v1.2.3 / chapter 10 of 19 / 01 jan 09 / greg goebel / public domain
* The smaller bodies of the solar system, particularly the asteroids, were long a lower priority for early planetary exploration efforts, and it has only been the last few decades that greater efforts have been made to learn more about them. This chapter provides a short survey of what is known about the asteroids and current efforts to explore them.
* Most asteroids reside in a belt between Mars and Jupiter. This "asteroid belt" was discovered two centuries ago through what eventually proved to be a combination of skill and luck.
Scientists have always noticed regularities and symmetries in numbers and calculations as possible clues to scientific insights. In 1766, a German astronomer named Johann D. Titius noticed that the distances of the planets known at the time followed something that resembled a neat sequence, except that the sequence indicated there should be a planet between Mars and Jupiter. Another German astronomer, Johann E. Bode, mathematically refined the idea, and it became known as "Bode's Law", or sometimes the "Bode-Titius Law".
The Bode-Titius is based on the sequence of values:
0 0.3 0.6 1.2 2.4 4.8 9.6 19.2 38.4
-- where each number, except 0 and 0.3, is twice the value of the preceding
number. Adding 0.4 to the sequence gives:
0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8
This sequence has an interesting match to the sequence of distances of the
planets from the Sun, as measured in astronomical units:
planet actual distance bode-titius law
___________________________________________
Mercury 0.39 0.4
Venus 0.72 0.7
Earth 1.0 1.0
Mars 1.52 1.6
? 2.8
Jupiter 5.20 5.2
Saturn 9.54 10.0
Uranus 19.19 19.6
___________________________________________
When the Bode-Titius law was originally published, the planet Uranus had not
been discovered. While the law was largely ignored at first, the
German-English astronomer Sir William Herschel's accidental discovery of
Uranus in 1781 at roughly the distance predicted by the law made astronomers
wonder if there was something to it after all.
The Bode-Titius law also implied that a planet should exist at 2.8 AU from the Sun, but nobody knew of any planet at that distance. Where was this "missing" planet? Finding a new planet was a challenge for astronomers of the time about as exciting as the prospect of winning the Nobel Prize is for a scientist today, and searches were conducted to find the missing planet.
A Hungarian baron named Franz X. von Zach attempted to calculate the orbit of the missing planet, organizing a group of astronomers who called themselves the "Celestial Police" to "arrest" the "culprit". The Celestial Policemen were each given a sector of the sky to search. The searches were conducted along the "ecliptic", the plane in which most of the planets orbit the Sun.
Other astronomers also conducted searches for the missing planet. On 1 January 1801, Giuseppe Piazzi of Palermo, Sicily, who was not a member of the Celestial Police at the time, discovered an object that he named "Ceres", after the matron goddess of Sicily. The missing planet had been found.
* Or had it? Ceres looked like a star through a telescope, not showing a visible disk. This meant that it was very small and not much of a planet. This was disappointing, and things became more puzzling in 1802, when Heinrich W.M. Olbers, one of the Celestial Police, discovered a second little planet that would be given the name of "Pallas". By 1807, two more little planets had been discovered and named "Juno" and "Vesta". Sir William Herschel proposed that these little planets be named "asteroids", meaning "starlike bodies", in accordance with their starlike appearance through the telescope, and the name stuck.
No more asteroids were discovered for decades, and the Celestial Police finally disbanded in 1815. Finally, in 1845, the asteroid "Astrea" was discovered, assisted by the availability of improved sky maps. This opened the floodgates. By 1852, 20 asteroids were known, and by 1870 the number had reached 110. With every major improvement in astronomical technology, an entirely new set of smaller and fainter asteroids was discovered. The proliferation of asteroids seemed to provide the answer to the question of what happened to the missing planet. Olbers suggested early on that the missing planet had actually existed, but had been destroyed in some cosmic disaster. All that was left of it were the asteroids, which formed such a clutter that some astronomers called them "vermin of the skies" that interfered with more worthwhile observations.
* About 18,000 asteroids are known at present, and orbits have been determined for about 5,000 of them. Asteroids with known orbits are listed in a catalog giving their order of discovery and, when available, the name they have been given by their discoverer. For example, "3 Juno" designates the asteroid Juno, the third asteroid discovered. Asteroids whose orbits are not known are simply identified by an entry giving the date of their discovery. By the way, asteroid orbits are tricky to calculate, since they are often "perturbed" by the strong pull of Jupiter's gravity. Asteroids can get lost if their positions are not checked every now and then.
Most asteroids occupy the asteroid belt. The average diameter of the orbits of these "main belt" asteroids ranges from 2.1 to 3.3 AU, and their orbits are generally inclined ten degrees of the ecliptic. The first three asteroids discovered -- Ceres, Pallas, and Vesta -- are the largest of the asteroids, with average diameters of 933, 523, and 501 kilometers respectively.
The number of asteroids with diameters greater than 100 kilometers is more than 200, and the number of asteroids with diameters greater than 30 kilometers is about 1,000. Estimates place the numbers of asteroids with diameters greater than a kilometer at 100,000 to a million. Despite these great numbers, the total mass of the asteroids is very small, about 2,000 times smaller than the mass of the Earth. Ceres, Pallas, and Vesta make up about half the total mass of all the asteroids. Vesta, by the way, is said to be the only asteroid that is ever visible to the naked eye from Earth. Although Ceres and Pallas are bigger, they are also darker and harder to spot.
There never was a true planet where the asteroid belt is now. The Bode-Titius law is now seen as dubious at best. It does seem to reflect some order in the way planets are formed, and is demonstrated to a degree in the order of the moons of the gas giant planets, but it has little predictive power. When the giant planet Neptune was discovered in 1840, it was entirely out of the sequence predicted by the Bode-Titius law.
* Although the large numbers of asteroids suggest STAR WARS movie images of Han Solo weaving the Millennium Falcon through fields of floating boulders with Darth Vader's Imperial Tie Fighters in screaming pursuit, the volume of space occupied by the asteroid belt is vast. The spacing between asteroids is typically millions of kilometers.
The layout of the asteroid belt is largely controlled by the gravitational effect of Jupiter. The first evidence of this was discovered in 1867, when an American astronomer named Daniel Kirkwood found rings in the asteroid belt where few or no asteroids were found. These depleted regions became known as "Kirkwood gaps". The Kirkwood gaps occur where the orbital period of an asteroid would have an exact integer ratio with Jupiter's orbital period. For example, an asteroid at a distance of 2.5 AU from the Sun would have an orbital period exactly three times that of Jupiter. In more formal terms, that asteroid would be said to be at the "3:1 resonance". The consistent tug of Jupiter's gravity generally tends to move any object in such a resonant orbit out of that orbit, keeping it relatively clear. In some cases, however, the resonances set up by Jupiter's gravity tend to cluster or concentrate asteroids.
In fact, the influence of Jupiter is likely the reason the asteroid belt exists in the first place. Astronomers now believe that Jupiter prevented a planet from ever being formed there. The giant planet's gravity jostled the small "protoplanets" in what is now the asteroid belt, preventing them from accumulating and causing collisions that fragmented them further. In some cases, objects were sent out of solar system completely by such interactions, and the mass of the asteroid belt today may be far smaller than it was in the distant past. Collisions continue today, if much more infrequently, with the impacts taking place at an average velocity of about 5 kilometers per second.
In 1918, the Japanese astronomer Kiyotsugu Hirayama discovered groupings of asteroids that shared common orbits and seemed to have been derived from the breakup of larger parent bodies. He called these groupings "families". Later astronomers have named as many as a hundred such families. Analysis of the families has been performed to study the evolution of the asteroid belt through collisions.
* Not all asteroids are found in the asteroid belt. There are also several families of "Trojan" asteroids, the best-known being those associated with Jupiter, and a number of families of "near Earth asteroids (NEAs)" whose orbits approach or even go inside of Earth's.
The Jovian Trojan asteroids exist at a position in Jupiter's orbit where they form a neat equilateral triangle with Jupiter and the Sun. There is a set of "leading" Jovian Trojans that precede Jupiter in its orbit, and a set of "trailing" Jovian Trojans that lag behind Jupiter in its orbit. The "Trojan points" two of the five "Lagrange points" identified by French celestial mechanic Louis Lagrange where objects can occupy stable orbits. About 2,000 Jovian Trojan asteroids have been catalogued so far, and it is suspected that there may be as many kilometer-sized asteroids in the Jovian Trojans as there are in the main asteroid belt.
It is also now known that other planets have sets of Trojan asteroids. Four asteroids have been found in the Trojan points of Mars, though nobody is expecting to find a large number of Martian Trojans. In early 2003, a team of astronomers announced that they had discovered an asteroid in Neptune's leading Trojan point. The object had been first spotted in 2001 and was designated "2001 QR322". It was estimated to be about 230 kilometers in diameter. Others have been found since then, and it is suspected there may be as many or more Neptunian Trojans as there are Jovian Trojans.
The NEA families include:
Astronomers believe that NEAs only survive in their orbits for 10 million to 100 million years. They are eventually eliminated either by collisions with the inner planets, or by being ejected from the solar system by near misses. Such processes should have eliminated them all long ago, but it appears they are resupplied on a regular basis.
Some of the NEAs with highly eccentric orbits appear to actually be extinct short-period comets that have lost all their volatiles, and in fact a few NEAs still show faint comet-like tails. These NEAs were likely derived from the distant "Kuiper Belt", a repository of comets residing beyond the orbit of Neptune. The rest appear to have been driven out of the asteroid belt by gravitational interactions near the resonance gaps.
A few hundred NEAs with diameters of 500 meters or more have been discovered. Estimates of the number that actually exist range up into the thousands. New NEAs are discovered every month by automated telescopic systems. An impact by a 10 kilometer wide NEA or comet in the Yucatan 65 million years ago is strongly suspected to have at least contributed to the extinction of the dinosaurs. NEA watchers believe that catastrophic Earth impacts by NEAs are not rare in geological terms, and that setting up an observational network to keep track of them is prudent. Given enough advance warning, it may be possible to mount a space mission to give a threatening NEA a slight nudge, shifting it out of a threatening orbit.
* In the late 1990s, astronomers discovered an interesting subclass of NEAs that actually more or less (in a very literal sense) share the Earth's orbit. The discovery of the first of these "Earth coorbital" asteroids, designated "3753 Cruithne", was announced in 1997.
3753 Cruithne's orbit is known as a "horseshoe" orbit, which describes the path its orbit traces out if the Earth is regarded as stationary. To visualize its simplest form, imagine an asteroid that is orbiting slightly inside the orbit of the Earth. Since it is in a tighter orbit, it orbiting faster than the Earth, and gradually catches up to it.
However, once it gets close to the Earth, the Earth's gravity nudges it into an orbit outside that of the Earth's. The asteroid is now moving more slowly than the Earth and falls behind. Eventually, the Earth will catch up to the asteroid in this exterior orbit and shunt it back to the original interior orbit. Since it is moving faster than the Earth in the interior orbit, it moves away from the Earth, to eventually come around behind it again ... and repeat the cycle once more. From the point of view of the Earth, the asteroid's path forms a "horseshoe", continuously approaching the Earth and being sent back on its path again. Since the Earth and the asteroid have similar orbital velocities, it takes centuries for the asteroid to complete a single horseshoe orbit.
The whole idea is very counterintuitive. It seems almost like the Earth's gravity is repelling the asteroid, but that's just an artifact of using the Earth's point of view. The scheme involves the interaction of the asteroid with the Earth and Sun, leading to the odd resonance.
In practice, this nice tidy scenario only applies if the eccentricity and inclination of the orbits of the Earth and the asteroid are a close match. If the asteroid's orbit is more elliptical than the Earth's, the asteroid will cross the Earth's orbital path during the course of their yearly orbits. 3573 Cruithne's orbit is more elliptical than that of the Earth and has a higher inclination, and the result is that, from the point of view of the Earth, the asteroid makes a series of many kidney-bean-shaped spirals along its horseshoe path. It performs one such spiral every year, with each spiral taking it one step towards or away from the Earth, and the cycle takes a total of 385 years.
When the asteroid reaches the Earth at the end of its cycle, its path actually does overlap that of the Earth, at least as seen from above. However, 3573 Cruithne's orbit is actually at an inclination to ours, so at one point in the spiral it's above our orbit, and at another it's below, passing the Earth well under our South Pole. There's no chance of an impact as long as nothing disrupts its orbit. In fact, the closest it gets is about 15 million kilometers, or 40 times the distance from the Earth to the Moon.
For the curious, "Cruithne" is the Celtic name for the first Celtic tribe to arrive in the British Isles several hundred years before Christ. They are better known as "Picts". The word is pronounced "croo-EEN-ya". Of course -- it's spelled exactly like it sounds.
The discovery of a second Earth coorbital asteroid, designated "2002 AA29", was announced in 2002. This object also features a horseshoe orbit, in this case one that brings it near the Earth every 95 years. It made a close approach on 8 January 2003, though it was still much farther away than the Moon. It will return to Earth on the "other side" of its orbit in 2098. In addition, analysis of its orbit showed that in 600 years it will appear to be a moonlet of the Earth, a "quasi-satellite", for about 50 years. The last time this happened was from 550 to 600 CE. It's never actually trapped by the Earth's gravity, it's just that for 50 years or so the motions of the two bodies coincide to make one move around the other until they drift out of synch again.
Two more Earth coorbital asteroids are known, including "1998 UP1" and "2000 PH5". As their dates show, they were discovered before 2002 AA29, but analysis of their horseshoe orbits was not complete at last notice. It is somewhat difficult to classify the Earth coorbital asteroids as either Atens or Apollos, since they are outside the Earth's orbit for about half the time and inside for the other half, and on the average the size of their orbit is about the same as that of the Earth's. Somewhat surprisingly, given the fact that relatively puny Mars has a small number of Trojan asteroids, nobody has discovered any evidence of any Earth Trojans. Possibly the relative proximity of Venus prevents asteroids from taking up a permanent station at the Earth Trojan points.
* There has long been a suspicion that there may be yet another population of asteroids, orbiting in a dynamically stable zone near the Sun inside the orbit of Mercury. If the "Vulcanoids", as they have been known, actually exist, they have so far escaped detection because they are small and could only be observed from the ground just after sunset or before sunrise, through the longest path through the atmosphere.
In 2002, a number of flight were conducted with an F/A-18D two-seat fighter operated by NASA to take pictures from high altitude over the horizon at these times of day to see if Vulcanoids could be spotted. The aircraft carried a video camera, the "SouthWest Ultraviolet Imaging System - Aircraft (SWUIS-A)", built by the SouthWest Research Institute (SWRI) in Boulder, Colorado, in cooperation with University of Colorado at Boulder astronomers. The camera was originally intended as a shuttle payload.
In early 2004, the SWRI / UC Boulder group began flights with a Black Brant sounding rocket that carried an ultraviolet spectrometer to observe Mercury, along with a "Vulcanoid Camera (VULCAM)" to hunt for Vulcanoids. VULCAM was an improved version of SWUIS-A and provided video imagery at 60 frames a second. So far, the research group has had to struggle with light interference from the Earth's limb and has not been able to spot any Vulcanoids.
* While asteroids have been observed and catalogued for two centuries, for nearly all that time they were nothing more than points of light in the sky. Even coarse studies of their properties did not begin until the 1950s, with details obtained from the intensity and spectral properties of reflected sunlight. The pioneer in asteroid physical studies was Gerard P. Kuiper of the University of Chicago, working with his students, particularly Tom Gehrels, now of the University of Arizona and a well-known NEA hunter.
The first item observed was that, as most asteroids are irregularly shaped, their brightness changes from a peak to a minimum and back to a peak as they spin. Their rotational period is normally in the range of 4 to 20 hours, and their variation in brightness is typically about 20%. However, the well-known NEA 433 Eros changes in brightness by a factor of four as it rotates.
Small asteroids tend to spin rapidly, since they tend to collide with objects a good fraction of their own size, with the impact kicking them into a spin, and have relatively small moments of inertia. Larger asteroids tend to spin more slowly, since the objects they collide with are usually much smaller than they are, and they have larger moments of inertia. However, there is likely another reason why larger asteroids tend to spin more slowly. No known asteroid larger than 200 meters in diameter spins with a rotational period shorter than 2.2 hours. This appears to be due to the fact that the larger asteroids are agglomerations of large masses and rubble, not monolithic chunks of rock. If they spin too fast, they simply disintegrate. The biggest asteroids, those with diameters larger than 125 kilometers, tend to spin faster in proportion to their size, probably because their stronger gravity keeps them together.
An asteroid's "light curve" can also be used to determine the orientation of its spin axis. If we obtain the light curve of an irregularly-shaped asteroid, the light variation will be a minimum if we are observing it along a line of sight that passes through its spin axis, or in other words are observing from directly over one of its poles. As the asteroid moves along its orbit around the Sun, the light variation will increase and reach a maximum when we are observing it at a right angle to its spin axis. A group of Italian astronomers has built a set of model asteroids and recorded 10,000 light curves to help match them to possible asteroid configurations.
* Light curves are useful, but they cannot provide a good estimate for the size of an asteroid. That information is usually derived from the thermal infrared emission from an asteroid. This is a bit tricky, however, since the infrared emission of an asteroid is proportional to its temperature. Asteroid temperatures are typically about 200 Kelvins, but the temperature is variable, a function of the asteroid's distance from the Sun and the asteroid's albedo.
Of course, the distance from the Sun is known if the asteroid's orbit has been calculated. Albedo can be determined by examining the ratio of reflected visible light to thermal emission. An asteroid with a high albedo will reflect most visible light, and its thermal emission will be low. An asteroid with a low albedo will absorb most visible light, and its thermal emission will be high. This means that if we observe two asteroids that are at the same distance and find they are equally bright, the one that has greater thermal emission has a lower albedo, and so a greater diameter.
Astronomers obtained a windfall of infrared data on asteroids after the launch of the Dutch-American "Infrared Astronomical Satellite (IRAS)" in 1983, which provided infrared measurement data for thousands of asteroids. The IRAS data seemed to show that the biggest asteroids have a different range of albedos than the smaller ones, which may be due to the fact that a high proportion of smaller asteroids may be fragments from the cores of larger parent bodies. IRAS also discovered dust bands within the asteroid belt that had been produced by collisions.
* Another method used, when luck permits it, to determine the diameter of an asteroid is a stellar occultation, with an asteroid observed as it passes in front of a star. A star is effectively at infinity in comparison with the distance between the asteroid and Earth, and so the shadow cast on the Earth by the star and the asteroid is effectively the same size as the asteroid. This means that dimensions of the asteroid can be determined by watching the asteroid with an array of widely-spaced telescopes and precisely timing the occultations at each telescope. Given that the orbit and orbital velocity of the asteroid are known, each timing gives the length of a slice through the asteroid.
One of the most successful occultation observations was performed on Pallas in 1983 by a group of hundreds of amateur astronomers working with a team of professionals. The result of the exercise, coupled with data from light curve and earlier occultation observations, indicated that Pallas was an ellipsoid with dimensions of 574 by 526 by 501 kilometers.
* Spectroscopic analysis of asteroids has provided clues to their composition. Astronomers have organized asteroids according to spectral properties, giving each class a letter designation, such as "S class", "C class", and "D class", which are the three most common classes of asteroids. Over a dozen classes have been defined. Spectral characteristics of meteorites found on Earth often match the spectral characteristics of different classes of asteroids, strongly pointing to a link between the two.
The details of the spectral classes are of no great interest to nonspecialists. In very broad terms, asteroids tend to range from those with stony or iron-metallic compositions, including the S class, to asteroids made of more primitive materials, including the C and D classes. Some of the primitive asteroids are "carbonaceous" and appear to be largely black lumps of soot. The very largest asteroids appear to have had molten cores early in their histories, and show some evidence of igneous (volcanic) structures. Fuzzy images of Vesta taken by the NASA Hubble Space Telescope show basaltic lava flows over the asteroid's surface.
Asteroids in the inner regions of the main belt, closest to the Sun, appear to be predominantly stony-metallic. Those in the outer regions of the main belt appear to be predominantly of primitive composition, suggesting that their composition may be little changed from the days when the planets were formed. Such primitive asteroids may provide clues to the makeup of the early Solar System.
In a particularly interesting set of observations, NASA's Hubble Space Telescope took a series of hundreds of images of Ceres, the largest asteroid, over a period of 9 hours, the length of Ceres' "day". The observations seem to show that Ceres is a round body that likely does have a differentiated structure, with a light surface layer around a denser core. Given the density of the asteroid, the mantle apparently contains a large proportion of water ice,
* The idea that most asteroids with diameters of a few hundred meters or more are agglomerations of fragmented components was proposed in the late 1970s by Don Davis and Clark Chapman, then at the Planetary Science Institute in Tucson, Arizona. Davis and Chapman originally believed that only the largest asteroids were likely such "flying rubble piles", but beginning in the 1990s, increasingly detailed images of asteroids showed that only the smallest asteroids were likely to be monolithic. In fact, the densities of some asteroids known in detail are so low that it is likely that there are voids in their interiors.
Interestingly, these images still often seem to show asteroids that appear to be a single chunk of rock and spin as a unit, not a cluster of masses jumbled together, as the notion of a "flying rubble pile" might suggest. However, the images also often show impact craters a substantial fraction of the diameter of the asteroid. Any solid asteroid would have simply been shattered by an impact big enough to make such a crater, while a loosely bound conglomerate of masses would be able to absorb the blow. It seems that even the weak gravity of an asteroid is strong enough to keep it together under ordinary circumstances, with its cohesion assisted by bonding processes that are poorly understood. However, under stress caused by, say, a close flyby of a planet, such an asteroid would break up easily.
* The first asteroid to ever be imaged in any detail was an NEA named 4769 Castalia. Radar observations of asteroids, even some main belt asteroids, were performed with increasing detail through the 1980s, and when Castalia passed by Earth in August 1989, Stephen J. Ostro and his colleagues at NASA JPL managed to obtain a radar image of it. Castalia proved to have a "peanut" shape, consistent with the idea that it was composed of two 800-meter chunks more or less glued together.
This led to further radar observations of asteroids. The NEA 1620 Geographos, originally spotted in 1951, was imaged by radar during a flyby in 1994, and turned out to be shaped something like a sweet potato with dimensions of 5.1 by 1.8 kilometers. The NEA 4179 Toutatis was imaged during an Earth flyby in 1992 and proved to be two masses stuck together, one about 4 kilometers across and the other about 2.5 kilometers across. Astronomers are beginning to suspect that such "contact binary" or "Siamese twin" configurations are common among asteroids.
This suspicion was reinforced by radar observations performed by Ostro and his colleagues of the main-belt asteroid 216 Kleopatra in November 1999. Analysis of the radar data showed Kleopatra to have a shape like a dumbbell or a dog's bone, with two large knobby ends connected by a narrow neck. Kleopatra, which was originally discovered in 1880, is estimated to have dimensions of 217 by 94 by 81 kilometers, and appears to be made largely of metal. It has an eccentric and highly inclined orbit by the standards of main-belt asteroids. Other asteroids have been since observed by radar, and it turns out that the contact binary configuration is common. Some of the asteroids observed by radar also had tiny moonlets.
* The first asteroid to be photographed close-up was the main belt asteroid 951 Gaspra, which was imaged in 1991 by the NASA Galileo spacecraft, discussed later, while on its way to Jupiter. 951 Gaspra has the appearance of a lumpy potato with dimensions of 19 by 12 by 11 kilometers.
In 1993, Galileo imaged the main belt asteroid 243 Ida, which looked like a lumpy rock with dimensions of 56 by 24 by 21 kilometers. The images were taken from 3,500 kilometers away, and had minimum feature resolutions of 35 meters, twice as good as those obtained from the Gaspra flyby. The images revealed a surprise as well: Ida proved to have a moonlet about 1.4 kilometers in diameter, which was named Dactyl. Analysis of Dactyl's orbit around Ida was used to obtain a value for the mass of Ida, which was so low that it strongly hinted there were voids inside the larger asteroid.
* The first successful space probe intended specifically to explore asteroids was the NASA JPL "Near Earth Asteroid Rendezvous (NEAR)" spacecraft, a Discovery-series spacecraft that was launched by a Delta 7925 booster on 17 February 1996. Its objective was to go into orbit around the well-known NEA 433 Eros, the first NEA to be discovered, back in 1898. The NEAR spacecraft was equipped with cameras, spectrometers, and other instruments to obtain images and perform chemical studies of the composition of Eros.
In 1997, while on the way to Eros, NEAR performed a flyby of the main belt asteroid 253 Mathilde. Mathilde is a primitive carbonaceous asteroid, blacker than coal, and has an eccentric orbit that takes to the outer reaches of the asteroid belt. Mathilde is the largest asteroid visited by a spacecraft to date, with dimensions of 66 by 48 by 46 kilometers. Mathilde's surface is gouged by huge craters that make it look as though large bites were taken out of it. The largest of the craters is about 30 kilometers across.
A good estimate of Mathilde's mass was obtained by its influence on NEAR's trajectory, and once again the density was surprisingly low, less than half that of the similar "carbonaceous chondrite" meteorites found on Earth. An astronaut trying to walk on Mathilde would only weigh about half a kilogram. Walking on Mathilde would be tricky at best, and the astronaut would likely get thoroughly black and dirty as well.
On Valentine's Day, 14 February 2000, NEAR reached its objective, Eros, and went into orbit around it. The probe was then given the name "NEAR Shoemaker", after the late great planetary astronomer Eugene Shoemaker. NEAR was to have gone into orbit around Eros over a year earlier, but when the probe was being prepared for orbital insertion in December 1998, its communications system decided to go on vacation for about 27 hours. However, mission controllers were able to react and save the mission.
Eros has dimensions of 33 by 13 by 13 kilometers, and looks like a long potato. It has two major gouges in it, almost certainly due to impacts. Eros is a particularly interesting NEA, since its orbit does cross that of the Earth, and some astronomers have calculated that it has a 1 in 20 chance of striking the Earth in the next billion years. The impact would be substantially more destructive than that which is believed to have exterminated the dinosaurs. By the late summer of 2000, NEAR's observations had made it clear that Eros was not a "flying rubble pile", instead consisting of a single piece of undifferentiated rock that seemed to date back to the origins of the solar system.
NEAR continued to orbit Eros until 12 February 2001, when mission controllers managed to ease the spacecraft down to a "soft landing" on the asteroid. Although the mission was supposed to end after this stunt, very much against predictions the probe was still working, and the mission was extended for ten days to permit close-up measurements with the probe's X-ray and gamma-ray spectrometer instruments. No other instruments were used during this final mission phase, since the "beached" space probe could not direct its high-gain antenna at Earth, meaning that all data had to be funneled back through the low-rate low-gain antenna. The data that could be returned by other instruments with the spacecraft in that position would have hardly been worth the bandwidth.
* In the meantime, other probes had obtained images of asteroids. In early 2000 the NASA Cassini probe, on its way to Saturn via a Jupiter flyby and also discussed later, imaged the asteroid 2685 Masursky. The asteroid was observed from a distance of 1,545,000 kilometers (960,000 miles). The observations were performed with the spacecraft's wide angle and narrow angle imagers, using various spectral and polarizing filters. The observations showed the asteroid to be from 14 to 19 kilometers across.
Similarly, the NASA Stardust comet probe, launched in 1999, imaged the 4-kilometer-wide asteroid AnneFrank on 2 November 2002; and the ESA Rosetta comet probe imaged the 4.6-kilometer-wide asteroid 2867 Steins on 5 September 2008. These two probes are also discussed in more detail in a later chapter.
* While NASA and the ESA sent probes to asteroids, Earth-based astronomers performed remote inspections of asteroids using new "adaptive optics" telescopes, with mirrors that are deformed under computer control to adjust to optical variations in the Earth's atmosphere and obtain a clearer picture.
In late 2000, a group of astronomers under Dr. William Merline of the Boulder office of SWRI released images of a large double asteroid, and an asteroid with a small moonlet. The double asteroid was the outer belt asteroid Antiope, which consists of two bodies about 80 kilometers in diameter with a separation of 160 kilometers. The images were obtained by the Keck telescope on Mauna Kea on the island of Hawaii.
Images obtained by the SWRI team from the Canada-France-Hawaii Telescope (CFHT), a neighbor of the Keck on Mauna Kea, showed that the 145-kilometer-wide asteroid Pulcova has a moonlet about a tenth its size, orbiting once every four days at a distance of about 800 kilometers. The SWRI group discovered a similar moonlet orbiting the 220 kilometer wide asteroid Eugenia in 1999. The SWRI teams cautions that they inspected 200 asteroids and only found two with moonlets. CFHT images were also obtained of 216 Kleopatra, and had an excellent correspondence to the radar images taken of the asteroid.
The new European Southern Observatory Very Large Telescope (ESO VLT), an array of four big telescopes in Chile, also got into the asteroid observation act, with astronomers announcing in 2005 that VLT imagery had revealed an asteroid with two moonlets. 87 Sylvia, a main belt asteroid, was discovered in 1866 and has a diameter of about 280 kilometers. The two moonlets were a few kilometers across; since Sylvia in Roman legend was the mother of the founders of Rome, Romulus and Remus, the two moonlets were given the names of her sons.
* Other space missions to asteroids are now in progress or being planned. The Japanese ISAS MUSES-C probe was launched on 9 May 2003 on a mission to the asteroid 1998 SF36, in an orbit between Earth and Mars. The spacecraft was put into space on an ISAS M-5 solid-fuel booster from the ISAS Kagoshima launch center. The probe's name loosely stands for "M-5 Space Engineering Spacecraft Mark C". Once in space, it was given the much more elegant name of "Hayabusa (Peregrine Falcon)".
Hayabusa had a launch mass of 530 kilograms and featured a high-efficiency ion propulsion system, featuring three ion thrusters, as well as a highly autonomous flight control system. After performing an Earth flyby in May 2004 to get a gravity assist, it then performed a rendezvous in mid-September 2005 with the asteroid 1998 SF36, which is about 700 by 300 meters in size; the asteroid was named "Itokawa", after Dr. Hideo Itokawa, one of the founders of the Japanese space program.
At the time of the rendezvous, asteroid Itokawa was about 1.3 times as far away from the Sun as is the Earth. The rendezvous was originally scheduled for June 2005, but an intense solar flare in 2003 damaged the spacecraft's solar panels, reducing the power for the ion thrusters and forcing a modification of the flight trajectory. One of the spacecraft's three reaction wheels had also failed before reaching the asteroid, but the other two were able to keep the probe stable.
The probe moved to a stationary position relative to the asteroid at an altitude of about 20 kilometers. It performed observations for about two months using its instrument suite, which included:
Following this surveillance period, the probe was to then descend to the asteroid to "touch down" momentarily and take samples by firing a metal ball into the surface and then using a retractable funnel-shaped collector to scoop up debris. To perform preliminary scouting, Hayabusa was to release a small hopping landing rover microprobe named MINERVA (Micro-nano Experimental Vehicle for Asteroid), with a mass of 590 grams, carrying three cameras and a set of thermometers. NASA had planned to contribute a larger rover, but it didn't survive budget cuts.
Mission actions following the surveillance period did not go well. MINERVA was released as planned on 12 November 2005, and then disappeared. Two attempts were made to touch down for samples, on 19 and 25 November, the first being a clear failure and the second being judged likely a failure. It did have the distinction of being the first spacecraft to land on an asteroid -- or for that matter, any other celestial body other than the Moon -- and take off again.
After the encounter, Hayabusa was to cruise back toward Earth, arriving in June 2007 and dropping a 20 kilogram sample-return capsule that will be recovered in Australia. However, communications were temporarily lost at a critical time and the probe missed the Earth-return window. Current plans see it coming home in 2010; ground controllers have had some success in keeping the spacecraft's damaged ion engine system in useful operation, so there is some hope that Hayabusa will make the Earth rendezvous -- though not great expectations that it will actually provide samples.
Despite the disappointments, Hayabusa was still a partial success, since the probe returned high-quality observations of Itokawa. It appears to be a contact binary with a "flying rubble pile" composition, consisting of about 40% voids. The surface was not heavily cratered, suggesting that the asteroid was not very old, far younger than other asteroids observed in close-up so far, though some contrarians believe that impacts may have shaken the object enough to fill in craters and so Itokawa is older than it looks.
* In late 2001, NASA approved another Discovery-series asteroid probe named "Dawn", which is to orbit Vesta and Ceres and perform detailed observations. The probe was named "Dawn" because Vesta and Ceres are seen as survivors of the first days of the solar system. After a near-death experience in 2005, when NASA considered cancelling the program due to cost overruns, Dawn was launched from Cape Canaveral on 27 September 2007 by a Delta II Heavy vehicle, featuring nine solid-rocket boosters.
The Dawn probe was built by Orbital Sciences Corporation, and had a launch mass of 1,218 kilograms (2,685 pounds). It featured a solar-powered xenon-ion electric propulsion system with three thrusters, the technology being derived from the solar-electric propulsion system used on the NASA-JPL Deep Space 1 probe, once again discussed later. Electric power was provided by Dutch-built solar arrays spanning 19.8 meters (65 feet). The instrument suite included:
The probe is to perform a gravity slingshot around Mars in February 2009 to enter orbit around the asteroid Vesta in August 2011 for nine months of observations. It is then to travel to Ceres, entering orbit in February 2015.
* Although a manned mission to an NEA would be technically practical, no manned mission to an asteroid has been seriously proposed or is in planning. Jeffrey Kargel, an astrogeologist with the US Geological Survey in Flagstaff, Arizona, has suggested there may be good reason to perform a manned asteroid mission one of these years: asteroids may be a source of massive quantities of valuable metals.
Kargel focused on metals that are relatively rare and precious on Earth, but which might be available in quantity in a metallic asteroid: platinum, and its relatives iridium, osmium, palladium, rhenium, and rubidium. He estimated that a metallic asteroid a kilometer in diameter should contain 400,000 tonnes of these metals. Kargel performed his analysis using 1990 US dollar prices for these metals, and concluded that their total value would be $5 trillion USD. Of course, introduction of large quantities of such rare metals into the world economy would cause their price to fall dramatically, but even taking that into consideration the value would still be over $300 billion USD.
NEAs orbit the Sun at velocities similar to that of the Earth, and so sending a spacecraft to one is relatively easy. A team of asteroid miners could fly to a suitable NEA, set up a solar power system and a smelter, and use an electromagnetic gun to shoot the ingots produced by the smelter back to Earth. The ingots could be carried in aerobraking shells to enter the Earth's atmosphere, where the aeroshells would then deploy parachutes to drop their payload into a desert or other remote location.
Transferring all 400,000 tonnes of metal would require three electromagnetic gun shots a day for 20 years. Given revenue of $15 billion USD a year, the profits would very likely justify the expense. Kargel's study estimates that there are likely to be about six candidate NEAs that fit the bill.
