Combining Electric and Sail Propulsion for Interplanetary Sample Return

Fast sample return from the outer Solar System would open an entirely new avenue for space science, but the vast distances make this a daunting task. The achievable transit velocity and the need for extra propellant on the return trip limit the feasibility of returning extraterrestrial samples to Earth. To keep the mission duration short enough to be of interest, sample return from objects farther out in the Solar System requires increasingly higher velocities. High specific impulse, electric propulsion reduces the propellant required for the outbound and return trips, but decelerating the spacecraft at the inner Solar System from high velocity still involves a long, inward spiral trajectory. The use of solar sails to rapidly decelerate incoming sample capsules and eliminate propellant is explored in this paper. The sail is essentially a “solar parachute ” used for braking at the end of the interplanetary return flight, permitting a higher transit speed and truncating the deceleration spiral. In this application the sail is relatively small and manageable since only the sample capsule and its sail are decelerated. A comparison is made between using all-electric propulsion versus combining electric propulsive acceleration with sail deceleration for sample return from the distances of Saturn, Uranus, and Pluto. Solar-sail braking dramatically reduces the return flight time by one-third or more compared to using electric rocket deceleration. To elucidate the technology requirements, wide ranges for both the loaded sail density and electric propulsion specific mass are considered in this initial parametric study

Radioisotope Electric Propulsion for Robotic Science Missions to Near-Interstellar Space

Abstract The use of radioisotope electric propulsion for sending small robotic probes on fast science missions several hundred astronomical units (AU) from the Sun is investigated. Such missions would address a large variety of solar, interstellar, galactic and cosmological science themes from unique vantage points at 100 to 600 AU, including parallaxdistance measurements for the entire Milky Way Galaxy, sampling of the interstellar medium and imaging of cosmological objects at the gravitational lens foci of the Sun (2 550 AU). Radioisotope electric propulsion (REP) systems are low-thrust, ion propulsion units based on multi-hundred watt, radioisotope electric generators and ion thrusters. In a previous work, the flight times for rendeavous missions to the outer planets (< 30 AU) using REP were found to be less than fifteen years. However fast prestellar missions to several hundred AU are not possible unless the probe's energy can be substantially increased in the inner Solar System so as to boost the final hyperbolic excess velocity. In this paper an economical hybrid propulsion scheme combining chemical propulsion and gravity assist in the inner Solar System and radioisotope electric propulsion in the outer Solar System is studied which enables fast prestellar missions. Total hyperbolic excess velocities of 15 AU/year and flight times to 550 AU of about 40 years are possible using REP technology that may be available in the next decade. *StafT Sdcntist, Accelerator Phyakr Department

Inductive and Electrostatic Acceleration in Relativistic Jet-Plasma Interactions

We report on the observation of rapid particle acceleration in numerical simulations of relativistic jet-plasma interactions and discuss the underlying mechanisms. The dynamics of a charge-neutral, narrow, electron-positron jet propagating through an unmagnetized electron-ion plasma was investigated using a three-dimensional, electromagnetic, particle-in-cell computer code. The interaction excited magnetic filamentation as well as electrostatic plasma instabilities. In some cases, the longitudinal electric fields generated inductively and electrostatically reached the cold plasma wave-breaking limit, and the longitudinal momentum of about half the positrons increased by 50% with a maximum gain exceeding a factor of 2 during the simulation period. Particle acceleration via these mechanisms occurred when the criteria for Weibel instability were satisfied.Comment: Revised for Phys. Rev. Lett. Please see publised version for best graphic

The rational development of molecularly imprinted polymer-based sensors for protein detection.

The detection of specific proteins as biomarkers of disease, health status, environmental monitoring, food quality, control of fermenters and civil defence purposes means that biosensors for these targets will become increasingly more important. Among the technologies used for building specific recognition properties, molecularly imprinted polymers (MIPs) are attracting much attention. In this critical review we describe many methods used for imprinting recognition for protein targets in polymers and their incorporation with a number of transducer platforms with the aim of identifying the most promising approaches for the preparation of MIP-based protein sensors (277 references)

Seasonal Dynamics and Defoliation Impact on Herbage Yield in Aspen Boreal Habitats of Alberta

Within the aspen boreal ecosystems, little information exists on the seasonal dynamics of available herbage and the effects of varying defoliation regimes on accumulated herbage growth and associated opportunities for animal production. We examined seasonal changes in herbage phytomass in conjunction with defoliation treatments in Bromus inermis-Poa pratensis grasslands in central Alberta. Changes in herbage pools were examined by sampling at five monthly intervals from April to September 1997 and 1998, inclusive. Vegetation was also subjected to a factorial experiment with an initial defoliation in late-May, June or July, at heights of 2.5, 7.5, or 15 cm, and repeated at 3-, 6- or 9-week intervals until the end of September. Green herbage, standing dead and fallen litter increased from spring to summer and decreased from summer to fall. Average growing conditions resulted in a peak phytomass of 350 g m-2, and varied by year. Weathering losses of green herbage, standing dead and fallen litter over winter were 34%, 52% and 51%, respectively. Dry matter losses of total herbage (all three pools) over winter were 58% of 1997 summer green phytomass. Initial timing, height, and frequency of clipping all affected accumulated herbage yield (P < 0.001). The greatest accumulated herbage yield was from clipping initiated in May. Light clipping resulted in less phytomass accumulation relative to moderate and heavy clipping. Clipping frequencies of six and nine weeks resulted in similar phytomass removal, but were greater than herbage removal associated with three weeks frequency. The interaction between clipping date and frequency of clipping demonstrates the importance of temporal rest and sensitivity of forage plants to defoliation, and lends support to the use of rotational grazing systems

Peto's paradox and human cancers

Peto's paradox is the lack of the expected trend in cancer incidence as a function of body size and lifespan across species. The leading hypothesis to explain this pattern is natural selection for differential cancer prevention in larger, longer lived species. We evaluate whether a similar effect exists within species, specifically humans. We begin by reanalysing a recently published dataset to separate the effects of stem cell number and replication rate, and show that each has an independent effect on cancer risk. When considering the lifetime number of stem cell divisions in an extended dataset, and removing cases associated with other diseases or carcinogens, we find that lifetime cancer risk per tissue saturates at approximately 0.3-1.3% for the types considered. We further demonstrate that grouping by anatomical site explains most of the remaining variation. Our results indicate that cancer risk depends not only on the number of stem cell divisions but varies enormously (approx. 10 000 times) depending on anatomical site. We conclude that variation in risk of human cancer types is analogous to the paradoxical lack of variation in cancer incidence among animal species and may likewise be understood as a result of evolution by natural selection