The Delivery of Water During Terrestrial Planet Formation

The planetary building blocks that formed in the terrestrial planet region were likely very dry, yet water is comparatively abundant on Earth. Here we review the various mechanisms proposed for the origin of water on the terrestrial planets. Various in-situ mechanisms have been suggested, which allow for the incorporation of water into the local planetesimals in the terrestrial planet region or into the planets themselves from local sources, although all of those mechanisms have difficulties. Comets have also been proposed as a source, although there may be problems fitting isotopic constraints, and the delivery efficiency is very low, such that it may be difficult to deliver even a single Earth ocean of water this way. The most promising route for water delivery is the accretion of material from beyond the snow line, similar to carbonaceous chondrites, that is scattered into the terrestrial planet region as the planets are growing. Two main scenarios are discussed in detail. First is the classical scenario in which the giant planets begin roughly in their final locations and the disk of planetesimals and embryos in the terrestrial planet region extends all the way into the outer asteroid belt region. Second is the Grand Tack scenario, where early inward and outward migration of the giant planets implants material from beyond the snow line into the asteroid belt and terrestrial planet region, where it can be accreted by the growing planets. Sufficient water is delivered to the terrestrial planets in both scenarios. While the Grand Tack scenario provides a better fit to most constraints, namely the small mass of Mars, planets may form too fast in the nominal case discussed here. This discrepancy may be reduced as a wider range of initial conditions is explored. Finally, we discuss several more recent models that may have important implications for water delivery to the terrestrial planets.     

Electromagnetic ion cyclotron resonance heating in the VASIMR

Plasma physics has found an increasing range of practical industrial applications, including the development of electric spacecraft propulsion systems. One of these systems, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine, both applies several important physical processes occurring in the magnetosphere. These processes include the mechanisms involved in the ion acceleration and heating that occur in the Birkeland currents of an auroral arc system. Auroral current region processes that are simulated in VASIMR include lower hybrid heating, parallel electric field acceleration and ion cyclotron acceleration. This paper will focus on using a physics demonstration model VASIMR to study ion cyclotron resonance heating (ICRH). The major purpose is to provide a VASIMR status report to the COSPAR community. The VASIMR uses a helicon antenna with up to 20 kW of power to generate plasma. This plasma is energized by an RF booster stage that uses left hand polarized slow mode waves launched from the high field side of the ion cyclotron resonance. The present setup for the booster uses 2–4 MHz waves with up to 20 kW of power. This process is similar to the ion cyclotron heating in tokamaks, but in the VASIMR the ions only pass through the resonance region once. The rapid absorption of ion cyclotron waves has been predicted in recent theoretical studies. These theoretical predictions have been supported with several independent measurements in this paper. The ICRH produced a substantial increase in ion velocity. Pitch angle distribution studies show that this increase takes place in the resonance region where the ion cyclotron frequency is equal to the frequency on the injected RF waves. Downstream of the resonance region the perpendicular velocity boost should be converted to axial flow velocity through the conservation of the first adiabatic invariant as the magnetic field decreases in the exhaust region of the VASIMR. In deuterium plasma, 80% efficient absorption of 20 kW of ICRH input power has been achieved. No evidence for power limiting instabilities in the exhaust beam has been observed.     

Methodology for Multiaircraft Minimum Noise Impact Landing Trajectories

The problem of optimizing landing trajectories with respect to noise impact index is addressed. In contrast with previous work, multilandings are considered. In order to make the problem tractable the trajectories are specified in functional form with certain parameters left free for selection by the optimization methods. Constraint of aircraft dynamic behavior, trajectory separation, pilot workload, passenger comfort, and maximum noise intensity all enter into the determination of what is an allowable trajectory. A version of the quasi-Newton iterative procedure is used to determine the optimum parameter values. The results show improvement in noise impact to the airport considered and the potential for even greater improvement at many airports.     

A Simple Formula for Radar Detection Probability on Swerling I Targets

A formula is presented for the detection probability on a single scan of a Swerling I target. The formula does not use the Gaussian probability function and is accurate to 1.5 dB for the integration between 10 to 1000 pulses and for false alarm probabilities between 0.693 × 10-3 and 0.693 × 10-6.     

Snow-covered lake ice in GPS multipath reception – Theory and measurement

Speculary reflected signals from the ground can significantly affect the performance of Global Positioning System receivers. For this type of multipath condition, the received powers are primarily the sum of the speculary reflected and direct signals. These reflected signals can provide useful information about the land-surface composition. In this paper, we discuss the special case of a snow-covered frozen lake, with incident energy at 1.57542 GHz with right-hand circularly polarization at elevation angles between 2° and 40°. The relative received powers are computed and measured for various thicknesses of lake ice. The received powers for both theory and measurement have the same behavior throughout a range of elevation angles. The potential for inferring lake ice thickness is explored for a snow-covered lake ice case study.