CONTOUR popular article 052002.doc CONTOUR - The Tale of two comets Launch anecdote or description Define Comet Nucleus Tour (CONTOUR) and mention APL, Cornell, JPL, Joe Veverka as P.I. CONTOUR OSERVING GOALS The genius of the CONTOUR mission design allows the spacecraft to employ a series of Earth swingby maneuvers to reshape its orbit to intercept at least two short period comets. Rather than the spacecraft chasing the comets around the other side of the solar system, these Earth swingbys maneuvers will allow the spacecraft to wait for the comets to come to it, thus ensuring that the comets are relatively near the Earth at the encounter times. This proximity to Earth will not only facilitate communication with the spacecraft but Earth-based observations will be that much easier. Professional and amateur astronomers are being asked to monitor the comet's large-scale atmospheric activity while the spacecraft is carefully examining the heart of the comet - its nucleus. Figure 1 is a schematic drawing showing the spacecraft's sequence of events beginning with its launch from Cape Canaveral on July 1, 2002. The spacecraft will then spend a month and a half in Earth orbit awaiting just the right celestial circumstances before departing Earth with an injection maneuver on August 15, 2002. The spacecraft's orbit returns it to Earth a year later and this first Earth swingby will reshape the spacecraft's orbit for the flyby of comet Encke on November 12, 2003. Any spacecraft departing Earth will initially have a trajectory that is similar to the nearly circular motion of the Earth about the Sun. Because comet Encke's orbit is quite eccentric and inclined to the Earth's orbital plane by about 12 degrees, the CONTOUR spacecraft will flyby Encke at the relatively high velocity of 28 km per second. The Encke flyby trajectory was carefully chosen to make the spacecraft return once again to the Earth's neighborhood in August 2004 and then two more times in February of 2005 and 2006 before flying past comet Schwassmann-Wachmann 3 on June 19, 2006. This time the flyby velocity is about 14 km per second, about half that at the Encke encounter. At these extraordinary velocities, the CONTOUR spacecraft must be carefully protected from even the tiniest cometary dust particle hits [see sidebar]. Both comets Encke and Schwassmann-Wachmann 3 are short period comets that orbit the sun every few years and whose motions are confined to the inner solar system. If the spacecraft is healthy after the second short period comet flyby and the resources are made available by NASA, CONTOUR could be targeted to a third comet that might be either another short period comet or possibly a long period comet that is arriving for the first time from the outermost edge of our solar system. This latter option would allow a comparison of two aging short period comets with a young fresh long-period comet. While a number of attractive old short periodic comets (e.g. comet d'Arrest) have been identified as viable third comet targets, a flyby of the CONTOUR spacecraft past a new comet will depend on the future discovery of a suitable comet with the appropriate orbital characteristics that would allow the CONTOUR spacecraft to intercept it. Each of the target bodies will be observed with the same imaging and chemical composition instruments making comparisons of the results that much easier. DIVERSITY OF THE CONTOUR TARGET COMETS The primary goal of the CONTOUR mission is to study the diversity of comets and the two target bodies, comets 2P/Encke and 73P/Schwassmann-Wachmann 3 could not be more different. In November of 2003 when the CONTOUR spacecraft flies within 100 miles of comet 2P/Encke, the images will reveal an old, stable, highly evolved comet that is in transition from an active comet to an ex-cometary asteroid that has lost its ability to outgas. Comet 73P/Schwassmann-Wachmann 3 is younger and far more unpredictable than Encke, having split off at least three pieces in late 1995 for no apparent reason. SW3 is likely to be a very fragile object and may offer a glimpse of a freshly cleaved interior as the CONTOUR spacecraft flies closely past in June of 2006. Comet 2P/Encke, the second comet after 1P/Halley to be identified as having a short orbital period about the sun, has the shortest period of any comet (i.e., 3.3 years) and its orbital motion can carry it no closer than 0.9 AU from the planet Jupiter. Without gravitational tugs by Jupiter, the orbit of comet Encke has changed very little over the previous several millennia. It routinely passes within 0.34 AU of the sun and, among short period comets, only comet 96P/Machholz 1 has a closer perihelion passage distance at 0.12 AU. As a result of is proximity to the sun, its short orbital period and its avoidance of Jupiter's gravitational effects, the nucleus of comet Encke is likely to be a highly evolved, relatively old, dark body that has lost most of its volatile ices. It is probably well on its way to becoming an inert ex-comet, indistinguishable from an asteroid. More than once during its history, comet Encke has been observed as a naked eye object but unlike the other naked-eye short periodic comets, 1P/Halley, 109P/Swift-Tuttle, and 55P/Tempel-Tuttle, there is no record of any sightings in the ancient Chinese records. It may be that comet Encke was dormant for several centuries prior to its discovery in 1786. Comet 73P/Schwassmann-Wachmann 3 (hereafter SW3) was discovered on 1930 May 2 by Arnold Schwassmann and Arno Arthur Wachmann at the Bergedorf observatory near Hamburg Germany (see figure ?). The intrinsic faintness of the comet, a less than secure orbit, and unfavorable observing conditions for the 1935-36 perihelion return prevented the comet from being recovered for the next eight returns to perihelion and it was not re-discovered until 1979 when the observing conditions were the most favorable since the 1930 discovery apparition. Because of its poor observing circumstances, the comet was missed at its next return to perihelion in 1985-86 but it has been observed during its last three apparitions in 1989-90, 1994-96, and 2000-01. In 1989, the JPL astronomer Zdenek Sekanina studied the coma fans of ejected gas and dust associated with comet SW3 and noted that the fan's axis was pointing in the general direction of, but not exactly at, the sun. Presumably, these active areas result when localized regions of ices become exposed to sunlight. Sekanina concluded that this type of feature (also noted for comet Encke) is characteristic of one or more active regions located near the sunlit rotation pole of the nucleus. The continuous nature of the fan-like structure requires that the nucleus spin axis be pointed continuously toward the sun near perihelion and hence the spin pole must be near the comet's orbital plane. However the motion of the nucleus fan-axis could not be fit to a model where the spin axis is fixed in space. It seems likely that this comet's rotation pole is not fixed in one direction but rather wandering. [Illustration of Encke fan structure] The fact that the nucleus of SW3 split into several pieces for no obvious reason in late 1995 suggests that this cometary nucleus is extremely fragile and that freshly cleaved surfaces may reveal the interior structure of this comet when the CONTOUR spacecraft flies by on June 19, 2006. The exposure of fresh ices on the comet might also explain the comet's apparent brightening of 2-3 magnitudes noted in the 2000 observations when compared with those made before the comet split in 1995. Based upon observations taken before the comet split in 1995, the radius of the nucleus was estimated to be less than 1.1 km. The surface of comet SW3 is likely crusted over to a large degree with only a few percent of its surface area active. The Earth close approach to within 0.08 AU on May 12, 2006 is of particular interest being one of only six known cometary close Earth approaches to within 0.1 AU in the twenty first century. This Earth close approach will allow an extensive ground-based observing campaign to complement the spacecraft observations. THE CONTOUR SPACECRAFT PAYLOAD The CONTOUR spacecraft is carrying two camera systems that will image the comets and allow detailed close up views of their nucleus' size, shape, surface features, and rotation characteristics. These cameras are also critically important for guiding the spacecraft to the proper trajectory to effect a close flyby on the sunward side of each nucleus. The larger of the two camera systems is the CONTOUR Forward Imager (CFI) with an aperture of 30 cm. It will be used both to image the cometary nuclei from great distances to navigate the spacecraft for a close approach and to image features in the cometary atmosphere, or coma. The CONTOUR Remote Imager and Spectrometer (CRISP) will undertake most of the high resolution imaging of the nucleus. The CRISP camera will be used to map the cometary nuclei with a clarity, or image resolution, several times better than that achieved by the spacecraft that flew past comet Halley in March 1986 and past comet Borrelly in September 2001. Both the CFI and CRISP cameras have the capability to image the comet's coma and nucleus at several different wavelength regions. These spectroscopic measurements will be used to discern whether particular regions of the coma and nucleus surface have the differing spectral responses that would indicate there are variations in the composition in various regions of the coma or at different locations on the surface of the comet. If there are compositional differences noted, the CFI and CRISP measurements will try to correlate these with jets or features in the coma or with specific regions on the surface of the nucleus. [Halley and Borrelly nucleus images] The two primary composition instruments on board the CONTOUR spacecraft are the Neutral Gas and Ion Mass Spectrometer (NGIMS) and the Comet Impact Dust Analyzer (CIDA). During the CONTOUR spacecraft fast flybys, the neutral gases from the comet's coma will enter the NIGMS instrument at high speed. The atoms and molecules in these gases are stripped of an electron (ionized) and then sent down a tube in the instrument that measures the quantity of each species that have a particular total count of protons and neutrons. That is to say, NIGMS keeps track of the number of atoms or molecules that it encounters by recording, for each particle, the ratio of its number of protons and neutrons (atomic weight) to its total electric charge. The CIDA instrument takes advantage of the extraordinary encounter velocities to break down tiny cometary dust particles into ionized atoms and molecules. These charged ions are then electrostatically accelerated down two drift tubes with the heavier ions taking a longer time to reach the detector at the end of the second tube. By counting the number of each species with the same time-of-flight to reach the detector, CIDA can determine the abundance of various atoms and molecules that originally made up the colliding dust particles. By comparing and interpreting the results of both the NIGMS and CIDA instruments, scientists will be able to determine the chemical makeup of the cometary dust and gas and hence infer the chemical composition of the nucleus itself. IMPORTANCE OF COMETS - THE BUILDING BLOCKS OF THE SOLAR SYSTEM AND LIFE Since comets are the leftover bits and pieces from the outer solar system formation process and they are among the least changed objects within our solar system, the CONTOUR spacecraft measurements will help establish the original chemical mix from which the major planets like Jupiter, Saturn, Uranus and Neptune formed some 4.6 billion years ago. With knowledge of the chemical mix from which these planets began, together with an understanding of their current compositions, planetary theorists will have the beginning and end points for these planet's compositions. Hence they should be able to better define the evolutionary processes that have taken place in each planet's lifetime. The CONTOUR mission will make key measurements that should help scientists to determine to what extent collisions of short period comets with the Earth have delivered the Earth's oceans and atmosphere and the carbon-based molecules that are necessary for the development of life. One of these measurements will be the determination of the ratio of deuterium to hydrogen in each of the target comets. Deuterium is a stable isotope of hydrogen that contains a neutron as well as a proton in the hydrogen atom's nucleus. This deuterium to hydrogen abundance ratio (D/H) has been measured for three comets to date (Halley, Hale-Bopp, and Hyakutake) and the results show that this D/H ratio is twice that found in the Earth's seawater. Hence comets like these three, which arrived from the Oort cloud at the very edge of our solar system, could not have contributed all the Earth's water supplies. Could Earth collisions of short period comets like Encke and SW3 have supplied a portion of Earth's water? Unlike comets Halley, Hale-Bopp and Hyakutake, the CONTOUR target comets arrived from the Kuiper belt near Pluto's orbit and not from the far more distant Oort cloud. The NIGMS instrument will also examine several abundance ratios in the cometary coma to see if these ratios match those found in the Earth's atmosphere. For example, if the abundance ratios of the very stable noble elements xenon and krypton are the same in both the comet's coma and in the Earth's atmosphere, that would be evidence suggesting that the early Earth's atmosphere was largely obtained from a series of cometary collisions. The CIDA and NIGMS results may also help identify whether or not the abundance ratios of various carbon-based molecules in the cometary dust and gas are consistent with those same ratios on the Earth's surface. Since all life on Earth is composed primarily of water and carbon-based molecules, CONTOUR may be able to identify cometary collisions as a vehicle by which Earth received its supply of life's building blocks. It may well be that comets were both the building blocks of the outer solar system planets and providers for the building blocks of life on Earth. THE IMPORTANCE OF COMETS - POTENTIAL FUTURE THREATS AND POSSIBLE RESOURCES Cometary collisions with Earth are a bit worrisome because there may not be sufficient warning time between the discovery of an assailant comet and its encounter with Earth. The number of close Earth encounters is far larger for asteroids than it is for the less numerous comets and the collision rate of near-Earth asteroids is about 10 times that for Earth approaching comets. However, the ongoing searches for near-Earth asteroids will likely allow them to be discovered many years prior to any predicted Earth collision, thus allowing time to deflect the assailant asteroid away from Earth. The same cannot be said for comets since many of them arrive, announced, from the very edge of our solar system and are not discovered until then begin to become active inside the orbit of Jupiter (see Sidebar). A comet on a nearly parabolic path will take but 9 months to travel the distance between the orbits of Jupiter and the Earth and this is probably not enough time to mount an effective mitigation campaign. To make matters worse, an average long period comet would be expected to encounter Earth at some 50 km per second versus 15 km per sec for an average near-Earth asteroid. Hence long period comets would have about 11 times the energy upon impact, as would a typical near-Earth asteroid of the same mass. The resulting damage would be far worse for the cometary collision. The situation is not so bad for the short periodic comets that spend their lives within the inner solar system but even these objects can collide with Earth at a velocity nearly double that for a typical near-Earth asteroid. If we are to provide the necessary lead-time to deal with Earth threatening comets, we must redouble our efforts to find the long-period comets farther out in the solar system. We must also understand their structures since in trying to deflect an Earth approaching comet, it would make a big difference whether we were dealing with a fragile, fluffy collections of snowballs or one large, solid, ice ball. We must get to know the enemy before a threatening object is identified. The imaging observations provided by the CONTOUR spacecraft will go a long way toward identifying the structures of two very different cometary nuclei. Are these cometary nuclei collections of loosely bound smaller cometesimals or are they monolithic dirty ice balls? It is ironic that those short periodic comets that can most closely approach the Earth are also the ones that are most accessible to spacecraft missions. As a rule of thumb, the more Earth-like an object's orbit, the easier it is to effect a rendezvous mission with that object. Evidence from the five spacecraft that flew past comet Halley in March 1986 suggested that this comet, and likely others, are about 50% water ice so that comets are a natural supply for life sustaining water when the colonization of the inner solar system begins. Water can be broken down into hydrogen and oxygen and thus provide the ingredients for the most efficient type of rocket fuel. When the colonization of the inner solar system begins, accessible short period comets like those visited by the CONTOUR spacecraft may well serve as the watering holes and fueling stations for interplanetary travelers. A very rare Earth collision by a large comet could end life as we know it one day but at the same time comets must be considered as likely sources for the raw materials that will allow the future colonization of the inner solar system. AMATEUR CONTRIBUTIONS SOLICITED The imaging cameras together with the gas and dust composition instruments on board the CONTOUR spacecraft will provide major improvements in our understanding of the cometary nuclei and the near-surface coma features. These close-up spacecraft observations will be that much more valuable if there is a corresponding set of ground-based observations of the comet's large-scale coma features to correlate with the spacecraft observations. For example, the combination of close-up spacecraft and remote ground-based observations offers a unique opportunity to identify the source regions and the mechanisms by which the coma's large-scale features are formed. How many and where are the active areas on the surface of comet Encke and what is the mechanism by which comet Encke's large scale fan structures are formed? Can the anomalous brightening of comet Schwassmann-Wachmann 3 in 2000 and 2001 be explained by the exposure of fresh ices to sunlight following the comet's splitting in late 1995? The comet Encke and Schwassmann 3 encounters take place fairly close to the Earth so there will be ample opportunities to image both comets using modest equipment. About the time of the comet Encke encounter on November 12, 2003, the comet will be in the constellation Cygnus observable as a 7.3 magnitude object in the pre-dawn sky. This encounter will take place some 48 days prior to perihelion and 5 days prior to its Earth close approach to within 0.26 AU on November 17, 2006. For the CONTOUR encounter with comet Schwassmann-Wachmann 3 on June 19, 2006 the 6th magnitude comet should be easily observable in Cetus in the pre-dawn sky. A month earlier on May 13, 2006, the comet could be more than two magnitudes brighter in the pre-dawn sky when it makes its Earth close approach to within 0.076 AU, the fifth closest known cometary Earth approach in the 21st century. Detailed ephemerides and interactive orbital movies for these comets and all other comets and asteroids are available on JPL's near-Earth object web site at: http://neo.jpl.nasa.gov Once the CONTOUR spacecraft is safely past its flyby of comet Schwassmann-Wachmann 3 in mid-2006, it would be possible to add another target body to the CONTOUR mission schedule. Ideally, a fresh long-period comet could be found prior to the February 2006 Earth close approach allowing the trajectory leading up to the Schwassmann-Wachmann 3 encounter to be modified to undertake a flyby of a long-period comet. NASA supported professional search teams currently dominate the discoveries of near-Earth asteroids. However by looking where the professionals rarely do, amateur astronomers observing in the pre-dawn and post-twilight skies are still making significant cometary discoveries. In fact, amateur astronomers discovered all the most impressive naked eye comets in recent memory, Hyakutake, Hale-Bopp, and Ikeya-Zhang. As an inducement to amateur comet hunters, the discoverer of the long-period comet that is approached by the CONTOUR spacecraft will be invited to join the CONTOUR science team to witness the excitement of that cometary encounter. Comets among the solar system's smallest bodies but their importance is in no way proportional to their sizes. Next to the sun itself, theirs is the most important realm SIDEBAR: A DECADE OF COMETARY EXPLORATION The comets to be visited by the CONTOUR spacecraft are not the only ones to be visited by spacecraft in the coming years. Beginning with the successful September 2001 encounter of Comet Borrelly by the Deep Space 1 spacecraft, there will be encounters with comet Encke in November 2003 and Schwassmann-Wachmann 3 in June 2006 by the CONTOUR spacecraft. The Stardust spacecraft will fly past comet Wild 2 in early January 2004 and return a dust sample to Earth two years later. A new crater will be formed on comet Tempel 1 when the Deep Impact impactor spacecraft collides with this comet on July 4, 2005. Finally, the Rosetta spacecraft of the European Space Agency will catch up with, and land upon the surface of, comet Wirtanen in late 2011. The Deep Space 1 spacecraft passed within 2140 km of short period comet 19P/Borrelly on September 22, 2001 and the spacecraft images (figure ??) revealed a potato-shaped nucleus emitting collimated jets of gas with the interior ices of the nucleus covered with a dry, jet-black matrix of carbon based materials. Compared to the Borrelly images, CONTOUR will provide more than a factor of ten improvement in the image resolution, or clarity, for comets Encke and Schwassmann-Wachmann 3. When the Stardust spacecraft flies past comet Wild 2, it will not only image this comet's nucleus but it will also capture entire dust particles in a very under-dense silicate foam material (aerogel) and bring these particles back to Earth for chemical analysis. Although lightweight, compact spacecraft instruments can go a long way toward identifying the chemical constituents of cometary dust, there is no substitute for the comprehensive and detailed chemical analysis provide by the far larger and more sophisticated instruments in Earth-based laboratories. The Deep Impact mission will not only investigate the surface of comet Tempel 1 as it flies by, but a separate instrumented impacter will actually run into the comet. Without using any explosives, the impacter's high speed collision will generate energy of about 40% that of the 1945 Hiroshima nuclear explosion and create its own crater the size of a football field on the comet's surface. Before being destroyed during the surface impact, an on board camera will image surface features down to a resolution of only 15 cm. The Deep Impact flyby spacecraft will follow the impactor to the comet, observe the crater being formed, and hopefully get a look at the interior regions of this comet. By running into a comet before one has a chance to run into us, we should learn about a comet's interior structure and hence have a better understanding on how to deal with any comet found on an Earth threatening trajectory. While the Deep Space 1, CONTOUR, Stardust and Deep Impact spacecraft will all fly quickly past their cometary targets, the Rosetta spacecraft will chase comet Wirtanen around the inner solar system for eight years and finally catch up with, orbit, then land upon the surface of this small, but active comet. The Rosetta science instruments should allow a very detailed, close up view of this comet and discern its surface characteristics, along with its structure, composition, mass, density, and porosity. With a total of 6 different comets to be visited by spacecraft, the first decade of the twenty first century will be recorded as the great period of exploration for the comets of our solar system Why water? - Water world Ubiquitous water in solar system. Hydrogen and helium make up 99 of solar system with O and Carbon having about 0.1 and 0.05% of H. He doesn't combine well with other elements so next most abundant element that does combine is O - hence lots of water everywhere where it is cold enough to condense out of water vapor before the water vapor is removed by sun's T Tauri phase. SIDEBAR OBSERVING STEALTH COMETS IN A COSMIC SHOOTING GALLERY Mother Nature has conspired against researchers who wish to study the nuclei, or cores, of comets. As these nuclei enter the inner solar system where they can be observed, they throw up gas and dust to hide their features from Earth based observers. The water ice that comprises much of a comet's nucleus becomes warm enough to vaporize between the orbits of Jupiter and Mars. On its way into the inner solar system, a comet's outgasing activity generates an atmosphere, or coma, which effectively hides its nucleus. On its way out of the Earth's neighborhood, this activity shuts down near the orbit of Jupiter exposing the nucleus but by then the comet is too far from Earth to allow comprehensive observations. Spacecraft observations are necessary to unveil the secrets of the cometary nuclei but even then Mother Nature conspires against these observations by presenting blacker than black nuclei that throw up clouds of dust that could easily cripple an unprotected flyby spacecraft. For example, the CONTOUR spacecraft will fly past the nucleus of comet Encke at some 28 km per second, or 63,000 miles per hour. At that speed you could fly coast to coast in just over two minutes! While the dust particles associated with comets are tiny, flyby spacecraft velocities are so great that even these tiny particles have extraordinarily destructive powers. The CONTOUR spacecraft has been provided with a dust shield designed to withstand a dust particle hit 150 times more energetic than a 22 caliber rifle shot. This dust protection device is termed a "Whipple shield", after a design by CONTOUR science team member and well-known cometary scientist, Fred Whipple. It was Fred Whipple, who first correctly suggested in 1950 that a comet's nucleus is an icy body with embedded dust particles. The CONTOUR Whipple shield consists of four sheets of Nextel ceramic fabric separated from one another by a few centimeters positioned out in front of seven closely layered sheets of Kevlar. Kevlar is the material used to manufacture bulletproof vests. The Nextel fabric sheets are not meant to stop a dust particle but simply to fragment it at each layer. Thus, the successive layers of Nextel fabric will have to deal with more and more, but smaller and smaller, fragments. For the fragments of the original dust particle that make it all the way through the final layer of Nextel fabric, the seven layers of Kevlar should provide the final line of defense for the CONTOUR spacecraft.