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Here we present the design and ground testing for a nanosatellite mission that will demonstrate terrestrial-to-space clock synchronization using a pulsed optical link to a low Earth orbiting nanosatellite. The 1 kg, 1 L nanosatellite payload comprises fully redundant chip-scale atomic clocks, microprocessor-based clock counters, picosecond event timers, and avalanche photodetectors. During flight operations, an experimental satellite laser ranging facility, located at the Kennedy Space Center in Florida, will emit nanosecond optical pulses of infrared laser light towards the nanosatellite. By reflecting the emitted pulses off of a retroreflector array mounted on the nadir face of the nanosatellite and returning the pulses back to the ground, the laser ranging facility will record the round-trip light-travel time of the laser pulses. At the same time, one of the avalanche photodetectors will record the arrival time of the pulses at the nanosatellite. By combining these data, the discrepancy between the ground and space clocks can be determined, in addition to the range to the satellite. Laboratory testing of the space instrument indicates a short term time-transfer precision of less than 200 ps, equivalent to a range accuracy of 6 cm. This flight instrument will comprise roughly 1U of a 3U CubeSat mission manifested for launch in the Fall of 2017 through NASA’s CubeSat Launch Initiative program.

fundamental physics test, Kennedy-Thorndike, clocks, ultra-stable cavities, iodine spectroscopy, space instrumentation

The gravity tractor concept is a proposed method to deflect an imminent asteroid impact through gravitational tugging over a time scale of years. In this study, we present mission scenarios and operational considerations for asteroid mitigation efforts involving multiple gravity tractors. We quantify the deflection performance improvement provided by a multiple gravity tractor campaign and assess its sensitivity to staggered launches. We next explore several proximity operation strategies to accommodate multiple gravity tractors at a single asteroid including formation-flying and mechanically-docked configurations. Finally, we utilize 99942 Apophis as an illustrative example to assess the performance of a multiple gravity tractor campaign.

The UV LED mission demonstrates the precise control of the potential of electrically isolated test masses that is essential for the operation of space accelerometers and drag free sensors. Accelerometers and drag free sensors were and remain at the core of geodesy, aeronomy, and precision navigation missions as well as gravitational science experiments and gravitational wave observatories. Charge management using photoelectrons generated by the 254 nm UV line of Hg was first demonstrated on Gravity Probe B and is presently part of the LISA Pathfinder technology demonstration. The UV LED mission and prior ground testing demonstrates that AlGaN UV LEDs operating at 255 nm are superior to Mercury vapor lamps because of their smaller size, lower draw, higher dynamic range, and higher control authority. We show flight data from a small satellite mission on a Saudi Satellite that demonstrates AC charge control (UV LEDs and bias are AC modulated with adjustable relative phase) between a spherical test mass and its housing. The result of the mission is to bring the UV LED device Technology Readiness Level (TRL) to TRL 9 and the charge management system to TRL 7. We demonstrate the ability to control the test mass potential on an 89 mm diameter spherical test mass over a 20 mm gap in a drag free system configuration. The test mass potential was measured with an ultra high impedance contact probe. Finally, the key electrical and optical characteristics of the UV LEDs showed less than 7.5 percent change in performance after 12 months in orbit., Comment: 22 pages, 21 figures

The proposed space mission mini Space-Time Asymmetry Research (mSTAR) aims at a test of special relativity by performing a clock-clock comparison experiment in a low-Earth orbit. Using clocks with instabilies at or below the 1·10−15 level at orbit time, the Kennedy-Thorndike coefficient will be measured with an up to two orders of magnitude higher accuracy than the current limit set by ground-based experiments. In the current baseline design, mSTAR utilizes an optical absolute frequency reference based on molecular iodine and a length-reference based on a high-finesse optical cavity. Current efforts aim at a space compatible design of the two clocks and improving the long-term stability of the cavity reference. In an ongoing Phase A study, the feasibility of accommodating the experiment on a SaudiSat 4 bus is investigated.

T HE Lambert problem offers a substantial way of determining theminimumenergy transfer between twoknownpoints along a Keplerian orbit. Most of the analysis for this problem relies on a geometrical approach, since the problem’s definition is attuned to the geometry [1,2]. The main idea of the Lambert minimum-energy problem starts by defining the length between two position vectors. Consequently, it states geometrically that the semimajor axis of the minimum-energy orbit is related to the chord length and length of the position vectors [1]. Currently, there is no other analytical approach besides the geometric analysis for solving the problem. In this note, however, an alternative analytical method for solving the Lambert minimum-energy problem is proposed. Theminimum velocity at the initial position is obtained by applying a constrained optimization tool. As the initial position vector in the problem is fixed, it is apparent that determining theminimum initial velocity is the same as obtaining theminimum-energy orbit. Using the alternative technique could give us new insight into solving various orbital problems.

This paper presents results of a program to investigate the magnitude, origin and parametric variations of destabilizing forces which arise in high power turbines due to blade-tip leakage effects. Five different unshrouded turbine configurations and one configuration shrouded with a labyrinth seal were tested with static offsets of the turbine shaft. The forces along and perpendicular to the offset were measured directly with a dynamometer, and were also inferred from velocity triangles and pressure distributions obtained from detailed flow surveys. These two routes yielded values in fair agreement in all cases. For unshrouded turbines, the cross-forces are seen to originate mainly (∼2/3) from the classical Alford mechanism (nonuniform work extraction due to varying blade efficiency with tip gap) and about 1/3 from a slightly skewed hub pressure pattern. The direct forces arise mainly (3/4) from this pressure pattern, with the rest due to a slight skewness of the Alford mechanism. The pressure nonuniformity (lower pressures near the widest gap) is seen to arise from a large-scale redistribution of the flow as it approaches the eccentric turbine. The cross-forces are found to increase substantially when the gap is reduced from 3.0% to 1.9% of blade height, probably due to viscous blade-tip effects. The forces also increase when the hub gap between stator and rotor decreases. The force coefficient decreases with operating flow coefficient. In the case of the shrouded turbine, most of the forces arise from nonuniform seal pressures. This includes about 80% of the transverse forces. The rest appears to come from uneven work extraction (Alford mechanism). Their level is about 50% higher than in the unshrouded cases.