Q: Why are you interested in gravity?

A: Two aspects of gravity interest scientists. The first is the static gravity field. It is relatively constant and doesn't change over time, but it does have small variations. Earth is a fairly uniform sphere. Gravity's pull is about the same anywhere on the surface: You weigh about the same anywhere on Earth as you walk around. But if you look more carefully, you find a bit of variation. Grace is going to make very precise measurements of these variations in the gravity field. This information is very important for oceanographers, for example. They want to be able to know how much of the ocean topography they see -- the hills and valleys-- on the ocean's surface is the result of gravity rather than ocean currents.

The second aspect of gravity that interests scientists is the part that it isn't static -- it varies over time. Some very slow processes inside Earth cause the gravity field to change. For example, the polar caps used to be much bigger. The weight of that ice flattened Earth a bit at the poles. Since that ice melted off, the land is now rebounding--Northern Canada and Scandinavia are rising. Earth is becoming more spherical again, and this change is reflected in the gravity field. These changes in gravity over time are one of the very new things that Grace is going to look at. As water moves around the world, Grace is going to be able to track it by mass. We'll be able to measure the depth of an aquifer and actually see sea level changing. We'll be able to weigh the ice sheets. This is a whole new branch of science, and we're just beginning to understand its applications.

Q: Why and how does gravity vary around the Earth?

A: Gravity is the attraction between two masses--it is a function of how big they are and how far apart they are. Earth isn't a perfect sphere--it's bumpy. The density of its mass and the distance to that mass varies as you move around Earth's surface. The effect on gravity is small and has mostly to do with the internal structure of Earth and to some extent with its topography.

There's big gravity low off the coast of India, where there are thought to be the remains of some old mantle features associated with the plate tectonics of India that led it to collide with the Himalayas. There's a big high in the South Pacific, also thought to be due to mantle structures.

Q: How do you measure gravity?

A: The basic way to measure gravity is to drop something and watch its trajectory as it falls. That's Newton's law. There's an instrument called a gravimeter that you can use to measure gravity. You can set it on the ground and measure the gravity right at that point. That works pretty well, but there are only so many places in the world where you can go. That's always a problem and why space is advantageous because it covers so much ground.

The way large-scale gravity has been mapped until now is by looking at the orbital motions of satellites, seeing the effects of gravity as they travel around Earth. In fact, one of the earliest discoveries of artificial satellites was the discovery of what we call J3 -- the pear-shaped part of Earth--the difference in gravity between the northern and southern hemispheres.

Community Email This Article Comment On This Article Related Links SpaceDaily Search SpaceDaily Subscribe To SpaceDaily Express Understanding Time and Space Memory Foam Mattress Review SPACE MEDIA NETWORK PROMOTIONS Solar Energy Solutions Tempur-Pedic Mattress Comparison ... Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News TIME AND SPACE Unified Physics Theory Explains Animals Running Flying And Swimming Durham NC (SPX) Jan 05, 2006 A single unifying physics theory can essentially describe how animals of every ilk, from flying insects to fish, get around, researchers at Duke University's Pratt School of Engineering and Pennsylvania State University have found. The team reports that all animals bear the same stamp of physics in their design.