More than 300 years after Newton started thinking about apples, gravity is on Dr. Michael Watkins' mind, too. Dr. Watkins is the project scientist for the Gravity Recovery and Climate Experiment, the Grace mission, getting ready for launch this fall. Grace will make very precise measurements of Earth's gravity field.

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.

Q: How is Grace going to work?

A: To measure gravity, what you really want to do is track a particle in space, namely a satellite, and track it very accurately. That's what Grace is going to do. We're going to track one satellite with another satellite with extremely high precision. The two Grace satellites are going to be about 220 kilometers (137 miles) apart. We're going to be able to measure the distance between them to within 1 micron. That's like measuring the distance between here and San Diego to within the thickness of a red blood cell.

Imagine a large lump of mass on Earth's surface like a big mountain range. We have two satellites coming toward it. The first one senses the mass a little bit before the second one. It's pulled a little towards the mountain range before the second satellite is, changing the separation between the two spacecraft. They continue over the mountain range, and at some point, they're both pulled toward it. Then as they go over, the second satellite will react more strongly. We will sense these separations with a radio link.

It turns out that there are some other things that affect the motion of satellites besides gravity by the way -- things like air drag. Even though the satellites are way up there, you can get a little lump of atmosphere that might cause the separation between the satellites to change a bit. To get rid of that, we'll carry one of the most precise accelerometers that has ever been built. It is so sensitive that it can sense the drag caused by something as small as a particle of smoke.

Q: Once you make these gravity measurements, what are you going to do with them?

A: Grace measurements of Earth's gravity field are going to be 100 times more accurate than those we now have. They are going to revolutionize our understanding of Earth's structure, oceans and climate and how they are changing.

People have jokingly referred to this as "remote sensing without photons." It is the only kind of remote sensing technique that doesn't scatter light or some part of the electromagnetic spectrum off an object but uses gravity directly as a way to measure and learn things about Earth.