Passive gravitational Imaging

We tend to assume in physics that bodies like planets give off perfectly curved fields, smooth arcs and curves that are mathematically precise, in reality this is not true, land masses such as mountain ranges and seas have there effect on the gravity ‘felt’ above them, so does the composition of the rock, a light continental granite will have less pull than an iron rich oceanic basalt. Though these effects may destroy the accuracy of our models, these effects are also of great benefit, and from these variations in field strength gravitational imaging was developed.

            Gravitational imaging allows a map to be drawn up of the irregularities in the gravitational field of a body, and also give clues to the composition and structure of the examined body. There are two main method used in imaging, one uses a single instrument to judge the field strength, this method is called relative measurement, the other uses more than one sensor and is called absolute measurement, the latter is more useful and more precise, but the former is good for measuring field strength of distant objects.

            In the heart of all the sensors used in these two methods is a material not dissimilar to driver coil material, when gravitational field is applied across it a physical property of the material changes, usually a pd forms across the material in parallel to the applied field. With calibration this can become a sensitive and accurate sensor of gravitational field, which can not only determine strength but also direction.

            Relative measurement uses one of these sensors, and can be used in two main ways. The first is for sweeping across areas of the sky, readings will change if the sensors passes over a region of increased matter density, this is very useful for detecting gas density within the galaxy and in interstellar space, and also useful for the detection of dark cold matter, or matter that is otherwise tricky to observe (phase space etc). The other way in which a single sensor can be used is to place it in orbit around the body of interest, irregularities in the field will be detected as a change of gravitational strength by the sensor, and assuming the orbit remains a constant radius useful ideas about density of the body can be measured. However the accuracy of this second method is limited by the fact that a change in gravitational field strength will alter the orbit of the sensor slightly, and so there is a loss of accuracy, this is where absolute measurement is useful.

            Absolute measurement uses more than one sensor, and because of this it can be used in a way that provides far more accurate mapping of the body. Several sensors are placed in similar orbits around the body, as well as measuring the field strength, they also measure with high accuracy the distance they are from each other. By comparing distances they can factor the perturbation of their orbits by the irregularities in the field, and give far more accurate measurements of field irregularity.

            Gravitational imaging provides cartographers and geographers with useful information about the structure and likely composition of the studied objects, it also provides mapping vehicles with an easy way of determining whether an icy body is made up of fragments of rocky material that might not be visible from the surface, or for working out the history of such an object, whether it has been shattered, or had been heavily bombarded in its history.

            This technology doe however have some fundamental limitations, although increased sensor size improves accuracy, the mass of such a sensor does begin to interfere with the readings taken. Also the sensors are prone to fluctuate as other bodies, such as moons, or even nearby stars, add their own gravitational field into the sensor. Some of these problems can be resolved using a different technique called active gravitational imaging.