A very large number of small (less than 1500 km maximum dimension) bodies composed of rock/dust, ice and condensed gases orbit around the Sun as asteroids and comets. The asteroids are composed mainly of rock, with some ice, and are detectable by solar light reflected from their low albedo surfaces. The comets generally are dominantly ice with some rocky material that are visible because solar light reflects from an envelope of gases collected around an ice/rock nucleus.Three good overviews of asteroids and comets are found at these sites: (1), (2), and (3).
About 10000 Asteroids have been discovered as individuals so far. Most are located either within the "Main Belt" or the "Kuiper Belt". Some astronomers have estimated that at least 400,000 bodies larger than 1 km exist within the Solar System; the total number, including small ones is likely well in excess of a million. More are being found each year owing to a stepped-up search program (see below). The number of known comets is smaller.
In the 20th Century, the asteroids, solid fragments of varying size that orbit in several regions of the inner Solar System, have had dramatic increases in interest, in part because they are the more frequent bodies that have collided with the Earth and the Moon, and most other larger solar bodies, producing impact craters which on our planet could have (and have had) diastrous consequences - castastrophic in terms of effect on life - of magnitudes only now being properly appreciated. We will start this review with the nature and distribution of asteroids.
Tens of thousands of asteroids occupy the Main Belt, an interval of mean distance 2.4 A.U., between Mars and Jupiter. This diagram shows the belt in relation to the planets, along with several other asteroid clusters closer to Jupiter.
Contrary to the above illustration, the asteroids are not uniformly dispersed. They form several belt swarms, with reduced numbers in between. These lower regions are called Kirkwood Gaps:
The average size of a Main Belt asteroid is about 100 km. Typically, the separation between individual asteroids in the belt is about 1600 km (1000 miles). Telescopic observations have indicated that about 75% of these asteroids have very low albedos, which is consistent with compositions that are similar to carbonaceous chondrite meteorites, generally considered to be the most primitive Solar System material. Other (about 17%) asteroid groups occur in different locations, such as the Apollo belt (Earth-crossing) and Trojan belt (Jupiter-crossing). Many of the Trojan's are darker and redder. As they orbit the Sun, they also rotate, some with periods of a few days to a week or so and others in a matter of hours ("tumble"). Mars and Neptune also have some associated asteroids that cross into their orbital zones.
Comets are easier to detect than smaller asteroids. Comets have higher albedos, and thus are brighter, and as they enter the region of the Solar System within the outer planets they develop luminous tails. Asteroids, on the other hand, present greater difficulties in finding them and determining their orbits. Most asteroids have such low albedos that they don't begin to reflect enough sunlight until relatively close to Earth. Only the larger ones in the Main Asteroid Belt are fairly easy to spot and measure. To illustrate detection techniques, let us describe how the Near Earth Asteroids (NEA), the Apollo belt, are found and monitored to determine their size and orbits.
Optical telescopes, with mirrors around a meter in diameter, are the instruments most commonly used. A portion of the sky (usually around 1 to 10 Moon diameters in width) is viewed over a short time interval (hours to a few days). Successive views are recorded on film or electronically using CCDs. Stars and galaxies will retain their fixed positions relative to each other during the interval. An asteroid will look much like these fixed distant stellar/galactic bodies if it is close enough to "shine". But being very close with respect to the distant background, it can move a measurable distance in that time. Consider this trio of telescope photos:
In the left and center photos, all the starlike objects appear to be in the same fixed positions. But one such body, in right center, actually moves to a slightly new position, towards the upper left, in the middle photo. When the two photos are superimposed, as shown in the right image, two "stars" appear instead of one. The left "star" in the pair is just the same body that has moved in this time interval (30 minutes). This is explained by assuming the body is so close to the Earth that its motion is discernible over short time periods. The presumption is that it is an asteroid. Working with that idea, the actual distance of the object from Earth is determined by parallax (using two telescopes). Once that has been calculated, the size of the body can be estimated (taking its albedo into account) and its velocity is likewise determined from the distance/time relation. Repeated observations allow its orbital parameters to be specified. The painstaking search efforts involved (if done manually) has been eased by computer programs that automatically seek out diagnostic displacements.
The Earth-crossing asteroids all are less than 10 km in maximum dimension. From actual counts, plus extrapolations, the number of Earth-crossing asteroids greater than 1 km in diameter is now placed between 700 and 1000; a collision with Earth for one of that size is estimated to be around 1 per 1-2 million years and for a 10 km asteroid once every 100 million years. The majority of all asteroids have irregular shapes.
Among the Main Belt asteroids the biggest is Ceres (933 km [583 miles]); second in size is Pallas (530 km [331 miles]) and Vesta is slightly smaller; these all reside in the Main Belt. The larger asteroids tend to approach spherical in shape; Ceres is the only one presently known to be almost spherical. At least some of these have melted and differentiated, so that they have developed iron-rich cores, and possibly a mantle with dispersed metallic iron (the pallasite meteorites may derive from this). One school of planetologists considers these to be "small planets".
Asteroids have been imaged by conventional telescopes, by radar, and by passing or orbiting space probes. This first image shows that ground telescopes do not normally produce good images, even of the larger asteroids, such as Ceres.
A recent Hubble Space Telescope (HST) image has been reprocessed to give the best look yet at Ceres. This is important because this asteroid has now been tentatively proposed as a dwarf planet within the Main Belt asteroids.
As implied on page 18-3, the dinosaur "final solution" from a huge asteroidal impact, followed by several movies that deal with world-threatening (and destroying) collisions with outer space debris, have led to a more organized search and inventory (and orbit-determination) for asteroids and comets. Optical telescope methods look for tiny light blips in photographs that move with significant displacements relative to fixed star backgrounds. This is turning up new asteroidal bodies - many quite small - with notable numbers added each year. Another approach uses radar: this image of Toutalis (4.6 x 2.4 x1.9 km [2.9 x 1.5 x 1.2 miles]) is a mosaic of several radar returns:
Radio astronomy is also adept at imaging larger asteroids. This image of asteroid 1999JM8 was made using the large dish at Arecibo in Puerto Rico:
Vesta, at 525 km (325 miles) in long dimension, has been imaged by the Hubble Space Telescope, as seen here:
Although not shown clearly, there appears to be a large impact crater with a central peak on Vesta. Spectral analysis of light from Vesta produces values that are very close to the mineral group of pyroxenes. In 1960, a meteorite was found in Western Australia that is entirely pyroxenite (making this a rare type). Many asteroid specialists believe this came from Vesta; if so, it is our first sample of an asteroid that we can analyze.
Both ground telescope and even HST images of small objects such as asteroids can be blurry. Thus, to effectively image in detail an asteroid a space probe needs to visit its vicinity. The Galileo spacecraft was programmed to transit close to two asteroids. It passed the first, Gaspra, in 1991. This asteroid, 19 x 12 x 11 km (~12 x 7.5 x 7 miles) in size, consists of iron-nickel and iron-magnesium-rich silicates. This is how it appears in color:
Galileo approached and imaged the second asteroid, Ida, on August 28, 1993, as shown here (at about 33 m [108 ft] resolution):
This asteroid, about 58 x 23 km (36 x 14 miles) in dimensions, is chondritic and, like Ida, pockmarked with craters. Totally unexpected was the presence of a small orbiting body, about 1.5 by 1.2 km (0.93 x 0.75 miles), named Dactyl–, making this pair the first known binary asteroids. Close-up, Dactyl has a small crater, causing it to look a bit like Saturn's Mimas in miniature:
The first visit exclusively to the asteroid belt has been made by NEAR, for Near Earth Asteroid Rendezvous mission, launched in June 1997 to reach the large asteroid Eros in January, 1999. After launch, the spacecraft had the name Shoemaker added to it, in memory of the Dean of Astrogeologists, Dr. Eugene M. Shoemaker (see bottom of next page). Enroute NEAR-Shoemaker took a close look at Mathilde (59 x 47 km; 37 x 29 miles) on June 22, 1999:
Mathilde has one of the lowest albedos, less than 4%, of any asteroidal object. It also has a low density, 1.4 g/cc, indicating that much of its interior is either ice and/or voids - thus it has been described as a "fluffy" ball. Its solid component has probably a carbonaceous chondrite composition.
In 2000, NEAR-Shoemaker successfully orbited Eros and has taken data on its composition. Below is a view of Eros, whose dimensions of 33 x 13 x 13 km give it a peanut-like shape. Eros is rapidly rotating, as evident from the different positions over a short time span on a December 1998 date.
NEAR-Shoemaker has operated successfully around Eros for several years, taking thousands of images at various resolutions from different orbital heights. This next image is one of the best, taken in a false color mode in which a green and two NIR bands are used.
Here are three views of parts of EROS:
Here is a close-up (resolution: just a few meters) of a crater on Eros:
After completion of the NEAR-Shoemaker mission, the mappers gave names to the major features on a flattened mosaic of its surface, as follows:
NEAR-Shoemaker made a close approach to Eros, reaching an altitude just 6 km (4 miles) above its surface, on October 27, 2000. Here is part of a mosaic of images made during that pass. This view shows a crater and rock debris as small as 1 meter across:
In early 2001, it was evident that the spacecraft's fuel was nearly depleted. The NEAR-Shoemaker scientists and managers pondered how to finish the mission and decided on a bold course - to land the spacecraft on EROS's surface. The asteroid was 196,000 miles from Earth; if successfull this would be the farthest any manmade object had set down on a solid body in the Solar System. Good operational maneuvering and some real luck were essential to the endeavor. Here is the target area chosen:
The landing attempt began in the afternoon (EST) of February 12. Five burns (fuel blasts) were involved over a 50 minute interval, each slowing the spacecraft and thus causing it to drop ever closer to the surface. The last burn dropped the descent speed to 6 km/hr (4 mph). All the while, images were taken and radioed to Earth. The spacecraft not only reached the surface but survived the touchdown and continued to send back signals.
Pictures obtained as the spacecraft moved ever closer to its landing resemble in their increasing detail those returned from Ranger impacts on the Moon. Here is a sequence obtained as NEAR-Shoemaker approached a large crater (again, named Shoemaker). The caption gives the spatial resolution achieved for each image.
Here is a NEAR-Shoemaker descent image taken when the spacecraft was just 250 m (820 ft) from the surface. Rocks less than a meter in dimension are scattered on the surface, as had been expected.
Astonishingly, the gamma ray spectrometer on the spacecraft was not damaged. When its solar panels were arrayed so as to pick up sunlight and hence provide power, the spectrometer was turned on and data were collected from two depths. Here is one of the plots:
This triumph, with its serendipitous landing results, augers well for future missions to asteroid surfaces; sampling these (either by instrumental analysis or by sampling collecting and return to Earth) will give scientists their first solid data on the composition of these asteroids which have the most primitive material in the condensed planetary rock system. This should thus confirm whether any of the meteorites fallen on Earth are truly samples of the planetesimals which, for Earth and the inner planets, were the primary constituents that eventually melted.
The first try at landing on an asteroid, collecting samples, and then returning those to Earth is being carried out by the Japanese space program. The small (535 x 294 x 209 meters) Apollo Belt asteroid Itokawa was reached by the Hayakawa spacecraft and then placed in near orbit around this irregular object - the asteroid Itokawa.
The initial attempt at landing failed but success occurred from a second attempt in November 2005. The spacecraft may have collected samples (although probably not more than small amounts of dust and then headed enroute back to Earth, with arrival in 2010. Here is the landing site:
Analysis of instrumental data on Hayabusa and other sources have now led scientists to conclude that Itokawa is an assemblage of rock particles ranging from dust to boulders, held together by weak gravity and electrostatic attraction.
The return trip of Hayabawa to Earth was fraught with difficulties, which were largely overcome by the daring and skills of the Japanese technicians. On June 12, 2010 the spacecraft reached Earth, entered its atmosphere, and released a small capsule which parachuted into the desert of central Australia and was quickly found and recovered.
NASAs Dawn mission to asteroids was launched in September of 2007. The probe will reach 4 Vesta in 2011 and 1 Ceres in 2015. An overview of the mission is given at this Wikipedia website. JPL is directing the mission; check its Dawn homepage at JPL index.
The redirected long-term space program announced by the Obama administration includes a possible manned mission to land on an asteroid. This would allow sampling similar to the Moon visits during the Apollo program. It is very important to fix the composition of a typical asteroid, as this would in principle determine the nature of the starting materials believed to dominate the rocky planets and probably the cores of the Giant planets.
Planetologists generally agree that they already know some basic facts about asteroid composition. Most meteorites are considered to be parts of asteroids or pieces broken off of asteroids. But until and unless the Hiyakawa mission is successful, no meteoritic material tied to a specific individual asteroid has been acquired - until 2008. That year a tiny asteroidal fragment (about 3 meters), named TC3, was discovered hurtling towards Earth. It was predicted to hit the atmosphere in northern Africa in September. This indeed happened, with the asteroidal body exploding at a height of about 32 km (20 miles). The event was observed by nomads in the Namib desert of Sudan. Scientists journeyed into the area below the fireball and found many pieces of the meteoritic material surviving the explosion. You can read details of this event at the Astronomy Now website.
Many more asteroidal bodies populate the Kuiper Belt. There are now several large objects that orbit the Sun from within or near that Belt. Their status as large asteroids or mini-planets remains unsettled. Here they are as depicted in an artist's conception:
The object named 2003 EL61 is peculiar. Its shape is ellipsoidal - almost like a football with rounded tips. It is the fastest rotating object in the Belt, tumbling in a full rotation every 3.9 hours.
With many thousands of asteroids in the Main Belt, it is not surprising to learn that occasionally two such bodies will collide and the smaller one may then disintegrate. This is the best explanation for the observation by the Hubble Space Telescope in late January of 2010 of what appears below as debris in the Belt. This rubble trails out from the moving asteroid much like the tail of a comet. Because the debris stream diminishes so rapidly, the collision is interpreted as having been very recent:
The question of the origin of asteroids is still debated. All agree that they represent primitive materials and are at least as old as the planets. A few planetologists still argue that some, perhaps all, of those in the Main Belt represent a disrupted planet. But most subscribe to the concept that these are really planetesimals (accretionary bodies built up of fragments of gravity-attracted small solids that condensed and organized during the first stages of solar history) that never succeeded in building up by accretion into (a) planet(s). The reason for this failure to reach planet-size is ascribed to the perturbing influence of gravity from the Giant Jupiter. Most of these bodies today have undergone repeated collisions that knock off chunks from the target body, some of which escape to become new asteroids but much (most?) re-assemble into the collided remnant or into a neighboring asteroid to form a new shape.
Some asteroids are solid in the conventional sense but may now consist of joined fragments (note shape of Toutatis, which appears to have three connected pieces); these have greater densities but may contain internal voids. Others, perhaps the majority, being lower in density, are thought to be comprised of smaller fragments. These indeed may even be a composite of sand, gravel, and small boulder sized material (probably the residue of the carbonaceous silicates that were the first solar dust aggregates). They are held together by gravity, electrostatic attraction, and possibly a form of ice. When collisions on them occur, pieces may be knocked off but these tend to rejoin the parent in a matter of days. Over billions of years many collsions have occurred, reordering the asteroids into new assemblages. Most asteroidal surfaces, whether solid or held together in a manner similar to "sand castles" made at a beach, are covered with loose debris or "regolith".
In the treatment of meteorites (page 19-2) it was pointed out that asteroids ranged from primitive carbonaceous chondrites in composition to those that seem to have a distinct set of concentric shells, with iron cores. The stony meteorites display varying degrees of thermal and shock metamorphism, with the irons being evidence that the asteroidal body may have grown large enough to melt and form an iron core before later disruption. Sources of heat energy are needed to produce these differences. Some of that energy was provided by the same radioisotopes of uranium, thorium, and potassium as currently continue to heat the Earth. But in the early Solar System there were also isotopes with short half-lifes that may have produced even more thermal energy. Two known to have been effective are Al26 and Fe60, both now naturally extinct. Collisions between asteroidal bodies also contributed to the heat content, causing at least part of the target body to overheat locally. Another source of heat could have been from the Sun itself, both as thermal radiation and solar wind.
Asteroids, and fragments therefrom, occasionally hit the Earth, as we showed on page 19-2 dealing with meteorites and page 18-1 dealing with impact craters. So do comets strike our planet: a possible example (although it could have been a stony meteorite) was the 1908 Tunguska event in Siberia, where trees were knocked down (in a radial pattern pointing to a center of blast) over many hundreds of square kilometers by an explosion just above the surface. Great amounts of dust were thrown into the atmosphere, producing abnormal red sunsets worldwide for the next several years.
Comets are among the most spectacular of the heavenly bodies with their long, icy tail, receiving mystical significance from early observers, until later observers determined their true nature. Interest has always been high regarding comets, balls of ice and rock, because of their spectacular appearances (glowing "stars" with tails); they are well represented in mythologies and astrologers' portents.
Scientists now know that comets are mainly ice balls of varying sizes (up to 10-30 km [6-19 miles]), mixed with rock debris to some extent. They travel in eccentric orbits (e.g., Kuiper Belt) within the Solar System, repeating appearances or simply flying through once, if not captured gravitationally. As seen when still far from Earth, the central coma appears as a glowing ball from 10,000 to 100,000 km (6,214 - 62,137 miles) in apparent diameter, around a much smaller solid nucleus. A good example is the comet Encke:
As a comet passes the Sun, the solar wind and other factors cause it to ablate, expanding the coma and creating a stream of particles that trail off as a long tail (up to 100 million km [62,137,000 miles] long) of dust and plasma. The tail points away from the Sun along a radial line. Most comets actually develop two tails, one consisting of debris reflecting sunglight and the other composed of ions excited by UV irradiation. Look first at the unnamed comet and then at the tail from famed comet Halley.
Scientists think that the nucleus (descriptively referred to as a "dirty snowball") consists of very primitive materials, organized during the Solar System's development. Spectroscopic studies indicate the presence of molecular compounds of carbon, nitrogen, and hydrogen, including CN, NH, NH2, and HCN, which break into ions carried into the tail. A telescope view of Comet Kudo-Fujikawa using a sensor that detects hydrogen ions produced this image, in which the H ions are colored reddish:
Astronomers know the orbits of some comets well enough (through observations) to predict when they will return. Most comets come from either the Kuiper Belt (beyond Pluto) or the vast Oort "Clouds" that extend to the outer reaches of the Solar System. Some may be intergalatic but that has yet to be demonstrated. As seen in this diagram, the Kuiper Belt is a toroidal configuration that begins beyond the orbit of Neptune. Short-Period (recur within the inner Solar System in less than 200 years) comets come from this Belt. The Oort Cloud, extending between 10000 and 50000 A.U., is the source of Long-Period (> 200 years) comets and is distributed in a spherical zone around the Solar System.
The Kuiper Belt contains Objects (KBO) that are primarily comets but seem to also include some asteroidal type bodies. The Belt was predicted to exist by Gerald Kuiper in the 1950s but the first direct imaging was not achieved until the 1990s. The example shown here indicates the difficulty in spotting such small objects far out in the Solar System; once located, the circled object moves relative to the background, allowing its orbital migration to be calculated. (The blocky squares are the individual pixels in the detector plate.)
The KBOs are clustered in a disk-shaped region near the ecliptic (plane in which most of the planets orbit the Sun), lie at distances from 30 to 100 A.U., and may contain up to (an estimated) 100,000 objects larger than 20 km in diameter. Centaur-class KBOs are relatively close to Neptune's orbit and include the icy body named Chiron (discovered in 1977; at first thought to be an asteroid but now proved to be a comet with a short tail); of the estimated 400 Centaur objects greater than 100 km in diameter, at least 9 are notably larger.
One of the largest known KBO asteroids is 2002 LM60, discovered first by a ground telescope and then confirmed and studied by the Hubble Space Telescope:
The discoverers have named this asteroid Quaoar (pronounced Kwa o whar), an American Indian name associated with the tribe that once occupied the Los Angeles Basin. The asteroid is spherical (indicating that it was once molten) and is close to 1250 km (780 miles in diameter). Quaoar is 6.5 billion kilometers from the Sun, around which it moves in a circular orbit. It lies 1.5 billion kilometers (about one billion miles) past Pluto. Another recently discovered asteroid is 2001 KX76, named Ixion, whose size ~1200 km (745 miles). Some other large objects in this belt (using Pluto and its moon Charon for comparison) are depicted in this schematic: