[10.0] Missions To The Asteroids (1)

v2.0.0 / chapter 10 of 20 / 01 jul 16 / greg goebel

* 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 survey of what is known about the asteroids.

Ceres from Dawn probe



* 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, 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 so 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 believing 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 Hirayama Kiyotsugu 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, of which more is said later. 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 suspected to have contributed to the extinction of the dinosaurs, though climatic shifts were already raising hell well before the impact. 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 co-orbital" 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 orbiting 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 co-orbital 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.

At least three more Earth co-orbital asteroids are known. Two of them, "1998 UP1" and "2000 PH5", have horseshoe orbits. As their dates show, they were discovered before 2002 AA29, but their orbits were not fully determined until later. The third, 2016 HO3, does not have a horseshoe orbit; its orbit around the Sun intersects with that of the Earth to turn the little asteroid into a "quasi-satellite" of the Earth, with an orbit around our world ranging from 38 to 100 times the distance of the Earth to the Moon. 2016 HO3, which is about 30 by 90 meters in size, has apparently been stuck in this holding pattern for at least a century; it may well move out of it again a few centuries from now.

It is somewhat difficult to classify the Earth co-orbital 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 Hornet 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. It failed to spot any Vulcanoids.

In 2006, NASA launched the twin "Solar Terrestrial Relations Observatory (STEREO)" solar observatories, one spacecraft leading the Earth in its orbit, the other trailing, with the two obtaining simultaneous observations of the Sun from different perspectives. The STEREO mission was intended for general solar observations -- but an analysis of its observations, published in 2013, did check for Vulcanoids, and ruled out any such bodies bigger than six kilometers in diameter.



* 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.

Other relatively recent observations have spotted a handful of smaller asteroids spewing out water or dust; originally it was thought the emissions were due to an impact, but now it seems more likely they are extinct comets that somehow ended up in the asteroid belt and have lost their surface ices. About a dozen such "asteroidal belt comets (ABC)" have been found. It seems that the emissions were caused when the orbits of these ABCs were perturbed, bringing them closer to the Sun and resulting in more heating.

* 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.