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A simplified model of nucleon-nucleus reaction is developed and some of its properties are examined. Comparisons with proton production measured for targets of Al-27, Ni-58, Zr-90, and Bi-209 show some hope for developing an accurate model for these complex reactions. It is suggested that binding effects are the next step required for further development.
A comet nucleus sample return mission has been proposed for implementation near the end of this century. The objective of the mission is to collect a sample of undisturbed material from beneath the surface of an active comet and return it to earth in a minimally altered state. Potential targets include the short-period comets Encke, Tempel 2 and Wild 2. This paper defines such a mission and describes its requirements with regard to science, sampling, thermal protection and performance. It has been determined that with present launch capabilities, Solar Electric Propulsion (SEP) is an enabling technology for this mission and total program costs would be in the 700 million to 1 billion (FY '84) dollar range.
Results of ten nongravitational orbital solutions are given for Comet Brooks 2, based on observations from the apparitions 1889-1980. The time history of the nongravitational parameters, including the transverse component A-sub-2, is vital for modeling the nucleus precession. The comet is obviously a fragment of a once larger object that split during a close encounter with Jupiter in 1886. Factors possibly responsible for the breakup are examined. A similarity between the orbital histories of P/Brooks 2 and P/Wild 2 is noted.
It is shown that information on the chemical composition of cometary snows can be inferred from the distance r(0) between sublimating states in the cometary nucleus. Consideration is given to three techniques for measuring r(0): estimation of the dependence on distance of non-gravitational forces (NGF); estimation of the dependence on distance of molecular emissions; and (3), analysis of the cometary light curve. The dependence on distance of the NGFs suggests that the observed sublimations of short-period coments are determined by water snow. Light curves of newly discovered comets appear to confirm this result. The large production rates of H and OH in cometary atmospheres suggest that they are due to dissociation of H2O in the vapor states. Estimates of r(0) for eleven different comets are given in a table.
The Comet Rendezvous Asteroid Flyby (CRAF) spacecraft, to be launched in 1991, was designed to study the physical and chemical properties of cometary and asteroid bodies. It is proposed that these properties can be determined by utilizing a penetrator experiment delivery system aboard the CRAF, which would deliver a passive gamma ray spectrometer to the comet and determine the composition of subsurface materials. Results of calculations for a model comet (50 percent CI carbonaceous chrondrite, 50 percent ices) provide data on the expected gamma ray fluxes, the minimum detectable limits, and the detection uncertainties for 35 isotopes. These results confirm that this technique can determine the composition of both the rocky and ice components, and the rock-to-ice ratio in the nucleus. In addition, useful information regarding the formation of the solar system is provided.
The detection of a strong, and possibly broad, Paschen-alpha line from the narrow-line radio galaxy 3C 219 is reported. The detected flux is larger than predicted from the H-alpha line and the case B recombination. This implies the presence of a highly reddened line-emitting region in the nucleus.
The flight dynamics of a Penetrator probe which, as part of a Comet Rendezvous and Asteroid Flyby mission will impact a known periodic comet, possibly Hempel 2, and penetrate beneath the surface to obtain measurements, is discussed. The mission is described, including the penetrator-separation dynamics, the dynamics during initial coast and rocket thrust phase, and the final coast to impact. The effects on the results that may be caused by the comet nucleus gravity field variations, uncertainty in the spacecraft state vector, the spacecraft pointing accuracy, initial separation errors, and rocket motor thrust errors are examined.
Three modes for obtaining samples of cometary material: remote sampling, hover mode sampling, and landed (anchored) sampling are discussed. For each mode, trade-offs involving complexity, risk factors, and cost must be analyzed for the spacecraft and the sampling system. Flyby results for comet Halley indicate heterogeneity in the cometary nucleus that could dictate sampling at several different locations. Coring drills, drive tubes, and specialized mantle sampling devices must operate at cold temperatures (100 to 180 K) without heating the sample. Sample selections must be accomodated by the sample return canister, which must also be protected from thermal excursions.
Utilizing the electron and magnetic field data from the ICE tail traversal of comet Giacobini-Zinner along with the MHD equations, a steady state, stress balance model of the cometary magnetotail was developed, and used to infer important but unmeasured ion properties within the magnetotail at ICE and upstream at the average point along each streamline where cometary ions are picked-up. The derived tailward ion flow speed at ICE is quite constant at approx. -20 to -30 km/sec across the entire tail. The flow velocity, ion temperature, density, and ion source rates upstream from the lobes (current sheet) at the average pick-up locations are approx. -75 km/sec (approx. -12), approx. 4 million K (approx. 100,000), approx. 20 cc (approx. 400), and approx. 15 cu cm/sec. Gradients in the plasma properties between the two regions are quite strong. Implications of inferred plasma properties for the near-nucleus region and for cometary magnetotail formation are examined.
Images obtained by the Halley multicolor camera were used to determine the projected size and shape of the nucleus. The location of the terminator and numerous surface features were determined. There is good correlation between the brightest surface features and the dust jets; however, many bright features are seen which are not associated with jets. Most of the observed features are circular and appear to be related to surface elevation. The angularity of the terminator gives an indication of the three-dimensional structure of the face which was observed.
Images obtained by the Halley multicolor camera using the clear filter with a pass band from 300 to 1000 nm were used to study dust activity in the comet nucleus. Comparisons with ground based observations confirm that dust production towards the Sun increases in activity relative to the southern background source while the Giotto spacecraft was approaching. This is in agreement with the assumption that the sunward activity becomes stronger when the source rotates towards the Sun. Estimated dust column density is 90 billion/sqm, with optical thickness less than or = 0.3. Surface reflectivity is less than 1%, indicating a very rough surface with large fractions of shadowed areas.
Numerical simulations of the structure of aggregates of grains in the solar system show they are fractals with low density and irregular shapes. Comet nuclei forming from a mixture of ice and dust grains are likely to have nonuniform composition as well as structure. Such a nucleus appears to account for observed characteristic of comets. The structure has significant implications for the evolution of cometary nuclei.
Simultaneous visible and infrared observations of comet P/Neujmin 1 1984c are presented which show that the comet has a large (mean radius 10 km), dark (geometric albedo 2-3 percent) nucleus with a surface which is mostly inert material but which still shows a low level of gaseous activity. This is the first physical evidence that cometary nuclei can leave behind an inert body after the coma activity ceases. No asteroid or asteroid class has been found to match the reflectance and albedo of this comet except possibly some D asteroids.
Simultaneous broadband visual and infrared photometry of comet P/Arend-Rigaux is reported. Its VJHK colors are similar to other periodic comets and the class D asteroids. The visual flux decreased by 0.5 mag within 2 hr, while the thermal flux at 10 microns continued to decrease by 0.7 mag over 3 hr. These observations are consistent with thermal models of a nucleus with a geometric albedo of 0.03 and equivalent radii of 5.1 and 3.8 km.
A penetrator system has been suggested as an approach for making in situ measurements of the composition and physical properties of the nucleus of a comet. This study has examined in detail the feasibility of implementing the penetrator concept. The penetrator system and mission designs have been developed and iterated in sufficient detail to provide a high level of confidence that the concept can be implemented within the constraints of the Mariner Mark 2 spacecraft.
A joint ESA/NASA proposal for the acquisition and delivery of samples of the cometary nucleus is considered, and the importance of such samples in providing input as to the origin of the solar system is underlined. Mission options employing all-chemical propulsion, solar electric propulsion, and a combination of the two are explored in detail. Conceptual spacecraft designs have been developed and mass performance estimates made for a wide range of possible targets. The importance of the use of some form of aerobraking during return to earth is noted. Methods for acquiring the nuclear samples are discussed, and characteristics of the mission operations are reviewed.
X-ray observations of M81 made with the Einstein satellite are analyzed, and the results are reported. Eight bright sources other than the nucleus were detected, with luminosities in excess of about 2 x 10 to the 38th ergs/s. These are probably young, massive, accreting binary systems. A comparison of M81 with the morphologically similar galaxy M31 shows that the former is overluminous in both the radio continuum and the X-ray emission and to a lesser extent the far-infared emission, relative to the optical luminosity. This points to a difference in the star-formation histories of the two galaxies. The individual X-ray sources detected in M81 are all more luminous than the most luminous sources of M31. The nuclear source in M81 has a very soft and absorbed X-ray spectrum which cannot be explained by either inverse Compton radiation of the compact nuclear radio source or by an extension of the radio synchrotron emission.
High-resolution far-infrared observations of the galaxy NGC 4945 have been obtained from the Kuiper Airborne Observatory. Using new observational techniques and nonlinear deconvolution routines, it is found that virtually all of the far-infrared luminosity originates from a nuclear source no larger than 12 arcsec x 9 arcsec (225 pc x 170 pc) in extent. This size constraint, coupled with the far-infrared dust temperature, indicates that the source is deeply embedded in dust: the lower limit for the 100 micron optical depth is 0.35, which is by far the largest yet measured in an external galaxy. Published optical spectra of NGC 4945 reveal a heavily obscured nonthermal source which exhibits broad line profiles typical of a Seyfert 2 active nucleus; it is concluded that the far-infrared emission is probably due to thermal radiation from dust grains surrounding the nonthermal nuclear source. A compact cluster of massive young stars may also contribute to the infrared luminosity, but the evidence for such star-forming activity is weak.