with a planet is an extraordinary, millennial event. The observatories of the world lined-up for a week of observations.
Fragments of comet P/Shoemaker-Levy 9 collided with Jupiter on July 16-22, 1994. The results were spectacular. At least 20 large fragments impacted the planet at 60 kilometers per second, causing plumes thousands of kilometers high. They left hot bubbles of gas in the atmosphere and great dark scars which lasted for months after the collision. The largest fragments were estimated at 2 kilometers in diameter.
The fragments impacted Jupiter at approximately 45 degrees south latitude and 6.5 degrees longitude from the limb, and 15 from the dawn terminator just out of view of the Earth. Eleven minutes after impact, the point in the atmosphere where the impact occurred would rotate across the limb and 14 minutes later would cross the dawn terminator. The fragments were moving at an angle of 83 degrees from the Jovian north to south axis and struck the cloud tops at about 45 degrees.
Impact from Viewpoint of Q3 Fragment
The aftermath of the impacts were visible on Jupiter's atmosphere for months.
