We make the case for the early development of a Mid-Frequency-Band (MFB) gravitational wave (GW) observatory in geosynchronous orbit (73,000 km arm), optimized for the frequency band 10 mHz to 1 Hz. MFB bridges the science acquisition frequencies between the ground observatories LIGO/VIRGO (4/3 km arm - as well as future planned ones 10/40 km arm), and the milli-hertz band of LISA (2.5 Gm arm)- with usable sensitivity extending to 10 Hz. We argue that this band will enable the timely development of this game-changing field of astrophysics, with observations of medium mass Binary Black Holes (BBH) and Binary Neutron Stars (BNS) sources prior to their mergers in the LIGO frequency range as well as Extreme Mass Ratio Inspirals (EMRI)s and mergers of supermassive BBH within the main detection band. MFB is better placed than LISA to access this exciting frequency region.

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

We present a method for 3D sub-nanometer displacement measurement using a set of differential optical shadow sensors. It is based on using pairs of collimated beams on opposite sides of an object that are partially blocked by it. Applied to a sphere, our 3-axis sensor module consists of 8 parallel beam-detector sets for redundancy. The sphere blocks half of each beam's power in the nominal centered position, and any displacement can be measured by the differential optical power changes amongst the pairs of detectors. We have experimentally demonstrated a displacement sensitivity of 0.87nm/Hz at 1 Hz and 0.39nm/Hz at 10 Hz. We describe the application of the module to the inertial sensor of a drag-free satellite, which can potentially be used for navigation, geodesy and fundamental science experiments as well as ground based applications.

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.

A Drag-Free CubeSat mission has been proposed to demonstrate the feasibility of a Gravitational Reference Sensor (GRS) with an optical readout for a 3 units (3U) spacecraft. A purely drag-free object is defined by the absence of all external forces other than gravity, which are shielded by the spacecraft. In a real case, the TM will still be affected by disturbances. Several of them are passively reduced by the design of the TM housing. This system is a thick-walled aluminium box that holds the shadow sensors and shields the TM. The housing has an effect on the mechanical, thermal and magnetic environment around the TM. All of them have been analysed. The mechanical vibrations have to fit the launch environment and the modes have to be outside of the measurement range (0.0001 - 1 Hz). The magnetic field has to be reduced by a 0.01 factor. The temperature difference between internal opposing surfaces, determining pressure on the TM, has to be below 10^-3(1 mHz/f)1/3 K Hz^-1/2. The housing, together with the TM, the sensors and the UV LEDs for charging control, constitutes the GRS, which would then fit into a 1U. The other 2Us are occupied by the caging mechanism that constraints the TM during launch, the thrusters, the Attitude Determination And Control System (ADACS) and the electronics. The Drag-Free CubeSat will be the result of the combined efforts of Stanford, University of Florida, KACST and NASA and will be the first drag-free mission with an optical readout and the first GRS designed within the limits of a 3U small satellite. In the first section, this paper briefly updates on the main characteristics and systems of the project. Particular emphasis is then given to the recently designed housing, its expected performance and the open issues.

Two key trends have the potential to revolutionize the way humans conduct spaceflight, namely, the miniaturization of satellites (e.g., microand nano-satellites) and the distribution of payload tasks among multiple coordinated units (e.g., spacecraft formation-flying, on-orbit servicing/robotics, fractionation, swarms). The combination of these approaches is leading to a new generation of space architectures, so-called distributed space systems, which promise breakthroughs in space science, planetary science, and exploration. Based on the experience gained from the most recent flight demonstrations of autonomous spacecraft rendezvous and formation-flying (e.g., PRISMA, TanDEM-X), this paper describes a first-ofa-kind space technology and science program under implementation by the KACST-Stanford’s Center for Excellence in Aeronautics and Astronautics in collaboration with national and international partners in the time frame of 10 years. The ultimate goals are to 1) develop a multi-purpose reconfigurable precise distributed space system based on microsatellite platforms, and 2) demonstrate its unprecedented capabilities in areas such as earth gravimetry and system dynamics, gravitational waves detection, and on-orbit servicing.

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.

Precise control over the potential of an electrically isolated proof mass is necessary for the operation of devices such as a Gravitational Reference Sensor (GRS) and satellite missions such as LISA. We show that AlGaN UV LEDs operating at 255 nm are an effective substitute for Mercury vapor lamps used in previous missions because of their ability to withstand space qualification levels of vibration and thermal cycling. After 27 thermal and thermal vacuum cycles and 9 minutes of 14.07 g RMS vibration, there is less than 3% change in current draw, less than 15% change in optical power, and no change in spectral peak or FWHM (full width at half maximum). We also demonstrate UV LED stimulated photoemission from a wide variety of thin film carbide proof mass coating candidates (SiC, Mo2C, TaC, TiC, ZrC) that were applied using electron beam evaporation on an Aluminum 6061-T6 substrate. All tested carbide films have measured quantum efficiencies of 3.8-6.8*10^-7 and reflectivities of 0.11-0.15, which compare favorably with the properties of previously used gold films. We demonstrate the ability to control proof mass potential on an 89 mm diameter spherical proof mass over a 20 mm gap in a GRS-like configuration. Proof mass potential was measured via a non-contact DC probe, which would allow control without introducing dynamic forcing of the spacecraft. Finally we provide a look ahead to an upcoming technology demonstration mission of UV LEDs and future applications toward charge control of electrically isolated proof masses.

A drag-free spacecraft utilizes a Gravitational Reference Sensor (GRS) to shield an internal free-floating test mass (TM) from (a) external disturbances and (b) from disturbances caused by the spacecraft itself. The GRS measures the position of the spacecraft with respect to the TM and a feedback control system commands thrusters to maintain that position. In principle, the test mass is then completely freed from non-gravitational disturbances so that it and its “tender” spacecraft follow a pure geodesic. To date, three drag-free spacecraft have flown: TRIAD I in 1972, which provided the first navigation by satellite, Gravity Probe B in 2004, which tested predictions of Einstein’s general relativity theory, and the 2009 geodesy mission, GOCE (Steady-State Ocean Circulation Explorer). Next generation GRS technology for geodesy, fundamental physics and gravitational wave detection in space, has been under development at Stanford since 2004. Most recently a small scale instrument, called the 1U GRS has been proposed for a 3U CubeSat primarily for Earth aeronomy and geodesy applications. The 1U GRS consists of a 25 mm diameter spherical test mass housed inside a 50 mm cubic cavity. The sphere's position is sensed with a LED-based differential optical shadow sensor, its electric charge is controlled by photoemission using UV LEDs, and the spacecraft position is maintained with respect to the sphere using a cold gas micro-propulsion system. This paper highlights the history, applications, design, and laboratory technology development for this proposed CubeSat mission.

The proposed space mission STAR (SpaceTime Asymmetry Research) aims for significantly improved tests of fundamental space-time symmetry and the foundations of special and general relativity. A series of missions is planned where the first mission focuses on a KennedyThorndike experiment. Later missions will additionally carry out Michelson-Morley experiments as well as precision measurements of gravitational redshift and Local Position Invariance. STAR targets an improvement of at least two orders of magnitude compared to previous experimental accuracy on ground.

This white paper describes a space gravitational wave mission concept consisting of three dragfree spacecraft at the Earth-moon L3, L4, and L5 Lagrange points. It also describes a Gravitational Reference Sensor (GRS) instrument concept, an Interferometric Measurement System (IMS) concept, and enabling technologies for drag-free propulsion, charge management and material coatings. The primary submitter is John W. Conklin for the team members listed above. We are willing to present this concept at the workshop and there is no sensitive or controlled information herein.