Gravitational Attraction

A little while ago the European Space Agency launched GOCE, its gravity mapping satellite. GOCE stands for Gravity field and steady-state Ocean Circulation Explorer. It is already in orbit but in a couple of weeks its batteries will be fully charged-up, its systems calibrated and its work will begin. Unlike a typical satellite, with spindly aerials, disks, and protruding panels, this satellite is streamlined and sleek. GOCE needs aerodynamic features because it will fly at only 250km above the surface of the Earth, lower than most Earth orbiting satellites, and will encounter wisps of atmosphere. It will plot the earth’s gravitational field to a higher level of accuracy than we are currently able.


GOCE's mission will require a high level of stability, hence the fins and its ion-propulsion engine. See an animation here: www.esa.int/SPECIALS/GOCE_animation/

Things have different weights when in different places. The gravitational attraction an object experiences, its desire to rush to the centre of the earth, is effected by the presence of irregularities in the shape of the earth, the uneven distribution of mass beneath the crust, and the presence of mountains or even large buildings. The intensity of the desire varies and the GOCE will map this variable terrain. It will draw a surface of equal gravitational force, sometimes called the geoid, as it dips above and below the Earth's physical surface. A science blogger gives this illustration:

Say you take a one-pound weight, and you walk along the surface of the Earth. If the density of the ground underneath you begins to increase, the gravitational force will increase and the weight will get heavier. To counteract this, you raise the weight up higher - therefore reducing gravity's influence. If, as you walk along, you continue to adjust the height of the weight so that it always weighs one pound, you will be following the surface of the geoid. www.scientificblogging.com/welcome_my_moon_base/goce_aerodynamic_satellite_will_map_geoid

So, those African women carrying supplies on their heads have actually found a way to make the load a fraction lighter. A very small fraction.

One of the most useful outcomes of GOCE’s work will be an increased understanding of the movement of the oceans. But, personally, I am looking forward to the maps. Gravitational forces at play have been mapped before. The difference between standard gravity and the actual gravity found in a certain place is sometimes called the Bouguer anomaly. I examined a Bouguer Anomaly Map of Northern Ireland recently.


From the Bouguer Anomaly map

This is a detail from southern Armagh. It can be seen that Slieve Gullion is a highly positive area. This would probably make a kind of sense to anyone familiar with this mountain. It is a proud and distinguished peak and seems to deserve any form of special attention. Slieve Gullion is an extinct volcano standing apart from other mountains but the highest in the county nonetheless. I wonder if this apartness, free from softening influences, contributes to the large positive Bouguer anomaly wrapped around the mountain. Geologists reckon the strength of this anomaly indicates a high-density mass of rock concealed under the mountain. I can report that, when climbing Slieve Gullion lately, my backpack sure seemed heavier as I approached the peak. But seriously, I wonder could a mountain with a large positive Bouguer anomaly, such as Slieve Gullion, be noticeably more tiring to climb?


Gullion at sunset, image taken from here.

Slieve Gullion’s powerful presence also means it has made frequent appearances in myths, legends, and more recent writings from Northern Ireland. It seems the mountain carries an unusually high cultural weight too. In the meantime we can watch for the GOCE, a fast-moving dot passing 250km above our heads.