A Comment on "The Far Future of Exoplanet Direct Characterization" - the Case for Interstellar Space Probes

Following on from ideas presented in a recent paper by Schneider et al. (2010) on "The Far Future of Exoplanet Direct Characterization", I argue that they have exaggerated the technical obstacles to performing such 'direct characterization' by means of fast (order 0.1c) interstellar space probes. A brief summary of rapid interstellar spaceflight concepts that may be found in the literature is presented. I argue that the presence of interstellar dust grains, while certainly something which will need to be allowed for in interstellar vehicle design, is unlikely to be the kind of 'show stopper' suggested by Schneider et al. Astrobiology as a discipline would be a major beneficiary of developing an interstellar spaceflight capability, albeit in the longer term, and I argue that astrobiologists should keep an open mind to the possibilities.Comment: Accepted for publication in Astrobiolog

X-ray spectra of Hercules X-1. 3: Pulse phase dependence in high energy continuum

Pulse phase-dependent spectral changes in the high energy (less than 20 keV) continuum of Hercules X-1 were observed. Cyclotron absorption of underlying continua can reproduce the observed angular dependence in the high energy cutoff. Implications of this model, which include the possibility of determining the angular separation between the line of sight and the neutron star magnetic field if the absorbing electron spectrum is known are discussed

IN VITRO STIMULATION OF ANTIBODY FORMATION BY PERITONEAL CELLS : II. CELL INTERACTIONS AND EFFECTS OF IMMUNOCHEMICAL OR METABOLIC INHIBITORS

Peritoneal cells (PC) from normal, unimmunized mice were placed in ultra-thin monolayer cultures containing carboxymethylcellulose (CMC), sheep red blood cells (SRBC), and complement, and tested for the appearance of plaques of lysis. The behavior of PC from young male mice and from female mice that had given birth to several litters (retired breeder mice) was studied. It was found that cells from spleen, mesenteric lymph node, thymus, bone marrow, thoracic duct lymph, or Peyer's patches could not form plaques in the CMC microcultures. Also, various combinations of these cells did not lead to plaque formation. When cells from any of these sources were mixed with PC, there was either no effect or an actual inhibition of plaque formation, the plaque counts being lower than would have been expected from the number of PC present in the mixture. Optimal plaque formation by peritoneal cells was found to be dependent on an optimal cell concentration, this optimum being around 5 x 106/ml for young male mice and 0.5 x 106/ml for retired breeders. Inhibition of plaque formation was found with either supra- or suboptimal cell concentrations. The inhibition by excess cell concentration may have been a simple nutritional or nonspecific overcrowding effect, as it could also be induced by an addition of an excess of spleen or lymph node cells. The failure of more dilute PC preparations to give adequate numbers of plaques appeared to be more specific, as plaque numbers could not be restored to normal by addition of spleen cells. The suggestion was that some cell to cell interaction between PC was involved. This dependence on cell concentration was not seen with immunized spleen PFC. Plaque appearance could be specifically and reversibly suppressed by placing PC in a medium containing rabbit anti-mouse IgM serum. Anti-IgG serum had no such effect. These experiments strengthened our view, expressed in the accompanying paper, that plaque formation was due to the formation of IgM, hemolytic antibody to SRBC by the PC. Metabolic inhibitors were incorporated into monolayer cultures and had different effects with the different types of PFC used. In the case of spleen cells from mice actively immunized against SRBC 4 days before killing, actinomycin D had no effect on plaque counts and puromycin reduced plaque numbers by a factor of 2. In the case of PC from young male mice, actinomycin D in concentrations above 0.01 µg/ml caused reductions down to < 2% of control values in plaque counts, and puromycin (10 µg/ml) had a similar effect. The PC from retired breeder mice occupied an intermediate position between the two cases just discussed. A compartment of cells, equal to about one-fifth of the total normal PFC compartment, was identified as resistant to high concentrations of either actinomycin D or puromycin, being similar in these respects to PFC from spleens of intentionally preimmunized mice. The mitotic poison, Colcemid, did not affect plaque counts in any situation tested. The theoretical implications of these results are briefly discussed

IN VITRO STIMULATION OF ANTIBODY FORMATION BY PERITONEAL CELLS : I. PLAQUE TECHNIQUE OF HIGH SENSITIVITY ENABLING ACCESS TO THE CELLS

An improved method for the short-term culture of mouse peritoneal cells in a medium containing carboxymethylcellulose (CMC), sheep erythrocytes (SRBC), and guinea pig complement is described. It involves preparation of microcultures, of thickness 12–15 µ and volume 3.6 µl, under paraffin oil. With such cultures, peritoneal cells from normal, unimmunized young male CBA mice give about 3000 hemolytic plaques per million cells cultured, this figure being attained within 24 hr. The plaque detection method is about four times as sensitive as the Jerne technique. A method is described whereby such plaque-forming cells (PFC) can be transferred, by micromanipulation, to fresh monolayer cultures containing SRBC, CMC, and complement. In this fashion, the secretory capacity and susceptibility to inhibitors of peritoneal PFC can be tested in detail. Using this technique, evidence is presented that the hemolytic substance responsible for plaque formation is actually secreted by the cell at the center of the plaque, and is not a complement component but probably an antibody. Studies on the time of plaque appearance after cell transfer, and the subsequent growth rate of the zone of hemolysis, have been performed. They speak against the idea that the PFC is either a reservoir of cytophilic antibody or a "background" PFC. Rather they suggest that active antibody secretion is induced in the cell at some defined time point in culture. Detailed kinetics of the rate of appearance of plaques in peritoneal cell cultures revealed an exponential phase lasting from about 3 to about 13 hr with a doubling time of 2 hr. The reasons for this are not known. A greatly heightened reactivity was shown in peritoneal cells of mice that had been pregnant several times. Cultures of such cells showed more rapid plaque appearance and a peak activity about 20 times higher than with cells from young male mice. Cultures in which 1 cell in 10 formed a plaque were not infrequent. A series of experiments on germ-free mice showed reactivity similar to that of conventional mice from the same strain and source. The significance of the findings for cellular immunology are discussed

The City Lost and Found: New Perspectives on Urban Representation and Activism, 1960-1980

The city of the 1960s and 1970s was in crisis. Buildings were on fire. Massive highways threatened vibrant neighborhoods. Citizens were taking to the streets in protest. In short, the future of the city was anything but assured. From the pages of Life magazine to municipal reports, these decades witnessed a widespread recognition of forces that threatened to destroy the social and physical fabric of American cities. Nowhere was this more evident than in the country’s three largest cities: New..

The City Lost and Found : nouvelles perspectives sur la représentation urbaine et l’activisme entre 1960 et 1980

Dans les années 1960 et 1970, la ville était en crise. Les bâtiments étaient en proie aux flammes. Des autoroutes gigantesques menaçaient d’éventrer des quartiers animés. Les citoyens descendaient dans la rue pour manifester. Pour résumer la situation, l’avenir de la ville semblait plus qu’incertain. Que l’on consulte les articles du magazine Life ou les rapports municipaux, ces décennies ont été marquées par l’essor de forces menaçant de détruire le tissu social et physique des villes améric..

Unstable states in QED of strong magnetic fields

We question the use of stable asymptotic scattering states in QED of strong magnetic fields. To correctly describe excited Landau states and photons above the pair creation threshold the asymptotic fields are chosen as generalized Licht fields. In this way the off-shell behavior of unstable particles is automatically taken into account, and the resonant divergences that occur in scattering cross sections in the presence of a strong external magnetic field are avoided. While in a limiting case the conventional electron propagator with Breit-Wigner form is obtained, in this formalism it is also possible to calculate $S$-matrix elements with external unstable particles.Comment: Revtex, 7 pages. To appear in Phys. Rev. D53(2

Chasing the immortal strand: evidence for nature's way of protecting the breast genome

Mutations arise during cell division at a predictable rate. Besides DNA repair mechanisms, the existence of cellular hierarchies that originate with a stem cell serve to reduce the number of divisions necessary for normal physiology. In a previous issue, Bussard and colleagues demonstrate that mammary stem cells have an additional remarkable trait; namely the ability to selectively retain a template DNA strand during self renewal. In doing so, they avoid the accumulation of mutations in that so called 'immortal strand'. The implications of this are discussed with reference to the development and treatment of cancer

BVRI Light Curves for 29 Type Ia Supernovae

BVRI light curves are presented for 27 Type Ia supernovae discovered during the course of the Calan/Tololo Survey and for two other SNe Ia observed during the same period. Estimates of the maximum light magnitudes in the B, V, and I bands and the initial decline rate parameter m15(B) are also given.Comment: 17 pages, figures and tables are not included (contact first author if needed), to appear in the Astronomical Journa