Comets and asteroids: A strategy for exploration
The initial phases in the genesis of the solar system are examined. Particular attention is given to asteroids and comets.
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Modest asteroid or comet flyby mission trajectories
The paper discusses modest comet or asteroid missions which require a launch energy of 20 sq km/sq s or less and allow only navigational post launch delta-V's. Objects with perihelia of 2.0 AU are available directly after an earth launch; this distance is extended to 2.4 AU by a Mars gravity assist and the flyby speeds can be reduced. Twenty-one asteroid targets have been identified for Mars gravity assist trajectories using the 1986 Mars opportunity. Two asteroids on direct trajectories with low asteroid flyby speeds and four Mars gravity assist targets are identified as possible rendezvous targets for launches with capability of carrying the rendezvous delta-V engines.
Bender, D. F.↗
The comet rendezvous asteroid flyby mission
The Comet Rendezvous Asteroid Flyby Mission (CRAF) is described. After gravity assists from Venus and Earth, the spacecraft will fly by the asteroid 46 Hestia en route to a rendezvous with P/Tempel 2 in Nov. 1996, when the comet is near aphelion. The scientific experiments for the CRAF mission are: an imaging system; a visual and infrared mapping spectrometer; an infrared radiometer; a penetrator carrying a gamma-ray spectrometer, a scanning differential calorimeter and evolved gas analyzer, accelerometers, and temperature probes; a neutral gas and thermal ion mass spectrometer; a secondary ion mass spectrometer for analyzing dust, gas, and thermal ions; a scanning electron microscope and particle analyzer; an X-ray fluorescence and gas chromatographic analyzer for collected dust and ice samples; a dust counter and velocity analyzer; a retarding potential ion mass spectrometer; a supra-thermal ion mass spectrometer and electron analyzer; a magnetometer; and a coordinated radio, electron, and plasma wave analyzer.
Neugebauer, M.↗
Trajectory options for the Comet Rendezvous Asteroid Flyby mission
The Comet Rendezvous Asteroid Flyby mission is strongly supported by the scientific community. Two major events during the past year have forced a re-analysis of the mission options, with the goal of selecting a baseline mission for launch in 1993. Venus and earth gravity assists can be used to allow rendezvous with comet Tempel 2 in 1996; the scientific potential of the resulting mission is excellent. Although backup mission opportunities have been identified, the significantly longer flight times weaken the scientific appeal of these missions.
Miller, S. L.↗
A possible comet and asteroid link in the formation of comets
IRAS data on particles in the asteroid belt and 'cirrus' clouds at large heliocentric distances are used as the bases for a model of ongoing comet production. It is noted that many particles in the solar system have a radiation pressure/gravitational attraction ratio of 0.5, a condition that holds true for particles in the asteroid belt. Collisionally disturbed into eccentric orbits, the particles could collect frosty grains in the outermost solar system. IRAS revealed the presence of many of these grains in the asteroid belt. It is numerically shown that the agglomeration of many subkilometer radius asteroids deflected into the outer solar system by orbital perturbations induced by Jupiter could provide a nucleating location for the outwardly-migrated grains. Techniques which could be employed to test the hypothesis are discussed.
Odell, C. R.↗
The comet rendezvous asteroid flyby mission
The Comet Rendezvous Asteroid Flyby (CRAF) mission was approved for a New Start by the United States Congress in 1989. CRAF will be developed in parallel with the Cassini (Saturn orbiter/Titan probe) mission. The two missions have been combined into a joint program because of the substantial cost savings (approximately $500 M, or greater than 25 percent) which can be realized by using a common spacecraft design, several identical science instruments, a single management team, and a joint ground operations and data handling system for the two missions. CRAF and Cassini will be the first users of the new Mariner Mark 2 spacecraft which has been designed to carry out the next generation of planetary missions to the outer planets and to small bodies. CRAF is a joint mission between the United States, Germany, and Italy. Each partner will provide both engineering hardware and science experiments. Cassini is a joint mission between the United States, Germany, Italy, and the European Space Agency (ESA), with ESA providing the Titan atmospheric entry probe, called Huygens.
Weissman, Paul↗
Relation of meteor orbits to the orbits of comets and asteroids
Meteor orbit distribution compared with comet and asteroid orbit distribution
COMET↗
Crater production on Venus and Earth by asteroid and comet impact
New calculations of the collision probabilities of asteroids and comets with Venus were carried out based on the orbits of the known Venus-crossing asteroids and comets. For comparison, asteroid and comet collision probabilities and cratering rates on the Earth and Moon were recalculated and the estimated cratering rates on Venus were normalized to those of the Earth.
Shoemaker, Eugene M.↗
Cometary delivery of organic molecules to the early earth
It has long been speculated that earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10 to the 9th to 3.8 x 10 to the 9th years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, it is found that 4.5 x 10 to the 9th years ago earth was accreting intact cometary organics at a rate of at least about 10 to the 6th to 10 to the 7th kilograms per year, a flux that thereafter declined with a half-life of about 10 to the 8th years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, about 3 x 10 to the 12th kilograms and about 6 x 10 to the 14th kilograms, respectively.
Chyba, Christopher F.↗
Deep Mapping of Small Solar System Bodies with Galactic Cosmic Ray Secondary Particle Showers
We will investigate the use of galactic cosmic ray (GCR) secondary particles to probe the deep interiors of small solar system bodies (SSBs), including comets, asteroids, and geologic structures on the surfaces of airless bodies. Applications include solar system science, planetary defense, and resource utilization. Our Phase I study demonstrated that muons, the long-range charged component of GCR showers, can penetrate SSBs up to a km in diameter, providing information on their interior structure. Muons produced in Earth’s atmosphere have been applied to image the interior of large objects for science and engineering. In Phase I, we found that the production of muons in the solid surfaces of airless bodies is much smaller than in Earth’s atmosphere. Nevertheless, the flux of transmitted muons is sufficient to detect inclusions within an asteroid or comet in a reasonable amount of time, ranging from hours to weeks, depending on the size of the SSB and the density contrast, position and size of the inclusion. For asteroids and comets, large density variations (e.g., porous soil or ice versus solid rock) are relatively easy to detect. The intrinsic spatial resolution of muon radiography (“muography”) is on the scale of a few meters. The spatial resolution that can be achieved in practice depends on signal intensity and integration time (counting statistics), the angular resolution of the muon tracker (hodoscope) and details of data reduction and analysis methodology. Our Phase II project will assess remaining unknowns for the application of muography to determining the interior structure of SSBs, assess risks for implementation, and provide a roadmap for development of SSB muography beyond the NIAC program. To achieve our objectives, we will focus on four interrelated tasks: Task1) Signal and background characterization: Characterize the production and transmission of muons and secondary particle backgrounds made by cosmic ray showers in SSBs; and near-surface features from radiographic and tomographic data; Task2) Imaging studies: Develop methods to determine the density structure of SSB interiors and near-surface features from radiographic and tomographic data; Task3) Instrument design: Using simulations and bench-top laboratory experiments, investigate specific concepts for the design of compact hodoscopes and components; Task4) Synthesis: Combine the results of the first three tasks to determine the range of applicability of the method, identify the steps needed for maturation of the concept, and explore concepts for a pilot muography mission.
Prettyman, Thomas H.↗
Studies of asteroids and comets
About 3000 accurate positions of asteroids and comets were measured concentrating particularly on close-Earth approachers, but also targeting asteroids in need for follow-up observations because of unusual orbital characteristics or poor observational history. In 1985, 36 of the discoveries were numbered. Very accurate + or - 0.1 arcsec?) positional measurements were made of P/Halley on four nights in November and December 1985 in support of Giotto mission navigation. The asteroid orbit files were maintained, and ephemeris data for observational planning were supplied to colleagues worldwide. Considerable progress was made in understanding how information on asteroid shapes, surface albedo variegation, and rotational states can be derived from lightcurve and phase curve data. The analytical formulation was completed of a new asteroid magnitude system for the International Astronomical Union; the system was adopted at the New Delhi General Assembly by IAU Commission 20. Photographic photometry of about 10000 images of faint asteroids observed during the course of the United Kingdom Schmidt-CalTech Asteroid Survey in 1981 was continued, completing the microdensitometric scanning and all process of software development for data handling.
Bowell, E. L. G.↗
Asteroid and comet orbits using radar data
For the 30 asteroids and 4 comets for which radar astrometric data were given by Ostro (1991), orbits have been computed using both the radar and the existing optical measurements. The techniques required to process radar data in orbit determination solutions are outlined, and future radar observation opportunities for asteroids and comets are identified. For asteroids and comets that have only short intervals of optical astrometric data, the additional use of only a few radar observations allows a far more accurate extrapolation of their future motions. The use of radar data can often ensure an object's successful recovery at future earth returns and greatly assist efforts in monitoring the motions of the rapidly growing population of known near-earth objects, including their future close-earth approaches.
Yeomans, D. K.↗
Environmental Perturbations Caused by the Impacts of Asteroids and Comets
We review the major mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary following an asteroid impact. We then discuss how the proposed extinction may relate to the impact of asteroids or comets in general. We discuss the limitations of these mechanisms in terms of the spatial scale that may be affected, and the time scale over which the effects may last. Our goal is to provide relatively simple prescriptions for evaluating the importance of colliding objects having a range of energies and compositions. We also identify the many uncertainties concerning the environmental effects of impacts. We conclude that, for impact energies below about 10(exp 4) Mts (megatons of TNT equivalent) - i.e., impact frequencies less than in 6 x 10(exp 4) yr, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively - blast damage, earthquakes, and fires should be important on a scale of 10(exp 4) or 10(exp 5) km (exp 2), which corresponds to the area damaged in many natural disasters of recent history. However, tsunami could be more damaging, flooding a kilometer of coastal plane over entire ocean basins. In the energy range of 10(exp 4) to 10 (exp 5) Mts (intervals up to 3 x 10(exp 5) yr; comets and asteroids with sizes up to 800 m and 1.5 km, respectively) water vapor injections and ozone loss become significant on the global scale. In the submicrometer dust injection fraction from the pulverized target material is much higher than is presently thought to be most likely, then dust injection could be important in this energy range.
Toon, Owen B.↗
Spectroscopy of faint asteroids, satellites, and comets
Nineteen asteroids with orbital elements comparable to those of short-period comets and the outer Jovian satellites J6 Himalia and J7 Elara have been observed with the 228-cm telescope and image-tube spectrograph of Steward Observatory. No activity indicating cometary outgassing was detected. If comets are being perturbed into asteroidal orbits, then this lack of activity can be explained by an apparently short transition time between active and extinct phase. In addition, spectra of the faint comets Sanguin 1977p, Chernykh 1977l, Arend-Rigaux 1950 VII, West 1978a, and van Biesbroeck 1954 IV were obtained, showing CN(0,0) and in some cases C2(1,0) and C3 emission. Comet Arend-Rigaux was active again in the fall of 1977 and Comet West showed on May 31, 1978 a weak tail at a distance from the sun of 6.0 AU.
Degewij, J.↗
The comet rendezvous asteroid flyby mission: A status report
The Comet Rendezvous Asteroid Flyby (CRAF) mission received a new start in fiscal year 1990. CRAF will match orbits with an active short-period comet and follow it around the Sun, making scientific measurements of the nucleus, coma, and tail. The Imaging system will map the nucleus surface at a resolution of 1 meter/line-pair or better, while Visible and Infrared Mapping Spectrometer (VIMS) and Thermal Infrared Radiometer Experiment (TIREX) will produce spectral and thermal maps of the surface. Onboard instruments will collect cometary dust, ice, and gases and perform elemental and molecular analysis. A suite of fields and particles instruments will observe the solar wind interaction with the cometary atmosphere and tail. Radio tracking of the spacecraft will provide an accurate measure of the nucleus mass and higher harmonics in the comet's gravity field. En route to the comet, the spacecraft will make a close flyby of a large asteroid, preferably a primitive type from the outer main belt. Observations at the asteroid include remote sensing mapping of the surface, detection of any solar wind interaction observable at the flyby distance, and measurement of the asteroid mass to better than 10 percent accuracy. Detailed design of the CRAF spacecraft is currently underway at the Jet Propulsion Laboratory (JPL). Recent mass growth has necessitated a switch to Venus-Earth gravity assist type trajectories, similar to that used by the Galileo spacecraft. These trajectories require longer flight times from launch to rendezvous with the target comet. The details of the current baseline mission, spacecraft design, and instrument payload will be reviewed.
Weissman, Paul R.↗
Asteroid and comet surfaces
Photometric and spectrophotometric studies of asteroids and comets are in progress to address questions about the mineralogical relationship between asteroids near the 3:1 Kirkwood gap and ordinary chondrite meteorites and between cometary nuclei and the surface of asteroids. Progress was made on a method to convert the measured excess UV flux in the spectrum of 2201 Oljato to column abundance of OH and CN. Spectral reflectance measurements of large asteroids near the 3:1 Kirkwood gap, which is expected to be the source of ordinary chondrite meteorites, were briefly examined and show no spectral signatures that are characteristic of ordinary chondrite meteorite powders measured in the lab.
Mcfadden, Lucy-Ann↗
Radar observations of asteroids and comets
Radar techniques for the observation of asteroids and comets are reviewed, emphasizing the logical basis for inferring physical properties from radar measurements. Results to date are reviewed, focusing on some recent highlights of the research to demonstrate the synergism between radar and other ground-based techniques. Particular attention is given to the asteroids 2 Pallas, 16 Psyche, 2101 Adonis, and the comet IRAS-Araki-Alcock.
Ostro, S. J.↗
Predicting Close Earth Approaches of Asteroids and Comets
The motions of all known Earth approaching asteroids and comets with reasonably secure orbits have been numerically integrated forward in time to A.D. 2200. Special care was taken to use the best available initial conditions including orbits based upon radar data.
Comets vaporizing ices Earth asteroid 2340 Hathor↗