Results of observations reported in 2003 by Rodney Gomez of Brazil support the presence of two distinct Kuiper belts. One lies largely within the solar ecliptic; its asteroids are grayish. A second belt has trajectories up to 40° off the ecliptic; its asteroids have a reddish tint.
The Oort Cloud (OC) is a widespread swarm of perhaps billions of comets that orbit farther from the Sun than the KBOs. These orbits are not confined close to the ecliptic but can follow paths that occur anywhere in the "sphere" of the Solar System with the Sun at its center. As yet, no OC object as it resides now within the Oort Cloud has been imaged by either ground-based telescopes or the HST owing to their small sizes and great distances from Earth. However, their existence is based on firm reasoning that predicts them to be a significant part of the bodies that formed from the nebula that organized into the Sun, planets, and other objects. The artist's sketch below shows roughly the relative sizes of solar region volume containing the Oort Cloud and the Kuiper Belt:
Individual Oort Cloud comets have been imaged because they have been perturbed from their distance locations and brought into the Solar System, as described by this diagram:
One of the most recent Long-Period comets to be discovered (in 1996) is Comet Hyakutake:
Kuiper Belt and Oort Cloud assemblages have been observed around other stars. These are usually referred to as Debris Disks (see page 20-11). Here is one example seen by the Hubble Telescope. A mask around the central star allows the reflecting debris, probably developed from or modified by collisions, to stand out.
As of 2005, 898 comets have been catalogued; most now have their orbits reasonably well calculated. Of these 184 have definitely been established as periodic - namely, those comets known to repeat their passage through the inner Solar System at intervals of years to centuries as they orbit the Sun; these are the aforementioned Short-Period (KBO) and Long-Period (OC) types). New comets and others of the 878 may also be periodic but more observations over time are needed to confirm this. The number of comets - most still undetected - within the Solar System may be in the billions; it is not yet known whether comets also reside in interstellar space.
Most comets positioned well away from the Sun are without pronounced tails, and are best found by looking for notable displacements of small bright objects (early stage comas) relative to fixed star backgrounds in film records taken days apart. Such motions delineate the advance of comets, as well as reflecting asteroids, at high speeds through solar space.
The best known of all comets is Halley’s Comet, observed and recorded in ancient times. Its periodicity, first predicted by Edmund Halley to verify Newton's Laws of Motion, causes it to reappear about every 76 (range 75 to 79) years. It is thus within the Short-period class. Having passed in 1909, as shown in this wide angle view that displays its magnificent tail, it reappeared in 1986, and many predicted it would provide a great celestial display (which generally fell short of expectations).
Debate raged in advance about sending one or more space probes to examine it close-up, since the next opportunity would not be until 2061. Although NASA decided against this adventure, Japan, the former Soviet Union, and the European Space Agency sent probes to gather data. In particular, the Italian government designed and launched a spacecraft named Giotto, which came within 540 km (336 mi) of the nucleus on March 13, 1986. Here is a close approach view:
Giotto found that Halley's nucleus, measured at 16 km x 8 km (10 x 5 miles), is very dark, lumpy and of low density (0.1-0.2 g/cc). This suggests that it was then very porous, with most of the ice having ablated or evaporated away, leaving carbon-rich dust as a residue. Although not as bright as anticipated and a disappointment to ground viewers, the comet, as seen through telescopes, provided exceptional displays. We can emphasize reflectivity variations in its coma and tail, representing particle density differences, by displaying them in false color:
A recent comet sensation is Comet Hale-Boggs, discovered on July 23, 1995, that passed Earth as close as 85 million miles on April 2, 1997. Hailed by many as the Comet of the Century because of its size (four times larger than Halley’s Comet) and brightness, it was visible in northern and southern hemispheres during much of the first half of 1997. Here is a typical view, taken on March 11, 1997, by Jerry Platt, one of many amateur astronomers who tracked this spectacular celestial visitor.
Another comet, Holmes, first noticed in 1997, began to flare up in November of 2007. Here is a ground telescope image of this comet; the inset shows the nucleus at the time of maximum flare up:
The Spitzer Space Telescope produced this pair of images; the left image shows the huge coma that appears to be made up of dust (imaged at the 22 micrometer wavelength so that this is a thermal view) that "exploded" outward during the flare up; the right image shows details at a different wavelength:
Here is a Hubble Space Telescope image of comet Holmes, again without a tail:
Most comets that are far from the Sun don’t have pronounced tails, so to find them we look for notable displacements of small bright objects (early stage comas) relative to fixed star backgrounds in film records, taken days apart. Such motions delineate the advance of comets, as well as reflecting asteroids, at high speeds through the solar system. The 6 panels in the next illustration show the progressive movement over hours of the comet Wirtanen; when imaged it was about 605 million km - or more than 4 A.U's - from Earth. The comet, a tiny dot is circled in each panel. Follow its displacement from right to left (in panel 4 it is directly in front of the reference background star).
In 1999, NASA sent a probe called Deep Space 1 to test new observation techniques and if possible to gather new information about comets it would approach. It has lasted beyond its planned lifetime. One of its most spectacular achievements was on Sept. 22, 2001 when it passed within 2200 km (1400 miles) of the Comet Borrelly to image its nucleus. Concern was high that the probe, suffering now from some malfunctions, would be damaged by cometary debris. However, it succeeded grandly in getting excellent images of the nucleus. Below is the last image received before planned shutdown in which the elongated nucleus, an 8 km (5 mile) long object imaged at 45 m resolution, displays ridgelike irregularities, faults and other geologically describable features; in some ways, it resembles an asteroid but its dark surface is composed of ice and dust (note the jets of material streaming off):
Sensors on Deep Space 1 produced these images, with information described in their captions.
A relatively pristine comet was discovered in January 1988 by the Swiss astronomer Paul Wild (its German pronunciation is "Vilt"). Comet Wild 2 was once beyond Jupiter but a close encounter with Jupiter has carried it into an orbit beyond Mars such that it passes Earth about every 6 years. Here is its appearance in December 1990:
In 2001, NASA launched a probe (the Stardust mission) to image Comet Wild 2 and to collect samples of dust and gas as it passed through the comet's tail. The experiment was successful. This drawing shows how the particles are collected:
Here is a view of Wild 2's nucleus (~5.4 km [3.3 miles] in maximum dimension) from a distance of 500 km (341 miles):
Features on Wild 2 have been named (with descriptive humor), as shown here, in which the spacecraft was somewhat closer.
Close-up images show some distinctive features, including depressions, fractures, and prominences
Still another image subjected to different processing showed glowing material coming off this nucleus. The cometary body apppears to consist of a mix of ice and rock together hard enought to serve as a strong solid that is not disrupted by repeated cratering action (some investigators think the pits are ablation - rather than impact - produced).
Using photometric filters, Stardust has obtained data in the visible and infrared allowing good estimates of the composition of materials at Wild 2's surface, as follows:
Based on the observations just described, the interpretation of Wild 2's makeup indicates that it has some similiarities to the Kuiper belt asteroids, suggesting a kinship.
Stardust successfully flew through the comet's tail and collected particles. It then fired rockets to begin the long journey home. In the middle of the night on January 15, it hit the atmosphere at about 40000 km/hr, slowed, deployed a parachute and landed saftely on the desert floor of the Dugway Proving Grounds in Utah. Here is a photo showing it at this resting place.
After removing the sample payload package, it was opened at JPL in Houston, with great hopes. The sample collection material is a network of rectangular "cells" placed within an aluminum framework. These cells are made up of a material called "Aerogel" which consists of a silica compound that is solid and rigid but supports a structure that gives the gel about a 95% porosity (open space) so that cometary particles can move through until finally becoming trapped in the solid component.
Note the black spot in one of the cells above. This is a cometary particle. Preliminary estimates suggest perhaps up to a million very tiny (millimeter to submillimeter) particles were collected. One noteworthy example, with associated tracks, is shown here.
Another eye-catching dust particle impact produced a pattern in the aerogel that is remarkably similar to pictures of the ejecta tossed out from much larger impacts during the growth phase of the forming crater; this Wild 2 example may have been caused by some different process:
The first mineral identified in the captured comet debris is the olivine end member Forsterite (Mg2SiO4) which is on Earth a gem variety known as Peridot. Here it is:
This mission is judged a huge success. Everything worked. Several hundred chosen investigators will subject recovered grains to a wide range of tests. As information is acquired, results will appear on this page.
The European Space Agency (ESA) is conducting a follow-up to Giotto with a mission (Rosetta) to the Comet Churyumov-Gerusmanko (6 km diameter) after a successful launch on March 2, 2004 and planned arrival in 2014. Heavily instrumented, Rosetta includes a lander on the comet's nucleus. As it travels through the asteroid belt, it has been observing several of the asteroids. Here is its view of the small asteroid Stein:
NASA's Deep Impact Mission was launched in January of 2005 to comet Tempel 1. In July of 2005 as it near Tempel'surface, it launched a probe designed to make a large impact of the surface. This took place with results summarized in this image:
Missions to comets and asteroids remain dynamic programs because they are oriented towards learning what these bodies can tell about the early Solar System. We will mention two now underway by citing their Web sites: 1) the Epoxi mission which is the continuing flight of the Deep Impact satellite now headed to comet Harley; and 2) the Stardust-NExT mission, launched in 2007 as a follow up to Stardust, to examine the modification made by impact of the spacecraft on the surface of comet Tempel 1
What is the ultimate fate of most comets? Two possibilties are most likely. First, they may just gradually "wear away" as their icy comas are ablated or otherwise destroyed as they repeatedly pass their parent star. Second, they may be pulled into that star or one of its planets (see next page) by gravitational attraction, thus becoming totally annihilated. Evidence for this latter fate has been indirectly retrieved from a ground telescope study (McDonald Observatory in Texas) of a Herbig Be type star, LKHa234 in nebular cloud NGC 7129, 3200 l.y. away from Earth in the Milky Way (see page 20-5 for a discussion of this star type). This 100000 year-old star (about 5 solar masses) was observed over 5 days. Spectra taken of it show dramatic changes, e.g., in sodium light, during that period. One reasonable interpretation is that a comet with sodium content crashed into the star, thus dispersing its source so that sodium spectral lines disappeared. Here is an image of LKHa234 and its surrounding nebula taken during this period.
In the last twenty years or so, concern has heightened that asteroids or comets might strike the Earth, as was proved in Section 18. Those whose orbits could cross Earth are collectively known as Near-Earth Objects (or NEOs). This possibility was heightened by the comet that struck Jupiter, as discussed on the next page. Some federal and private funding has been provided to conduct a systematic search for NEOs. To date hundreds of new asteroids and a few comets have been detected and their orbits calculated. If one is shown to have a good chance to hit the Earth, several ways to stop this have been considered. Sending a space probe with a nuclear device onto the NEO might break it up, but the pieces could continue (like buckshot) on a collision course. The best strategy would be find a way to deflect the orbital path so that the asteroid or comet would have a much reduced chance of meeting the Earth in its orbit. A nuclear explosion away from the NEO could exert enough pressure to modify its orbit. Since calculations have now indicated that an Earth-striking NEO's impact strong enough to cause serious damage may be as frequent as once every thousand years, the NEO detection program has taken on a new urgency.
All this new knowledge about asteroids and comets roaming about out there has raised troubling concerns within the Planetology community about the possibilities of impacts from these projectiles onto the Earth. This has been mention before in Section 18. Proposals to mount a systematic inventory of "heavenly bodies" and their trajectories are now given serious evaluations. The U.S. Congress has called for an effort that would give the world enough warning for our technology to devise schemes to intercept approaching asteroids and comets, steering them to a non-collision course. One group - Pan-STARRS (Panoramic Survey Telescope and Rapid Response Systems) - will utilize four telescopes (3 meter mirrors) at different localities to constantly search for these potentially dangerous projectiles.
A helpful overview of spacecraft visits to asteroids and comets is found during a JPL lecture series webcast. Access this at von Karman lectures, choosing the topic "Comets and Asteroids", June 20, 2002.