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Getting to Know Your Solar System (15): Asteroids (Vol. 1)

by Troubadour

Community

(This content is not subject to review by Daily Kos staff prior to publication.)

Friday, Jun. 01, 2012 Friday, Jun. 01, 2012 at 1:02:37am PDT

Throughout the solar system, countless objects of widely varying sizes, bizarre geometries, and diverse compositions orbit the Sun independently or attend gas giant planets in swarms. Those which overwhelmingly consist of rock and metal are called asteroids, and threaten Earth-bound human civilization at the same time they hold out the promise of a free, unimaginably wealthy, and unbounded future history. They bear a stark message for mankind: Come to us in hope, before we come to you in fire.

The progress of our adventure so far (current in bold):

1. The Sun
2. Mercury
3. Venus
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
10. Luna
11. Mars (Vol. 1)
12. Mars (Vol. 2)
13. Mars (Vol. 3)
14. Phobos & Deimos
15. Asteroids (Vol. 1)
16. Asteroids (Vol. 2)
17. Ceres
18. Jupiter
19. Io
20. Europa
21. Ganymede
22. Callisto
23. Saturn
24. Mimas
25. Enceladus
26. Tethys, Dione, and Rhea
27. Titan
28. Iapetus
29. Rings & Minor Moons of Saturn
30. Uranus
31. Moons of Uranus
32. Neptune
33. Triton
34. The Kuiper Belt & Scattered Disk
35. Comets
36. The Interstellar Neighborhood

Allow me to introduce the individual asteroids we will be exploring on this part of our journey, in addition to more general discussions of asteroids throughout the solar system. For the sake of keeping things straight, their names will occur in bold throughout this entry in the series, since a number of other objects are also named. In rough order of increasing size:

25143 Itokawa:

951 Gaspra:

433 Eros:

243 Ida and its moon Dactyl:

253 Mathilde:

21 Lutetia:

4 Vesta:

These are not necessarily the most interesting or significant asteroids - we won't have a good idea of that until we have visited a much larger number of objects - but simply the ones for which we have relatively clear photographs today. Nevertheless, the amount of material to cover is massive, so I have decided to break the Asteroids entry into two volumes. Topics covered in Volume 1 (in bold):

I. Context
II. Population Groups, Families, and Spectral Types
III. History
IV. Properties
V. Past Relevance to Humanity
VI. Modern Relevance to Humanity
VII. Future Relevance to Humanity
VIII. Future of The Asteroids
IX. Catalog of Exploration

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I. Context

Countless asteroids exist throughout the solar system in random orbits, but major concentrations occur in more regular solar orbits between Mars and Jupiter, and at the L4 and L5 Lagrange points of Jupiter - locations of gravitational stability in empty space 60° ahead of and behind the planet in its solar orbit. Several other significant concentrations exist to a lesser extent, but the former overwhelmingly dominate in number, size, and total mass. Orbital diagrams from a receding, solar Northern perspective:

Orbital diagrams for each of our example asteroids:

You'll notice right away that asteroids are not necessarily well-behaved in terms of the shape and inclination of their orbits: Quite a few cross the orbits of several planets, tend to have relatively eccentric (i.e., elliptical) orbits, and may exhibit pronounced inclinations outside the plane of the ecliptic where planetary orbits are clustered. Although it is not classified as an asteroid - it's far too massive, distant, and icy - such orbit-crossing behavior and inclined orbital plane were two of the factors that led to Pluto's demotion from planet status.

Depictions of asteroid belts in film and television are completely false: Despite the relatively high number-density of objects in these regions, the space between them is still so vast that you could not see one object from another except as points of light unless it was a rare case of one asteroid orbiting another like Ida and its moon Dactyl. From the surface of an asteroid, the surrounding space would look no different than anywhere else in the solar system - i.e., empty. Passing through a region dense with asteroids still carries risks, but only because spacecraft are typically traveling at enormous speeds for years on end, and even then the only significant impact risk is from very small objects - sand grains and pebbles.

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II. Population Groups, Families, and Spectral Types

Asteroids are generally classified according to three pieces of information: Where its orbit is located, whether it was created by being blasted off of another asteroid, and what its observed spectra indicate about composition.

1. Population groups:

These categories are based on the shape and/or location of an orbit, which are typically named after the first member of the group to be discovered or hypothesized. Groups based on crossing of planetary orbits may overlap to some degree, since one object may cross several such orbits, and some objects cross an orbit at perihelion (close approach to the Sun) while others will do so at aphelion (far point from the Sun). Here is a diagram depicting the major inner solar system groups, including those co-orbital with Jupiter:

Atens: Asteroids whose semi-major axis is less than the distance from Earth to the Sun, 1 astronomical unit (AU). Because their orbits may be eccentric, some Atens cross the orbit of Earth and are monitored by international efforts focused on detecting potential impact threats. There are about 722 Atens currently listed by the International Astronomical Union (IAU) Minor Planet Center. The most famous Aten is 99942 Apophis, a 240-meter asteroid with a 1 in several-hundred-thousand chance of impacting Earth in 2036.

Apollos: Asteroids whose semi-major axis is greater than 1 AU, but whose perihelion (closest approach to the Sun) lies within Earth's orbit. They are all Earth-crossers by definition, some are considered potential planetary threats, and some may additionally cross the orbits of Venus and Mercury, or planets outward of Earth. Itokawa is an Apollo asteroid that crosses the orbits of both Earth and Mars. Another noteworthy Apollo is 4179 Toutatis, a 4.5 km-long asteroid considered potentially hazardous to Earth due to chaotic gravitational perturbations it experiences - however, these are far more likely to eject the asteroid from the solar system than direct it to Earth. Toutatis as it passed near Earth in 1996 - quite a monstrous-looking creature:

Mercury-crossers: These are asteroids whose orbits cross that of Mercury, and they may also cross the orbits of other planets if the object has an eccentric solar orbit. There are about 581 members of this group, some of which are Atens and Apollos - others come from farther out in the solar system.

Venus-crossers: Asteroids that cross the orbit of Venus, including some Atens and Apollos. This group includes some Mercury-crossers, and some that continue outward to cross the orbits of other planets. I haven't been able to find a comprehensive list, but as an inner-system population, the numbers are probably in the hundreds.

Earth-crossers: Several hundred asteroids have been identified that cross the orbit of Earth, including both Atens and Apollos, as well as objects whose orbits take them much farther out than Earth. Some Earth-crossers are also Venus- and Mercury-crossers, and plenty have their most distant reaches beyond Mars. As noted, Itokawa belongs to this group as well.

Amors: Asteroids that approach Earth from the outside, but do not cross our orbit. Many of these are Mars-crossers. Eros is an Amor asteroid. Phobos and Deimos may have originally been Amors that were captured by Mars at some point. A few Amors are considered potentially hazardous to Earth, not because of their regular orbits, but because they come close enough that later perturbations could cause them to change into Earth-crossers.

Earth Trojans: These are semi-hypothetical populations at the L4 and L5 Sun-Earth Lagrange points, 60° ahead of and behind Earth in its solar orbit. I say "semi-hypothetical" because only one asteroid has been observed in such a location - an unnamed 300m object at Sun-Earth L4 (SEL-4) catalogued as 2010 TK₇. Generally speaking, asteroids located at the triangular Lagrange points of a system (L4 and L5) are known as Trojan asteroids - a convention that came about due to the fact that such asteroids found in association with Jupiter came to be named after Greeks and Trojans from Homeric poetry, but the latter name stuck for both. Objects at a body's L4 are called "leading Trojans" and L5 "trailing Trojans." It is possible there may be Venus or Mercury Trojans, but none have yet been observed. An illustration of Sun-Earth Lagrange points, with the location of the Earth Trojan indicated:

You can also see above why L4 and L5 are called "triangular" Lagrange points - because they form one vertex of equilateral triangles with two masses of a system. These are the most stable Lagrange points, because the dynamic balance of gravitational forces between the two objects causes masses near these points to be directed back toward them if perturbed away from them. This looks like gravity fields centered on points in empty space, but is actually an illusion based on changing directions of force as objects move in orbit.

So when you're close to SEL-4 or -5 and move a little bit away from it toward the Sun, Earth's gravity will cause you to move back toward the point; and when you move a little bit away from it toward Earth, the Sun's gravity will cause you to move back toward the point. It takes very little energy to escape if you want to - the point is that you can also stay put easily if you want to: Like a sunken conversation pit on top of a hill. Trojans are technically included as planetary crossers because they do in fact cross the orbit of their associated planet by circuiting the triangular Lagrange point - however, they will never intercept the planet while they remain Trojan.

Near-Earth Asteroids (NEAs): This a broad grouping that includes the Earth-crossers, Amors, Apollos, and most observed Atens. It is a sub-group of the Near-Earth Objects (NEOs) - a broader category that includes Earth-crossing comets. There are several thousand NEAs, including Eros and Itokawa.

Mars-crossers: Can include any of the other types of planetary crossers, and is strongly (but not definitively) associated with the Amors. Eros and Itokawa belong to this group as well.

Mars Trojans: It's the same concept as with Earth Trojans, only there are at least three confirmed asteroids in this group - one at Sun-Mars L4, and two at Sun-Mars L5. As noted, these are technically also Mars-crossers even though they will never intercept Mars without being perturbed out of Lagrangian orbit.

The Main Belt: Hundreds of thousands of asteroids are known to exist between the orbits of Mars and Jupiter, and very likely the number of significant objects extends into the millions. Gaspra, Ida/Dactyl, Mathilde, Lutetia, and Vesta are all Main Belt objects, as is the dwarf planet Ceres. The gravitational influence of Jupiter creates four relatively empty orbital zones in the Belt called Kirkwood gaps: Objects whose orbits end up in these zones become orbitally unstable and will end up being accelerated by resonance with Jupiter, most likely out of the Belt and possibly out of the solar system.

These gaps, and the continuing destructive role played by objects whose orbits become associated with them, are consequences of the same process responsible for the existence of the Belt: Without the division and destruction caused by Jovian influence, the mass of the region would have accreted into a planet early in the history of the solar system. Instead there is only one stunted dwarf planet, several large asteroids, and a multitude of shattered fragments wandering around. One of these fragments was shot into the inner solar system and destroyed the dinosaurs, and another may yet endanger our existence.

Hildas: These asteroids migrate over the course of their orbits along a three-lobed path between the Main Belt and the orbit of Jupiter - an unusual motion caused by resonance with the planet. There are about 1,100 Hildas known. Hildas against a background of the Main Belt and Jupiter Trojans:

Jupiter Trojans: Asteroids at the Sun-Jupiter L4 and L5 points. These are the second most significant asteroid concentrations in the solar system with 5,253 major objects identified to date, although there are thought to be around a million over 1 km in diameter - comparable in magnitude to the Main Belt. Trojan groups with significant numbers are commonly known as swarms. The two swarms of Jupiter Trojans are roughly equal in number and composition. The L4 Jupiter Trojans are sometimes called "Greeks" due to their namesakes being Greek figures from the Iliad, but this has generally fallen out of favor and both swarms are called Trojans, distinguished either as "leading" (L4) or "trailing" (L5). There are very likely also Saturn and Uranus Trojans, but no objects have yet been observed in these locations.

Centaurs: These objects straddle the boundary between comets and asteroids, and exhibit highly elliptical, unstable orbits between Jupiter and Neptune that may cross the orbits of any of the gas giant planets. There are thought to be about 44,000 Centaurs over 1 km in diameter, although the largest currently known, 10199 Chariklo, is about 260 km - halfway in size between Lutetia and Vesta. A chart of the largest known Centaurs, with red lines through them indicating the distance between perihelion and aphelion:

Neptune Trojans: There are six known Neptune Trojans, with five at the L4 point and one at L5. There are likely many more. At this location in the solar system, solid bodies tend to have a mass increasingly dominated by ice, so I only include Neptune Trojans as asteroids because of their orbital behavior rather than what kind of objects they are. We could likewise describe objects even further out as icy asteroids because they aren't close enough to the Sun to outgas, and thus technically are not comets, but I will arbitrarily limit asteroids to bodies at or within the orbit of Neptune and leave the rest for later diaries in this series.

There are several other, less significant asteroid population groups that occur due to various planetary resonances - overwhelmingly Jupiter, but any planet can produce a resonance effect that herds some number of asteroids into a given orbit. Hungaria asteroids, for instance, fell into the innermost Kirkwood Gap and were perturbed into highly eccentric orbits that put them in resonance with Mars - a resonance that is progressively ejecting them from the solar system. Alindas are accelerated by Jupiter into increasingly eccentric orbits that bring them further and further into the inner solar system until they become resonant with an inner planet, which makes some of them dangerous. And Damocloids are a group of objects thought to be extinct comets who've vented all their ices and have only rock left. Asteroids can also be grouped as Jupiter-crossers, Saturn-crossers, Uranus-crossers, and Neptune-crossers, with major overlap.

2. Families:

An asteroid family is a group of objects thought to be impact fragments of the same original object, which in some cases still exists as the largest member of the family. Family members share very similar solar orbits, have the same apparent composition, and occur in statistical distributions indicative of being part of one original object. About a third of Main Belt objects are thought to belong to families, although this may be comparatively high relative to other groups due to the frequency of collisions caused by Jupiter's disruptive gravitational influence on the Belt. Small families or sub-families - i.e., when a fragment is itself shattered by subsequent collision - may be called clusters. Terms for more ambiguous familial relationships include "clumps," "clans," and "tribes."

Families can only be discerned within a limited period of time after collision because their orbits, though similar, will gradually spread apart and undergo divergent influences that will eventually make it impossible to derive their origin based on orbits. This time period can be arbitrarily large or small depending on the particulars of an orbital situation, so many of the objects not discernable as family members are in fact fragments that have simply dispersed too greatly. Complicating matters is the fact that, although family members ultimately come from the same parent body, not all were necessarily created in the same impact, so there can be significant complexity in determining the "family tree" of a related group of objects.

Gaspra is a member of the Florian family - the third largest in the Main Belt, accounting for 4-5% of Belt objects. The parent asteroid is a 140 km object called 8 Flora, so Gaspra is thought to be an ejecta remnant from this other asteroid about 8 times larger than itself. The asteroid that killed the dinosaurs may have been a Florian. Ida and Dactyl are members of the Koronidian family, which is another large Main Belt family. The Koronidians are named after 158 Koronis, but that is neither the parent body nor the largest currently known member of the family - the original impact is thought to have been catastrophic enough that the parent no longer exists. Vesta is the parent of its own family, the Vestoids. It is the largest known, accounting for approximately 6% of all Main Belt asteroids.

However, it should be stated that not all mid-sized and large asteroids in families were directly shaped by the impacts that led to their existence: Many are thought to be later agglomerations of smaller rubble that occurred from those impacts. So, for instance, if you were to witness the impact that ultimately produced Ida, you would not necessarily see an Ida-shaped body emerging from it - rather, you might see a number of objects orbiting each other that over time would heap together into Ida. A chart of Main Belt asteroids plotting semi-major axis against orbital inclination reveals several major families:

3. Spectral Types:

Asteroids as individual objects are classified according to spectra and albedo (i.e., reflectivity), both of which are indicators of composition. Some types are categorized together in broad groups due to significant shared properties. Asteroid spectral types are not as advanced as the classification schemes for meteorites - small asteroid fragments recovered on Earth's surface. This is because we have access to detailed chemical and structural analysis of meteorite samples, while data concerning asteroids is limited to spectra, albedo, and broad physical characteristics. Only one asteroid sample has ever been retrieved - a few grains of dust from Itokawa brought back to Earth by Japan's Hayabusa spacecraft.

It should be noted that spectral classifications are based on arbitrary divisions, because actual spectra form a continuum that blends seamlessly into other categories. This is due to the fact that compositions are likewise continuous - i.e., one object might have 1% of a given mineral, another might have 2%, another 3%, and all the way up to arbitrarily high values. Since there are a substantial number of minerals found in asteroid spectra, the potential combinations are numerous. However, a few broad categories suffice to give a general understanding of these objects, and three types overwhelmingly dominate the asteroid population:

C-type asteroids are relatively dark, carbonaceous objects that account for 75% of all known asteroids. C-types are most common in the outer Main Belt, and Mathilde is a member of this spectral class. A carbonaceous meteorite probably similar to some C-type asteroids:
S-type asteroids are bright, stony, silicate-rich objects most abundant in the inner Main Belt, and thus are also relatively common in the inner solar system. This category comprises 17% of all asteroids. Itokawa, Gaspra, Eros, and Ida are all members of this class. A meteorite thought to originate from an S-type asteroid:
M-type asteroids are the third most abundant spectral class, and are thought to be dominated (though not necessarily defined) by dense metallic bodies - particularly those with heavy concentrations of nickel and iron. Nickel-iron asteroids are the most devastating objects when they become impactors because they are denser, deliver more kinetic energy in the same-sized object compared to other types, and are less likely to be stopped by the Earth's atmosphere. Such objects are thought to originate from the highly metallic cores of larger bodies that had undergone differentiation and were subsequently shattered by massive impact. However, many M-type asteroids are not dense or highly metallic at all, so it's a case of similar spectra coming from diverse types of compositions. Lutetia is M-type, though data from Europe's Rosetta probe indicate it is carbonaceous rather than metallic. M-type comprises the majority of the remaining percentage of all asteroids. A nickel-iron meteorite on Mars:

A visual summary of spectral type distributions:

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III. History

In the earliest period of the solar system 4.57 billion years ago, it consisted of a hot soup of gas and metallic vapor called a protoplanetary disk. As this material flowed into the Sun and made the center of the solar system hotter, the rest began to thin out and cool from the outside in. The metal vapor precipitated into clouds of liquid droplets that flowed into each other, cohered, and broke apart on a continuing basis, but over time, as the environment cooled further, they were able to stick together longer and form larger agglomerations before being dissociated again. Eventually these droplets solidified together, forming objects called chondrules - grainy structures of some meteorites that are the oldest solid objects in the solar system. Chondrules give their name to chondrite meteorites, which are defined by the fact they exhibit these ancient features.

It is amazing that we can set eyes on the solidified remnants of these primordial droplets, which have not changed since the moment they stuck together and solidified billions of years ago. The very same objects you see embedded in the meteorite below had orbited the Sun freely for millions of years before any of the planets had formed:

Not long after this process began, the bulk of mass in the protoplanetary disk came to form Jupiter, and its gravitational influence began ejecting material out of the region that is now the Main Belt. More than 99% of all objects that had formed there would ultimately be thrown out of the solar system, into the Sun, or into other planets by this process. Meanwhile, some primordial chondrite objects ended up accreting into asteroids massive enough for gravity to differentiate their internal structures, obliterating the chondrules and creating grained rock mantles and solid nickel-iron cores. Vesta is an example of such an object, having fully formed only a few hundred million years after solids began precipitating from the disk.

Some planetary scientists hypothesize that about 4.53-4.48 billion years ago, a large Earth Trojan accumulated mass to the point of becoming a co-orbital planet dubbed Theia and was perturbed into colliding with Proto-Earth. This is one hypothetical origin of the impactor generally agreed to have been responsible for the creation of the Moon.

Mathilde formed around 4-2 billion years ago, mostly likely as an impact fragment from a larger, presently unknown C-type asteroid. Its features have been preserved since this time because its low density dampens seismic waves from subsequent impacts, preventing them from shaking out earlier craters like an Etch-A-Sketch. This is perhaps how the asteroid was able to survive the creation of its largest feature - the huge crater that gouges into an entire face and would easily have shattered a more cohesive body. Instead of being blasted apart by the shockwave, it may simply have been absorbed by the low-density material, sending some of it into orbits that would subsequently decay and return the material back to the surface. Around the same time, the oldest craters on Lutetia are about 3.6 billion years old, so this is likely when it formed.

Then about 1 billion years ago, the unnamed parent body of the Koronis family was shattered, which led to the formation of Ida. Sometime 300-200 million years ago, an impact blows a chunk off Flora that becomes Gaspra. About 100 mya, Dactyl is blasted off Ida. Unfortunately, Eros, unlike Mathilde, is highly cohesive, and so a major impact some tens of millions of years ago shook the asteroid sufficiently to erase much of its prior surface-visible history. However, landings and sample return missions would still be able to find data that could shed light on the date of the asteroid. The dinosaurs are wiped out on Earth by the impact of a Main Belt family fragment asteroid 65 million years ago.

Eight million years ago or thereabouts, rubble was blasted off an undifferentiated S-type chondrite asteroid and some of its chunkier pieces recombined to form Itokawa. The asteroid is small enough that you can actually see the pieces that comprise it - particularly the massive boulders that protrude from its surface, about the size of Olympic swimming pools. Relatively smooth regions cover cracks or depressions between the major chunks where smaller rubble and dust has accumulated. We will explore these and other features of the various example asteroids in more detail in the next volume.

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Topics I will be covering in Volume 2 include, but are not limited to: Orbital dynamics, rotation, size, mass, surface features, temperature, structure, mythology surrounding meteorites, past Earth impacts, planetary defense, asteroid mining, and asteroid colonization. Until then, here are some blurry images of asteroids not chosen as examples due to the paucity of material about them. The vast majority of asteroids don't even have images this good - they're just points of light against a star background. Starting with more images of Toutatis: