Ice Moon 4 Return to Enceladus

by Brandon Q Morris

  How Asteroids Came into Being

  For a long time it was assumed asteroids were the remnants of a destroyed planet that once orbited the sun in the area between Mars and Jupiter, as a planet seemed to be missing there. The area is now called the ‘asteroid belt.’ The missing planet was even given a name—Phaethon. However, the total mass of the asteroids orbiting there, as we know today, amounts to only about 5 percent of the mass of Earth’s moon. Therefore, this hypothetical former planet would have been much smaller than our moon.

  During the 19th century, the search for Phaethon caused a lot of attention to be paid to the area between Mars and Jupiter. This at least had the advantage that the first asteroids there were quickly discovered. Ceres, Pallas, Juno, and Vesta were found in turn and initially believed to be planets. When Neptune was discovered in 1846, it was therefore considered not the eighth, but the thirteenth planet. Soon, though, so many celestial bodies were added that a new category of minor planets was created. They were labeled as asteroids.

  Today, asteroids are assumed to occupy intermediate positions in the creation of planets, so to speak: Bodies still in the process of developing into planets. In the asteroid belt, however, the strong gravitational influence of Jupiter prevented the development of a planet. Later, the asteroids collided with each other and other planets. This formed fragments, which collided again, leading to a whole menagerie of asteroids with diverse structures. Only the largest of these developed differentiated structure by melting, so the components were distributed into core and crust. When one of the large asteroids collided with another one, its core created fragments with an especially high metal content, while the splinters of the crust were rich in silicates.

  For many years it was believed that asteroids were compact, monolithic bodies. Since then, some surprisingly-low densities have been measured and some huge impact craters have been discovered. Many asteroids might represent a kind of cosmic debris pile, the components of which are loose, held together only by their own gravitational force. Such a body is relatively resistant to collisions. Monolithic objects, on the other hand, would be torn apart by the shockwaves of major impacts. Another support for this fact is the relatively low rotational speed of large asteroids. During fast rotation around its own axis, centrifugal forces would tear apart an object that is only loosely held together.

  The Naming of Minor Planets

  Because there are so many minor planets, their nomenclatures follow a methodology that seems rather complicated at first. Let’s start with the discovery of a new asteroid. Initially, its orbital parameters might not be known with certainty, and the object then receives a provisional designation that begins with the year of its discovery and is followed by a space and then a letter that indicates the half month of its discovery. A marks the first half of January, B the second half of January, C the first half of February, and so on. The letter I is left out, and Z is not needed. Then a second letter follows: A for the first one discovered in this half month, B for the second, and so forth. Once again, I is omitted, but Z is used. Therefore, 2015 AA would be the first asteroid discovered in the first half of January 2015.

  Nowadays, more than 25 asteroids are discovered in half a month, so the designations go from A through Z and then begin again at A... The number of times the alphabet was completed is indicated by a subscripted number. The first set of letters would have the subscript 0, though this is omitted in print. Accordingly, the asteroid 2015 AA67 would be the (68*25)+1=1701st asteroid discovered during the first half of January 2015.

  At some point, the orbit of the asteroid is determined more precisely, so it can always be found again if necessary. The reward for all of these efforts: Now the asteroid receives a ‘real’ number (starting at 1)! In previous years you could see the sequence of discoveries through these numbers, but that doesn’t work anymore. In addition, the asteroid now has the right to a name. For ten years, the discoverer has the naming rights, but the International Astronomical Union must confirm it, and this process can take a while. For instance, naming asteroids after pets or using advertising messages is no longer permitted. A name should not contain more than 16 characters, must consist of a single, pronounceable word, and should not be too similar to existing names. Politicians or military personalities must have been dead for at least 100 years before an asteroid can be named after them. Because of these limitations, there are many asteroids that have been numbered but remain unnamed.

  Types of Asteroids

  If you look closely in the telescope and determine which wavelength ranges of light an asteroid reflects, the branch of science called astronomical spectroscopy, you can often draw conclusions about the asteroid’s composition. Of course ‘often’ does not mean ‘always,’ and points toward the problems involved in this spectral classification. The most frequently used classification scheme employing this technique was developed in 1984 by the American astronomer David Tholen. It consists of three larger groups and six smaller classes that cannot be assigned to any of the groups.

  The asteroids of the C-group contain a relatively large—up to 3 percent—share of carbon compounds. Therefore they reflect relatively little light. These are probably the most ‘primeval’ asteroids. During their long life they were exposed to little heat, and they reflect the composition of the planetary cloud from which they developed. The group includes multiple types:

  C-type: At 75 percent, this is the most common. They consist of silicon minerals with a relatively high content of carbon and organic materials, which partially condensed out of the primeval solar nebula. Therefore, they give us some insight into the early period of the solar system. Examples are (10) Hygiea, (54) Alexandra, (164) Eva.

  B-type: Reflect differently in the UV range compared to the C type, but are otherwise similar. Example: (2) Pallas.

  F-type: Similar to C, but with differences in the UV range. In addition, lacking absorption lines in the wavelength range of water. Example: (704) Interamnia.

  G-type: Also similar to C, but with different absorption lines in the UV range. Example: (1) Ceres.

  At 15 percent, the S-group is the second most common type. The S stands for silicates, i.e. silicon minerals like olivine or pyroxene of which these bodies are made. It is assumed they developed inside large asteroids that were later destroyed. According to their metal content they were located near the outside, indicating little iron and nickel, or more deeply inside, indicating more iron and nickel. The only representative is the S-type. Example: (15) Eunomia, (3) Juno.

  In the Tholen classification, the X-group consists of the objects displaying a very similar spectrum but probably having a quite different composition. The visible difference mostly relates to their reflectivity, called albedo. Asteroids for which this value is unknown are directly assigned to the X-type. The other types belonging to this group are as follows:

  M-type: Medium albedo. The M-type often—but not always—has a relatively high metal content. Presumably these are the former cores of larger asteroids. Example: (16) Psyche.

  E-type: High albedo, came probably from the crust of a larger asteroid. Examples: (44) Nysa, (55) Pandora.

  P-type: Low albedo, among the darkest asteroids. Examples: (259) Aletheia, (190) Ismene.

  The following other types of asteroids occur relatively rarely, are not classified within any group, and some of them are seen only in certain parts of the solar system.

  A-type: Reddish spectrum, fewer than 20 known specimens, probably came from the crust of a larger body. Example: (246) Asporina.

  D-type: Very dark and reddish, might have originated from the Kuiper belt beyond Neptune. The Martian moon Phobos might be related to them. Example: (624) Hektor.

  T-type: Dark and reddish, only a few specimens, might be related to the C-type. Example: (96) Aegle.

  Q-type: Lines of olivine and pyroxene as well as metal in their spectrum, might be more common than hitherto assumed. Example: (1862) Apollo.

  R-type: Medium bright, slig
htly reddish, with lines of olivine and pyroxene in their spectrum. Example: (349) Dembowska.

  V-type: Similar to S-type, but with more pyroxene. Due to the fact that the V-type asteroids often have orbits very similar orbit to Vesta, it is believed they might have been torn off the crust of Vesta through collisions. Example: (4) Vesta.

  Where One Can Find Asteroids

  Generally, the asteroid belt is considered the home of asteroids, and it contains ninety percent of the them. But these objects also zoom around in other areas of the solar system. Only within the orbit of Mercury, relatively close to the sun—less than a third of the distance between the Earth and the sun—are asteroids yet to be discovered. There the so-called vulcanoids are suspected to exist. If they do exist—as meteorite impacts on Mercury seem to suggest—they would have diameters below 50 kilometers, because otherwise they would have shown up on photos. Discovering them wouldn’t be easy, since they get so close to the sun. During the search, large telescopes would run the risk of being destroyed by the bright sunlight. The asteroids that come closest to the sun are (1566) Icarus (up to 0.19 AU) and (3200) Phaethon (up to 0.14 AU). Both move around the sun in strongly elliptical orbits.

  The most relevant ones for our daily life are undoubtedly the so-called Near-Earth Asteroids or NEA. These celestial bodies usually don’t get farther away from the sun than Mars. Four types can be distinguished: Amor, Apohele, Apollo, and Aten.

  The Amor class crosses the orbit of Mars toward Earth, but without ever reaching Earth. Among them are (433) Eros, which approaches the orbit of Earth to as close as 0.15 AU, as well as the eponymous asteroid (1221) Amor, which was discovered in 1932. Amor asteroids do not cross the path of Earth and therefore cannot hit it, just like asteroids of the Apohele (or Atira) class, which move around the sun completely within Earth’s orbit.

  But there are also the so-called ‘Earth crossers.’ These are asteroids with orbits that cross the orbital path of Earth, which might eventually lead to a collision. They either belong to the Apollo class, where the orbit is wider than that of Earth, or the Aten class, where the orbit is narrower than the one of Earth. Currently, none of the known objects seem to pose a real danger. Nevertheless, they are studied intensively. Statistically speaking, once a year Earth is hit by an object with a diameter of less than four meters, and once in every five years a seven-meter object hits us, usually disintegrating in the atmosphere. Every 2,000 or 3,000 years there is an impact with effects similar to the Tunguska event of 1908. And about twice in a million years we might have to be prepared for the impact of an asteroid with a diameter of up to a kilometer. Even larger objects, with up to five-kilometer diameter, hit us every 20 million years. Currently no such asteroid is on a collision course, but this could change. Occasionally, objects from the asteroid belt change their trajectories due to disturbances caused by Jupiter.

  The asteroid belt is the reservoir for all the asteroids roaming through the inner and outer solar system. Its distance from the sun is about 2 to 3.4 Astronomical Units—an AU is the distance from the Earth to the sun. However, this area is not evenly filled with objects. Quite the opposite. The strong interaction with Jupiter causes some orbits to be unstable whenever the ratio of the orbital periods is a whole number: resonance. This creates dips, which are called ‘Kirkwood gaps’ after the American astronomer Daniel Kirkwood, who noticed them in 1866. Among them is the 4:1 resonance at 2.06 AU, the inner limit of the main belt, afterward the Hestia gap (3:1), a resonance zone at 5:2, and the Hecuba gap with a 2:1 resonance where the main belt ends at 3.4 AU. The majority of asteroids orbit between the 4:1 and the 2:1 resonance.

  In general, the distances between asteroids are huge, even in the asteroid belt. Hitting an asteroid with a spaceship isn’t easy, and accidental collisions are almost impossible. In the course of millions of years some collisions between asteroids have happened, though. This caused the formation of groups with similar orbits, which are called ‘families.’ Over time these can become very large. The Flora and Eunomia families, for example, contain up to 5 percent of all objects in the main belt, several thousand asteroids each.

  On the orbit of each planet there are points where the gravitational forces of the sun and the planet cancel each other. Those are the so-called Lagrange points. There one finds stable orbits used by asteroids. Such companions were first found near Jupiter, where they run 60 degrees ahead of the planet (Greeks) or 60 degrees behind (Trojans). Later, similar asteroids were found near other planets, and the term was expanded accordingly. By now, there are over 7,000 known Jupiter Trojans. Their number might be comparable to that of the asteroids in the main belt, but as they are farther away, not as many have been individually discovered yet. Venus and Earth have one known Trojan each (2013 ND15 and 2010 TK7). For Mars, nine objects have been identified. In the orbits of Uranus and Neptune, which are difficult to observe due to the enormous distances from Earth, one Trojan each has been discovered.

  Asteroids between the orbits of Jupiter and Neptune are also called Centaurs. It is assumed that these are inactive comets. The largest known Centaur, (10199) Chariklo, has a diameter of almost 250 kilometers and might even possess a ring system. The double Centaur (65489) Ceto and its moon Phorcys form a double planetoid system in which two components of similar size orbit each other.

  But the solar system certainly does not end beyond Neptune. Everything beyond it is simply called a trans-Neptunian object (TNO). This probably includes tens of thousands of bodies—dwarf planets like Pluto or Sedna, comets, but also many, many asteroids. Compared to their counterparts in the main belt, these are rather dark. Many contain a core of dust and ice—in that case the transition to comets is fluid. Their orbits are sometimes influenced by that of Neptune (resonances), as is the case with the Plutinos (with Pluto among them), which move in a 2:3 resonance with Neptune. Others, such as the Cubewanos, follow paths that are tilted against the normal rotational plane of the planets.

  Wrong Way Drivers in the Solar System

  In the solar system, everyone drives to the left: Seen from the North Pole of the ecliptic, the plane of the solar system, all planets, and other heavenly bodies move to the left, or counterclockwise. All of them? Almost all... The planets and dwarf planets obey this rule, but we are aware of 82 asteroids that are wrong-way drivers.

  Just like on a highway, this won’t work for long. If an object is hurled by some strange accident from the Oort Cloud into the interior of the solar system, and like some bad driver from the boondocks, disregards all traffic rules, the end might come after a few thousand years. But if these bad drivers choose their courses cleverly, they can last a few million years.

  The asteroid 2015 BZ509 is a particularly smart representative of this group. This object, with a diameter of about three kilometers, moves around the sun approximately along the path of the gas giant Jupiter. Twice per revolution it even encounters the planet—without a collision. The reason is 2015 BZ509 selected a course that astronomer call a trisectrix.

  So far we know of no other asteroid moving around the sun on such a path that consists of two circles merging with each other. Therefore the asteroid once encounters the great Jupiter while being closer to the sun, and the other time farther away. If only the gravitational force of Jupiter influenced its course, it could be absolutely stable. But the effects of the other planets reduce this time to a few million years.

  We do not exactly know where 2015 BZ509 originated. It could be a former comet that is no longer active. It is probably one of the Damocloids, bodies similar to Halley’s Comet that have already lost their volatile material. They often have retrograde (clockwise) orbits and originally arrived as visitors from the Oort Cloud, that ‘pile of debris’ at the outermost limits of our solar system.

  The Ten Most Interesting Asteroids

  1. (1) Ceres, with a diameter of 945 kilometers, is the largest object in the asteroid belt, and the only dwarf planet inside the orbit of Neptune. In the entire solar system it rea
ches the 33rd position when it comes to size. In 1801, Ceres was the first newly discovered dwarf planet. Similar to a large terrestrial planet, it seems to have a core and a crust, and there may be an ice ocean between them, like on Enceladus. Water vapor emissions were detected in 2014.

  2. 2014 RC approached Earth in 2014 to at least one-tenth of the distance to the moon, i.e. less than 40,000 kilometers. The special feature of this asteroid, with a size of 30 to 50 meters, is its rotational speed. No other one rotates this fast—a rotation takes only 15.8 seconds. Due to the centrifugal forces generated, the object must be massive.

  3. (216) Kleopatra stands out primarily due to its shape—the asteroid looks like a bone. It was discovered in 1880, and in 2008 two companions, Alexhelios and Cleoselene, were found. The asteroid is about 217 kilometers long and 90 kilometers thick. It appears to consist of loosely strung-together rocks and is 30 to 50 percent empty space. The two satellites have a size of 3 and 5 kilometers, respectively.

  4. (243) Ida is an asteroid in the main belt. It is shaped irregularly and about 31 kilometers long. More interesting is its companion, Dactyl. Ida proved to be the first asteroid with its own moon. Ida belongs to the S type, as does the egg-shaped Dactyl, which is about 20 times smaller in diameter.

  5. Among all known asteroids, (1566) Icarus comes closest to the sun, hence its name. Every 9, 19, or 28 years the asteroid, which has a diameter of about 1,440 meters, also gets near Earth, so it could potentially become a danger. In 1968 it was as close as 16 times the distance to the moon. Back then it was the first asteroid to be observed by radar.