The solar system formed 4.6 billion years ago from a cloud of dust and gas called a molecular cloud (1). The molecular cloud was composed predominantly of hydrogen and helium gases and a small amount of heavier atoms and molecules that had been formed during the lifetimes of previous generations of stars. In addition, refractory (heat resisting) dust grains were also present in the cloud. Something, perhaps a nearby supernova, caused increased density regions to form within this molecular cloud. If these cloudlets became massive enough, then they would gravitationally attract the nearby gas, causing the cloudlets to increase in mass and contract. Any residual motion or small rotation within the gas would cause these cloudlets to spin more rapidly as they contracted under gravity. These spinning cloudlets formed the center of the protosolar nebula. The cloud collapsed from the inside out. The rotation caused the nebula to flatten into a disk with most of the mass in the disk and in a bulge at the center. However, there was still some gas in a halo around the disk.
As the central bulge collapsed, it added more material from the surrounding disk and halo. Gravitationally, the center collapsed to a smaller sphere, and the pressure and temperature increased. When the pressure and temperature were sufficiently large (temperature of a few million degrees), the hydrogen atoms in the core of the sphere started to fuse into helium, and the cloud became a new star. At this point, the star was still surrounded by a disk of material and a halo of gas. The high luminosity of the infant star propelled a “wind” of material flowing outward along its rotational axis, creating a bipolar flow. This blew away much of the material in the surrounding halo, stopping the infall onto the disk. It is within this disk that the planets themselves formed. We can estimate the minimum mass that this disk must have been to form the current planets by several methods: the percentage of rocky material in the gas (0.4% of the total) or the mass-loss rates of young stars (3×10~8 MQ/year for 3 million years or a total mass loss of 0.06 MQ where MQ is the current mass of the Sun). Thus, the disk from which the planets formed must have had a substantial fraction of the mass of the Sun and much of it was lost after the planets formed.
About this time, clumps of rock and ice started to grow in the disk. The clumps began to collide with one another and accrete (stick together), causing them to grow larger. Larger objects have greater gravitational attraction, so the biggest clumps grew bigger and ”gobbled up” the surrounding material. The clumps grew until a critical size was reached, and then they collapsed into the cores of the planets. The largest core became our current-day Jupiter and was 20-30 times the mass of Earth after a few million years (it is currently 300 times the mass of Earth). Around the time the cores achieved such large masses, they began to have profound influences on the surrounding leftover material.
The timescales for these processes are short: the cloud collapse took around 10,000 years, the disk/wind clearing took around 100,000 years, and the initial building of planets was complete in around 10 million years (2).
As the Sun settled into the main part of its life as a normal star, the gravitational attraction from the protoplanets, along with the wind from the Sun, gradually removed the residual material from the disk. Some time after the molecular cloud started to collapse, our solar system consisted of the Sun, the major planets, and small quantities of leftover building blocks of planets, called planetesimals. The nature of these leftover planetesimals depended on where they were in the solar system relative to the Sun. Those nearer the Sun, in the region from Mercury through the asteroid belt (the so-called terrestrial planet zone), were predominantly rocky bodies. In the region from Jupiter onward (the so-called giant planet zone), the bodies were a combination of ices and rocky material. Note that ”ice” in this context does not refer only to water ice, but includes ices of other more volatile materials such as carbon dioxide, ammonia, methane, and formaldehyde. The leftover planetesimals from this early period that still reside in our solar system (more on their fate later) are called comets. The comets of today are mostly unchanged from the time they were formed. Thus, comets today preserve information on conditions when the solar system was formed and are of prime importance for providing constraints on conditions in the early solar nebula.

Physical Properties

Comets present a striking appearance in the night sky. Their presence has been noted since ancient times; their name is Greek meaning ”hairy ones.” Our modern understanding of their nature dates to the 1950s (3). Basically, a comet is a ”dirty snowball.” It is a small body; typical comets have nuclear dimensions of 1-20 km. This nucleus is composed of ices and dirt. As the comet approaches the Sun in its orbit, the nucleus is heated, causing the ices to sublime (sublimation is the process whereby materials go from the ice stage to the gas stage directly without ever becoming liquid). These gases form a coma around the nucleus as they flow outward from the nucleus into the vacuum of space. As the ices sublime and flow outward, they carry dust with them into the coma. The coma of a comet is not a bound atmosphere because the gravitational pull of the tiny nucleus is small. Instead, the coma gases are constantly being replenished via sublimation, and the gases in the outer coma are being lost into interplanetary space.
Once the coma has formed, it is extremely difficult to image the nucleus of the comet with ground-based or Earth-orbital telescopes because it is shrouded by the coma. Only two comets have had their nuclei fully resolved by imaging. Comet Halley was imaged by the Giotto and Vega spacecrafts in 1986. Using the high-quality Giotto images, we measured the nucleus as 8 x 8 x 16 km, but it was actually quite an irregular shape. Comet Borrelly was imaged by the Deep Space 1 spacecraft with a size of 4 x 4 x 8 km and, again, was quite irregular. Other comets have had nucleus sizes estimated by various other techniques. Unfortunately, the spacecraft did not travel close enough to Halley or Borrelly for the comet’s gravity to influence the spacecraft orbits, so we do not have any direct measurement of the density or mass of a comet. Various assumptions have led to the concept that the density is between 0.2 and 1.2 g cm ~ 3 (water has a density of 1.0gcm ” 3).
As the comet continues in its orbit around the Sun, the Sun is exerting an outward force known as radiative pressure on the dust, pushing the small dust grains away from the Sun. Thus, the dust ends up lagging the comet in its orbit around the Sun. The combination of the radiative pressure of the Sun and the Keplerian motion of these particles as they continue in orbit around the Sun causes the dust to curve away from the comet on a track outside of the cometary orbit and form a tail known as a dust tail. However, not all comets develop dust tails.
Interplanetary space is not empty, and the comet interacts with its local environment. In addition to emitting radiation at most wavelengths, the Sun sheds ionized material in the form of the solar wind. The solar wind is a hot (~ 100,000 K) plasma consisting of an electrically neutral mixture of ions (mostly electrons and protons) that originates in the solar corona, is accelerated into interplanetary space, and drags along the solar magnetic field. By Earth’s orbit, the bulk velocity of the solar wind reaches a maximum of 200-800 km sec ~1. Comets have no intrinsic magnetic fields or ionospheres, but they are continually releasing neutral gases through sublimation. Some of these gases become pho-toionized. Some tens of thousands of kilometers on the sunward side of the nucleus, a bow shock forms (a sharp boundary separating the ionized material of the comet, called the magnetosphere, from the solar wind material which is forced to flow around the comet) at the distance where the kinetic pressure of the solar wind particles equals the magnetic pressure of the ionized gases. As the solar wind magnetic field drapes around the comet, the solar wind exerts a tangential force on the boundary on the side away from the Sun, dragging the plasma into a long tail called the magnetotail. On opposite sides of the magnetotail, the current travels in opposite directions. Inside the magnetotail, where the opposing currents meet, is a neutral region called the neutral current sheet. In addition, the solar wind reacts with the ionized coma gases, pushing the ionized gases outward. This forms what is known as the ion tail of a comet. The ion tail is straight and always points directly away from the Sun, which means that it sometimes leads the comet in its orbit. Not all comets display ion tails.
Figure 1 shows a picture of comet Hale-Bopp. This comet was discovered in 1995 by Alan Hale and Thomas Bopp (independently) and came closest to the Sun on 1 April 1997. This image was obtained with a CCD camera (an electronic detector) and special filters on the 0.76-m telescope of McDonald Observatory. The various parts of the comet are labeled in this picture.
The nucleus is quite small, but the dimensions of the rest of the comet are substantial. A typical cometary coma is more than 1 million km in radius, or substantially bigger than the Sun. However, as large as the coma is, it is quite insubstantial and has a density that qualifies it as a better vacuum than can be created in most laboratories (a pressure of 10 ” 8 – 10 “9torr)! The tail of the comet can stretch to several hundred million km behind the comet, but the tail has densities even lower than the coma.
A comet does not shine from its own radiation. Instead, the light from the Sun is reflected from the dusty material, and the gas generally “glows” by a process known as fluorescence. (Fluorescence occurs when an atom or molecule reaches an excited state by absorbing a photon and then returns to a lower state by emitting a photon. Fluorescence is termed resonance fluorescence when the wavelength of the exciting photon has the same energy as that of the downward transition and the atom or molecule goes back to the energy level from which it started).
An image of comet Hale-Bopp obtained by the 0.76-m telescope of McDonald Observatory. The image is approximately 40 arcmin on a side. The parts of the comet are labeled on the picture. This is an image obtained by a CCD camera.
Figure 1. An image of comet Hale-Bopp obtained by the 0.76-m telescope of McDonald Observatory. The image is approximately 40 arcmin on a side. The parts of the comet are labeled on the picture. This is an image obtained by a CCD camera.
Very far from the Sun, there is not enough heat to sublimate the gases of the comet. Therefore, if a comet were viewed at large distances from the Sun, it would appear just as a small, rocky, icy body that has no coma. However, comae have been detected around cometary nuclei at distances past Saturn’s orbit [e.g., (4)].

Cometary Orbits and Origins

A comet is generally discovered as a fuzzy object that is moving with respect to the background stars. About half of the comets are discovered by amateurs, who scan the skies looking specifically for comets, and the other half are discovered by professional astronomers who are obtaining images of the sky when they discover the comet. Discoveries of comets are relayed to the International Astronomical Union’s (IAU) Central Bureau for Astronomical Telegrams for confirmation and assignment of a designation.
Each cometary discovery is given an official designation by the IAU that consists of the year of discovery followed by a letter identifying in which half month of the year the discovery is made (e.g., January 1-15 is A, January 16-31 is B,…, December 16-31 is Y; the letter I is not used for half month designations) and by a consecutive number to indicate the order of discovery in that half month. Thus, the fourth comet discovered in the first half of March 1996 would have been designated 1996 E4. This gives each comet a unique name. In addition, comets are named for their discoverer(s); up to three names are assigned to the comet. Thus, the comet that is shown in Fig. 1 is officially comet 1995 O1 (Hale-Bopp) and was the first comet discovered in the second half of July 1995. It was discovered by Alan Hale and Thomas Bopp.
The position of the comet needs to be measured carefully with respect to a fixed reference frame of distant stars or quasars. These positions are then used to compute an orbit around the Sun, taking into account the gravitational influence of all of the planets. All known cometary orbits are elliptical; the eccentricity e of the orbit ranges from close to circular, in the case of comet Schwassmann-Wachmann 1, to parabolic. (Note that the orbits of the planets are very close to circular, except for Pluto.) If it takes the comet fewer than 200 years to orbit the Sun, a comet is termed to be in a periodic, or short-period, orbit. Strictly speaking, all comets are in periodic orbits, in the sense that they are gravitationally bound to the Sun, but some of the comets have periods that are so long compared with a human lifetime that they are designated as long-period or nonperiodic comets. The division at 200 years is purely arbitrary and historical.
The orbits of the major planets are all confined, more or less, to a single plane. If we define the plane of the ecliptic as the plane of Earth’s orbit around the Sun, we find that, except for Pluto, all of the major planets are in orbits that are inclined by no more than 7° from the plane of the ecliptic. Even Pluto is inclined only 17° from the plane of the ecliptic. All of the planets also travel around the Sun in the same direction, which we call the prograde or direct sense. Prograde orbits confined near the plane of the ecliptic are consistent with our picture of the formation of the solar system from a disk of material surrounding the newly forming Sun. For convenience in describing the location of objects in our solar system, astronomers use a distance known as an Astronomical Unit or AU, which is defined as the mean distance of Earth from the Sun. It is approximately 1.5 x 108km. The heliocentric distance of an object is its distance from the Sun.
Cometary orbits have properties very different from the orbits of the planets. Taken in aggregate, there is no preferred inclination of cometary orbits to the plane of the ecliptic. Some comets have orbits that travel around the Sun in the direction opposite from the planets, or retrograde. An example of a comet that has a retrograde orbit is comet Halley.
For now, we will ignore the short-period comets and concentrate on the orbits of the comets that have long- or non-periodic orbits. The semimajor axis of an ellipse is designated a. The inverse 1/a of the semimajor axis is proportional to the orbital binding energy, which is the additional energy a comet would need to escape the gravitational attraction of the Sun completely. If 1/a<0, the comet is not bound to the Sun.
For long-period comets, we can compute a quantity 1/a0, the inverse semi-major axis of the original orbit, where the original orbit is the orbit of the comet integrated backward in time to the point before it entered the planetary system and referenced to the barycenter of the solar system. Figure 2 is a histogram of 1/a0 (data from Reference 5 with updates). Note the pronounced peak in the histogram around a ~ 20,000 AU. This peak was first noticed by Oort in 1950 (6) using a sample of only 19 cometary orbits. From such a histogram, Oort concluded that the solar system must be surrounded by a spherically symmetrical halo of comets, gravitationally bound to the Sun, and at distances greater than 10,000 AU. This halo of comets is currently called the Oort cloud. The position of its inner edge is quite uncertain, but the position of its outer edge is defined by the Sun’s tidal truncation radius of 100,000-200,000 AU (7).
Oort realized that the typical change in energy that a comet receives when it passes through the planetary system (the part of the solar system within Pluto’s orbit) is approximately +0.0005 AU ~1. Yet the peak in Fig. 2 is fairly narrow. It is unlikely that a comet that started out in the peak on its first passage into the planetary system would have a value of 1/a which would leave it in the peak on subsequent passages. Thus, we refer to comets where a > 10,000 AU as dynamically ”new” comets because it is likely that they are on their first passage into the planetary system. Comets not in the peak in the histogram have most likely passed through the planetary system before. On a first passage into the inner solar system, gravitational perturbations from Jupiter would eject roughly half of the new comets and capture the other half to more tightly bound (smaller values of a), less eccentric orbits. Only 5% of new comets would be returned to Oort cloud distances (8).
Returning to the short-period comets, Fig. 3 plots the inclination i as a function of semimajor axis a for the 130 short-period comets that are currently cataloged. Note that the vast majority of these objects have semimajor axes between 3 and 4 AU. This group of objects tend to have their aphelions (farthest distance from the Sun) near Jupiter and have small encounter velocities with Jupiter. The orbits of this group of comets are highly influenced by Jupiter. These comets have come to be known as Jupiter-family comets (JFCs). Typically, there has been an arbitrary cutoff for JFCs at an orbital period of 20 years. This limit is shown as a dotted line in the plot. Comets whose orbital periods are between 20 and 200 years are referred to as Halley-type comets (HTCs), after their most famous member that has a 76-year orbital period.
The number of comets as a function of inverse original semimajor axis. The data for this histogram are from Reference 5 with further updates from Marsden (personal communication). Note the sharp peak at 1/o0 slightly larger than 0. This is the evidence that there is a spherical reservoir of comets known as the Oort cloud.
Figure 2. The number of comets as a function of inverse original semimajor axis. The data for this histogram are from Reference 5 with further updates from Marsden (personal communication). Note the sharp peak at 1/o0 slightly larger than 0. This is the evidence that there is a spherical reservoir of comets known as the Oort cloud.
In Fig. 3, note that all of the JFCs are in prograde orbits that have relatively low inclinations (a mean of ~ 13°) and some of the HTCs are in retrograde orbits (i>90°; the HTCs have a mean inclination of ~78°). These differences in properties suggest that the JFCs and HTCs may be dynamically different and perhaps have different origins.
Most of the JFCs are clustered at inclinations of less than 35°, suggesting that the three comets whose periods are less than 20 years but inclinations are greater than 40° may be different from the other JFCs. Carusi et al. (9) have offered an alternate, less arbitrary, definition for the division between JFCs and HTCs. They suggest that the boundary should be based on a parameter known as the Tisserand parameter T, which is defined as
tmp4F3_thumbThe inclination as a function of the semimajor axis for the short-period comets. In this figure, the orbits for the 130 cataloged short-period comets are shown. The dotted line divides the plot into two segments based on the traditional separation between Jupiter-family and Halley-type comets of orbital period — 20 years. The open circles are those comets whose Tisserand parameter (see text) T>2; the closed circles are comets where T<2. Jupiter-family comets are confined to the plane of the ecliptic, preferentially, whereas Halley-type comets are not.
Figure 3. The inclination as a function of the semimajor axis for the short-period comets. In this figure, the orbits for the 130 cataloged short-period comets are shown. The dotted line divides the plot into two segments based on the traditional separation between Jupiter-family and Halley-type comets of orbital period — 20 years. The open circles are those comets whose Tisserand parameter (see text) T>2; the closed circles are comets where T<2. Jupiter-family comets are confined to the plane of the ecliptic, preferentially, whereas Halley-type comets are not.
where aJ is the semimajor axis of Jupiter’s orbit and a, e, and i refer to the semimajor axis, eccentricity, and inclination of the cometary orbit. In their scheme, the division between JFCs and HTCs would be T — 2. The Tisserand parameter is a measure of the relative velocity between Jupiter and a comet
Jupiter s orbit and are confined to orbits either totally interior to (the case for Comet Encke) or totally exterior to (the case for Chiron) Jupiter’s orbit. In Fig. 3, the value of the Tisserand parameter is coded as an open circle for comets where T>2, or JFCs, and closed circles for T<2, or HTCs. Inspection of this figure now shows that the JFCs in this definition all have low inclinations with a mean of 11.6°, whereas the HTCs seem to have no preferred inclination. Note that this definition is an improvement over the 20-year period definition because it takes into account Jupiter’s gravitational influence on the comets. Thus, this definition, along with some modifications (10), is now being widely adopted to differentiate between different types of comets.
Note that the orbits of the HTCs are reminiscent of the long-period and nonperiodic comets in that there are no preferred inclinations for their orbits. Thus, both of these types of comets appear to come from a spherical reservoir ofa form different from the disk envisioned for forming the planets. JFCs, on the other hand, have low inclination orbits. For this reason, they are sometimes referred to as ecliptic comets.
How do comets get from the outer solar system to the inner solar system? The most important force acting on Oort cloud comets is the gravitational effects of the disk of our galaxy (11,12). The gravitational tidal forces of the galactic disk will cause the Oort cloud to be a prolate spheroid whose long axis points toward the center of the Galaxy. This force acts like a torque on each comet and leaves the semimajor axis virtually unchanged but causes the perihelion distance (closest approach distance to the Sun) to undergo a random walk. When the perihelion distance crosses into the planetary system, the planets (especially Jupiter) begin to have a major influence on the orbit.
Other external forces that influence cometary orbits are passing giant molecular clouds (GMCs) or stars passing through the Oort cloud. These are much less important than the Galactic gravitational field. Passing GMCs are rare events that have a mean interval of occurrence of 300 million years (8). However, when they do occur, they have a major effect on the part of the Oort cloud they pass.
During the lifetime of the solar system, about 5500 stars have passed within 100,000 AU of the Sun. A star of the same mass as our Sun tunneling through the Oort cloud with a velocity of 20 km sec ~1 will eject all comets within about 450 AU of the star. Passing stars have probably ejected about 10% of the total population of the Oort cloud (8).
It is estimated that the mean dynamic lifetime of comets in the Oort cloud is only about 60% of the age of the solar system. Thus, the Oort cloud must somehow be replenished for Oort cloud comets to exist still. One possibility for replenishment is the capture of comets that originally formed around other stars and then were ejected into interstellar space. This process is very unlikely because the relative velocity of these comets with respect to our solar system makes them difficult to capture.
A more likely scenario for replenishing the Oort cloud is that an inner Oort cloud exists at a few thousand AU from the Sun whose cometary orbits are perturbed to move them from the inner Oort cloud to the outer Oort cloud (13). When these inner Oort cloud comets are at aphelion, Galactic and stellar perturbations would be sufficient to raise their perihelia from the inner to the outer Oort cloud. However, normal effects of the Galactic gravitational field would not affect them other than at aphelion, so that the orbits of inner Oort cloud comets would otherwise be stable for the entire lifetime ofthe solar system. There is no physical demarcation for the inner and outer Oort clouds, and the differences between these clouds is only a matter of definition. The inner Oort cloud is, in general, just the part of the Oort cloud that remains relatively inactive, whereas the outer Oort cloud is the principal source of comets that enter the inner solar system.
Let us return to the question of why the Oort cloud appears spherical although we theorize that comets are leftover debris from the formation of the planets in a disk of material surrounding the young Sun. Current dynamic simulations have shown that the comets did not form originally at the location of the present Oort cloud. Instead, the comets are leftovers from the region of the giant planets, Jupiter through Neptune (14). Small objects that remained in the region of the giant planets after the planets formed would come under the gravitational influence of these planets. As a planetesimal comes near a planet, the planet exerts a gravitational tug on the planetesimal. This excess force is strongest at the closest approach to the larger body, but because the two bodies are in different orbits that have different velocities around the Sun, the excess force is exerted only for a short time. This excess force changes the orbit of the planetesimal (because of conservation of momentum, it also changes the orbit of the larger planet, but by a very much smaller amount than the change in the planetesimal’s orbit because the mass of the planet is much larger than the mass of the planetesimal). Thus, the orbit of the planetesimal is changed in a random-walk fashion. When the planet that causes the change in orbit is very massive, such as Jupiter or Saturn, then the excess velocity imparted to the planetesimal is most often large enough to eject the planetesimal from the solar system on a hyperbolic orbit. Some small fraction of objects would have their orbits changed to orbits near Uranus and Neptune or out to the inner Oort cloud.
Uranus and Neptune are not massive enough to impart such a large increase in velocity. Instead, the influence of Uranus and Neptune would increase eccentricities, perihelia, and inclinations of the orbits. Ultimately, most of the objects that start in the solar nebular disk at the distance of the current Uranus and Neptune would end up in orbits whose semimajor axes are several thousand AU but their orbits would no longer have inclinations that were small with respect to the plane of the ecliptic. Instead, the perturbations of the orbits by Uranus and Neptune would randomize the inclinations, and the planetesimals would form a spherical cloud at the distance of the inner Oort cloud. From there, they would populate the outer Oort cloud.
Thus, we are able to start with a disk of material in the protoplanetary system and, through the influence of gravity, the major planets perturb the orbits into the spherical Oort cloud. Most of the Oort cloud comets were formed in the Uranus/Neptune region, whereas Jupiter and Saturn ejected most of the leftover debris that existed near their orbits.
Let us return to the short-period comets. In Fig. 3, it was shown that there are two dynamic classes of short-period comets, the JFCs and HTCs. Using the Tisserand parameter definition for delineating these two groups, we noted that the average inclination of the JFCs was low (11.6°) and no orbits had inclinations greater than 35°. This is quite reminiscent of the original protosolar disk. The HTCs, however, show no preferred inclinations. In 1988, Duncan et al. (15) showed, by computer simulations, that it was impossible to reproduce the low-inclination, short-period comet orbits from perturbations of the spherical Oort cloud because comets that enter the planetary system from the Oort cloud would preserve their random inclinations. Duncan et al. showed that a cometary source that has a low orbital inclination distribution was far more consistent with the observed orbits. They posited that a belt of comets must exist outside Neptune’s orbit that were the source of the short-period comets. They named this reservoir the Kuiper belt because it was first suggested by Kuiper in 1951 and Edgeworth in 1949 that the disk of the protosolar nebula would not be truncated at Neptune’s orbit. (Some scientists think that the reservoir for short-period comets should be named the Edgeworth-Kuiper belt, but this is not universally accepted.) Further computer simulations have shown that a significant fraction of the orbits of objects in the Kuiper belt would be stable for the lifetime of the solar system. Perturbations of the giant planets would be sufficient occasionally to stir up the objects near the inner edge of the Kuiper belt and bring these objects into the inner solar system. However, the inclinations of the perturbed objects would not be changed enough to lose the signature of a disk.
In summary, from dynamic considerations, our understanding of the structure of the outer solar system is that it consists of two parts. Starting from around the orbit of Neptune, there is a disk of objects, known as the Kuiper belt. This disk has a thickness of order +25°. This disk is a primordial disk of objects (it formed at the same time as the planetary system), and a significant fraction of the orbits of objects in this belt is stable for the lifetime of the solar system. Starting at a few thousand AU from the Sun, there is a separate reservoir of comets known as the Oort cloud. This reservoir is spherical in shape, not disklike. The Oort cloud consists of two regions, the inner and outer Oort clouds. The inner cloud is relatively unperturbed and replenishes the outer Oort cloud, but the orbits of objects in the outer Oort cloud are stable only for about 60% of the lifetime of the solar system. Objects in the Oort cloud did not form in the cloud but instead formed in the Uranus/Neptune zone, and planetary perturbations changed their orbits to the Oort cloud. Thus, the Oort cloud is not a primordial reservoir of comets.
Objects that reside in the Oort cloud are much too faint to be detected by current techniques. Our only evidence for the Oort cloud, therefore, is the peak in the histogram of the inverse semimajor axis of long-period comets shown in Fig. 2. Our concept of the mass of material in the Oort cloud comes about from simulations and is quite uncertain (estimates range from a few to 50 Earth masses).
We have hard observational evidence for the existence of the Kuiper belt. The first Kuiper belt object was discovered in 1992 (16) and since that time, 241 objects whose orbits are in the Kuiper belt have been discovered (as of 21 February 2000—more are being discovered regularly). The objects that have been discovered are relatively large; diameters range from 50 to 500 km. Even so, they are up to 100,000 times fainter than can be seen with the naked eye and require very difficult, dedicated searches. However, these objects are clearly much larger than the comets that are seen in the inner solar system as active comets. One study (17) using the Hubble Space Telescope to image 100 times fainter than ground-based surveys reported the discovery of objects of more typical cometary dimensions. However, this study is controversial and has not yet been duplicated.
Figure 4 shows the inclinations of the orbits of the current catalog of Kuiper belt objects as a function of the semimajor axis. Included on this plot are several dashed vertical lines that mark the location of orbits that resonate with Neptune’s orbit. The 2:3 mean motion resonance refers to an orbit in which the object orbits the Sun twice for every three times that Neptune orbits the Sun (that is, the object is in general exterior to Neptune’s orbit). These resonances are very special dynamic places. If an object starts out in orbit nearest to Neptune in a 2:3 resonance, then every time it orbits the Sun twice, it will come back nearest to Neptune. Thus, Neptune will have an excellent opportunity to perturb the object repeatedly, and this orbit is unlikely to remain stable for very long. On the other hand, if an object is in a 2:3 resonance and starts off away from Neptune, it can never get near Neptune, and it is protected from Neptune’s influence, even if the eccentricity of the orbit is such that the object crosses Neptune’s orbit. Pluto’s orbit is an example of an orbit that crosses Neptune’s orbit, but Pluto is in a 2:3 resonance with Neptune and thus, is protected. Inspection of Fig. 4 shows that many of the Kuiper belt objects that have been discovered are in protected resonances, especially those whose orbits cross Neptune’s. Nonresonant orbits inside Neptune’s orbit have been cleared of objects.
The inclination as a function of the semimajor axis for the known Kuiper belt objects. In this figure, 228 Kuiper belt orbits are shown. Thirteen objects have semimajor axes > 55 AU. Mean motion resonances of Neptune are denoted by vertical dashed lines.
Figure 4. The inclination as a function of the semimajor axis for the known Kuiper belt objects. In this figure, 228 Kuiper belt orbits are shown. Thirteen objects have semimajor axes > 55 AU. Mean motion resonances of Neptune are denoted by vertical dashed lines.
From the properties of the discovered objects, we can attempt to estimate the number and mass of Kuiper belt objects. While doing this, we must keep certain difficulties in mind. We can discover only the larger, brighter objects in the inner part of the Kuiper belt. Thus, we have no knowledge of the density of small objects, nor of the density of objects in the outer part of the Kuiper belt. Indeed, we have no real knowledge of the distance to the outer edge of the Kuiper belt. When objects are discovered, we can measure their brightness. From this, we need to figure out their sizes and masses. The Kuiper belt objects do not shine on their own, but, instead, they reflect light from the Sun. We know how much light the Sun emits at all wavelengths, and we know how far the objects are from the Sun. However, to convert from brightness to size, we need to know what percentage of the incoming light is reflected from the surface of the object. This percentage is known as the albedo. The albedo of these objects is generally unknown, though we have an idea of the albedo of some active comets, especially Halley’s comet. Typical active comets have albedos of 4%, that is, only 4% of the incident light is reflected. If comets are ice, why is the albedo so low? After all, ice is bright white! Think in terms of snow that has fallen and been plowed and has had dirt mixed into it. That snow will be dirty and gray and much less bright than freshly fallen snow. The same is true of comets. The dirt in the nucleus mixes with the ice and makes the ice much less reflective than if it were pure. If we assume that comets in the Kuiper belt also have 4% albedo, we can convert from brightness to size. These estimates of size are only as good as our estimate of the albedo. Pluto has an albedo of 40%, which may be due to frost deposition, but may tell us something about albedos in the same region as the Kuiper belt. If Kuiper belt objects had similar albedos, they would be much smaller than our current estimates. From the size, we can determine the volume. Then, if we know the density of the body, we can determine the mass. However, as discussed above, the density of comets is very uncertain. Taking all of these factors into account, we believe that the Kuiper belt has only a fraction of an Earth mass of material.


To date, several spacecraft have visited comets and have made measurements in situ, but we have yet to retrieve samples and return them for study in our laboratories. The vast majority of our knowledge of cometary composition comes, however, from remote observations obtained with ground-based or Earth-orbital telescopes. These data are gathered using spectroscopic and photometric techniques on a wide variety of comets. The spectrum of a comet is characterized predominantly by molecular emission coupled with an underlying continuum of sunlight reflecting from the dust.
As indicated earlier, the nucleus of a comet is a combination of ices and refractory (dusty) material. About 50% of the mass of the comet is ice and the rest is dust. As the comet approaches the sun and sublimes and gas flows away from the nucleus into the coma, gas phase chemical reactions take place. Some of the chemical reactions are between two molecules or between molecules and dust, but the vast majority of the reactions are photochemical reactions resulting from the interaction of solar radiation and gases in the coma. The gas species become changed quickly. We term the original gaseous species that are the same as the ices the parent species. Species that are produced by chemical reactions of all types are termed daughter species (though some are granddaughter, great-granddaughter, etc.). Optical spectra of comets have been studied for more than 100 years. Until recently, only daughter species were detected. In the past two decades we have detected directly parent species with observations in the IR and mm. Table 1 contains a list of species that have been observed in the spectra of comets. Obviously, some of the species listed here are radicals and cannot be parent species. Others are stable species that could be parent species or could be the result of chemical reactions.
Gas. Spectra of comets can be used for determining the bulk composition of coma gases. However, it requires sophisticated models to use observations of daughter species to determine the nature of parent species. The importance of understanding the nature of ices is to understand which species were formed in the interstellar medium, before incorporation into the protosolar nebula and which species are the result of chemical processing in the early protosolar nebula. The exact composition of the species, the ratio of one species to another, and the isotopic ratios yield information about the conditions in which the ices formed.
Table 1. Atoms and Molecules Detected in Comets

Type species Species
Ions Solids
Atoms and Molecules Detected in Comets

Our understanding of the chemical composition of comets has increased tremendously in the past few decades because of improvements in our ability to observe comets with an ever increasing array of highly sensitive instruments. Originally, cometary spectra were confined to the optical region of the spectrum, using photographic plates as the detector. These observations were limited in spectral coverage and in detail because the dynamic range of plates is too limited to record properly the range of abundances in comets.
In the 1980s, we added charge-coupled devices (CCDs) to our arsenal of optical detectors. These allowed including the near-infrared region of the spectrum, and they possessed the dynamic range necessary to study coma densities that range across many orders of magnitude.
In the 1980s, the International Ultraviolet Explorer (IUE) spacecraft was launched. This spacecraft made it routine to observe comets in the ultraviolet, where several important atomic and molecular transitions, such as hydrogen Lyman a and Lyman b,CS, CO, S2, etc., are found. Systematic studies of comets in the UV by the IUE yielded important understanding of the relative abundances in comets. These observations have been continued with the Hubble Space Telescope (HST) since the demise of the IUE. The HST is much more sensitive than the IUE and can cover the optical and IR wavelengths, in addition to the UV. It also has much higher spatial resolution and yields important information about the distribution of gases within the coma.
Radio observations of comets were added in the 1970s but lacked the sensitivity to observe all except the OH radical. In the 1980s, came more sensitive radio detectors and the advent of millimeter-wave observations of comets. In addition, sensitive IR array detectors were developed, allowing IR observations of comets. The radio, millimeter, submillimeter, and IR bandpasses are extremely critical to our understanding of comets because it is only in these bandpasses that we can detect parent molecules directly.
All of these detector improvements, coupled with the appearance of the bright comets Hyakutake and Hale-Bopp in the mid-1990s, have allowed a manyfold increase in our understanding of the chemistry of comets.
Inspection of Table 1 shows that a wide range of species is detected, from very simple atoms and molecules to quite complex molecules. However, 80% of the ices are water ice. The presence of fully oxygenated species alongside fully reduced species (i.e., CO versus CH4 and C2H2) indicates that the comets could not form in a thermochemically equilibrated region of the solar nebula.
HNC is a species that could yield clues to the amount of chemical processing in the protosolar nebula (HCN is hydrogen cyanide, whereas HNC is hydrogen isocyanide, a differently bonded combination of the three atoms). HNC is seen in the interstellar medium at a ratio HNC/HCN o 1/T (where T is the temperature of the interstellar region). HNC could be a parent molecule and would be indicative of the formation temperature. If HNC were a parent, then the ratio HNC/ HCN would be constant as the heliocentric distance of the comet changed. Irvine et al. (18) measured both HNC and HCN in comet Hale-Bopp as a function of heliocentric distance and found that the ratio was not constant with heliocentric distance. They concluded that this was clear indication of ion-molecule chemistry in the coma and that HNC was not a parent and could not yield information on deposition temperatures.
Formaldehyde (H2CO) is ubiquitous in the interstellar medium, and its presence in comets would be expected if comets incorporated interstellar ices. However, it is also possible to produce formaldehyde through chemical processing in the protosolar nebula, so the fact that we detect formaldehyde in comets does not necessarily indicate that ices survived from the interstellar medium until they were incorporated in comets. The detailed abundance of formaldehyde can lend a clue to this question. However, we have evidence that the formaldehyde may be contained in inhomogeneous inclusions in the nucleus, so our present understanding of the quantity of H2CO is insufficient to determine its origins.
Earth’s atmosphere is more than 70% nitrogen. This nitrogen is important for life. How does the nitrogen abundance of comets compare with that of Earth? This is mostly an unanswered question because nitrogen-bearing species are very difficult to measure. The most likely nitrogen-bearing parent species in a comet are the dust, which does have some nitrogen, N2,NH3, and HCN. Of these, only HCN is easily detected, but it is expected to be a trace species. Wyckoff et al. (19) reviewed the various observations that bear on the question of the abundance of nitrogen in comets and concluded that the total nitrogen relative to other species is depleted from solar by about 6-10 times. Until more data become available, this is still an open question.
The frosts of noble gases are extremely volatile, so observations of noble gases would represent very sensitive thermometers of the formation temperatures and subsequent evolution of comets. Measurements of the noble gases would indicate the warmest temperatures the comet had ever reached. Unfortunately, noble gases are extremely difficult to detect, and none was identified in the in situ measurements of Halley’s comet. So far, only weak upper limits have been obtained for any noble gases (20).
Much can be learned about the fractionation history of the solar nebula by looking at the isotopic ratios of the comets. (Isotopes of an element have the same number of protons but different numbers of neutrons in their nuclei, e.g., hydrogen and deuterium). The fractionation history is a function of temperature, density, and pressure. If we can determine the isotopic ratios as a function of formational distance from the Sun, we have accurate values for these parameters. In general, the isotopic ratios for comets bear much more resemblance to those of the Sun and meteorites than they do to the values in the interstellar medium.
The isotopic ratio D/H is of particular importance to our understanding of the evolution of the solar nebula and its bodies, and particularly to our understanding of Earth. The protosolar nebula started with a value of D/H which was representative of the interstellar medium at the time it was formed. Various processes cause the loss of deuterium and hydrogen from a planet’s atmosphere. Less massive bodies lose more of their light elements as they evolve because their gravitational fields are smaller than the more massive bodies. Because hydrogen is lighter than deuterium, hydrogen is lost first. It is, thus, easy to deplete both of these elements but almost impossible to enrich them. Because H is lost faster than D, the ratio of D/H will tend to increase as a result of loss processes in a planet. Material at different heliocentric distances will also have different D/H values because of the temperature gradients in the solar nebula. Figure 5 shows measured values for D/H in various solar system bodies. These are difficult measurements, so for some bodies there are multiple values, and they do not always agree. As one moves upward on this plot, one is measuring an enriched D/H value compared with the original protosolar value.
The D/H ratio of a number of solar system bodies. The data for the giant planets and Titan come from Table 3 of Lecluse et al. (34). The comet values are, from left to right, for Halley (35), Hyakutake (36) and Hale-Bopp (37). Also marked on the plot are values for D/H in the interstellar medium and a range of values for D/H from protosolar nebula models. A heavy line marks the D/H ratio for standard mean ocean water (SMOW), a measure from Earth's oceans. For details of the individual numbers, see the sources cited.
Figure 5. The D/H ratio of a number of solar system bodies. The data for the giant planets and Titan come from Table 3 of Lecluse et al. (34). The comet values are, from left to right, for Halley (35), Hyakutake (36) and Hale-Bopp (37). Also marked on the plot are values for D/H in the interstellar medium and a range of values for D/H from protosolar nebula models. A heavy line marks the D/H ratio for standard mean ocean water (SMOW), a measure from Earth’s oceans. For details of the individual numbers, see the sources cited.
In Fig. 5, D/H values are plotted for three different comets. The agreement in their ratios is quite good, but a great deal more data are necessary before we can understand the degree of homogeneity in the comet formation region. Recall that comets formed in two distinctly different regions of the protosolar nebula. All three comets are Oort cloud comets, and the agreement suggests that the value of D/H measured may be typical of Oort cloud comets. It will be necessary to sample comets from both reservoirs to understand the conditions better. However, no Kuiper belt comets are bright enough to measure the D/H ratio with current equipment. As detectors and telescopes improve, this situation should change. Note, too, that the plotted values for cometary D/H are obtained from observing H2O and HDO. For comet Hale-Bopp, the D/H ratio was also measured by observing HCN and DCN. The ratio obtained this way was about a factor of 7 higher than that measured using the water species. We will return to this later.
The D/H ratio of Earth is measured from oceanic water and is shown on the plot as a heavy line marked SMOW (standard mean ocean water). Note that it is heavily enriched compared to the protosolar nebula. Comets show a value about twice the value for SMOW. It has been suggested that the enrichment over protosolar measured for Earth results from comets having been the source of the water on Earth. Although it is sure that a number of comets have bombarded the earth, thus contributing to the water content of Earth, the fact that the D/H ratio of comets is twice as high as Earth’s means that comets could not have been the most significant source of water on Earth. If comets were a major contributor to the oceans, the D/H ratio for SMOW would be closer to the D/H ratio for comets. Thus, D/H ratios can point to comets as only a minor source of Earth’s oceans and limits the rate of bombardment by cometary bodies.
In addition to measurements of D/H in comets, isotopic ratios of 12C/13C (in the gas) for about half a dozen comets and 14N/15N (for Hale-Bopp) have been measured. These isotopic ratios are similar to the value measured in the Sun and are vastly different from interstellar values. Measurements of 12C/13C in dust particles collected by the various Halley flybys show that this ratio is highly variable in the dust.
We have gathered various pieces of evidence; some of them indicate that comets contain material from the interstellar medium, but others indicate that there was substantial chemical processing in the protosolar nebula. Can any of these pieces of evidence indicate the temperature at which the cometary ices were deposited? Several lines of evidence yield such deposition temperatures.
The ratio of H2O/CO is indicative of the deposition temperature. However, extreme caution must be taken when computing this ratio because of the relative volatility of the two species. If observations are made of a comet at 5AU, then any sublimation is being driven by the CO, not the H2O. However, at 1AU, both species are sublimated, so a measure of H2O/CO gas at 1 AU will yield a value indicative of the ratio in the ices and thus gives information on sublimation temperatures. Laboratory experiments have been conducted by Bar-Nun and co-workers to determine the order in which the ices are deposited on grains and the subsequent ratio of sublimed gas. Comparison with observations of Halley, Hale-Bopp, and Hyakutake indicate that all three of these comets have deposition temperatures of ~ 50 K.
Recall that the D/H ratio, as measured by DCN, was a factor of 7 higher than the D/H ratio measured for HDO. The disparity between these ratios is also an indicator of the deposition temperature and implies a deposition temperature of 25-50 K. This is consistent with the H2O/CO temperature given before. Dust. Before the spacecraft flybys of comet Halley in 1986, our picture of the dust component in comets was entirely shaped by indirect evidence. The color of the continuum and the structure of the dust tail gave indications of the sizes of the particles in the coma. In addition, it was considered likely that some dust particles collected in Earth’s upper stratosphere originated from comets, although it was impossible to link any of the collected particles to a particular comet.
Our picture of the dust was that it was a component entirely separate from the gas. Our ideas changed dramatically with comet Halley and with subsequent IR observations of other comets. Three of the five spacecraft that visited comet Halley had instruments on them to measure the dust. Giotto, Vega 1, and Vega 2 all carried instruments to measure the composition of the cometary dust by having the dust hit a target surface and performing time-of-flight spectroscopy on the resultant components. These instruments were called PIA (particle impact analyzer) on Giotto and PUMA on the two Vega spacecraft. Based on these experiments, it was determined that the dust component of the coma had two different parts: an organic component rich in carbon, hydrogen, oxygen, and nitrogen (dubbed CHON particles) made up around 30% of the sample; Mg-rich silicate particles constituted another third of the particles; the remainder of the particles consisted of a mix of the two types of particles (21). The CHON particles, it was shown, are fragile and showed evidence of fragmentation during their outflow into the coma. The volatile nature of these grains allows them to be converted into gaseous species as they flow outward. Thus, the CHON particles turned out to be an extended source of gas in the coma.
The PIA/PUMA instruments sampled only small grains in the range of 10 ~ 16-10 ~11 g. This would correspond to spherical grains 0.06-3 mm in diameter. However, grains this size represent only a small fraction of the total dust mass. Another instrument on the Giotto spacecraft, DIDSY (dust impact detection system), measured grains as large as 10 ~ 5 kg (22). In addition, during the close approach of the Giotto spacecraft to the comet, an impact of a single grain on the spacecraft sent the spacecraft wobbling and damaged the optics. Models of the necessary size of the impactor indicate that the grains were as large as
The PIA/PUMA instruments measured the mass of the detected particles, their densities, and their compositions. They found that the CHON particles had a density of ~1gcm~3 and the silicate particles had a density of ~2.5gcm~3. This implies that the CHON-dominated grains are quite fluffy, whereas the silicate-dominated grains are more compact. However, the heavier grains of a particular type are as fluffy as the lighter grains. The bulk composition of the dust analyzed by PIA/PUMA shows that the dust in Halley’s coma has an elemental composition which is similar to the solar system composition (of course, the dust is depleted in hydrogen from the solar value but that is because it is easily lost, as explained before). This implies that Halley’s comet was not fractionated with respect to the protosolar nebula (21).
The analysis of the composition of individual grains shows a lack of chemical equilibrium among the grains. This was deduced from examining the ratios Fe/(Fe + Mg) and Fe/Si versus Mg/Si, two quantities that have been used for laboratory samples ofdust to distinguish different types ofparticles. The analysis also found very few, if any, Ca-Al-rich grains in Halley’s dust. One startling discovery was that the isotopic ratio 12C/13C was not the same at the grain level but varied from 1 to 5000 for individual grains (the normal solar system value is 89, and meteorites have ranges from 4-120). The low cometary dust values may be an instrumental artifact, but the high values appear to be real. The only explanation for such high 12C/13C ratios is that these particular grains were produced in a nucleosynthetic process, probably before the protosolar nebula was formed, and were incorporated into the comet unchanged.
Recently, the Infrared Space Observatory (ISO) observed the IR spectrum of comet Hale-Bopp in the spectral region from 7-45 mm. The spectrum obtained when the comet was at around 3 AU shows features that can be matched well by crystalline magnesium-rich olivine (fosterite, Mg2SiO4) (23). Other ISO spectra, obtained when the comet was closer to the Sun, might also contain features of pyroxene. In the interstellar medium, any observed silicates are only in their amorphous form, so the detection of crystalline fosterite is quite significant. At the temperatures and pressures of comet formation, silicates would form in their amorphous state, not in a crystalline form, so the detection of crystalline silicates in comets requires that these crystalline silicates formed elsewhere and were incorporated into comets when already in their crystalline form. ISO also detected, for the first time, crystalline silicates in evolved stars (so-called AGB stars and planetary nebulae), as well as in young stellar objects (24). This suggests that the crystalline silicates might have been formed before the protosolar nebula was created and were incorporated into comets when they were formed.
Diversity. Detailed study of individual comets such as Halley or Hale-Bopp tells us much about one comet. However, to characterize the early protosolar nebula, we need to study a large sample of comets and determine if the compositions are similar. If all comets have the same composition, this indicates that the protosolar nebula in the region where the comets formed was homogeneous. If all of the comets are not the same, then there could be a primordial difference, implying that the protosolar nebula was lumpy, or there could be an evolutionary difference. Extensive ground-based telescopic surveys have been conducted to determine the degree of heterogeneity of cometary abundances. These studies were carried out by Cochran and co-workers at The University of Texas, McDonald Observatory; by Newburn and Spinrad at The University of California, Lick Observatory; and the largest survey was completed by A’Hearn, Millis and co-workers, primarily at Lowell Observatory.
A’Hearn et al. (25) compared observations of 85 comets and looked at relative gas abundances. They found that the vast majority of comets seems to have similar relative abundance (one species relative to another). However, they found that a group of comets exists that is depleted in carbon-chain molecules, such as C2 and C3, but not in other carbon-bearing molecules, such as CN (comet Giaco-bini-Zinner is the prototype for this group of depleted comets). Although the carbon-chain-depleted comets represent a minority of their sample, all of the carbon-chain-depleted comets are Jupiter-family comets. Recall that JFCs formed in situ in the Kuiper belt, but other comets formed interior to the Kui-per belt and were ejected to the Oort cloud. Thus, if all of the carbon-chain-depleted comets are JFCs and none of the other comets is depleted, this points to differences in the conditions in the two zones offormation. It should be noted that not all JFCs show the carbon-chain-molecule depletion. This still leaves the question whether there are Oort cloud comet interlopers in the JFC sample, whether we are seeing compositional differences in the Kuiper belt region, or whether there are evolutionary differences in Kuiper belt comets.

Evolution of Cometary Nuclei

Comets represent the least altered components left over from the formation of the solar system, but they are probably not totally pristine. Though the Oort cloud is sparsely populated, collisions between Oort cloud comets can still occur occasionally. Several physical processes can also modify the outer layers of cometary nuclei while they reside in the Oort cloud.
The ices of the nucleus are subjected to various forms of radiation, including the radiative flux from passing stars, UV flux from forming stars while the protosolar nebula was still in a star-forming region, X-ray photons from the Galactic background and nearby exploding supernovae, and Galactic cosmic rays. These forms of radiation can profoundly alter the nature of the ices by altering the chemical bonds, radiation darkening and polymerizing ices (26,27). The radiation damage can increase the density and porosity of the ices and can turn simple ices into more complex ones. These processes can penetrate to different depths within the nucleus; UV-induced changes occur in only the outermost layers (a few to hundreds of atoms or 1-10 nm deep), and X-ray induced changes as deep as 10-100 mm. The depth of the alteration for charged particle radiation would depend on the energy of the particles; low-energy solar protons hardly penetrate, and high-energy protons penetrate deeply. If cometary surfaces have low density, the radiation damaged crust might extend many meters deep (28). Depending on the depth of radiation penetration, the altered ices may survive one or more passages into the inner solar system before being lost to sublimation and subsequent coma outflow.
Collisions between comets are responsible both for increasing the size of objects through accretion and reducing some objects to dust via grinding. Calculations by Stern (29,30) show the frequency of collisions in each region. The Kuiper belt comets which have already been discovered are large and have volumes some 1000-10,000 times that of a Halley-sized comet. To build each object, it was necessary for many collisions to take place (30). Comet-comet collisions in the Oort cloud are less important than in the Kuiper belt and probably only 1 in 10,000 comets suffers a catastrophic collision in the age of the solar system (29). However, small debris in the region (some of which is the result of previous collisions) will collide with the comets, gardening, polluting, and roughening the surface.
When the comets pass into the inner solar system and are heated, sublimation of the gas will carry away much of the embedded dust. However, some of the dust grains are sufficiently large that they cannot escape even the weak gravitational field of the nucleus. These grains will fall back upon the surface. In time, unless some event catastrophically ejects the entire surface, these larger grains will form a mantle on the surface ofthe comet, insulating the ices and choking the ability of the ices to sublime (31). Indeed, this scenario of sealing the surface of the nucleus has been proposed as one ending for the life of the comet.

Cometary Impacts

Every single day, about 100 tons of interplanetary material hits the upper atmosphere of Earth, and some of that material reaches the surface. A large percentage of the smallest particles originates in comets as dust grains that are released when ices sublime. These small cometary particles represent no hazard to Earth. Many of them are fluffy aggregates that “float” in the upper stratosphere for long periods of time and are occasionally collected for study by specially equipped high-flying aircraft.
The present gentle rain of small particles on the upper atmosphere has not always been the pattern for impacts on Earth. In the early stages of the formation of the solar system, many planetesimals still remained, and the young solar system was a much more chaotic place. Before they were ejected from the solar system, the icy planetesimals’ orbits were perturbed so that they often impacted the planets in the inner solar system. We see this record on the surfaces of atmosphereless and geologically inactive bodies such as the Moon or Mercury. Soon after the planets were first formed, they were being bombarded by icy and rocky planetesimals (comet and asteroid pieces) at a rate which was about 1000 times greater than the current rate; the bombardment rate fell dramatically to “only” 30 times the current rate around 3.5 billion years ago (32). While the planets were being so heavily bombarded, they were in constant upheaval. It was impossible for life to form. Around 3.8 billion years ago, a period we call the late heavy bombardment, conditions began to change for the better for life. The oldest currently known fossils that indicate life forms are about 3.5 billion years old. Thus, the heavy rate of cometary impacts must have frustrated the origins of life in the earliest days of the solar system.
Did comets also serve as a catalyst for life on Earth? Comets contain large quantities of water, a necessary ingredient for life, along with other more volatile materials. It has been proposed that these organics, along with water, may represent the important prebiotic building blocks necessary for life (33). The arrival of large quantities of cometary material in the early evolution of Earth may have made available necessary raw materials for the formation of amino acids. Thus, once the late heavy bombardment had ended, the materials which were delivered to Earth from the bombardment might have played a critical role in the origins of life on Earth.
Once life had begun, the future interactions of Earth and comets was not entirely beneficial or even benign. We know that, even in recent times, objects impact Earth, as evidenced by locales such as the Barringer meteor crater in Arizona, the Chicxulub crater in the Yucatan (the remains of the impact event which, it is thought, destroyed the dinosaurs at the time of the Cretaceous-Tertiary (K/T) boundary 65 million years ago), or the Tunguska impact zone where an impactor fell in 1908. Many of these impactors are not icy, cometary bodies, but are asteroidal in origin. Rocky materials are stronger than the fragile cometary nuclei and can more easily survive passage through the atmosphere to impact the ground. However, comet impacts are still possible. A recent spectacular example of a cometary impact was the Shoemaker-Levy 9 impacts (in 1994) on Jupiter. Though these impacts had no effect on Earth, they serve as a reminder of the magnitude of effects that are caused by impacts on planetary bodies. Indeed, cometary impacts can be more extreme relative to the mass of the impactor than asteroidal impacts because of the random orientations of the cometary orbits relative to the plane of the ecliptic. If an impact occurs with the impactor in a retrograde orbit, the relative velocity of the comet and the Earth can be very large, and the impact energy is proportional to the square of that relative velocity.
The best defense against impactors of any type is to know in advance that the body exists and is potentially hazardous. NASA is currently coordinating large search efforts for near-Earth objects to catalog as many objects as possible. This effort is likely to be relatively complete for bodies of asteroidal origins, but many cometary bodies will go undetected in these searches because the random inclinations of the cometary orbits make systematic searches difficult and expensive. Thus, we could be faced at any time with the threat of a cometary impact and the resultant damage. Our best hopes for planetary protection are to be as vigilant as possible and to enhance our understanding of the nature of cometary nuclei so that we understand the properties of potential impactors.

Missions To Comets

Much can be learned about the nature of comets via observations from the ground and Earth orbit. However, we can never see the nucleus of the comet in this way, and we must infer information about its nature by using models. To study the nuclei of comets, it is necessary to send spacecraft to them. These spacecraft missions can range from simple flybys, to rendezvous missions, to landings and sample returns. In this section, the completed and planned come-tary missions of the U.S. and other nations are described.

Completed Missions

ICE. The first cometary mission did not start its life as a mission to a comet. The International Sun Earth Explorer (ISEE) 3 was a mission to study the interaction of the solar wind with Earth’s magnetosphere. It was launched in 1978. After excursions into Earth’s magnetotail and five swingbys of the Moon, the spacecraft was retargeted to flyby comet Giacobini-Zinner and its name was changed to the International Cometary Explorer (ICE). The flyby of comet Giacobini-Zinner occurred on 11 September 1985, making this mission the first to encounter a comet.
Because the mission was intended to study the plasma environment of Earth, no cameras were carried by the spacecraft. The goal of ICE was to study the interaction of a comet with the solar wind. The spacecraft was targeted to fly through the tails of Giacobini-Zinner to gather data on the changing magnetic field and plasma. The experiments on board ICE found a thin, dense, and cold plasma sheet. The magnetic field strength was low, and the spacecraft detected a change in polarity as it crossed the comet axis. The temperature of the plasma increased with increasing distance down the tail until about 20,000 km from the nucleus. Because the mission was not intended to sample a comet, very little compositional information was obtained.
Sakigake, Susei, Vega 1, Vega 2, and Giotto. Comet Halley is one of the most famous comets, and so it was not surprising that there was great interest in missions to this comet. An international armada of spacecraft visited this comet: the European Space Agency sent the Giotto mission; the USSR sent two spacecraft, Vega 1 and 2; and Japan sent two spacecraft, Sakigake and Susei. A detailed description of these missions and their first science results can be found in a special issue of Nature (Volume 321, May 1986).
The two Japanese spacecraft were officially part of the Planet-A project of the Institute of Space and Astronautical Science (ISAS); one spacecraft was an engineering test spacecraft dubbed MS-T5 (renamed Sakigake, Japanese for ”forerunner”, after launch), and the other the Planet-A main spacecraft (renamed Susei, Japanese for ”comet”, after launch). They were launched in January and August 1985, respectively. The two spacecraft were identical but carried different instrument complements: Sakigake carried a plasma-wave probe, whereas Susei carried an ultraviolet imager and charged particle analyzer. Concerns about the dust environment near the nucleus caused the probes to be targeted well sunward of the nucleus; Sakigake passed 7 million km away on 11 March 1986, and Susei 151,000 km away on 8 March 1986. The relative velocity of Susei and the comet was 73 km sec ~1.
The two Vega missions were combination missions, whose goals were studying Venus with balloons and landers on their way to a rendezvous with comet Halley. They were launched 6 days apart in December 1984 and flew by Venus 4 days apart in June 1985. They flew by comet Halley on 6 March and 9 March 1986 at around 8,000 km on the sunward side of the nucleus. The two three-axis stabilized spacecraft were identical, including instrumentation. Each carried a complement of 15 scientific instruments to measure the plasma, the dust, the composition, and to obtain images of the comet. The encounter velocities were 79.2 and 76.8 km sec “1.
The Giotto mission was launched by the European Space Agency (ESA) on 2 July 1985; it was the first ESA interplanetary mission. It achieved closest approach to the nucleus on 14 March 1986 at an encounter distance of 600 km and a flyby velocity of 68.4 km sec ~1. The spacecraft was spin-stabilized and carried a complement of 10 instruments, including a camera, mass spectrometers, dust detectors, and plasma instruments. The close encounter distance coupled with the fast flyby velocity were a calculated risk to the spacecraft but were driven by the desire to get close for the best imaging and for sampling by the various instruments. The biggest risk came ifthe spacecraft entered a dust jet (a dust jet is a region of the coma in which the flux of 1- to 10-mm particles is enhanced by a factor of 3-10 over the ambient coma). Fourteen seconds before the closest approach, the spacecraft apparently was hit by a “large” dust grain, causing the spacecraft’s angular momentum vector to shift by 0.9° and to wobble. Because the spacecraft had no onboard data storage devices, this interrupted communications and data transfer, though some data were transmitted intermittently for the next 32 min. Some of the instruments were damaged at this time, but in general, the spacecraft remained healthy. Thus, ESA was able to retarget the spacecraft for a flyby of comet Grigg-Skjellerup on 10 July 1992, but some instruments, such as the cameras, were not functional.
The Giotto mission’s Halley multicolor camera achieved the milestone of the first excellent images of the nucleus of a comet, supplemented by images from the Vega cameras. The images showed a lumpy, elongated object; the dimensions were 8 x 8 x 16 km with several very active jets, and the rest of the surface was relatively quiescent; only 20-25% of the surface appears to be active. The nucleus had a very dark albedo of 2-4%, comparable to the darkest known bodies in the solar system. The comet does not rotate simply around a single axis but has a complex wobble-spin tumbling motion. The dust environment was quite severe, especially at the high encounter velocities. It was this armada of spacecraft that finally confirmed our general picture that 80% of cometary ice is H2O ice.
Deep Space 1. This mission is the first of the NASA New Millennium missions intended primarily as demonstrators of cutting-edge technology never before flown. The Deep Space 1 mission was launched on 24 October 1998. It was primarily an engineering mission that had a science component planned to be executed ifpossible. This mission was intended to test 12 new technologies; the most important was ion propulsion, but included such technologies as autonomous navigation, low power electronics and new instruments. The nominal scientific mission was a flyby of asteroid 1992 KD (Braille) at 10-km altitude on 28 July 1999. The mission was extended to flyby comet 19P/Borrelly on 22 September 2001 at a closest distance of 2170 km from the nucleus. During the course of the flyby, 52 visible-wavelength images and 45 infrared spectra were obtained.
Deep Space 1 observed an irregularly-shaped cometary body with a long axis of 8 km and a 2:1 axis ratio (38). The surface had a low albedo of 3% and true albedo variations from 1-3%. Only about 10% of the surface appeared to show activity and no ices were detected on the surface. A jet appeared to emanate directly from the rotational pole.
Current Missions. Great interest continues in understanding comets and their constraints for models of the protosolar nebula. Thus, several missions are currently planned and launched or to be launched within the next decade. This section will describe all of these missions.
Stardust. This is the fourth mission in the NASA Discovery program of small focused planetary science missions, and it is the first robotic return of material from beyond the Moon’s orbit. The Stardust spacecraft was launched on 7 February 1999 (Principal Investigator Dr. Donald Brownlee of the University of Washington). The Stardust mission will encounter comet Wild 2 in June 2004 at a relative encounter velocity of 6.1 km sec ~1 and an encounter distance as close as 150 km. Wild 2 is expected to be much less active than comet Halley, and the much lower encounter velocity should protect the spacecraft. In addition, the encounter with Wild 2 will take place at a heliocentric distance of 1.86 AU, where comets are less active than the 0.9 AU heliocentric distance of Halley at encounter. Stardust will carry a camera for imaging and a dust particle analyzer (CIDA—a similar instrument to the PIA/PUMA instruments discussed earlier).
The most important goal for Stardust is to capture samples of interplanetary dust (collected in 2000) and cometary dust and return them to Earth for analysis. It will accomplish this goal by using a capture target built of aerogel attached to panels on the spacecraft. Aerogel is an ultra-low-density microporous substance which has been tested on previous shuttle missions. Once the flyby of the comet has been achieved, the aerogel target will be closed into a sealed sample return capsule (SRC), and the spacecraft will return to the vicinity of Earth. When the spacecraft approaches Earth, the SRC will be released from the spacecraft and will descend into the atmosphere on a parachute. It is planned that the sample will land in the Utah Test and Training Range, where the SRC will be recovered by helicopter or ground vehicle and transported to the planetary materials curatorial facility at the Johnson Space Center. The Earth return of the SRC is scheduled for 2006. More information can be found at http://
CONTOUR. The comet nucleus tour (CONTOUR) is another NASA Discovery mission; its goal is studying the diversity of cometary nuclei (Principal Investigator Dr. Joseph Veverka of Cornell University). CONTOUR includes detailed investigation of two diverse short-period comets: Encke (November 2003) and Schwassmann-Wachmann 3 (June 2006). However, the mission profile is extremely flexible and can be modified to include the first-ever study of a ”new” comet (such as Hale-Bopp) should one be discovered during the mission.
The spacecraft is scheduled for launch in July 2002 and will carry four instruments with it: a wide-field and a high-resolution camera, a dust analyzer (CIDA—a copy of the Stardust instrument), and a neutral and ion mass spectrometer. To maintain its extreme orbital flexibility, the CONTOUR spacecraft uses repeated Earth-return trajectories and multiple gravity-assist maneuvers. A unique feature of CONTOUR is that it will be placed into a spin-stabilized “hibernation” mode, requiring no ground contact, during its cruise intervals between comet encounters and Earth swingby maneuvers. The spacecraft will be “awake” for 75 days around encounters and 50 days around each Earth maneuver. More information about CONTOUR can be found at http://www.
Rosetta. The Rosetta mission is the third Cornerstone mission of the European Space Agency (ESA). This mission is to be launched January 2003 from Kourou, French Guiana, aboard an Ariane-5 rocket. Rosetta is designed to rendezvous with comet P/Wirtanen in August 2011, orbit around the comet, making observations of the nucleus, and eventually, to land a package of instruments called the Rosetta Lander (or RoLand) on its surface in August 2012. The long cruise phase (eight years) is necessary to rendezvous with the comet far from the Sun and at a low enough encounter velocity.
Rosetta is larger than the previous three missions. It carries 12 instruments in addition to RoLand. The orbiter instruments include a camera, several spectrographs, a neutral and ion mass spectrometer, a dust analyzer similar to that on Stardust and CONTOUR, a dust detector, and a plasma instrument. RoLand carries a suite ofeight additional instruments to evaluate samples of the comet in situ. No samples will be returned to Earth. RoLand will include an instrument to take a core sample from 200 mm under the nucleus’ surface.
One unique problem faced by the Rosetta mission is to land successfully on the surface when we do not currently have any information about the nature of the material on which RoLand will land. RoLand will land with a relative velocity of less than 1 m sec ~1 and will fire an anchoring harpoon into the nucleus to secure the lander to the surface (recall that the gravity of a comet is weak because it has such low mass). Once tethered to the surface, RoLand will begin its measurements, transmitting the data to the orbiting spacecraft for relay to Earth. More information can be obtained about Rosetta at rosetta/.
Deep Impact. The final cometary mission is another in the Discovery program. Deep Impact is to be launched in January 2004 and will encounter comet Tempel 1 on 4 July 2005 (Principal Investigator Dr. Michael A’Hearn of the University of Maryland). Deep Impact’s goal is to sample the deep interior of a cometary nucleus. It would be quite impossible to do this by trying to drill a core sample, so the Deep Impact approach is quite different. Deep Impact will expose material deep in the interior by creating a crater of > 100-m diameter and > 20-m deep. This will be done by releasing an autonomously targeted mass of roughly 1/2 metric ton (mostly composed of copper) one day before the impact. Cratering is a natural process in space. The difference between a naturally created crater and that created by the Deep Impact spacecraft is that we will know the size and mass of the impactor and exactly when it hit.
The impactor will be carried by the flyby spacecraft. After the release of the impactor, the flyby spacecraft will decelerate slightly so that its closest approach to the nucleus will occur after the impact at an interval roughly twice the predicted time to form the crater. The impactor will carry a camera to obtain high-resolution images of the surface of the impact site just before impact. The flyby spacecraft will carry two cameras and two spectrometers to observe all phases of the crater-forming process, the resultant crater, the debris plume, and the changes induced in outgassing. The comet will be observed simultaneously from Earth to record data about the changing comet. For more information, see http://


Comets are small icy/dusty bodies left over from the formation of the solar system and represent the least altered bodies from the epoch of formation. They formed in the outer parts of our planetary system; some still reside in their primordial reservoir known as the Kuiper belt; those formed in the Uranus-Neptune region were ejected to a spherical reservoir known as the Oort cloud; those formed near Jupiter and Saturn were ejected from our solar system entirely.
The nuclei of comets are small (typically a few km) bodies composed of ices and dust. Of the ices 80% is water ice. The dust is composed of organics and silicates. As the nucleus is heated, the ices sublime and flow outward from the nucleus. The dust/gas mixture forms a large coma and even larger tails.
We are on the verge of increasing our knowledge of comets tremendously with a series of spacecraft already launched and about to be launched. The next decade should revolutionize our understanding of comets and thus, the solar nebula.

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