Week 6 Pluto Comets and Space Debris Pluto

Week #6 Pluto, Comets, and Space Debris

Pluto n n Pluto, the outermost known planet, is a deviant. Its elliptical orbit is the most out of round (eccentric) and is inclined by the greatest angle with respect to the Earth’s orbital plane, near which the other planets revolve. n Pluto’s elliptical orbit is so eccentric that part lies inside the orbit of Neptune. Pluto was closest to the Sun in 1989 and moved farther away from the Sun than Neptune in 1999. So Pluto is still relatively near its closest approach to the Sun out of its 248 -year period, and it appears about as bright as it ever does to viewers on Earth. n It hasn’t been as bright for over 200 years. It is barely visible through a medium-sized telescope under dark-sky conditions.

Pluto’s Orbit:

Pluto n n The discovery of Pluto was the result of a long search for an additional planet that, together with Neptune, was believed to be slightly distorting the orbit of Uranus. Finally, in 1930, Clyde Tombaugh, hired at age 23 to search for a new planet because of his experience as an amateur astronomer, found the dot of light that is Pluto (see figure). n n It took him a year of diligent study of the photographic plates he obtained at the Lowell Observatory in Arizona. From its slow motion with respect to the stars over the course of many nights, he identified Pluto as a new planet.

Pluto’s Mass and Size n n n Even such basics as the mass and diameter of Pluto are very difficult to determine. Moreover, Pluto has made less than one revolution around the Sun since its discovery, thus providing little of its path for detailed study. As recently as 1968, it was mistakenly concluded that Pluto had 91 per cent the mass of the Earth, instead of the correct value of 0. 2 per cent.

Pluto’s Mass and Size n n The situation changed drastically in 1978 with the surprise discovery (see figure) that Pluto has a satellite. The moon was named Charon, The presence of a satellite allows us to deduce the mass of the planet by applying Newton’s form of Kepler’s third law. Charon is 5 to 10 per cent of Pluto’s mass, and Pluto is only 1/500 the mass of the Earth, ten times less than had been suspected just before the discovery of Charon.

Pluto’s Mass and Size n n n Pluto’s rotation axis is nearly in the ecliptic, like that of Uranus. This is also the axis about which Charon orbits Pluto every 6. 4 days. Consequently, there are two five year intervals during Pluto’s 248 -year orbit when the two objects pass in front of (that is, occult) each other every 3. 2 days, as seen from Earth. n Such mutual occultations were the case from 1985 through 1990.

Pluto’s Mass and Size n n From the duration of fading, we deduced how large they are. Pluto is 2300 km in diameter, smaller than expected, and Charon is 1200 km in diameter. n n Charon is thus half the size of Pluto. Further, it is separated from Pluto by only about 8 Pluto diameters, compared with the 30 Earth diameters that separate the Earth and the Moon. n So Pluto/Charon is almost a “double-planet” system.

Pluto’s Mass and Size n n n The rate at which the light from Pluto/Charon faded also gave us information that revealed the reflectivities (albedoes) of their surfaces, since part of the surface of the blocked object remained visible most of the time. The surfaces of both vary in brightness (see figure). Pluto seems to have a dark band near its equator, some markings on that band, and bright polar caps.

Pluto’s Mass and Size n n In 1990, the Hubble Space Telescope took an image that showed Pluto and Charon as distinct and separated objects for the first time, and they can now be viewed individually by telescopes on Mauna Kea in Hawaii (see figure, top) and elsewhere the “seeing” is exceptional. The latest Hubble views show that Pluto has a dozen areas of bright and dark, the finest detail ever seen on Pluto, whose diameter is smaller than that of the United States (see figure, below).

Pluto’s Mass and Size n If we were standing on Pluto, the Sun would appear over a thousand times fainter than it does to us on Earth. n Consequently, Pluto is very cold; infrared measurements show that its temperature is less than 60 K. From Pluto, we would need a telescope to see the solar disk, which would be about the same size that Jupiter appears from Earth.

Pluto’s Atmosphere n n n Pluto occulted — passed in front of and hid—a star on one night in 1988. Astronomers observed this occultation to learn about Pluto’s atmosphere. If Pluto had no atmosphere, the starlight would wink out abruptly. n n Any atmosphere would make the starlight diminish more gradually. The observations showed that the starlight diminished gradually and unevenly. n Thus Pluto’s atmosphere has layers in it.

Pluto’s Atmosphere n From the 1988 occultation, astronomers were also able to conclude that the bulk of Pluto’s atmosphere is nitrogen. n n A trace of methane must also be present, since the methane ice on Pluto’s surface, detected from its spectrum, must be evaporating. Still, Pluto’s atmospheric pressure is very low, only 1/100, 000 of Earth’s.

What Is Pluto? n From Pluto’s mass and radius, we calculate its density. n n It turns out to be about 2 g/cm 3, twice the density of water and less than half the density of Earth. Since ices have even lower densities than Pluto, Pluto must be made of a mixture of ices and rock. n Its composition is more similar to that of the satellites of the giant planets, especially Neptune’s large moon Triton, than to that of Earth or the other inner planets.

What Is Pluto? n n Pluto remains strange in that it is so small next to the giants, and that its orbit is so eccentric and so highly inclined to the ecliptic. Increasingly, Pluto is being identified with a newly discovered set of objects in the outer Solar System, which we will now study. n n Is Pluto even a planet? It is so small, so low in mass, and in such an inclined orbit with respect to the eight inner planets that perhaps it should only be called an asteroid, a “Kuiper-belt object, ” or a “Trans-Neptunian Object. ”

Kuiper-belt Objects n n n Beyond the orbit of Neptune, a population of icy objects with diameters of a few tens or hundreds of kilometers is increasingly being found. The planetary astronomer Gerard Kuiper (pronounced koy´per) suggested a few decades ago that these objects would exist and should be the source of many of the comets that we see. As a result, these objects are now known as the Kuiperbelt objects, or, less often, Trans-Neptunian Objects.

Kuiper-belt Objects n n n The Kuiper belt is probably about 10 A. U. thick and extends from the orbit of Neptune about twice as far out (see figure). About 1000 Kuiper-belt objects have been found so far, and tens of thousands larger than 100 km across are thought to exist. The objects may be left over from the formation of the Solar System.

Kuiper-belt Objects n They are generally very dark, with albedoes of only about 4 per cent. n n n Pluto, by contrast, has an albedo of about 60 per cent. Still, Pluto is one of the largest of the Kuiper belt objects, so much larger than most of the others that it is covered with frost. Triton may have initially been a similar object, subsequently captured by Neptune. A Kuiper-belt object larger than Pluto’s moon Charon was found in 2001, about half of Pluto’s diameter. One that may be even somewhat larger was found in 2002, though the uncertainty limits of these two Kuiper-belt objects overlap. n The newer one, tentatively and unofficially named Quaoar (pronounced “kwa-whar”)

Kuiper-belt Objects n David Jewitt of the University of Hawaii and Jane Luu, now at MIT’s Lincoln Lab, have been the discoverers of most of the known Kuiper-belt objects. n n They found the first one in 1992 and they and several other astronomers are looking for more. Michael Brown of Caltech and his colleagues stunned the world in July 2005, with their discovery of an outer-solar-system object even larger than Pluto (see figures). n Initially named 2003 UB 313, it was first sighted in 2003 but not confirmed until 2005.

Kuiper-belt Objects n UB 313 is now 97 A. U. out from the Sun, more than twice as far out as Pluto. n n It takes over 500 years to orbit the Sun. Its orbit is tilted an incredible 44°, taking it so high out of the ecliptic that no previous planet hunter found it. Undoubtedly, it was thrown into that highly inclined orbit after a close gravitational encounter with Neptune. Is it a 10 th planet?

Comets n n Nearly every decade, a bright comet appears in our sky. From a small, bright area called the head, a tail may extend gracefully over one-sixth (30°) or more of the sky. n n The tail of a comet is always directed roughly away from the Sun, even when the comet is moving outward through the Solar System. Although the tail may give an impression of motion because it extends out only to one side, the comet does not move noticeably with respect to the stars as we casually watch during the course of a night.

Comets n n Still, both comets and stars rise and set more or less together (see figure). Within days, weeks, or (even less often) months, a bright comet will have become too faint to be seen with the naked eye.

Comets n Most comets are much fainter than the one we have just described. n n About two dozen new comets are discovered each year, and most become known only to astronomers. If you should ever discover a comet, and are among the first three people to report it to the International Astronomical Union Central Bureau for Astronomical Telegrams at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, it will be named after you.

The Composition of Comets n n At the center of a comet’s head is its nucleus, which is composed of chunks of matter. The most widely accepted theory of the composition of comets, is that the nucleus is like a “dirty snowball. ” n It may be made of ices of such molecules as water (H 2 O), carbon dioxide (CO 2), ammonia (NH 3), and methane (CH 4), with dust mixed in.

The Composition of Comets n n n The nucleus itself is so small that we cannot observe it directly from Earth. Radar observations have verified in several cases that it is a few kilometers across. The rest of the head is the coma (pronounced coh´ma), which may grow to be as large as 100, 000 km or so across (see figure). n The coma shines partly because its gas and dust are reflecting sunlight toward us and partly because gases liberated from the nucleus get enough energy from sunlight to radiate.

The Composition of Comets n n The tail can extend 1 A. U. (150, 000 km), so comets can be the largest objects in the Solar System. But the amount of matter in the tail is very small—the tail is a much better vacuum than we can make in laboratories on Earth. n n The dust tail is caused by dust particles released from the ices of the nucleus when they are vaporized. n n Many comets actually have two tails The dust particles are left behind in the comet’s orbit, blown slightly away from the Sun by the pressure of sunlight hitting the particles. As a result of the comet’s orbital motion, the dust tail usually curves smoothly behind the comet.

The Composition of Comets n The gas tail is composed of gas blown outward from the comet, at high speed, by the “solar wind” of particles emitted by the Sun. n n As puffs of gas are blown out and as the solar wind varies, the gas tail takes on a structured appearance. n n It follows the interplanetary magnetic field. Each puff of matter can be seen. A comet—head and tail together—contains less than a billionth of the mass of the Earth.

The Origin and Evolution of Comets n It is now generally accepted that trillions of tail-less comets surround the Solar System in a sphere perhaps 50, 000 A. U. (that is, 50, 000 times the distance from the Sun to the Earth, or almost 1 light-year) in radius. n n This sphere, far outside Pluto’s orbit, is the Oort comet cloud (named after the Dutch scientist Jan Oort). The total mass of matter in the cloud may be only 1 to 10 times the mass of the Earth. n In current models, most of the Oort cloud’s mass is in the inner 1000 to 10, 000 A. U.

The Origin and Evolution of Comets n n Occasionally one of these comets leaves the comet cloud. Currently, astronomers tend to think that gravity from the disk of our Milky Way Galaxy does most of the tugging. The comet’s orbit may be altered, sometimes into an elliptical orbit, if it passes near a giant planet, most frequently Jupiter. Because the comet cloud is spherical, comets are not limited to the plane of the ecliptic, which explains why one major class of comets comes in randomly from all directions.

The Origin and Evolution of Comets n n n Another group of comets has orbits that are much more limited to the plane of the Solar System (Earth’s orbital plane). They probably come from the Kuiper belt beyond the orbit of Neptune, a flatter distribution of objects ranging from about 25 to 50 A. U. We seem to discover more of these Kuiper-belt-origin comets than we expect compared with Oort-cloud-origin comets.

The Origin and Evolution of Comets n n Until recently, astronomers tended to say that the long-period comets, those with orbital periods longer than 200 years, came from the Oort cloud while comets with periods shorter than 200 years came from the Kuiper belt (see figure). Most of the long-period comets have semimajor axes close to 20, 000 A. U. , 5000 times the 40 A. U. semimajor axis of Pluto’s orbit.

The Origin and Evolution of Comets n The short-period comets, those with periods less than 200 years, were divided into “Jupiter-family” comets, whose orbits were made so small by encounters with Jupiter that their periods were less than 20 years, and “Halley-type” comets, which suffered less influence by Jupiter.

The Origin and Evolution of Comets n As a comet gets closer to the Sun than those distant regions, the solar radiation begins to vaporize the ice in the nucleus. n n n The tail forms, and grows longer as more of the nucleus is vaporized. Even though the tail can be millions of kilometers long, it is still so tenuous that only 1/500 of the mass of the nucleus may be lost each time it visits the solar neighborhood. Thus a comet may last for many passages around the Sun. n But some comets hit the Sun and are destroyed (see figure).

Halley’s Comet n In 1705, the English astronomer Edmond Halley (Halley is pronounced to rhyme with “Sally, ” and not with “say´lee”) (see figure) applied a new method developed by his friend Isaac Newton to determine the orbits of comets from observations of their positions in the sky. n n He reported that the orbits of the bright comets that had appeared in 1531, 1607, and 1682 were about the same. Moreover, the intervals between appearances were approximately equal, so Halley suggested that we were observing a single comet orbiting the Sun, and he accounted for the slightly different periods with Newton’s law of gravity from interactions with planets.

Halley’s Comet n n n Halley predicted that this bright comet would again return in 1758. Its reappearance on Christmas night of that year, 16 years after Halley’s death, was the proof of Halley’s hypothesis (and Newton’s method). n The comet has thereafter been known as Halley’s Comet ( see figure). Since it was the first known “periodic comet” (i. e. , the first comet found to repeatedly visit the inner parts of the Solar System), it is officially called 1 P, number 1 in the list of periodic (P) comets.

Halley’s Comet n It seems probable that the bright comets reported every 74 to 79 years since 240 b. c. were earlier appearances of Halley’s Comet. n n Halley’s Comet came especially close to the Earth during its 1910 return, and the Earth actually passed through its tail. n n The fact that it has been observed dozens of times endorses the calculations that show that less than 1 per cent of a cometary nucleus’s mass is lost at each passage near the Sun. Many people had been frightened that the tail would somehow damage the Earth or its atmosphere, but the tail had no noticeable effect. Even then, most scientists knew that the gas and dust in the tail were too tenuous to harm our environment.

Halley’s Comet n n The most astounding observations were undoubtedly the photographs showing the nucleus itself (see figure, bottom left), which turns out to be potato-shaped (see figure, bottom right). It is about 16 km in its longest dimension, half the size of Manhattan Island.

Halley’s Comet n The “dirty snowball” theory of comets was confirmed in general, but the snowball is darker than expected. n n Further, the evaporating gas and dust is localized into jets that are stronger than expected. n n It is as black as velvet, with an albedo of only about 3 per cent. They come out of fissures in the dark crust. We now realize that comets may shut off not when they have lost all their material but rather when the fissures in their crusts close.

Halley’s Comet n n n About 30 per cent of Halley’s dust particles are made only of hydrogen, carbon, nitrogen, and oxygen (see figure). This simple composition resembles that of the oldest type of meteorite. It thus indicates that these particles may be from the earliest years of the Solar System.

Halley’s Comet n The next appearance of Halley’s Comet, in 2061, again won’t be spectacular. n n n Not until the one after that, in 2134, will the comet show a long tail to earthbound observers. Fortunately, though Halley’s Comet is predictably interesting, a more spectacular comet appears every 10 years or so. When you read in the newspaper that a bright comet is here, don’t wait to see it another time. n Some bright comets are at their best for only a few days or a week.

Comet Shoemaker-Levy 9 n n n A very unusual comet gave thrills to people around the world. In 1993, Eugene Shoemaker, Carolyn Shoemaker, and David Levy discovered their ninth comet in a search with a wide-field telescope at the Palomar Observatory. This comet looked weird—it seemed squashed.

Comet Shoemaker-Levy 9 n Higher-resolution images taken with other telescopes, including the Hubble Space Telescope (see figure), showed that the comet had broken into bits, forming a chain that resembled beads on a string. n n Even stranger, the comet was in orbit not around the Sun but around Jupiter, and would hit Jupiter a year later. Apparently, several decades earlier the comet was captured in a highly eccentric orbit around Jupiter, and in 1992, during its previous close approach, it was torn apart into more than 20 pieces by Jupiter’s tidal forces.

Comet Shoemaker-Levy 9 n n Telescopes all around the world and in space were trained on Jupiter when the first bit of comet hit. The site was slightly around the back side of Jupiter, but rotated to where we could see it from Earth after about 15 minutes. n n Even before then, scientists were enthralled by a plume rising above Jupiter’s edge. Over a period of almost a week, one bit of the comet after another hit Jupiter, leaving a series of Earth-sized rings and spots as Jupiter rotated. n The largest dark spots could be seen for a few months even with small backyard telescopes.

Comet Shoemaker-Levy 9

Comet Shoemaker-Levy 9 n n The dark material showed us the hydrocarbons and other constituents of the comet. Spectra showed sulfur and other elements, presumably dredged up from lower levels of Jupiter’s atmosphere than we normally see. The biggest comet chunk released the equivalent of 6 million megatons of TNT— 100, 000 times more than the largest hydrogen bomb. So Comet Shoemaker-Levy 9 made us even more wary about what may be coming at us from space.

Recently Observed Comets n n In 1995, Alan Hale and Thomas Bopp independently found a faint comet, which was soon discovered to be quite far out in the Solar System. Its orbit was to bring it into the inner Solar System, and it was already bright enough that it was likely to be spectacular when it came close to Earth in 1997. n It lived up to its advance billing (see figure).

Recently Observed Comets n n n Telescopes of all kinds were trained on Comet Hale-Bopp, and hundreds of millions of people were thrilled to step outside at night and see a comet just by looking up. Modern powerful radio telescopes were able to detect many kinds of molecules that had not previously been recorded in a comet. Occasionally, other bright comets, such as C /2002 C 1, Comet Ikeya-Zhang (see figure), turn up and are fun to watch.

Spacecraft to Comets n n NASA’s Deep Space 1 mission flew close to Comet 19 P/Borrelly in 2001. n It obtained more detailed images of the bowling-pin-shaped nucleus ( see figure) than even Giotto’s views of Halley’s nucleus. This comet’s surface, and therefore probably the surfaces of comet nuclei in general, was rougher and more dramatic than expected.

Spacecraft to Comets n NASA’s Stardust mission, launched in 1999, went to Comet Wild 2 (pronounced Vilt-too), a periodic comet with a sixyear orbit. n n When it got there in 2004, it not only photographed the comet but also gathered some of its dust. It carries an extremely lightweight material called aerogel, and flew through the comet with the aerogel exposed so that the comet dust could stick in it.

Meteoroids n There are many small chunks of matter orbiting in the Solar System, ranging up to tens of meters across and sometimes even larger. n n When these chunks are in space, they are called meteoroids. When one hits the Earth’s atmosphere, friction and the compression of air in front of it heat it up—usually at a height of about 100 km—until all or most of it is vaporized. n Such events result in streaks of light in the sky, which we call meteors (popularly, and incorrectly, known as shooting stars or falling stars). n When a fragment of a meteoroid survives its passage through the Earth’s atmosphere, the remnant that we find on Earth is called a meteorite. n Counting even tiny meteorites, whose masses are typically a milligram, some 10, 000 tons of this interplanetary matter land on Earth’s surface each year.

Types and Sizes of Meteorites n Space is full of meteoroids of all sizes, with the smallest being most abundant. n n Most of the small particles, less than 1 mm across, may come from comets. The large particles, more than 1 cm across, may generally come from collisions of asteroids in the asteroid belt (see Section 8. 5). Tiny meteorites less than a millimeter across, micrometeorites, are the major cause of erosion on the Moon. Micrometeorites also hit the Earth’s upper atmosphere all the time, and remnants can be collected for analysis from balloons or airplanes or from deep-sea sediments.

Types and Sizes of Meteorites n n Some of the meteorites that are found have a very high iron content (about 90 per cent); the rest is nickel. These iron meteorites are thus very dense— that is, they weigh quite a lot for their volume (see figure).

Types and Sizes of Meteorites n Most meteorites that hit the Earth are stony in nature. Because they resemble ordinary rocks (see figure) and disintegrate with weathering, they are not easily discovered unless their fall is observed. n n That difference explains why most meteorites discovered at random are made of iron. But when a fall is observed, most meteorites recovered are made of stone. n Some meteorites are rich in carbon, and some of these even have complex molecules like amino acids.

Types and Sizes of Meteorites n A large terrestrial crater that is obviously meteoritic in origin is the Barringer Meteor Crater in Arizona (see figure, left). n n n It resulted from what was perhaps the most recent large meteoroid to hit the Earth, for it was formed only about 50, 000 years ago. Every few years a meteorite is discovered on Earth immediately after its fall. The chance of a meteorite landing on someone’s house or car is very small, but it has happened (see figure, below)!

Types and Sizes of Meteorites n n n Some odd Antarctic meteorites are now known to have come from the Moon or even from Mars. Meteorites that have been examined were formed up to 4. 6 billion years ago, the beginning of the Solar System. The relative abundances of the elements in meteorites thus tell us about the solar nebula from which the Solar System formed. n In fact, up to the time of the Moon landings, meteorites and cosmic rays (charged particles from outer space) were the only extraterrestrial material we could get our hands on.

Meteor Showers n n Meteors sometimes occur in showers, when meteors are seen at a rate far above average. Meteor showers are named after the constellation in which the radiant, the point from which the meteors appear to come, is located. n n The most widely observed—the Perseids, whose radiant is in Perseus—takes place each summer around August 12 and the nights on either side of that date. The best winter show is the Geminids, which takes place around December 14 and whose radiant is in Gemini.

Meteor Showers n On any clear night a naked-eye observer with a dark sky may see a few sporadic meteors an hour—that is, meteors that are not part of a shower. (Just try going out to a field in the country and watching the sky for an hour. ) n n During a shower, on the other hand, you may typically see one every few minutes. Meteor showers generally result from the Earth’s passing through the orbits of defunct or disintegrating comets and hitting the meteoroids left behind. (One meteor shower comes from an asteroid orbit. )

Meteor Showers n n Leonid meteor shower (whose radiant is in Leo) peaks every 33 years, when the Earth crosses the main clump of debris from Comet Tempel. Tuttle. On November 17/18, 1998, one fireball (a meteor brighter than Venus) was visible each minute for a while (see figure), and on November 17/18, 1999 through 2001, thousands of meteors were seen in the peak hour. n n We will now have to wait until about 2031 for the next Leonid peak. The visibility of meteors in a shower depends in large part on how bright the Moon is; you want as dark a sky as possible. n Meteors are best seen with the naked eye; using a telescope or binoculars merely restricts your field of view.

Asteroids n The nine known planets were not the only bodies to result from the gas and dust cloud that collapsed to form the Solar System 4. 6 billion years ago. n n n Thousands of minor planets, called asteroids, also resulted. We detect them by their small motions in the sky relative to the stars. Most of the asteroids have elliptical orbits between the orbits of Mars and Jupiter, in a zone called the asteroid belt. n It is thought that Jupiter’s gravitational tugs perturbed the orbits of asteroids, leading to collisions among them that were too violent to form a planet.

Asteroids n Asteroids are assigned a number in order of discovery and then a name: (1) Ceres, (16) Psyche, and (433) Eros, for example. n n Often the number is omitted when discussing well-known asteroids. Though the concept of the asteroid belt may seem to imply a lot of asteroids close together, asteroids rarely come within a million kilometers of each other. n Occasionally, collisions do occur, producing the small chips that make meteoroids.

General Properties of Asteroids n n Only about 6 asteroids are larger than 300 km in diameter. Hundreds are over 100 km across (see figure), roughly the size of some of the moons of the planets, but most are small, less than 10 km in diameter. Perhaps 100, 000 asteroids could be detected with Earthbased telescopes; automated searches are now discovering asteroids at a prodigious rate. n Yet all the asteroids together contain less mass than the Moon.

General Properties of Asteroids n Spacecraft en route to Jupiter and beyond travelled through the asteroid belt for many months and showed that the amount of dust among the asteroids is not much greater than the amount of interplanetary dust in the vicinity of the Earth. n n So the asteroid belt is not a significant hazard for space travel to the outer parts of the Solar System. Asteroids are made of different materials from each other, and represent the chemical compositions of different regions of space.

General Properties of Asteroids n The differences may be telling us about conditions in the early Solar System as it was forming and how the conditions varied with distance from the young Sun. n Many of the asteroids must have broken off from larger, partly “differentiated” bodies in which dense material sank to the center (as in the case of the terrestrial planets; see our discussion in Chapter 6). n The path of the Galileo spacecraft to Jupiter sent it near the asteroid (951) Gaspra in 1991 (see figure). n It detected a magnetic field from Gaspra, which means that the asteroid is probably made of metal and is magnetized.

General Properties of Asteroids n n Galileo passed the asteroid (243) Ida in 1993, and discovered that the asteroid has an even smaller satellite (see figure), which was then named Dactyl. Other double asteroids have since been discovered, and astronomers newly recognize the frequency of such pairs.

Near-Earth Objects n Some asteroids are far from the asteroid belt; their orbits approach or cross that of Earth. n n n We have observed only a small fraction of these types of Near-Earth Objects, bodies that come within 1. 3 A. U. of Earth. The Near Earth Asteroid Rendezvous (NEAR) mission passed and photographed the main-belt asteroid (253) Mathilde in 1997. The existence of big craters that would have torn a solid rock apart, and the asteroid’s low density, lead scientists to conclude that Mathilde is a giant “rubble pile, ” rocks held together by mutual gravity.

Near-Earth Objects n n NEAR went into orbit around (433) Eros on Valentine’s Day, 2000 (see figures), when it was renamed NEAR Shoemaker after the planetary geologist Eugene Shoemaker. Eros was the first near-Earth asteroid that had been discovered. n n It is 33 km by 13 km in size. NEAR Shoemaker photographed craters, grooves, layers, housesized boulders, and a 20 -km-long surface ridge.

Near-Earth Objects n n n The existence of the craters and ridge, which indicates that Eros must be a solid body, disagrees with the previous suggestions of some scientists that most asteroids are mere rubble piles as Mathilde seems to be. Eros’s density, 2. 4 g /cm 3, is comparable to that of the Earth’s crust, about the same as Ida’s, and twice Mathilde’s. From orbit, NEAR Shoemaker’s infrared, x-ray, and gamma-ray spectrometers measured how the minerals vary from place to place on Eros’s surface. n The last of these even survived the spacecraft’s landing on Eros (see figures), and radioed back information about the composition of surface rocks.

Near-Earth Objects Near-Earth asteroids (see figure) may well be the source of most meteorites, which could be debris of collisions that occurred when these asteroids visit the asteroid belt. Eventually, most Earth-crossing asteroids will probably collide with the Earth. n n Statistics show that there is a 1 per cent chance of a collision of this tremendous magnitude per millennium. n n Over 1000 of them are greater than 1 km in diameter, and none are known to be larger than 10 km across. This rate is pretty high on a cosmic scale. Such collisions would have drastic consequences for life on Earth.

Near-Earth Objects n Smaller objects are a hundred times more common, with a 1 per cent chance that an asteroid greater than 300 m in diameter would hit the Earth in the next century. n n Such a collision could kill thousands or millions of people, depending on where it lands. The question of how much we should worry about Near. Earth Objects hitting us is increasingly discussed, including at a meeting sponsored by the United Nations. n Even Hollywood movies have been devoted to the topic, though at present we can’t send out astronauts to deflect or break up the objects the way the movies showed.

Near-Earth Objects n n n Several projects are under way to find as many Near-Earth Objects as possible. Current plans are to map 90 per cent of them in the next couple of decades, and the pace of discovery is accelerating. Several projects use CCD detectors, repetitive scanning, and computers to locate asteroids and are discovering thousands each year, some of which are Near-Earth Objects.