This technique for compensating the gravitational attraction experienced by a test-mass freely floating onboard a satellite is new, and solves an important problem that all gravitational wave missions face. Its application to the geostationary Laser Interferometer Space Antenna (gLISA) mission concept addresses and completely solves an important noise source: the gravity-gradient noise.
Each of these masses must be specified in number, and each of them will have to be characterized in magnitude, shape, and location. For the purpose of answering these questions, and to test quantitatively the validity of the solutions through numerical analysis, perfect knowledge of the Comsat mass distribution is assumed, and the one provided to the innovators by Space Systems Loral is adopted. This allows an estimate both of the gravitational acceleration, and the gravity gradient exerted by the Comsat at the TM nominal location, o.
In order to determine the number of compensating masses to be added onboard the Comsat, the gravitational gradient, (gg)ij, is equal to the second partial derivatives of the gravitational potential, V(r), generated by the Comsat.
In order to identify the location and values of the compensating masses, for simplicity, it is assumed that they are of spherical shape and constant mass distribution. In the coordinate system given by the eigenvectors of the gravity gradient, it is then easy to see that the direction, along which the five masses will have to lie, coincides with the three coordinate axes (x, y, z). In particular, if two pairs of spheres on the x and y axes, respectively, are added and located in such a way to “bracket” point o, it should be possible to compensate both the acceleration and gravity gradient components along these two directions. The remaining mass will instead need to be located on the positive z axis in order to counter-balance the negative z component of the gravitational acceleration from the Comsat.
If the distance to the point o of each compensating mass is fixed, one can then solve for the values of the masses (that simultaneously cancel the acceleration and gravity gradient) by solving a nonhomogeneous linear system of five equations in five unknowns. Note that the closer the compensating masses can be to point o, the lighter their values will result. For a typical Comsat design weighing 3,200 kg, it was found that masses ranging between 7 kg and 13 kg would each need to be located about 25 cm away from point o in order to simultaneously cancel the acceleration and gravity-gradient exerted on the TM by the Comsat.
This work was done by Massimo Tinto of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49599
This Brief includes a Technical Support Package (TSP).
Gravitational Compensation Onboard a Comsat
(reference NPO49599) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA, specifically referencing NPO-49599 of NASA Tech Briefs, which discusses advancements in gravitational compensation technology for communications satellites (comsats). It aims to disseminate aerospace-related developments that have broader technological, scientific, or commercial applications.
The primary focus of the document is on a dedicated geosynchronous mission designed for gravitational wave (GW) detection, referred to as the geostationary Laser Interferometer Space Antenna (gLISA). The mission is structured to optimize science data collection while managing costs effectively. Key features of the mission include:
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Cost Reduction Strategies: The mission proposes a single launch using the Falcon 9 rocket, which helps minimize launch costs. It also suggests using a main spacecraft to provide orbit positioning for two free-flyers, simplifying operations and reducing the need for complex propulsion systems.
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Data Collection Efficiency: The mission is designed to maximize science data return, achieving over 90% collection times daily, except during eclipse seasons at equinoxes. To avoid complications during these periods, the payloads will be temporarily shut down for 45 days each spring and fall, allowing for simplified power management.
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Operational Coordination: The main spacecraft will serve as a hub for coordinating activities among the three satellites, facilitating data relay and ground control operations. This streamlined approach is intended to enhance the efficiency of mission operations.
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Mission Duration and Science Output: The planned on-orbit period is four years, with an expectation of yielding three years of science data. The document outlines that routine scheduling could allow for extended detection windows and efficient orbit adjustments during non-science periods.
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Technical Challenges: The document also addresses the challenges associated with achieving the required level of drag-free operation and suppressing gravity-gradient noise, which are critical for the success of the GW detection mission.
Overall, the document emphasizes the importance of tailored mission design to meet the specific needs of gravitational wave research while ensuring cost-effectiveness and operational efficiency. It serves as a resource for understanding the technological advancements and strategic planning involved in the development of the gLISA mission, highlighting NASA's commitment to innovative aerospace research and technology transfer.
